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History of Operational Use of Weather by U.S. Weather Services. Part I: The Pre-NEXRAD Era

ROGER C. WHITON* AND PAUL L. SMITHϩ Air Weather Service, Scott Air Force Base, Illinois

STUART G. BIGLER , Washington, D.C.

KENNETH E. WILK National Severe Storms Laboratory, Norman, Oklahoma

ALBERT C. HARBUCK# Air Weather Service, Scott Air Force Base, Illinois (Manuscript received 14 March 1997, in ®nal form 19 February 1998)

ABSTRACT The ®rst part of a history of the use of storm surveillance by operational military and civil weather services in the is presented. The meteorological research is long and distinguished but already well described. Hence, this paper and its companion focus on the history of operational radar from its birth in World War II through the introduction of the ®rst two operational Doppler weather radars. This part deals with the pre-Next-Generation era. An appendix to this part contains what is known by the authors about the principal technical characteristics of most of the radars discussed in both parts.

1. Introduction (1962) and Bigler (1981) summarize the history and status of the weather radar program conducted by what This and the companion paper describe the history of was then called the U.S. Weather Bureau; that material the operational use of storm surveillance radars by U.S. is updated and expanded in this paper. Research con- weather services. The papers are based on the experi- ducted by operational weather agencies is discussed ence of some of those who, at various times, have par- here, as are research threads that have found their way ticipated in or led operational weather radar programs. into operational use or have been of great bene®t to The use of radar to observe the weather developed operational radar meteorology. Here we concentrate on as an outcome of the intensive work on radar technology during World War II. The history of those early devel- the history of application of storm detection radar for opments, and of the research aspects of radar meteor- operational purposes, such as severe storm identi®ca- ology, is well described in Hitschfeld (1986), Atlas tion. Length restrictions prevented addressing the his- (1990a), and Rogers and Smith (1996). Bigler et al. tory of the operational use of cloud detection radars, wind pro®lers, and most other clear-air applications, the exception being the widely used single-Doppler clear- air wind measurement technique. Commercial applica- * Current af®liation: Science Applications International Corpora- tions of weather radar could not be covered in the space tion, O'Fallon, Illinois. available; Jorgensen and Gerdes (1951) present a good ϩ Current af®liation: Institute of Atmospheric Sciences, South Da- kota School of Mines and Technology, Rapid City, South Dakota. example. # Current af®liation: Amherst Systems, Inc., Warner Robins, Geor- Initially the security classi®cation attached to radar gia. systems of all kinds limited their use to the military weather services. Later the cost and complexity of these Corresponding author address: Dr. Roger C. Whiton, SAIC, 619 systems limited their operational use to government W. Hwy 50, O'Fallon, IL 62269. agencies, principally the military and civil weather ser- E-mail: [email protected] vices; however, remote displays from weather radar sys-

᭧ 1998 American Meteorological Society

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC 220 WEATHER AND FORECASTING VOLUME 13 tems became affordable at airline weather of®ces, com- TABLE 1. Frequency bands in the electromagnetic spectrum (IEEE mercial weather services, and broadcast weather facil- Standard 521-1984). ities. In the 1960s, increased availability of lighter- Band weight, solid-state electronics made it practical to man- designa- Nominal frequency Nominal wavelength ufacture a storm avoidance radar for use in commercial tion range range (cm) and eventually private aircraft. Some of these systems S 2000±4000 MHz 7.5±15 were adapted for use on the ground. The capabilities of C 4000±8000 MHz 3.7±7.5 aircraft weather radars have steadily grown, and they X 8000±12 000 MHz 2.5±3.7 Ku 12±18 GHz 1.7±2.5 are now widely available. By 1969, a few television K 18±27 GHz 1.1±1.7 stations in the midwestern United States and along the Ka 27±40 GHz 0.7±1.1 southeast coast had installed radars for use in the weath- er segments of their news broadcasts; the trend broad- ened through the 1970s, as ground-based weather radars World War II delayed reporting of important ®ndings became more capable and more affordable. From the until 1945 and later. At the outbreak of the war, the late 1960s through the present time, demand continued maturity of the combatants' radio-location technology for remote radar weather information, originating from differed among themselves only by about two or three radars not under local control. The sophistication of the years. The British work was more advanced than the remote information provided in response to this demand others, largely due to the efforts of Sir Robert A. Wat- grew signi®cantly, from the simple, facsimile-based sys- son-. A Scottish physicist and , Wat- tems of the 1960s to the -based techniques used son-Watt was a fellow of the Royal Meteorological So- today.1 ciety by 1915, published a paper on sferics by 1922, In the late 1980s and 1990s, responding to the suc- and delivered the Symons Memorial Lecture in 1929 on cessful development of techniques for employing sin- ``Weather and Wireless.'' In the ®rst of a number of gle-Doppler weather radar observations, the Department positions he held in the British government from 1915 of Commerce, Department of Defense (DOD), and De- to 1952, Watson-Watt developed crude radio-location, partment of Transportation (DOT) jointly ®elded two direction-®nding devices that could locate thunder- highly sophisticated, ground-based Doppler weather ra- storms based on the sferics they emitted. By 1935, as dar systems, the Next-Generation Weather Radar (NEX- head of the radio department of the National Physical RAD), now called the WSR-88D, and the DOT's Ter- Laboratory, he turned to the problem of radio location minal Doppler Weather Radar. Remote single-radar and of military targets by measuring the distance between multiradar composite data service is provided by value- the transmitter and those targets. In 1935, he started added NEXRAD Imagery Dissemination System investigating the use of electromagnetic waves to locate (NIDS) vendors. Recently, the lower cost of reasonably aircraft, work that in¯uenced the design of Britain's and sophisticated ground-based weather radars, some Dopp- the world's ®rst operational radar system, the Chain ler capable, has made them affordable by a broader Home radars. That system was in place before the Battle range of commercial weather services and broadcast of Britain and is credited with being one of the most weather facilities. Some types of weather radar data are important factors enabling the outnumbered Royal Air available, although not in real time, on the World Wide Force to turn back the over the skies of En- Web. gland early in the war. The letter designators used to designate electromag- Beginning in July 1940, a radar of 10-cm wavelength netic frequency ranges originated because of the need was operated at the Corporation Re- for secrecy during World War II. The band designators search Laboratory in Wembley, England, where Dr. J. were ®nally standardized by the Institute of Electrical W. Ryde worked (Doviak and Zrnic 1993). It is likely and Electronics Engineers in 1984. Those of interest to that the ®rst weather was seen on this radar or meteorology (some are used in this paper) are contained another like it in England, probably in late 1940 or in Table 1. possibly as late as February 1941. Perhaps to explain A table in the appendix of this paper contains what these weather echoes, which might interfere with de- is known by the authors about the principal technical tection of aircraft, Ryde was asked to investigate the characteristics of most of the radars discussed. attenuation and backscattering properties of clouds and rain (Probert-Jones 1990). Ryde reported this wartime work in the open literature later (Ryde 1946). Similar 2. Early origins studies conducted from 1942 to 1944 at the Massachu- The earliest origins of radar meteorology are dif®cult setts Institute of Technology's (MIT) Radiation Labo- to discern because the secrecy surrounding radar in ratory (the Rad Lab, as it was called), largely by Bent (1946), showed that weather could be detected on cer- tain types of radars out to ranges of 150 mi at 3- and 10-cm wavelengths. 1 Terms such as ``today'' and ``currently'' refer to 13 March 1997. During the ®rst half of 1943, Major J. Fletcher of the

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Army Air Forces Weather Service worked at the Rad contacts at universities and laboratories, such as the Rad Lab and, about a year later, established a program for Lab, to help them solve the problem. In this way, prob- use of weather radar within the Army Air Forces Weath- lems could be solved expeditiously and wartime oper- er Service (Wexler and Swingle 1947). Early efforts to ational needs met quickly. Just as important, radar use radar on the ground for operational meteorological weather of®cers' education and training enabled them purposes were of two kinds (Fletcher 1990). On the one to follow and understand research in radar meteorology hand, operational use was made of radars installed for and implement promising results locally. Under these nonweather missions, such as point and area defense, circumstances, local weather radar programs had a high bombing, navigation, gunnery, and aircraft control and technical quality even by today's standards (see, e.g., warning. On the other hand, some radars were dedicated Air Weather Service 1945). To some extent, the re- to or converted for use principally in operational weath- quirement for training in both meteorology and radar er support, such as the radars in weather reconnaissance has continued from World War II until the present day, aircraft and ground-based radars at weather stations. although the mix of educational degrees and technical training has varied. 3. First radar operations at individual stations Beginning in September 1943, air traf®c control and 4. First radar networks used for weather harbor defense radars already installed on the Atlantic surveillance and Paci®c sides of the Isthmus of Panama were used, The ®rst radar network used for weather surveillance on a noninterference basis, for weather surveillance pur- was formed in Panama in April 1944, when weather poses (Best 1973). These were single-station operations. observing and reporting began at two Harbor Defense By 1943, scientists at MIT's Rad Lab, where most radar Cristobal installations, facing the Atlantic Ocean (Best research and development in the United States was con- 1973). In May 1944, the network was augmented by ducted during World War II, had completed a series of using two higher-power aircraft-warning radars on Ta- visits to many of these radar sites. The visits were per- boga Island, near Balboa, on the Paci®c side and Fort formed to determine the effects of the atmosphere on Sherman on the Atlantic. The network took radar weath- radar propagation (today called radio meteorology) and er observations regularly, encoded them in a special to assess the usefulness of these radars in observing and radar reporting code, and transmitted them on Teletype to some extent forecasting atmospheric phenomena (to- communications. These reports were called RAREPs day called radar meteorology). (radar reports), much as present-day observations are. In most of these single-station operations and in the The operations and research activities of the Panama ®rst weather radar networks, an expert trained in both radar network were managed by Lieutenant Myron G. meteorology and radar served as a radar weather of®cer H. ``Herb'' Ligda, who was assigned in February 1944 and had operational, technique development, and re- as the 6th Weather Region's Radar Weather Of®cer. Un- search duties. During the course of World War II, weath- der study at the time were relationships between echo er of®cers received 15 months of intensive training in ``intensity'' and surface visibility, differences between meteorology and other subjects (nine months of mete- echoes over land and those over water, effects of the orology training preceded by six months of preparatory land±sea boundary on the movement of storms, the life studies for those who did not have two years of college cycle of convective storms, the effects of topography mathematics and physics). One hundred graduates from on storm movement and intensity, steering of storms by that program were sent to Harvard University for four winds aloft, and detection of by radar. An early months of intensive training in electrical engineering radar climatology study was completed, showing storm and basic radar theory and then to MIT for three months genesis regions and storm motion patterns in Panama. of training on speci®c radar systems (Atlas 1990b; These investigations were conducted to improve the op- Fletcher 1990). Some were given special familiarization erational value of radar weather information to weather at the Rad Lab. Many of the early leaders of research forecasters and their customers. in radar meteorology had this same education and train- The second weather radar network (located in India) ing. Until 1947, Air Weather Service (AWS), formerly grew by unifying several operations at individual sta- called the Army Air Forces Weather Service, had an tions; that network used radars assigned principally to explicit research mission: if in the conduct of operations performing a weather surveillance function. In the sum- it was found necessary to advance the state of meteo- mer of 1944, the 2nd Weather Reconnaissance Squad- rological knowledge or engineering practice or develop ron's B-25 aircraft were modi®ed to carry the ``radio new techniques to apply that knowledge to customers' set'' AN/APQ-13,2 which was actually an X-band radar. weather support problems, the on-site personnel often had the education, training, and ability to do so locally. When a problem exceeded the depth of their under- standing or capabilities to solve, these radar weather 2 Standardized nomenclature of military (and optionally civil) elec- of®cers had the background to recognize that and the tronics systems grew out of the experience of World War II. The

