GPS + LORAN-C Performance Analysis of an Integrated Tracking System

INNOVATION | Navigation & Tracking

James Carroll

he rapidly increasing use of satellite BEFORE GPS, EVEN BEFORE SATELLITES, there was LOng RAnge Navigation, navigation encompasses a broad or LORAN. Using terrestrial radio transmitters, it was developed during World Trange of civilian users. But not all War II for aircraft navigation. The wartime system evolved by the mid-1950s into users are sufficiently aware of important op- the present day 100 kHz LORAN-C system. LORAN's standard principle of operational risks in using, say, the Global Posi- eration is hyperbolic positioning. A receiver measures the difference in times of tioning System (GPS) for positioning, nav- arrival of pulses transmitted by a chain of three to six synchronized stations sep- igation, and timing (PNT) applications. arated by hundreds of kilometers. The time-difference measurement derived Systems that use GPS work very well when from the signals of two stations, when multiplied by the speed of propagation designed properly, and when sufficient ro- of the signals, forms a line of position (LOP); the receiver could be anywhere on bust ranging signals from the satellites are this line and give the same measurement. The available. When the GPS signal cannot be geometrical form of this LOP is a hyperbola. used for PNT, appropriate backup systems Measurements using a third station provide an- and procedures should be used. Service other hyperbola, which intersects the first at the providers are increasingly aware of the need position of the receiver. There are many LORAN to provide adequate system integrity, and to chains around the globe. provide a seamless procedure for timely The LORAN system is being modernized to switches to and from the backup procedures. INNOVATION INSIGHTS enhance its accuracy, integrity, availability, and As a result, there is increasing interest in in- with Richard Langley continuity. Vacuum-tube transmitters are being tegrated PNT systems. replaced with solid-state designs and new pri- The U.S. Department of Transportation's LORAN can help overcome mary frequency standards are being installed at Volpe National Transportation Systems Cen- transmitting stations. Manufacturers have deter in Cambridge, Massachusetts, has devel- some big-city navigation veloped compact LORAN receivers able to oped and installed vessel surveillance and track multiple transmitters simultaneously and tracking systems for maritime applications to automatically apply propagation bias correc- and is now applying this evolving technol- problems. tions. Some receivers are integrated with GPS ogy to surface applications in urban areas or other sensors. Receivers also feature im- in the U.S. Positioning and timing informa- proved antenna designs. Collectively, these improvements are known as En- tion is provided by a system that integrates hanced LORAN or eLORAN for short. Additionally, LORAN signals can be used GPS with enhanced LORAN-C (eLORAN) to convey differential GPS corrections. Such a system is already operational in and dead reckoning (DR). Europe. An important issue for the integrated Supported by the Coast Guard and the Federal Aviation Administration in the GPS/LORAN system is how well LORAN United States, a goal of eLORAN is to provide nonprecision approach for avia- and DR systems can supplement GPS when tion users and harbor entrance and approach for marine users. Land users will the GPS signal is not usable. Performance benefit, too. Since LORAN has different signal characteristics from those of requirements vary with application, but a GPS, it can be used in locations where GPS cannot — by itself or in conjunction basic requirement in urban areas is reliable with GPS and other sensors. In this month's column, we look at a system that location of the vehicle at an intersection. combines eLORAN with GPS and dead reckoning to overcome some of the This article describes the design and uses problems in navigating in big cities. of an integrated GPS/LORAN/DR tracking system, and presents results from per- Innovation features discussions about recent advances in GPS technology and its applications as well as formance evaluations conducted in New York the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of City, including the lower Manhattan finan- Geodesy and Geomatics Engineering at the University of New Brunswick, who welcomes your comments cial district. The results obtained show that and topic ideas. To contact him, see the “Contributing Editors” section on page 8 of this issue. eLORAN can be an effective backup to GPS in many areas of Manhattan, including some

40 GPS World | July 2006 www.gpsworld.com Navigation & Tracking | INNOVATION

dominated by tall buildings or open tunnels where GPS is not able to function well.

LORAN-C For the past several years, the U.S. Congress has authorized a total of about $140 million to enhance the U.S. LORAN-C system (FIGURE 1). At the same time, the evolving modernized LORAN-C system is being eval- uated for potential use as a backup to GPS in many transportation infrastructure applications. These include navigation, positioning, surveillance, and also timing- and frequency-based applications in the air, at sea, and on land. Some of these extend beyond transportation into the communications, emergency response, and security domains. ᮡ FIGURE 1 North American LORAN-C system architecture circa 2003. SSX: solid-state Here, we will look at another extension transmitter; TTX: tube-type transmitter; LSU: LORAN Support Unit. All transmitters in the of this technology to land applications in conterminous 48 states are now of solid-state design. urban areas, using positioning and timing information provided by an integrated The transmitters in the LORAN-C chain new communication modulation method GPS/LORAN-C system. The original transmit in a fixed time sequence. The length that enables operations that satisfy the accu- LORAN-C system, now about 60 years old, of time in tens of microseconds over which racy, availability, integrity, and continuity per- is nearing completion of a significant up- this sequence takes place is termed the group formance requirements for non-precision grade and enhancement effort funded by the repetition interval (GRI) of the chain. Chains aircraft approaches and harbor entrance and Federal Aviation Administration (FAA). are identified, differentiated, and discussed approaches, as well as the requirements of Known as Enhanced LORAN or eLORAN, in terms of their GRI. non