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The APQ-13 was developed jointly by the Bell Tele- be deployed. Tests conducted in 1946 favored the APQ- phone Laboratories and the MIT Rad Lab. It was man- 13 (Miller 1947), which was then ®elded as the ®rst ufactured in great numbers by , the widely distributed ground-based radar used for storm manufacturing arm of the Bell System of the day. It was detection. As is often the case with interim solutions, soon realized that the APQ-13, although originally de- the APQ-13 continued in operation at many military signed as a bombing and navigation system for the B- base and post weather stations until a follow-on weather 17, B-24, B-25, and B-29, could readily detect weather. radar was deployed in the 1960s. At the height of the Thereafter, the APQ-13 was often used for storm de- APQ-13 program, AWS had more than 60 in operation tection. In the fall of 1944, the second Weather Recon- at base and post weather stations worldwide. In October naissance Squadron was deployed to a location near 1977, the last operational APQ-13 was removed from Calcutta, India. The aircraft radar worked well for storm the Fort Sill, Oklahoma, post weather station. It was detection, but it was dif®cult to use the heavily tasked shipped to the Air Force Museum at Wright-Patterson weather reconnaissance aircraft and aircrews to stand Air Force Base (AFB), Ohio, which intended to display watch for fast-moving, squall-line-type systems called it in its original con®guration as a bombing and navi- nor'westers, for which operational warnings were de- gation radar. The museum indicated that it would de- sired. To meet this challenge, some APQ-13s were mod- scribe, in lineage notes, not only the radar's history as i®ed for ground use and installed on towers at weather a bombing and navigation system but also its much stations. A single-station operation was tried success- longer history as an operational weather radar, even fully at Guskara, India. Radars were added at Chabua though not built for the purpose. (June 1945) and Tezgoan (July 1945) near the Burma± Meanwhile, the Weather Bureau, now called the Na- India border to form a network covering the Assam tional Weather Service (NWS), obtained 25 AN/APS- Valley (Best 1973). Radar weather observations were 2F aircraft radars from the in 1946. These radars taken, recorded, and transmitted so that weather fore- had S-band wavelengths, so attenuation by rain was casters could synthesize storm data from multiple radars almost entirely avoided (Atlas and Banks 1951); how- and brief aircrews ¯ying in the ``little hump'' area of ever, detection of rain and snow was minimal due the China±Burma±India theater. to system performance limitations. The radars were Early use of radars individually and in networks for modi®ed for meteorological use and put into operation weather detection led to recognition of many basic fea- at a rate of about ®ve per year. The modi®cations were tures of storm structure and organization and of the performed by the Weather Bureau, which called the value of the information for operational purposes (e.g., modi®ed APS-2F radars WSR-1s, -1As, -3s, and -4s. Maynard 1945). These wartime successes set the stage These early WSR-series systems were all basically the for postwar use of radar in meteorology and the accom- same radar, differing among themselves principally in panying growth of the new science of radar meteorol- their indicators and controls. The WSR-1 had a plan ogy. position indicator (PPI, a horizontal, maplike display of echoes) and an A-scope (display of echo amplitude vs range or time), along with receiver and indicator ele- 5. Post-war use of World War II radars at ments, arranged vertically in a tall, narrow rack. The weather stations WSR-3 and -4 indicators were arranged in a three-panel In 1945, with development of the ®rst true weather horizontal console, and both these radars had a PPI, an radar already under way (see next section), it was de- A-scope, and a range±height indicator (RHI, a display cided to test whether the widely available APQ-13, or of echoes in vertical cross section). The WSR-1A con- an alternative, the AN/APS-10, would be more effective sisted of the receiver±indicator assembly of the WSR- as an interim radar until the ®rst weather radars could 3 plus the A-scope of the WSR-1, arranged in the ver- tically oriented rack of the WSR-1. The WSR-4 was essentially a WSR-3 with a traveling wave tube assem- bly that improved the system sensitivity. Modi®cations nomenclature is positional. The two letters before the slash, often dropped, indicate army, air force, and navy. The ®rst letter after the included replacing the small, aircraft-mountable slash indicates the class of installation or platform [AÐairborne, CÐ with a larger one and adding a power converter to permit air-transportable (no longer used), FÐground ®xed, TÐground trans- operation on conventional power (V. Rockney 1997, portable]. The second letter indicates the type of equipment (MÐ personal communication). The ®rst of these radars was meteorological, nonradar; PÐradar). The third letter indicates the commissioned at Washington, D.C. (Washington Na- purpose of the system (NÐnavigational aids; QÐspecial or combi- nation of several purposes; RÐreceiving such as passive detection tional Airport), on 12 March 1947 (National Climatic systems; SÐdetecting or establishing azimuth and range, i.e., search; Data Center 1997, personal communication). On 1 June TÐtransmitting). This is not an exhaustive list. The Federal Aviation 1947, the second such radar installed at a Weather Bu- Administration (FAA) uses the military nomenclature for radars it reau of®ce was commissioned at Wichita, Kansas, in shares with the military but has developed its own naming system (ARSRÐair route surveillance radar, ASDEÐairport surface detec- the heart of ``tornado alley.'' In May 1949, reports from tion equipment, ASRÐairport surveillance radar, and PARÐpreci- the Wichita radar were used to help guide an aircraft in sion approach radar). distress and threatened by surrounding severe weather

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC JUNE 1998 WHITON ET AL. 223 to an area free of severe , where it could a5-␮s pulse duration, and good sensitivity to hydro- be landed safely. Only three months after its commis- meteors. It also had a 0.5-␮s short-pulse mode for higher sioning in August 1947, the WSR-1 at Norfolk, Ne- resolution of nearby targets. Its receiver was linear, with braska, had paid for itself in cost-avoidance actions a limited dynamic range. Distance was measured in stat- taken by the Elkhorn Valley power system based on ute miles (mi) rather than nautical miles (n mi). The warnings of approaching electrical storms. These radars antenna, a little less than 8 ft in diameter, required no formed a part of the ¯edgling U.S. Basic Weather Radar radome, and the entire radio frequency (RF) transmitter± Network (see later section on that topic). In the 1950s, receiver package rode on the back of the antenna. These the Weather Bureau added to the number of such radars features precluded the need for trouble-prone rotary in service for storm detection purposes (see section 7). joints and reduced the waveguide and radome losses. Figure 1 shows the console of a CPS-9 radar. 6. Acquisition, deployment, and operational use of Production models were fabricated by Raytheon from the ®rst weather radar 1953 to 1954 and installed at military bases worldwide. CPS-9s were also installed at laboratories such as the The AN/CPS-9 Storm Detection Radar was the ®rst Air Force Cambridge Research Center [later renamed radar designed, developed, and deployed speci®cally for the Air Force Cambridge Research Laboratories use by as a weather observing and short- (AFCRL)], the Air Force Geophysics Laboratory range forecasting tool. Commensurate with the army (AFGL), and the Phillips Laboratory (PL)], and all origins of today's air force weather organizations, the weather training facilities and universities. The original Army Signal Corps was asked to acquire the CPS-9. In plan for employment of the CPS-9 called for a closely 1943±45, the Signal Corps Engineering Laboratories es- spaced network of radars at almost every military tablished a special radar weather section at Evans Signal weather station in the continental United States and a Laboratory, Belmar, New Jersey. That group designed limited number at military bases overseas. In fact, 56 the CPS-9 to meet requirements developed by the Army CPS-9s were produced for all services combined (Wil- Air Forces Weather Service. Some of those requirements liams 1953), and less than 50 went into operational use placed limits on the size and cost of the system; for in the Air Force; APQ-13s had to be kept in operation example, one requirement was that the radar should be at facilities that did not receive a CPS-9. The ®rst op- able to travel in a single C-54 aircraft. Studies were erational CPS-9 was installed at Maxwell AFB, Ala- conducted to determine the utility of wavelengths from bama, on 20 June 1954; that radar remained operational 1 to 10 cm in detecting precipitation and the availability for 30 yr before ®nally being replaced on 14 July 1984 of components at those wavelengths. It was by a more modern radar, the AN/FPS-77 (Fuller 1990a). found that, within the desired wavelength range, com- ponents were available only in the X band and S band In 1966 the CPS-9 was modi®ed by addition of the (Air Weather Service 1955). Developing components at Calibrated Echo Intensity Control (CEICON). This de- other wavelengths would have protracted development vice allowed insertion of known amounts of attenuation of the radar for an additional two to three years, so the into the receiver ampli®cation chain until the apparent choice of wavelength for the CPS-9 was reduced to two. radar echo amplitude matched a power reference line While it was possible to meet the technical requirements on the A-scope. This procedure enabled estimation of for radar resolution using an S-band radar, such a sys- the average power backscattered from weather targets tem, if produced, would have exceeded the limits on and thus the radar re¯ectivity factor of those targets. size, weight, and power. Accordingly, an X-band wave- The method was adapted from a similar technique de- length was selected for the CPS-9, thus determining the veloped for Weather Bureau radars by Bigler and Brooks other characteristics of the radar and its cost. (1963) and based upon concepts ®rst described in Lan- Development models of the CPS-9 were produced by gille and Gunn (1948). The quality of measurements the Raytheon Manufacturing Company, Waltham, Mas- taken using the CEICON was limited by the coarseness sachusetts, the same company that would later produce of the attenuator steps, receiver saturation, and other the Weather Bureau's WSR-57 radar and the DOT FAA's problems. In 1970, two AWS CPS-9s were modi®ed by Terminal Doppler Weather Radar. One of the develop- addition of the NWS's video integrator and processor ment models was put in place at MIT's Weather Radar (VIP) (see section 8). It was too late to include the VIP Research Project (Austin and Geotis 1990). Other de- in the acquisition of AWS's next storm detection radar, velopment copies received engineering testing at the the AN/FPS-77. In 1966, AWS still had 40 CPS-9s in Signal Laboratory and service testing by the Air Proving operation. By 1974, the number was reduced to 11. Ground of the air force. The Signal Laboratory also used None are in the operational inventory today. its development copy of the CPS-9 to conduct research Applied research was directed toward the develop- on operational use of the system from 1950 to 1953. ment of techniques for using the CPS-9 operationally. Design of the CPS-9 was re®ned based on the results Technical reports emerging from these efforts were dis- of testing (Williams 1953). tributed by AWS to CPS-9 sites from 1952 to 1955. An The CPS-9, an X-band system, had a 1Њ beamwidth, operator's manual for the CPS-9, written under the di-

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FIG. 1. CPS-9 radar console. rection of Pauline Austin of MIT, was made available from a strong cell. The storm was reported in 1955. by Stout and Huff (1953) and later analyzed in detail The CPS-9 proved particularly adaptable to investi- by Huff et al. (1954). Within two months, two more gations of the properties of synoptic-scale precipitation hook-shaped echoes associated with tornadic storms and cloud systems and development of early short-range were observed and photographed, one at Waco, Texas, forecasting techniques. It was known during design of the other at Worcester, Massachusetts. These three the system that attenuation by intervening rainfall would events provided an answer to the oft-asked question, place serious limitations on use of the radar for quan- ``Would a tornado be identi®able on a radar scope?'' titative measurement of precipitation. It was suspected The prevailing opinion at the time was no, because the also that hail, lying outside the Rayleigh scattering re- tornado itself is small in horizontal dimension. gion for an X-band wavelength, would cause underes- These three severe weather events led to the formation timation of the backscattering cross section of such tar- of a Texas Tornado Warning Network in which com- gets compared to the Rayleigh cross section. The un- munications between Weather Bureau of®ces and local derestimation factor would be much greater for an X- public of®cials were established. Major cities in Texas band radar than for an S-band system at typical hailstone were approached for funds (some from the private sector diameters. Oddly, these characteristics later proved use- and some from the public sector) to modify and install ful to Donaldson (1961), who analyzed pro®les of the the APS-2F, designated the WSR-1, -1A, -3, or -4, in radar re¯ectivity factor with height, with knees in these Weather Bureau of®ces. The Weather Bureau agreed to pro®les indicating the potential for hail and tornadoes. operate and maintain the radars and provide warnings to the public when con®rmed sightings were made. Vol- unteer spotter networks were established. In some cases, 7. Expansion of Weather Bureau radar and a spotter would report a tornado before it was identi®ed warning capabilities by radar, and sometimes identi®cation was made from The 1950s brought not only an expansion of military radar alone (Bigler 1956). The Texas Agricultural and weather radar capabilities, but also a major expansion Mechanical (A&M) Research Foundation handled the of the Weather Bureau's radar systems. On 9 April 1953, funds, arranged for the radar modi®cations to be per- a major tornado occurred in central Illinois north of formed in the laboratories of the Texas A&M University Champaign±Urbana. An Illinois State Water Survey ra- Department of Electrical Engineering, and ensured that dar was being operated for maintenance and test pur- antenna towers were erected and cables installed in poses by its electronics technician, D. Staggs, during Weather Bureau of®ces. Formation of the network began passage of the storm about 25 miles to the north. Staggs at a kickoff meeting held on 24 June 1953 (Kahan 1953); turned on a 35-mm camera and recorded a remarkable approximately six years were required before the net- echo with a hooklike appendage extending southward work attained full strength. About 17 radars were mod-

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC JUNE 1998 WHITON ET AL. 225 i®ed and installed under this joint effort by local gov- sembly. These radars proved to be a major asset for ernment, state, and federal agencies, and a university. tracking hurricanes. Modi®cations made to the radars included a new antenna The extensive damage caused by hurricane-force pedestal to support a 6-ft parabolic re¯ector, a rack- winds and heavy ¯ooding in two consecutive years re- mounted PPI and A-scope, and a ®berglass radome to versed the declining budgetary fortunes of the Weather protect the antenna and allow operation without wind Bureau in Congress. Moving quickly, the Weather Bu- and ice loading. reau senior staff developed a major budget proposal for The modi®ed APS-2F at Texas A&M University, al- ®scal year 1956 to improve hurricane and tornado warn- though not formally a part of the Texas Tornado Warning ing services. A sympathetic Congress approved the Network, was used at least once for warning purposes funding increases, and the bureau launched a major ef- (Bigler 1956). On 5 April 1956, a tornado that produced fort to improve its warning services. The budget pack- damage in Bryan and College Station, Texas, was de- age included funds for design, procurement, installation, tected by the Texas A&M University radar. At noon that and staf®ng of what would eventually become the day, the Weather Bureau Forecast Center at Kansas City, Weather Bureau's ¯agship radar, the WSR-57. The orig- Missouri, had issued what we would today call a tornado inal budget package provided for acquisition of 31 ra- watch for an area just to the north of Bryan. The Texas dars, including one system set aside for electronics tech- A&M University radar observed strong, tall, hook- nician training. shaped echoes with V-notch signatures after 1400 LT. In order to minimize the effects of attenuation by At 1445 LT, Texas A&M University meteorologists rainfall, the Weather Bureau speci®ed an S-band wave- called the Bryan Police Department and forecast that a length for the WSR-57. Design was complete by 1957. tornado would touch down 30 min. later. Actual damage The Weather Bureau selected the Raytheon Manufac- started at 1509 LT. Texas A&M University also warned turing Company, which earlier had produced the CPS- the College Station Consolidated School System, which 9, as prime contractor for the WSR-57 and ordered an decided to keep the children in their school buildings initial quantity of 31 radars in 1958. The navy ordered instead of releasing them at the scheduled time of 1500 eight radars and applied to the radar its military no- LT. This is probably the ®rst warning based solely on menclature, AN/FPS-41 (Rockney 1958). A staff of six interpretation of radar data and is a good example of (®ve radar meteorologists and one electronics techni- effective interaction between warning meteorologists cian) was funded at each radar station. Since the WSR- and the local community. Today, with improved warning 57s were installed in existing Weather Bureau of®ces, dissemination methods, increased community prepar- in almost all cases one or two electronics technicians edness, and better radar capabilities and coverage, it were already assigned there. All of the electronics tech- would be less likely that a research team would be is- nicians received a complete course of instruction in ra- suing warnings to communities directly. dar maintenance, so they were able to help each other After 1956, the task of modifying the APS-2F radars solve dif®cult problems and ensure continuous daily so they could be ®elded as WSR-1s, -1As, -3s, and -4s coverage even during periods of temporary absence. The was transferred to Weather Bureau headquarters, which radar meteorologists and electronics technicians quickly had to relocate some antennas that had been mounted became a team to ensure high-quality radar operations. in locations where they were dif®cult to maintain. At The WSR-57 retained some features of the CPS-9 the height of the program in April 1975, 82 of the WSR- design, including an off-center PPI and an RF package 1s, -1As, -3s, and -4s were operational. A few were mounted on the antenna pedestal. The WSR-57 had to replaced by the WSR-57, but most continued in service use a larger antenna than the CPS-9 in order to achieve until replaced by the WSR-74C over an extended period a2Њ beamwidth at an S-band wavelength. The antenna from 1976 to 1980. None are in service today. was enclosed in a ®berglass radome for protection from the weather elements and to permit continuous operation in high wind, freezing rain, and hail. The WSR-57 had 8. First Weather Bureau weather radars a somewhat higher peak transmitted power than the CPS-9 and similar pulse durations. The receiver chain Hurricanes became a major factor in Weather Bureau included a choice of linear or logarithmic response and planning and budgeting in the mid-1950s. Hurricanes an adjustable step attenuator similar to the CPS-9's CEI- Carol and Edna struck the U.S. Atlantic coast within 11 CON. The WSR-57's wider beamwidth and longer days of each other in 1954. In 1955, three more hur- wavelength made it less sensitive to hydrometeors than ricanes struck the East Coast, but this time the Weather the CPS-9, but this was not considered a serious limi- Bureau was better prepared. Long-range, high-powered tation. The WSR-57's improved ability to detect storms (by 1950s' standards) SP-1M S-band radars were in- behind intervening rainfall and to observe hurricanes at stalled on Nantucket Island, Massachusetts; Hatteras, great distances were considered more important design North Carolina; and San Juan, Puerto Rico. The SP-1M objectives. Its main PPI had a re¯ection plotter that radars were navy search systems modi®ed for meteo- mirrored onto the PPI without parallax any annotations rological use by addition of a traveling wave tube as- drawn by the operator on a faceplate above the PPI.

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC 226 WEATHER AND FORECASTING VOLUME 13

FIG. 2. WSR-57 radar console.

This feature made it easy to outline the areas and lines stations. The Weather Bureau started making ®xes in constituting a RAREP and measure the associated azi- trouble-prone components such as the sensitivity time muths and ranges that de®ned those echo features. The control (STC) circuits soon after deployment of the ra- re¯ection plotter could also be used to track signi®cant dars. In 1963, the Weather Bureau took steps to stan- individual echoes, areas, lines, and other features as they dardize performance of the WSR-57s. Starting in 1964, formed, moved, and dissipated over time. The tracks sunrise and sunset observations of the sun were used to could be used to obtain movement data needed for trans- calibrate the antenna azimuth position at sites lacking mission in the RAREP and to make short-term forecasts suitable, surveyed ground targets. By 1982, the solar of echo movement for severe storm warning and ¯ash- procedure was applied also to the WSR-74Cs and -74Ss ¯ood forecasting purposes. Figure 2 shows the console but limited to times when the sun was more than 10Њ of a WSR-57 radar. above the horizon. Included in the WSR-57 equipment package was a The VIP (Shreeve and Erdahl 1968) was added to the repeater PPI display, used exclusively for radarscope WSR-57 in about 1968, and the Digital VIP (DVIP) photography. A 35-mm and a Polaroid camera were became available for use at Digital Radar Experiment permanently mounted over the PPI scope. Included in (D/RADEX) stations in 1974 (Shreeve 1974). A sample the photographic ®eld were indicators and text showing of the output of the WSR-57's logarithmic ampli®er was station identi®er, date and time, attenuator control set- provided to the VIP, which had the ability to average, tings, range marker settings, pulse duration, linear or or integrate, the instantaneous backscattered power re- logarithmic receiver selection, and a frame number. The turned by distributed targets such as weather echoes. antenna elevation angle was displayed by a strobe line Thus it could obtain in a reproducible and automatic on the PPI (Rockney 1958). Thousands of feet of ®lm, fashion an estimate of the average backscattered power produced by these cameras for almost four decades, are and infer automatically from that and other quantities on ®le at the National Climatic Data Center (NCDC). the radar re¯ectivity factor Z of weather targets. The The ®lms served as a component of aircraft accident VIP enabled the radar operator to display contours of and incident investigations when it was suspected that Z directly on the radar's indicators. The six VIP inter- thunderstorm-associated aviation hazards might have vals, which from an engineering point of view are sim- been a contributing factor. The scope photography ca- ply intervals of range-normalized average backscattered pabilities built into these early radars led to the multi- power, could have been set in terms of convenient in- level digital data archiving systems of today's WSR- crements of the logarithm of Z. However, the VIP also 88D. had to provide some compatibility with the existing ®ve- An emergency power system was installed at all the category RAREP echo intensity code, which were de- WSR-57 and SP-1M stations to ensure uninterrupted ®ned using rainfall rates R, and also had to meet some operation if commercial power failed. Back-up voice specialized hydrometeorological needs. To meet those radio communications were also installed at the coastal objectives, the six VIP intervals were de®ned by sub-

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC JUNE 1998 WHITON ET AL. 227 dividing the existing ®ve-category system in terms of at least one occasion, to 230 n mi away, no matter how R and then associating with each threshold a value of much intervening rain was occurring. Z using the relationship of Marshall and Palmer (1948). Eleven of the WSR-57s were installed in the Midwest Eventually, meteorologists became so familiar with the to detect severe local storms. Two radars were installed VIP levels they began to use them instead of the radar in the mountainous west. These installations were at re¯ectivity factor in conversations and even in scienti®c Sacramento, California, principally for support to state papers. Initially, DVIPs were purchased only for D/ and federal hydrologic efforts in water-thirsty Califor- RADEX and Radar Data Processor (RADAP) II sites; nia, and at Point Six Mountain (elevation about 8000 in 1978 DVIPs were bought and installed on all WSR- ft) near Missoula, Montana, to support state and federal 57 radars. efforts to ®ght forest ®res by helping to locate areas of Some engineering details of the WSR-57's design left high probability of cloud-to-ground lightning strikes. much to be desired. For example, the elevation control Three WSR-57s were installed inland of the East Coast for tracking heavy rain areas as hurricanes decayed was open-loop, in which the antenna followed a com- when passing over land. mand signal but provided no feedback to indicate the Funding for 14 additional WSR-57 radars was ob- true antenna position, making it impossible at the radar tained in 1966 and 1967 to expand the network east of console to distinguish between commanded elevation the Rocky Mountains, staffed as the earlier stations angle and actual elevation angle. The WSR-57 had no were. Some of these additional sites were in locations tachometer feedback for rate control in the elevation where no Weather Bureau of®ce already existed, estab- drives. These shortcomings could easily have affected lishing a concept of network design based on optimum the accuracy of the WSR-57's observations of radar spacing rather than availability of an existing of®ce. For echo tops. The WSR-57's pedestal and antenna design example, a site was established 60 mi east of Denver, were inadequate in a number of ways, including an off- in Limon, Colorado. This site ensured network conti- set center-of-gravity that caused preloading and addi- nuity with the WSR-57 radars at Garden City, Kansas, tional wear on the bull-gear driving the antenna, and and Grand Island, Nebraska. In addition, the Limon site lubrication of the elevation drive that occurred only was far enough east to avoid major ground clutter and while the antenna was actually moving. The biggest occultation problems caused by the mountains south- problem was excess heat in the radome, which decreased west through northwest of Denver. the lifetime of receiver±transmitter-modulator compo- For use in Southeast Asia, the Air Force later ordered nents located there. The original linear receiver proved three FPS-41s, which were not returned to the United inadequate and was eventually taken out of service at States (see section 13). The Weather Bureau agreed to most sites. The original logarithmic receiver was re- operate or otherwise accept responsibility for the eight tained for many years but was eventually replaced by FPS-41s bought by the navy. At the height of the pro- a solid-state logarithmic receiver as soon as reliable log gram, NWS owned or operated 53 WSR-57/FPS-41 ra- dars, of which two were used for electronics technician receivers became available at an affordable price. training and the rest used operationally as primary sta- On fair-weather days, the WSR-57 staffs had ample tions on the Basic Weather Radar Network. time for analysis of data for local research studies (Hex- An important element of the new network design was ter 1963). Progress reports were exchanged to cross- near real-time telephone-line transmission of PPI data feed preliminary results between of®ces conducting and handwritten alphanumeric annotations to nearby of- similar studies. Since many of the research organizations ®ces. The equipment was designated Radar to Telephone of the time used X-band radars, it was important for all Transmission System (RATTS-65) and later as Weather to learn of any differences in data interpretation caused Bureau Radar Remote (WBRR) (Hilton and Hoag by the WSR-57's S-band wavelength. 1966). Later, dial-in capability was added to permit ac- The ®rst operational WSR-57 was installed at Miami, cess by a wide range of users, including military sta- Florida, in June 1959. The rest of the ®rst 31 radars tions, airline of®ces, and television stations (Bigler were installed during the early 1960s. All had to be 1969). Those early efforts at providing radar data re- located in existing Weather Bureau of®ces. The primary motely led to the dial-in or nonassociated principal user purpose of the network design was tracking hurricanes processor (PUP) and NIDS capabilities of today's WSR- as they approached and crossed the Atlantic and Gulf 88D. Coasts (14 WSR-57s). The spacing between the radars With improvements made over the years, despite its was approximately 200 n mi. The three SP-1Ms were weaknesses, the WSR-57 remained NWS's ¯agship ra- to remain in place at least temporarily. Cooperative dar until deployment of the WSR-88D in the 1990s. The agreements were reached with the Dow Chemical Com- last operational WSR-57 was removed from service at pany of Freeport, Texas, and the Copano Research Foun- Charleston, South Carolina, on 2 December 1996. dation of Victoria, Texas, for reporting weather ob- served by their radars. Subsequent experience demon- 9. Follow-on Weather Bureau radars strated that the eye of a well-developed hurricane could The 1960s were a period of great change in the elec- reliably be observed to a distance of 200 n mi and, on tronics industry. Led by the space program, miniaturi-

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC 228 WEATHER AND FORECASTING VOLUME 13 zation of components made possible by the transistor tems. NWS also needed four more local-warning radars caused a dramatic move away from the vacuum tube with special requirements such as hurricane and heavy technology of the early years of radio, television, and precipitation detection, which indicated S-band radars the radar technology of the 1940s. Within the Weather (P. Hexter 1997, personal communication). To meet Bureau, the need for replacement of the 1940s vintage these needs, NWS decided to buy 16 operational S-band WSR-1, -1A, -3, and -4 radars was becoming urgent. radars (none for the NWSTC) as the last purchase before Spare parts left over from wartime stockpiles were van- NEXRAD. Enterprise Electronics was selected as the ishing. Manufacture of components using the now-out- source of the radar known as the WSR-74S. The WSR- dated vacuum-tube technology was diminishing as com- 74Ss were purchased with an integral DVIP. panies changed their designs and production lines to From the operator's point of view, the WSR-74C and remain competitive by using the new technology. WSR-74S were quite similar. They had a single PPI that The 1967 Federal Plan for Weather Radars and Re- could be equipped either with a re¯ection plotter or an mote Displays, written by the Of®ce of the Federal Co- illuminated map overlay. Their RHI was earth curva- ordinator for Meteorological Services and Supporting ture±corrected like that of its predecessor, the AN/FPS- Research, identi®ed a continuing need for weather ra- 77. The RHI was almost as large as the PPI, making it dars of three basic types: synoptic weather radars, local- easy to use operationally. These radars had an A-scope use or local-warning radars, and remote displays. The and digital readouts of range, elevation, azimuth, and 1969 edition of that same plan indicated the Weather time. Thirteen WSR-74Cs still have not been decom- Bureau intended to buy modern local-warning radars to missioned and, of those, eight remain in active use to- replace the aging WSR-1s, -1As, -3s, and -4s. day. No WSR-74Ss are in the NWS inventory today, A small group of engineers from the Sperry Rand having been replaced by the WSR-88D. Some of these Company and Vitro, Inc., formed a company of their radars are in commercial use. own, Enterprise Electronics Corporation, to design and manufacture a new generation of C-band weather radars. 10. Acquisition, deployment, and employment of By 1969, they delivered their ®rst production model to AN/FPS-77 storm-detection radar and interim a television station in Tampa, Florida. A year later, a replacement second unit was installed in a station in Jackson, Mis- sissippi. In ®scal year 1976, NWS received funding over In the early 1960s, the air force recognized the need 3 ®scal years to replace 82 aging local-warning radars for a new weather radar system to replace the APQ-13 with 66 modern, C-band local-warning radars manu- (Paulsen and Petrocchi 1966), by then having exceeded factured by Enterprise Electronics; within NWS, the ra- its expected operational life. By 1966, AWS had decided dar was called the WSR-74C. An additional two systems to replace not only the APQ-13s but also the aging CPS- were purchased, one for use at the NWS Training Center 9s, which were increasingly dif®cult and expensive to (NWSTC) and the other for use at NWS headquarters, maintain. At that time, air force weather units' require- bringing the total buy to 68. The WSR-74Cs were pur- ments for weather radars vastly exceeded the number chased with an integral DVIP. The radar had a coaxial available, and AWS allocated radars using a formula magnetron and a completely enclosed, oil-bath modu- that included mission and weather. lator, and it made maximum use of mid-1970s solid- C-band components, which had become much more state electronics. It used a lightweight pedestal because prevalent by the time it was necessary to select the new the antenna was shielded by a radome and had per- radar's wavelength, were a candidate. Experience with manently lubricated antenna gears. NWS made arrange- the CPS-9 had shown that for most applications, the ments for cooperative use of one forerunner of the WSR- radar wavelength should be longer than X band. Weather 74C, called the WR100-5, also built by Enterprise. Five Bureau experience with the WSR-57 pointed toward use of the more capable S-band WSR-74S radars (see fol- of the S band. S-band antenna and transmitting system lowing paragraph) were actually used as local-warning components, being much larger than equivalent X- and radars, bringing the total number of NWS local-warning C-band components, were considered too expensive for radars to 71 in 1982. the advantage of less attenuation of radar energy by In 1976, the Basic Weather Radar Network still had precipitation. For that reason, a compromise, the C band, a few state-size gaps and a slightly larger number of was selected for the new radar, one of whose program smaller-size gaps. At that time, the WSR-57 production objectives was to be cheaper than the CPS-9. line was closed, and the WSR-57's old, vacuum-tube The air force procured 103 AN/FPS-77(V) Storm De- design was no longer modern enough to justify buying tection Radars, with a C-band wavelength, from Lear- more of them. At the time, Enterprise Electronics of- Siegler Electronics. These systems were purchased in fered an S-band weather radar similar in many respects two increments from 1964 to 1966. Test of the ®rst to the WSR-74C. To close the ®ve remaining gaps in production system was accomplished at Grif®ss AFB, the network and to replace seven WSR-57s that failed New York, in November 1964. Full-scale production between 1981 and 1985, NWS needed 12 ``synoptic'' began in 1965. The ®rst operational FPS-77 was in- radars that by network standards should be S-band sys- stalled in March 1966. Almost all the FPS-77s had been

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC JUNE 1998 WHITON ET AL. 229 installed by 1969. One was destroyed, found rusting on a loading dock at the long-abandoned County AFB on Long Island, New York. Of the remaining 102 systems, 78 were in operation at base weather stations and test ranges by 1977; nine more were programmed for installation at similar operational facilities; nine were in use as mockups at maintenance shops, and six were located at weather training and weather equipment maintenance training facilities. At the height of the pro- gram, after installation of the contingency reserve, 87 would eventually be installed at operational base weath- er stations, the rest of the 102 radars going to mainte- nance facilities and technical training schools. Two FPS- 77 radars remain in operational use today at (RAF) Base Mildenhall, England, and Katterbach Air Base (AB), . Because the number of requirements far exceeded the number of available radars, AWS was considering the use of radar remoting options variously called RATTS and WBRR. These remoting systems had the capability of making available in weather stations an annotated copy of the PPI at nearby radar facilities. The plan in- cluded using not only NWS WSR-57s as the source radar, but also some AWS CPS-9s and FPS-77s. The remote radar thrust was driven by the economy of pro- FIG. 3. FPS-77 radar console. viding data remotely versus installing a local radar. Shortfalls in the operational capabilities of the WBRR in meeting the demanding needs of weather units pro- FPS-77 had only one pulse duration, a compromise viding operational weather support to customers, tech- about midway between the long pulse and short pulse nical inadequacies of the WBRR itself, and concern over of other weather radar systems. The FPS-77's PPI used how long FPS-77 antenna pedestals would last if used an interesting but not totally successful dark-trace stor- continuously in the mode needed to update the WBRR, age tube that could be operated in lighted environments. caused the airforce to terminate its WBRR program. The The FPS-77's console is shown in Fig. 3. option was kept open for individual base weather sta- Operation and maintenance of the FPS-77 were trou- tions to establish their own remote connections to NWS bled by shortcomings in the radar's capabilities and de- WSR-57 radars. Shortfalls in the RATTS and WBRR sign, lack of an operator's manual, no initial operator systems created a niche market that ®rms such as Ka- training at installation time, lack of follow-on training, vouras, Inc., Alden Electronics, Inc., WSI, and others and limitations in maintenance test equipment and pro- eventually ®lled by selling services that provided timely cedures. Although billed as a radar capable of taking access to attractive, color, remote radar displays at an quantitative measurements, the FPS-77 had signi®cant affordable price (E. Dash 1996, personal communica- shortcomings in the antenna design, logarithmic ampli- tion). The wide availability of radar data on television ®er, iso-echo, and STC circuits. The radar had not been weathercasts created a demand among the operational equipped with a signal integrator such as the VIP, mak- customers of military weather services for the same sort ing it necessary for operators to make estimates of the of service. In the years between ®elding the FPS-77 and average backscattered power manually using the gain NEXRAD, many military weather stations, National reduction technique to measure the radar re¯ectivity fac- Guard and Reserve facilities, and weather centers ac- tor of weather echoes. quired remote radar displays linked to NWS radars. Re- In the late 1960s, the AWS leadership noted a lack mote access to radar weather data was incorporated into of readiness in operational use of radar. Initially, it was the design of NEXRAD by making the data available thought that simply providing an operator's manual for to value-added resellers under the NIDS. the FPS-77 would do the job. An operator's manual was The FPS-77's C-band wavelength allowed attaining published as AWS' Part C of Federal Meteorological a 1.6Њ beamwidth with an antenna diameter of 8 ft, much Handbook 7, the Weather Radar Manual, by 1973. AWS smaller than that required by the WSR-57. It had a low- units with weather radar had appointed radar coordi- cost antenna design with loosely joined mechanical el- nators and increased their emphasis on radar training evation drive linkages that introduced considerable un- and certi®cation. Nevertheless, the Weather Radar Man- certainty in measurement of radar echo heights. Its peak ual proved dif®cult to implement. When improved radar transmitted power was comparable to the CPS-9's. The re¯ectivity factor measurement procedures were put into

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC 230 WEATHER AND FORECASTING VOLUME 13 operational use, technical dif®culties were identi®ed. By be equipped at least with a signal integrator such as the this time it was clear that an electrical engineer who VIP or equivalent (Doppler radars require more signal was at the same time a radar meteorologist would be processing capability); and solid, practical, and sustain- needed to address the most serious of these problems. able calibration technology and techniques must be a One of the authors (PLS) took a year's sabbatical and part of any radar purchased or developed. A ®nal lesson, served as AWS Chief Scientist from summer 1974 to not necessarily learned as well, is that the best calibra- summer 1975. Together with a support team including tion ideas and most effective maintenance come about two other authors (RCW and ACH), he visited a number when the operators and maintainers work as a team in of FPS-77 facilities at technical training schools, main- a collaborative environment. tenance facilities, and operational sites, taking funda- To replace the remaining AN/FPS-103s (see section mental measurements at these radars. Analysis of the 14), to replace some FPS-77s after that radar was de- data showed lack of ®delity in the logarithmic ampli®er, clared unsupportable and was being phased out of the STC, and iso-echo circuitry. A separate set of problems inventory, and to meet a few additional weather radar was discovered in the antenna positioning system, as requirements at a time when the NEXRAD procurement shown by angular measures made using a gunner's quad- was slipped, AWS established the Operational Radar rant and solar boresighting. No records could be found Replacement (ORR) program. Approved by the air staff indicating that any FPS-77's effective antenna system in 1984, the ORR program procured a ``gap-®ller'' radar, gain had been measured, so standard gain horn mea- the AN/FPQ-21, from Enterprise Electronics; the con- surements were taken at every site that had the capa- tract award occurred in early 1986. Twenty-four systems bility to radiate. Many of the calibration techniques de- were procured, of which ®ve are still in operation today. scribed in Smith (1968, 1974) were applied to FPS-77 The FPQ-21 was similar but not identical to the NWS's radars. This intensive period of interest in calibration WSR-74C. All the systems procured by the air force of weather radars included the American Meteorological had an antenna re¯ector 12 ft in diameter, giving a beam- Society's Weather Radar Calibration Workshop, hosted width of 1.1Њ. All were equipped with an integral DVIP by one of the authors (KEW) at the National Severe and had color displays. Only commercial manuals, not Storms Laboratory (NSSL) in 1974. During the work- military-type technical orders and documentation, were shop, the views of all calibration experts could be con- procured with this system. No training was procured sidered and practical experiments conducted using the with the systems. The ®rst FPQ-21 was installed at Fort NSSL radars and a leased, transportable radar. A tech- Sill, Oklahoma, in February 1986. nique for solar boresighting, suggested by discussions at the workshop, was applied to the FPS-77 to calibrate 11. U.S. Basic Weather Radar Network antenna, beam- and display-positioning, and effective antenna system gain (Whiton et al. 1976); the technique In 1946, the Weather Bureau established the U.S. Ba- was later improved and extended to multiparameter ra- sic Weather Radar Network, although it was not called dars like the National Center for Atmospheric Research that at the time. Initially, the network consisted of early CP-2 Doppler system by Frush (1984). More recently, WSR-series systems, and a few air force, civil govern- Rinehart (1991) provided an in-depth treatment of solar ment, and cooperative radars. The network grew slowly position as a function of time. in the 1950s and remained an amalgam of heterogeneous By the summer of 1975, a priority-ordered list of systems (Rockney and Jay 1953). The air force agreed operational and maintenance actions was developed, in- to add CPS-9 radars to the network as they became cluding continuation of the calibration visits to radar available. In 1956, the Weather Bureau established the sites. Despite the transfer of responsibility for mainte- Radar Analysis and Development Unit (RADU) as a nance of weather equipment to the Air Force Com- part of the forecast center in Kansas City, Missouri. The munications Command, replacement of the logarithmic RADU was formed to resolve the problems associated ampli®er with a reliable, solid-state version was per- with using a radar network consisting of such a diverse formed in 1979. A further outcome of this work was array of radars and reporting practices, to prepare and the formation of an AWS radar calibration team, which transmit a Teletype summary of the radar echo distri- visited radar sites and made improvements in the FPS- bution over the United States, and eventually to prepare 77's meteorological measurement capability. The radar and transmit a graphical radar summary chart from all calibration visits continued until most of the FPS-77s the radar weather data collected. Beginning in 1959, were replaced by NEXRAD, whose technical require- WSR-57s began to replace WSR-1s, -1As, -3s, and -4s ments emphasized calibration both by self-test and ex- and AWS stations on the network. A few joint-use sites ternally when necessary and by a calibration visit team were established at Air Defense Command (ADC) radar whose services can be requested when needed. sites that ®lled gaps in network coverage (Foster 1957; Over time, improvements were seen. Lessons learned Rockney 1960; Bigler 1961). Beginning in 1966, 22 from this undertaking are that radar, no less than a ba- FAA air traf®c control radars were, in effect, added to rometer, is a quantitative instrument by which critical the network. Data from these radars were collected by atmospheric variables are measured; all radars should NWS meteorologists serving at four western U.S. air

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC JUNE 1998 WHITON ET AL. 231 route traf®c control centers (ARTCC) (see section 15). echo tops, which were included in the former RAREP Many primary stations on the network had alternates to code but are not found in the RCM, can be obtained by provide backup in case of outages; many AWS radars using the WSR-88D PUP or the products of the NIDS had alternate network responsibilities in the 1960s and vendors (P. Hexter 1997, personal communication). 1970s. 13. Wartime use of ®xed weather radars 12. Radar reporting code and radar summary During the Vietnam War, CPS-9 radars were relocated chart to Tan Son Nhut AB, Republic of Vietnam (RVN), and The RAREP code, used to transmit radar weather Nakhon Phanom AB, Thailand. Three WSR-57M Storm observations, originated in World War II in a form not Detection Radars, almost identical to the WSR-57 radars unlike the later Manually Digitized Radar (MDR) code, then in active use by the Weather Bureau, were pur- and evolved to the SD or ``azran'' (azimuth±range) code chased from Raytheon, retitled AN/FPS-41s, and in- in the 1960s and 1970s. These early ``plain-language'' stalled at Udorn Royal Thai Air Force Base (RTAFB) codes were used by the RADU in Kansas City, Missouri, and Ubon RTAFB, Thailand, and Pleiku AB, RVN. For to prepare manual analyses of radar weather data in the most part, these radars were used conventionally, order to produce and transmit hourly radar summary for observing, analysis, and forecasting at individual bulletins on Teletype. In 1960, the RADU also began weather stations, as they did not form a very good net- transmitting a manual radar summary chart, also called work. The FPS-41s at Udorn and Ubon and the CPS-9 a radar composite chart, at three-hourly intervals (Bigler at Nakhon Phanom were also used to collect half-hourly 1961). The utility of the radar summary charts was soon radar scope photography to assess the effectiveness of recognized (e.g., Wilson and Kessler 1963), and sug- cloud seeding operations conducted from 1967 to 1972 gested improvements were gradually implemented. In by the First Weather Group along the Ho Chi Minh Trail the 1970s, NWS began making use of the MDR code, in Laos (U.S. Congress 1974; Fuller 1990b). Later the in which numerical echo intensity data were reported at FPS-41 at Pleiku was relocated to Nakhon Phanom, each position in a grid system; the MDR data were more where it was also used to collect scope photography. easily assimilated into computerized applications of ra- These operations were intensive but odd by the stan- dar weather data (Moore et al. 1974). dards of today's computer-oriented weather operations. In 1976, in order to facilitate production of an au- Photographs were collected using a repeater scope and tomated radar summary (ARS) chart, a new digital radar photography equipment. The resulting ®lm packs were code was implemented as a separate section of the RA- then rushed to military photographic processing facil- REP, supplementing the human-readable azran data. ities at the bases, developed, and the transparencies Production of the ARS began one month later. In 1978, ¯own nightly to Tan Son Nhut by courier aircraft. Then, the ARS production system, including the new, numer- in an all-night operation, weather forecasters in a clas- ical RAREP code, allowed NWS to cancel their MDR si®ed facility projected the photographs, analyzed them program. The frequency of the ARS increased to hourly, by densitometer, and attempted to correlate changes in compared to the three-hourly products available when the appearance of photographed precipitating storms the chart was being produced manually. Timeliness of with the known time and location of cloud seeding ac- the chart was also improved. tivities, which had been sent to Tan Son Nhut by clas- The RAREP code, in whatever form, required exten- si®ed message. While records were kept and attempts sive manual work by the observer. It made no sense to were made, using radar and other data, to quantify the continue these manual methods with the advent of the effects of cloud seeding on rainfall and enemy activities, sophisticated, partially automated NEXRAD or WSR- the cloud seeding efforts were designed as a weather 88D. NEXRAD's automatically generated Radar Coded operation, not a statistical experiment capable of prov- Message (RCM) was in part devised as a replacement ing the effects of weather modi®cation. The United for the RAREP. Editing of the RCM, originally contem- States is now a signatory to United Nations conventions plated as a way to disseminate key radar data, including prohibiting environmental war, and operations of this severe storms information, was never implemented be- sort are no longer conducted. cause of the manpower requirements involved in the Many operational customers had emerged for the rain- editing process. An unedited form of the RCM is trans- fall data, including limited interest by the 7th Air Force mitted over WSR-88D communications channels and is Intelligence and strong interest by army terrain analysts used as the basis for preparation of the ARS graphic. at the Combined Intelligence Center Vietnam. Begin- That product is still disseminated to the Automation of ning in 1970, the First Weather Group, which managed Field Operations and Services system and transmitted air force weather support in Southeast Asia, started col- on the facsimile circuits. Software running at the Avi- lecting rainfall data from the radars quantitatively as ation Weather Center in Kansas City, Missouri, prepares well as by photograph. Hourly radar scope tracings were the ARS from RCM data, removing as much ground taken over a 10-n mi ϫ 10-n mi grid system at partic- clutter and anomalous propagation as possible. Radar ipating radars, and the radar re¯ectivity factor was es-

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC 232 WEATHER AND FORECASTING VOLUME 13 timated manually using a power reference line com- onics Center Indianapolis (NACI) to adapt a radar it parison technique. Echoes were labeled according to was designing for the marine corps using the DOD Stan- their intensity, summed over 6- and 24-h intervals and dard Electronic Module for use as an air force tactical the totals relayed to the analysis center by Teletype bul- weather radar. The marines' version of this radar did letin. By 1971, VIP-contoured scope images were traced not meet all the air force's needs; among other short- and the VIP levels used to determine the radar re¯ec- comings, the marines' version could not measure the tivity factor and estimated rainfall rate. The data were radar re¯ectivity factor. Two of the present authors (PLS keypunched and analyzed by computer. Customers re- and ACH) spent many hours informing the NACI team ceived this new capability so enthusiastically that the about characteristics of a weather radar that make it 1st Weather Group rated the effort as the single most effective for quantitative measurements. The air force successful in 1970 from a customer satisfaction point ordered six systems. The program encountered delays of view. Interest in the effects of precipitation on war while NACI attempted to meet all the air force needs. remains strong today. As late as the Gulf War, the army Problems surfaced during acceptance testing, and a sec- terrain analysis team at U.S. Central Command's Army ond series of tests was scheduled. Major R. Snell, HQ Component Command Headquarters said they needed AWS, spent three months between the two acceptance information about areas where there was a signi®cant tests helping the engineers at NACI correct de®ciencies potential for ¯ash ¯ooding (Air Force 1993). in the system. The corrections required a major system redesign. It was only through Snell's efforts that the radar was ®nally able to pass the second acceptance test. 14. Tactical weather radars No shelters for these radars had actually been accepted Experience has shown that many military operations due to contractual problems with the manufacturer, so are radar permissive, in the sense that radars can be used the radars were placed in bonded storage at NACI. Fur- without signi®cant threat that they will become a target ther delays were encountered in manufacture and inte- attracting enemy attack. In such situations, particularly gration of the radar's shelter. In 1977, the system was when the wartime weather conditions are expected to designated as the AN/TPS-68 Tactical Weather Radar. include signi®cant precipitation, a weather radar might In late 1982, the air force accepted delivery of the ®rst very well be useful. This was certainly true in southeast six systems, with acceptance having been delayed a full Asia, where the demand for weather data that radars three years, mostly due to the shelter problems. could provide far outstripped the operational weather From the user's perspective, the TPS-68 was like the personnel's ability to provide useful products. For these FPS-77, having many of the same technical character- practical reasons, air force weather personnel have al- istics (such as a C-band wavelength), features, and con- ways had either a true tactical weather radar, relatively trols. The TPS-68's electronics were solid state, more lightweight and con®gured to facilitate transportability, modern than those of the FPS-77, and some limited use or an equivalent like the APQ-13 in World War II. was made of digital electronics and light-emitting diode In early 1968, the 7th Air Force, Air Component of displays. The radar had a conventional, light-trace PPI the Military Assistance Command Vietnam, purchased with center blanking and a conventional RHI. It used from Bendix (now Allied Signal) 16 WTR-1s, similar an oscilloscope as an A-scope. The 6-ft antenna was to the commercial RDR-1 sold by Bendix mostly for stored in the shelter during transport and relocated to use in aircraft. The WTR-1, an X-band radar, included the roof of the shelter for operation. No operator's man- a receiver/transmitter, antenna, radome, and operator's ual or training was purchased with the TPS-68. In 1987, console (Allied Signal 1978). Of the 16 WTR-1s, all after a KC-135 crash destroyed the FPS-77 tower at but one were initially installed in Vietnam. The no- Fairchild AFB, Washington, a TPS-68 was used there menclature center applied the designation AN/FPS-103 successfully for a year until a new tower could be in- Tactical Weather Radar to all WTR-1 models. The FPS- stalled (D. Michalewicz 1996, personal communica- 103 was a nonquantitative radar with an echo intensity tion). The TPS-68 was also used successfully at Diego contouring capability where the contours did not rep- Garcia, an island in the Indian Ocean, in Operations resent known thresholds of the radar re¯ectivity factor. Desert Shield and Desert Storm. Thunderstorms at Diego A small radar that could be readily moved and mounted were interfering with takeoff of B-52s prepositioned there on primitive towers or the tops of buildings, the FPS- for employment in the Gulf War. The commander of the 103 met wartime needs for ¯exibility and ease of main- air force bomber unit at Diego credited skillful use of tenance. After 1971, FPS-103s were shipped to other the TPS-68 by assigned weather personnel with much of locations in the Paci®c and the United States or turned the success of the ®rst night's bombing in Desert Storm, over to the Republic of Vietnam Air Force. None are saying he would not have met his bombing schedule in the operational inventory today. without using the radar to pick out usable ``holes'' in the In 1974, the Air Force's Electronic Systems Division thunderstorm activity in which to generate, launch, and (ESD) responded to a Tactical Air Command required recover the B-52s. The timing was intricate because the operational capability for a tactical weather radar. ESD unit had to fuel the aircraft, load ordnance, launch, and asked the Naval Air Systems Command's Naval Avi- recover B-52s returning from the ®rst wave, all in the

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC JUNE 1998 WHITON ET AL. 233 short time available between thunderstorms. Of the six trollers regularly used several features of their equipment systems procured, ®ve remain in the inventory today; to suppress echoes that might mask aircraft targets. The however, four are used to provide parts for the one op- ®rst of these features, MTI, reduced ground clutter by erational system, installed at Taif, Saudi Arabia (R. Kan- eliminating stationary targets but also reduced the dler 1996, personal communication). strength of weather echoes. The second feature, CP, par- The TPS-68 has been declared unsupportable from a tially canceled weather echoes. The third feature used to maintenance point of view. The number of requirements suppress masking echoes was STC, which reduced the for tactical weather radars now far exceeds the number signal strength of targets depending on their range from of available systems, and the air force is considering the radar; in operation, the effect of STC was to virtually buying a replacement tactical weather radar from com- eliminate all precipitation echoes within 30 n mi of the mercial off-the-shelf sources. Acquisition of the replace- radar. To collect useful weather data, these features would ment system is a lengthy process, so four Special Op- have to be disabled, at least temporarily. erations Forces Tactical Weather Radars (Ellason 400-P After a successful test in 1965, Weather Bureau per- systems) have been purchased, and four Interim Tactical sonnel, beginning operationally in 1966, were assigned Weather Radars (Kavouras 2070-C equivalents) are under to FAA ARTCCs located near Salt Lake City, Utah; Los contract. The navy is also considering purchase of a lim- Angeles, California; Albuquerque, New Mexico; and Se- ited number of tactical weather radars for navy and ma- attle, Washington. Data from four to seven long-range rine corps use; the requirements for the navy/marines radars were available in these centers, for a total of 22 tactical weather radar differ from those of the air force radars, providing virtually complete coverage of the west- system. ern United States and offshore. Data from Weather Bu- reau radars located at Sacramento, California; Missoula, Montana; Catalina Island, California (for a few years); 15. Use of air defense and air traf®c control radars and Medford, Oregon (after 1971) were combined with for weather detection data from the ARTCCs and disseminated in a very dif- The high cost of installing, operating, and maintaining ferent way from that used east of the Rocky Mountains. radars in mountainous terrain and in remote locations With four to seven overlapping radars available in each caused the Weather Bureau to shy away from installations center, outlines of individual echoes were drawn with the there and seek cooperative programs with other agencies. greatest detail possible on acetate overlays especially pre- It had already been demonstrated that ADC radars could pared for each radar's surveillance area. The overlays be effective at detecting precipitation (Ligda 1957; Bigler were then composited to produce one chart per center. 1957). A pilot effort to employ such observations was Annotations describing echo motion, pertinent data from set up at an ADC long-range radar site in Tennessee in pilot reports obtained by the controllers, and data from 1960. In 1961, a similar effort was established at a site conventional hourly observations were added to the in West Virginia. In both cases, a staff of ®ve was as- charts before transmission. The composite chart from signed for round-the-clock preparation of RAREPs and each center was then transmitted by facsimile to the Salt other data. These programs were phased out by 1968 but Lake ARTCC, where all the western U.S. data were as- were cost-effective interim solutions to ®lling gaps in sembled into a single chart for distribution, once again weather radar coverage in the eastern United States. Sum- by facsimile. Eventually the western U.S. radar data were mer thunderstorms cause hundreds of wild®res every year added to the radar summary chart prepared for dissem- in the central region of . From 1968 to 1975, NWS ination over the national weather facsimile systems. The personnel were assigned to Alaskan ADC radar sites dur- data proved to be extremely useful to forecasters through- ing the summer to report, using the standard RAREP out the data-sparse western United States. code, the locations of weather echoes and associated in- When the joint-use program began in the western creased likelihood of lightning to ®re®ghting agencies. United States, switching between linear po- Thunderstorms were regularly observed from the Arctic larization (LP) and CP was a problem because con- Circle south to the Alaska range, with echo tops some- trollers did not want to work around the clutter, and times approaching 40 000 ft. As lightning detection net- changing polarization had to be done at each of the works grew, the need for this specialized service dimin- individual radar sites. As the controllers and radar me- ished. teorologists gained experience in working together, the Wilk et al. (1965) showed that the long-range, long- radars were operated more and more in the LP mode. wavelength ARSR-1D radars used by the FAA for air The controllers learned that most weather echoes did traf®c control were approximately equal to the WSR-57 not cause them serious control problems. Frequently it in precipitation detection capability as long as the moving was desirable to display the precipitation echoes, since target indicator (MTI) and circular polarization (CP) fea- aircraft approaching or ®nding themselves in precipi- tures of these radars were turned off; however, opera- tation could request and receive a heading out of the tional problems caused by equipment designed and pri- weather. This example of effective interagency coop- marily used for aircraft detection caused some serious eration continued into the 1990s, until NEXRAD radars compromises (Benner and Smith 1970). Air traf®c con- were installed in the west.

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC 234 WEATHER AND FORECASTING VOLUME 13

16. Advances in operational radar data processing extend the usefulness of quantitative radar weather data and digital applications out to ranges at which the beam totally overshoots the weather. The spacing of the present-day weather radar The treatment of weather echoes in a radar system network also helps limit the consequences of violations can be divided conveniently into two parts, signal pro- of the assumptions of the weather radar equation at any cessing and data processing. The demarcation between one radar, as well as to help solve the cone-of-silence the two is not always distinct, either conceptually or in problem, in which data are not available directly above terms of the implementation. In broad terms signal pro- the radar antenna due to antenna motion limits or vol- cessing refers to the manipulation of echoes from mul- ume scan restrictions. tiple transmitted pulses and multiple spatial locations The discovery that raindrop-size distributions often (usually range bins) to derive basic quantities such as followed an analytically convenient negative exponen- the radar re¯ectivity factor or mean Doppler velocity tial form with an associated power±law relationship be- for each range bin. This signal processing can be carried tween the radar re¯ectivity factor Z and the rainfall rate out in a stand-alone, special-purpose unit such as the R (Marshall and Palmer 1948) was initially greeted with VIP or its digital equivalent, or the processing can be some skepticism, triggering additional research. In the carried out in a general-purpose computer. Data pro- 1950s and 1960s, the U.S. Army and others commis- cessing, which follows signal processing in the data sioned considerable research on the relationship be- stream, refers to any further processing of these ``base tween Z and R and the associated drop-size distributions. data'' (to use a NEXRAD term) needed to perform func- The large body of such Z±R relations (see, e.g., Battan tions such as determining rainfall rates, applying al- 1973) proved fundamentally useful to operational radar gorithms to infer storm characteristics, preparing prod- meteorologists in measuring precipitation using radar, ucts, and displaying data or products. although the range of the variation can be perplexing. Weather radar equations are the underpinning for An unknown and perhaps substantial part of the vari- quantitative use of radar echo intensity data and are ability in the reported relationships can be ascribed to especially important in automated processing of radar sampling limitations (Smith et al. 1993). echoes to obtain meteorological quantities such as the Off-line postprocessing and analysis of radar data, radar re¯ectivity factor and rainfall rate. Early investi- digitized either manually or electronically (e.g., Kessler gations refer to underestimation by several decibels of and Russo 1963; Wilk et al. 1967; Wilk and Kessler the average backscattered power by the equation of the 1970), provided strong indications that computer pro- time (Marshall et al. 1955). Various investigators grad- cessing of the data could facilitate operational use of ually whittled the discrepancy down. Probert-Jones the radar information. Research applications of small, (1962) found that part of the remaining difference could digital , attached directly to weather radars be attributed to an overly simpli®ed model of the radar for data processing purposes, began in the 1960s. At beam as having a uniform distribution of power between ®rst, special-purpose computers designed for the appli- the half-power points. Probert-Jones's rederivation of cation were used (e.g., Atlas et al. 1963). The more the radar equation, using a Gaussian beam model, ex- versatile, general-purpose minicomputers that appeared plained most of the remaining discrepancy. Some sys- on the market in the mid-1960s soon proved superior tematic error sources may remain, such as the ®nite for this application (e.g., Smith and Boardman 1968). bandwidth error of Smith and Nathanson (1972), un- By the early 1970s, it was apparent to most that a weath- determined losses within the radar system, and propa- er radar without attached computer data processing and gation losses through the atmosphere. The matter is not applications is, almost by de®nition, an underutilized fully resolved, and further improvements in the weather system. Many uses of weather radar, especially hydro- radar equations are still a possibility. Except for large meteorological applications, severe weather detection, errors caused by violations of the underlying assump- and storm movement forecasting, are greatly facilitated tions made in deriving the weather radar equations, the by an attached processor and applications software. By errors are small enough that they are dif®cult to detect the 1970s, all research radars had been connected to relative to the measurement uncertainty of the radar sys- computer systems. It was apparent that, with some in- tems. The larger errors resulting from violations of the vestment in computer hardware and software, opera- assumptions (e.g., the beam not always being ®lled uni- tional radars could provide much greater bene®t to cus- formly with homogeneous precipitation) have serious tomers. limiting consequences on the usefulness of quantitative NWS conducted D/RADEX (McGrew 1972) from radar weather data beyond a range of about 200 km. 1971 to 1976 to investigate radar digitizing hardware Today, almost every radar meteorologist has at his or and applications software designed to exploit radar's her disposal data from a high-resolution, multispectral potential usefulness in meteorology and hydrology. Five meteorological system, the Geostationary Op- network WSR-57s were equipped with the D/RADEX erational Environmental Satellite-Next Generation se- facilities, including minicomputers, storage devices, and ries of . It is possible to fuse the information displays. D/RADEX gave rise to the vertically inte- from radar and satellite sources in such a way as to grated liquid water (VIL) technique, a useful thunder-

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC JUNE 1998 WHITON ET AL. 235 storm intensity and ¯ash-¯ood indicator (Greene and Later the availability of the same techniques on the Clark 1972). VIL experienced increased use with the WSR-88D helped give forecasters con®dence in making wider availability of computer data processing systems. the transition from non-Doppler to . D/RADEX also showed the operational feasibility of The advent of computer-driven color monitor tech- processing in near-real time the large amounts of data nology in the 1970s made it easier for meteorologists that are generated by using antenna tilt sequences to to recognize characteristic weather echo features. In the create a three-dimensional volume scan of the atmo- summer of 1982, a test of the usefulness of color graph- sphere. D/RADEX produced, from the volume scan ics displays to hydrometeorological forecasters was con- data, radar echo tops maps, real-time accumulated pre- ducted at the Pittsburgh Weather Service Forecast Of- cipitation, and a ¯ash-¯ood monitor. The system also ®ce. An Interactive Color Radar Display system was provided an implementation of Lemon's weak echo re- developed to display computer-generated radar products gion technique, an early severe storm identi®cation al- to forecasters in color. An expanded test of full oper- gorithm (Saf¯e 1976), and data retrieval by off-site users ational use of automated color displays for radar ob- using digital modems instead of remote scopes and servations and severe weather warnings was conducted slow-scan television-based remoting systems. Each D/ at , Oklahoma, in spring/summer 1983. RADEX system sent accumulated precipitation totals to The results were encouraging, and today color displays the relevant River Forecast Center (RFC) every 3 h. are an important component of the WSR-88D and other From 1976 to 1980, NWS pursued acquisition of a weather radar systems that rely on interpretation by a new, automatic RADAP, which would replace the aging meteorologist. D/RADEX equipment and expand the former D/RAD- EX system to include all network radars and some local- 17. Interpretation of radar weather data warning radars. In mid-1980, RADAP procurement was halted due to higher-priority requirements. At that time, Interpreting radar weather echoes and making judg- NWS decided to go ahead with a scaled-back RADAP ments about the character of the weather phenomena II program, an interim system that would replace the D/ associated with those echoes is the operational radar RADEX equipment and add an additional ®ve sites to meteorologist's daily work. The earliest such techniques the automated network. RADAP II was acquired in attempted to link the shapes and con®gurations of radar 1983, and the system was expected to meet NWS needs echoes to the underlying weather phenomena. The clas- until deployment of NEXRAD (Shreeve 1980; Greene sic example is the hook-shaped echo, often indicative et al. 1983). Eventually the number of RADAP II sites of tornadoes. As mentioned earlier, the ®rst hook echo grew to 12, including one in Panama. The VIL technique was photographed in 1953 by D. Staggs of the Illinois and echo tops map, developed in D/RADEX, made it State Water Survey (Stout and Huff 1953). Other char- into RADAP II and eventually into NEXRAD. The acteristic echo shapes supposedly associated with severe availability of RADAP II through the mid-1980s pro- weather were quickly reported. The horizontal con®g- vided NWS with a computer-based radar test bed in uration of echoes in lines and line echo wave patterns which non-Doppler techniques such as VIL could be was said to be indicative of the potential for severe used by operational weather forecasters. The bene®ts in weather. Characteristic echo motion was also thought to this approach were twofold. First, the techniques them- be an indicator. The vertical extent of radar echoes, the selves could be evaluated in a practical, operational set- presence of weak echo regions and bounded weak echo ting. Second, forecasters could be exposed to and trained regions (echo-free vaults), and particularly the extent to in the use of digital radar algorithms of the sort they which these echoes penetrated the tropopause were other would eventually experience in NEXRAD. The work of indicators. Donaldson (1965) not only reviewed these Elvander (1977, 1980), Winston and Ruthi (1986), and and other indicators, but also collected data on the like- others to de®ne objectively severe weather echo sig- lihood of experiencing severe weather, given their pres- natures based on digital radar data was implemented as ence. The spiral rainbands associated with tropical the severe-weather probability (SWP) algorithm of D/ storms are another recognizable echo characteristic and RADEX, a technique that later made its way into RA- can be useful in identifying the location of the storm's DAP II. In the mid-1980s, in unpublished work, El- eye or center of circulation. vander used a generalized operator approach to develop Research radars almost from the start had the ability the version of the SWP algorithm that is actually im- to measure the radar re¯ectivity factor Z of weather plemented in the WSR-88D today; coef®cients have so echoes. In the late 1950s, the AFCRL conducted in- far been derived only for Oklahoma thunderstorms (R. house studies and sponsored research at the Illinois State Elvander 1997, personal communication). The use of Water Survey and Texas A&M University in hail iden- re¯ectivity-only algorithms such as VIL and SWP led ti®cation techniques for the CPS-9. Based on the results to measurable improvements in storm warning statistics of this research (largely not reported in the journals but during the mid-1980s at RADAP II sites and eventually rather in technical reports), investigators such as Don- elsewhere, as forecasters learned to apply them to quan- aldson (1961) reported an association between pro®les tify storm assessments made using non-Doppler radar. of Z with height in the storm and the probability of hail,

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC 236 WEATHER AND FORECASTING VOLUME 13 severe thunderstorms, and tornadoes. Two-predictor It was intended that additional development of the techniques proved to be powerful, such as echo tops radar meteorologists' skills would be facilitated by use combined with the Z pro®le, and early algorithms were of radar scope photography. Many of the WSR-1 and -3 reported along with associated probabilities of detection radars and all the WSR-57s were equipped with 16- or and false alarm ratios (Boyd and Musil 1970). Opera- 35-mm cameras and Polaroid cameras for the purpose. tional radar meteorologists soon adopted these tech- The scienti®c curiosity of the radar staffs was encour- niques also. In the mid-1970s, Lemon's techniques aged. Copies of the 16- and 35-mm photographs could (Lemon 1977, 1980), which exploit the organization of be obtained for use in poststorm analysis. Emphasis was model severe storms, began receiving greater accep- placed on accurate diagnosis of echoes and the reasons tance by operational meteorologists, as they could be for them, not merely the routine reporting of their pres- used with a non-Doppler radar. ence. Doppler-based techniques for identifying severe These early activities in the enthusiastically devel- storms had been in development by research groups and oping ®eld of radar meteorology eventually triggered the Weather Bureau since the mid-1950s. Some such an effort by the Weather Bureau, air force, and navy to techniques are truly research-grade, in the sense that assemble and publish the most useful technical training applying them requires multiple Doppler radars. Others, material and operational procedures in the form of a such as the tangential shear of the radial velocity and standard and ``of®cial'' weather radar manual. The ®rst single-Doppler vortex recognition, are inherently more of these was called the Weather Bureau±Air Force± suitable for operational application in an affordable net- Navy (WBAN) Weather Radar Manual, ®rst published work. Donaldson et al. (1975) applied measures such in the 1960s. In the 1970s the WBAN Weather Radar as probability of detection, false alarm ratio, and a new Manual was replaced by FMH-7. metric called the critical success index to a large col- Because operational weather personnel do not always lection of cases and attempted to show whether Doppler have time to follow radar meteorological research, AWS radar is superior to conventional radar in detecting se- collected relevant results in technical reports, agency vere storms. A similar assessment of the utility of Dopp- recurring publications, and letters. The material was pro- ler radar in storm warning was reported by Lemon et vided to weather stations equipped with radar and to al. (1977). The results of both these studies, showing schools responsible for presenting technical training to that the Doppler techniques were superior to non-Dopp- weather and equipment maintenance personnel. These ler methods for tornado and severe thunderstorm de- included distribution of Donaldson's (1965) review, tection, were available to the participating agencies in publication of a tornado case study based on meteoro- time to be used in deciding the characteristics of the logical analysis and data from the FPS-77 radar (Finley nation's next-generation weather radar. In fact, these and et al. 1973), a digest of the signatures of severe weather related studies were probably the major factor, before as seen on the radars of the day (Whiton 1971; Whiton the Joint Doppler Operational Project (see Part II of this and Hamilton 1976), and a report of Lemon's (1977, paper), in establishing the operational utility of single- 1980) techniques. Doppler radar observations. The NWSTC (the ``technical'' was eventually dropped from the name) in Kansas City, Missouri, of- fered three specialized courses. The foundation course, 18. Education, training, and professional radar meteorology, provided four weeks of instruction development activities to meteorologists assigned to of®ces having a weather With deployment of the WSR-57, the Weather Bu- radar. NWSTC also presented a three-week version of reau's initial planning was that radar meteorologists the foundation course. Finally, NWSTC offered a users' would enhance their careers by working one or two course, three weeks in length, for meteorologists desir- years as a WSR-57 radar meteorologist as they pro- ing to use radar data remotely (Covey 1984). The gressed up their career ladder. As the network expanded, NWSTC also offered an intensive ``short'' course on a shortage of interested meteorologists developed, and radar maintenance for NWS electronics technicians. some of the best meteorological technicians were as- The air force technical training facility formerly at signed instead. All radar staff personnel successfully Chanute AFB, Illinois, and now at Keesler AFB, Mis- completed a four-week undergraduate-level training sissippi, offered short courses on radar meteorology and course at the University of Miami, receiving ®ve uni- the operational use of weather radars. The air force's versity course credits for their efforts. The lecture notes short course in radar meteorology had, since the 1960s, of the Miami course are in Hiser (1970). The training been about one training week in length. From about at Miami, which began in 1959, continued into the 1970 to about 1975, the air force merged its radar me- 1970s, when it was moved to what was then called the teorology course with its satellite interpretation course, NWS Technical Training Center (NWSTTC) in Kansas but the number of training days devoted to radar was City, Missouri. The electronics technicians received not affected. Shrinking air force budgets and a draw- eight weeks of training at the NWSTTC and were re- down frustrated attempts made in the mid-1970s to in- sponsible for complete maintenance of the WSR-57. crease radar content and course length in the short

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC JUNE 1998 WHITON ET AL. 237 course. The air force also taught radar weather obser- The RFCs in the 1960s found it dif®cult to apply vation and scope interpretation as a part of its weather radar data, as their computer models were more suited technician course. In addition, the air force taught radar to the use of river stage reports and 24-h precipitation maintenance as one of the largest units in its weather totals as reported by the conventional rain gauge net- equipment maintenance technician course. work. Attempts were made by some RFCs to apply Only to a limited extent can these courses and study MDR data and digital information from D/RADEX and materials provide the breadth and depth of understand- RADAP II when they became available in the 1970s ing of the whole ®eld of radar meteorology that we and 1980s, but success was spotty, and practices were might desire for ourselves as students of the masters not uniform across the RFC system. As was the case who established radar meteorology so successfully in with ¯ash-¯ood warnings, it was not until the WSR- the 1940s. The ®eld has now grown, formed alliances 88D was ®elded that these problems were addressed. with the cloud physics, mesoscale meteorology and se- Today the WSR-88D network provides digital precipi- vere storms communities, and developed subspecialties tation data to the RFCs. to such a degree as to make comprehensive understand- In the 1950s and early 1960s, radar meteorologists ing of it a lifetime's work. were sometimes required to give pilot weather brie®ngs. Most pilots were very aware of the weather radar sys- tem, as it was widely advertised in the aviation literature 19. Civil applications of weather radar and in ground school instruction. These pilots often At a typical WSR-57 site in the early 1960s, the radar asked the meteorologist for a summary of the weather meteorologist would take and transmit a RAREP every as observed by radar along their route of ¯ight. Later, hour via Teletype to the RADU. This entailed such ac- airline meteorology of®ces and dispatch facilities had tivities as identifying echo areas or lines, measuring access to information provided by the radar remoting maximum echo intensities and representative echo top systems, as did the FAA's Flight Service Stations, and heights, recognizing important ``signature'' features, us- the need for radar meteorologists to brief pilots de- ing the re¯ection plotter to determine cell and storm creased. motions, and encoding the information into the RAREP Radar scope photographs were taken routinely at all format. The RADU would then assemble the RAREPs the WSR-57 sites and at other network stations. These from participating radar sites and prepare composite ra- scope photographs were provided to NCDC for use in dar summary products. Radar operations were closely research and studies. Copies of the ®lm were maintained attuned to severe weather diagnosis and forecasting. Ra- at each radar site so radar meteorologists could conduct dar often provided key information on which to base and publish professional investigations of signi®cant weather warnings. When severe weather was observed, weather events and understand local weather processes the radar meteorologist took and transmitted special ra- characteristic of their location. dar weather observations. When a hurricane or typhoon came within detection distance of a weather radar, the 20. Military applications of radar weather data radar meteorologist was expected to take and transmit special observations of the location, eye size, move- Typically-military weather forecasters use radar ment, and other attributes needed by tropical cyclone weather data to improve the quality of severe weather forecasters. warnings, making them more useful to military deci- Local radar summaries were disseminated to the news sion-makers for resource protection purposes. In addi- media at some stations. With the ®elding of radar re- tion, radar weather data are used to improve observa- moting equipment in the late 1960s, remote radar weath- tions and forecasts vital to the conduct of military op- er data became more popular than such summaries. erations affected by weather. Two examples tell these The mission and capabilities of the RFCs did not stories. include ¯ash-¯ood forecasting; therefore, the local radar At Kelly AFB, Texas, site of the vast San Antonio meteorologist issued ¯ash-¯ood warnings, subjectively Air Logistics Center, inspections, modi®cations and re- assessing the intensity, movement, and duration of rain- pairs are made to the air force's C-5 transport and B- fall events as seen on radar. This was a tricky business, 52 bomber ¯eet. These aircraft are too large to be housed as a ¯ash ¯ood could arise from a combination of several in any of Kelly's hangars, and much of the work has to circumstances, including high rainfall rates, slow storm be done outside. To permit access by repair crews, the movement, or long duration. Peculiarities of soil type, aircraft are lightened and jacked while repairs are made. slope, and basin disposition of the terrain relative to the These aircraft offer a lot of tail surface and so are always rainfall patterns; existing soil moisture; expected ab- somewhat susceptible to being turned by wind. In the sorption and runoff; and a number of other conditions lightened and jacked con®guration, they are even more compounded the dif®culties. Today, the WSR-88D pre- vulnerable to these effects. If any of the aircraft are cipitation processing subsystem can be useful in ¯ash- turned while jacked, they fall off the jacks and the jacks ¯ood forecasting despite the system's relative imma- penetrate the airframe, causing great damage to multi- turity (see, e.g., Hunter 1996). million dollar aircraft. Forecasters at Kelly are expected

Unauthenticated | Downloaded 09/27/21 01:10 AM UTC 238 WEATHER AND FORECASTING VOLUME 13 to provide 1-h warning of damaging winds from thun- Doppler techniques. Operational radar meteorologists derstorms and other sources, as it takes that time to ``de- found recent research increasingly dif®cult to apply. jack'' the aircraft. These forecasters have to use radar Leaders of operational civil and military weather ra- not only to help forecast the arrival and severity of dar programs recognized an opportunity for change but thunderstorms but also to make inferences about wind were trapped by budgetary realities. AWS was caught from radar data to prepare the precise warnings needed. in the air force's protracted, post-Vietnam drawdown. Traditionally, Kelly forecasters used wind inferences NWS had a somewhat more positive outlook on the from echo motion and ®ne line data obtained from non- possibility of ®elding a high-quality replacement radar. Doppler radars. Now they can use wind data obtained Neither AWS nor NWS corporately favored the alter- directly from Doppler weather radars to make their native of a jointly acquired, next-generation weather warnings. radar. Resource protection is not the only military appli- In the second part of this history, we show how a cation of weather radar. Some military operations are few courageous leaders of operational weather radar particularly vulnerable to weather, and forecasts can ac- programs in both the services, backed by years of dif- tually improve the success ratio of these operations. An ®cult, intensive research in radar meteorology and in- example is airforce undergraduate pilot training, where spired by leading senior scientists in radar meteorology inexperienced student pilots ®rst earn their wings. At who were proactive in wanting to apply research results, each stage in the pilot training curriculum, the students were able to show the nation the way to ®eld a fully have different quali®cations and, with them, different capable, next-generation weather radar system. weather limits. At some stages of their training, student pilots can ¯y only under visual ¯ight rules; increasing Acknowledgments. The authors thank R. Donaldson cloudiness and weather in the training areas is cause for Jr., consultant to Hughes STX Corporation, for his thor- bringing these students back to home base. Bringing ough review of an early version of this paper. We also students back from a training area requires time because appreciate the assistance provided by Allied Signal, H. the approach pattern at home base is usually far busier Benner, C. Bjerkaas, E. Dash, R. Elvander, P. Hexter, than that at the busiest commercial airports. Moreover, R. Kandler, R. Miller, the National Climatic Data Center, the direction of arrival of advancing weather can have V. Rockney, and R. Saf¯e for providing valuable in- a devastating effect. If the base becomes ``weathered- formation. The contribution of one of the authors (PLS) in'' in an unforecast way before the ¯ying areas are was supported in part by National Science Foundation affected, the result can be a dangerous ``divert'' of mar- Grant ATM-9221528. ginally quali®ed pilots to locations without the proper fuel and maintenance facilities. From there, the aircraft and pilots have to be recovered later, with tremendous APPENDIX A impact to the ¯ying training schedule. Something that List of Acronyms and Abbreviations would have almost no effect at a less busy air base can have tremendous effect at a busy pilot training base. An AB Air base example is an unforecasted change in wind direction ADC Air Defense Command that requires the supervisor of ¯ying to ``turn around AFB Air Force base the pattern.'' With so many aircraft taking off, landing, AFCRL Air Force Cambridge Research Laborato- on approach to the runway, in the pattern, entering it, ries and departing from it, such changes can require time, AFGL Air Force Geophysics Laboratory valuable time during which the pattern cannot be used AGC Automatic gain control to recover aircraft. Most student jet training areas are A&M Agricultural and mechanical adequately covered by a single weather radar, so radar ARS Automated radar summary can be used effectively to improve ¯ying safety while ARSR Air route surveillance radar helping to maintain the ¯ying schedule. ARTCC Air route traf®c control center A-scope Type-A indicator, display of signal ampli- tude vs range or time 21. Summary ASDE Airport surface detection equipment By the mid-1970s, U.S. weather services had ®elded ASR Airport surveillance radar a variety of conventional, non-Doppler weather radars AWS Air Weather Service and were considering the technology with which to re- az Azimuth place them. All NWS radars except the most primitive AZRAN Azimuth/range were equipped with hardware-based integrators for sig- CEICON Calibrated echo intensity control nal processing; a few had attached data processors. AWS CP Circular polarization radars were, almost without exception, totally manual. Diam Diameter Research in radar meteorology, especially for severe DOD Department of Defense weather identi®cation, had turned toward the use of DOT Department of Transportation

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D/RADEX Digital radar experiment PL Phillips Laboratory DSP Digital signal processing or processor PPI Plan position indicator DVIP Digital video integrator and processor PUP Principal user processor ESD Electronic Systems Division Rad Lab Radiation Laboratory FAA Federal Aviation Administration RADAP Radar data processor FMH Federal Meteorological Handbook RADU Radar Analysis and Development Unit HQ Headquarters RAF Royal Air Force IEEE Institute of Electrical and Electronics En- RAREP Radar report gineers RATTS Radar to Telephone Transmission System IF Intermediate frequency RCM Radar coded message Ln Pulse Long pulse Rcv Receive LP Linear polarization RF Radio-frequency MDS Minimum discernible signal RFC River forecast center MDR Manually digitized radar RHI Range±height indicator MIT Massachusetts Institute of Technology RTAFB Royal Thai Air Force Base MTI Moving target indicator RVN Republic of Vietnam N/A Not applicable SD Radar code NACI Naval Avionics Center, Indianapolis Sh Pulse Short pulse NCDC National Climatic Data Center STC Sensitivity time control NEXRAD Next-Generation Weather Radar SWP Severe weather probability NIDS NEXRAD imagery dissemination system TDWR Terminal Doppler Weather Radar NSSL National Severe Storms Laboratory VIL Vertically integrated liquid NWS National Weather Service VIP Video integrator and processor NWSTC NWS Training Center WBAN Weather Bureau±Air Force±Navy NWSTTC NWS Technical Training Center WBRR Weather Bureau Radar Remote ORR Operational radar replacement WSR Weather surveillance radar PAR Precision approach radar Xmt Transmit

APPENDIX B Technical Characteristics

TABLE B1. Technical characteristics of operational weather radars (azÐazimuth, elÐelevation).

Characteristic AN/APQ-13 AN/CPS-9 Decca-41 AN/FPS-103 WTR-1 WSR-74C Wavelength (cm) 3.2 3.2 3.2 3.2 5.3 Peak transmitted power 40 250 30 45 250 (kW) (at transmitter) Pulse duration (␮s) MD-12: 0.5, 1.125, Sh pulse: 0.5 Sh Pulse: 0.2 2.25 3 2.25 Ln pulse: 5 Ln pulse: 2 MD-38: 0.5, 0.75, 2.25 Pulse repetition frequency 1350, 675, 270 Sh pulse: 931 250 400 259 (sϪ1) Ln Pulse: 186 Linear IF ampli®er and Yes 0±15 dB above Yes No response noise Logarithmic IF ampli®er No No No Ϯ1 dB from 0 to 78 and response dB above noise Range normalization or No Off, 100 mi Off, 35 n mi In DVIP STC Iso-echo No No Uncalibrated con- No touring feature IF attenuator, CEICON or No CEICON No Selectable DVIP or equivalent IF attenuator (3, 6, 12, 24, 48 db) VIP or DVIP No Normally no No DVIP Nominal noise ®gure 33 12 10 9 dB max (dB) Nominal MDS (dBm) Ϫ83 Ϫ103 Ϫ106 Ϫ104 Antenna re¯ector diam 2.5 7.75 14 wide ϫ 2.6 high 2.5 8 (ft) Beamwidth, azimuth (Њ) 3 1 0.6 3.6 1.6

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APPENDIX B (Continued)

Characteristic AN/APQ-13 AN/CPS-9 Decca-41 AN/FPS-103 WTR-1 WSR-74C Beamwidth, elevation (Њ) 3 1 2.8 3.6 1.6 Antenna mobility rng (Њ) 360 az 360 az 360 az Ϫ10, 0, 30 el 0to90el Ϫ5, 0, 25 el Antenna positioning ac- Ϯ1 az, el curacy (Њ) Effective antenna system 33 41.5 30 40 gain (dB) Characteristic AN/FPQ-21 AN/FPS-77 AN/TPS-68 TDWR ASR-4 Wavelength (cm) 5.3 5.4 5.4 5.6 10.3 Peak transmitted power 250 250 165 250 425 (kW) (at transmitter) Pulse duration (␮s) 3 2 2 1.1 0.833 Pulse repetition frequency 259 324 375 235±2000 1040, 1170, 1200 (sϪ1) Linear IF ampli®er and No Ϯ1 dB from 0 to 15 0±30 dB above Dynamic range: 129 response dB above noise noise dB Logarithmic IF ampli®er Ϯ1 dB from 0 to 78 Ϯ1 dB from 4 to 64 0±90 dB above and response dB above noise dB above noise noise Range normalization or In DVIP Off, 30, 60, 120 n 5±100 n mi STC mi Iso-echo No 10±30 dB (10) 10 dB; 30 dB; 36± 33±60 dB (3) 78 dB (3) matched to log amp IF attenuator, CEICON or Selectable DVIP or 0±109 dB (1) 0±90 dB (10) equivalent IF attenuator (1, 0±9dB(1) 2, 4, 8, 16, 32, 64 dB) VIP or DVIP DVIP No No Nominal noise ®gure 3.5 10 12 2.3 13.5 (dB) Nominal MDS (dBm) Ϫ108 Ϫ104 Ϫ104 Ϫ113 Ϫ102 Antenna re¯ector diam 12 8 6.5 25 9 wide ϫ 17 high (ft) Beamwidth, azimuth (Њ) 1.1 1.6 2 0.55 1.5 Beamwidth, elevation (Њ) 1.1 1.6 2 0.55 5±30 csc2 Antenna mobility rng (Њ) 360 az 360 az 360 az Ϫ2to60el Ϫ2to60el Ϫ2to60el Antenna positioning ac- Ϯ0.25 az, el Ϯ0.5 az, el Ϯ0.5 az, el curacy (Њ) Effective antenna system 40 36.5 38 50 34 gain (dB) Characteristic WSR-1 WSR-1A WSR-3 WSR-4 WSR-57M AN/FPS-41 Wavelength (cm) 10.5 10.5 10.5 10.5 10.5 Peak transmitted power 50 50 50 50 500 (kW) (at transmitter) Pulse duration (␮s) Sh pulse: 1 Sh pulse: 1 Sh pulse: 1 Sh pulse: 1 Sh pulse: 0.5 Ln Pulse: 2 Ln pulse: 2 Ln pulse: 2 Ln pulse: 4 Ln pulse: 4 Pulse repetition frequency Sh pulse: 650 Sh pulse: 650 Sh pulse: 650 Sh pulse: 650 Sh pulse: 658 (sϪ1) Ln Pulse: 325 Ln pulse: 325 Ln pulse: 325 Ln pulse: 325 Ln pulse: 164 Linear IF ampli®er and response Logarithmic IF ampli®er and response Range normalization or No No No No Off, 125 n mi STC Iso-echo No No No No 10 position IF attenuator, CEICON or Uncalibrated gain Uncalibrated gain Uncalibrated gain Uncalibrated gain Log atten ®ne: 0±33 equivalent control control control control dB (3) Log atten coarse: 33, 66 dB Identical lin atten controls VIP or DVIP No No No No Yes after 1968

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APPENDIX B (Continued) Characteristic WSR-1 WSR-1A WSR-3 WSR-4 WSR-57M AN/FPS-41 Nominal noise ®gure 8 (dB) Nominal MDS (dBm) Ϫ108 Antenna re¯ector diam 666612 (ft) Beamwidth, azimuth (Њ)44442 Beamwidth, elevation (Њ)44442 Antenna mobility rng (Њ) 360 az 360 az 360 az 360 az 360 az 3-position tilt Ϫ2to50el Ϫ2to50el Ϫ2to50el Ϫ10 to 45 el Antenna positioning ac- curacy (Њ) Effective antenna system 38.1 gain (dB) Characteristic WSR-88D SP-1M WSR-74S ARSR-1D ARSR-2 Wavelength (cm) 10.5 10.6 10.7 23 23 Peak transmitted power 750 700 500 4000 5000 (kW) (at transmitter) Pulse duration (␮s) Sh pulse: 1.57 Sh pulse: 1 Sh pulse: 1 22 Ln pulse: 4.7 Ln pulse: 5 Ln pulse: 4 Pulse repetition frequency Sh pulse: 8 Sh pulse: 600 Sh pulse: 545 360 360 (sϪ1) selectable 318±1304 Ln pulse: 120 Ln pulse: 162 Ln pulse: 318±452 Linear IF ampli®er and Dynamic range: 93 response dB with AGC Logarithmic IF ampli®er and response Range normalization or DSP STC Iso-echo DSP IF attenuator, CEICON or DSP Selectable DVIP or equivalent IF attenuator (3, 6, 12, 24, 48 dB) VIP or DVIP DSP DVIP Nominal noise ®gure 2 9 dB max ( dB max 6.5 (dB) with parametric ampli®er) Nominal MDS (dBm) Ϫ113 Ϫ104 Ϫ109 Antenna re¯ector diam 28 12 12 40 wide ϫ 11 high 47 wide ϫ 23 high (ft) Beamwidth, azimuth (Њ) 1 2 2 1.35 1.2 Beamwidth, elevation (Њ) 1 2 6.2 csc2 3.75 Antenna mobility rng (Њ) 360 az Ϫ1±60 el Antenna positioning ac- curacy (Њ) Effective antenna system 45 38 34.3 gain (dB)

Where an item is blank, that means no information could be found about it. Where a feature is missing, the corresponding item should read ``No'' or ``N/A,'' indicating not available. The abbreviation ``Sh pulse'' indicates short pulse, whereas ``Ln pulse'' indicates long pulse. The abbreviation ``Xmt'' indicates transmit, while ``Rcv'' indicates receive. The abbreviation ``az'' indicates azimuth, while ``el'' indicates elevation. ``AGC'' indicates automatic gain control, and ``DSP'' indicates digital signal processing.

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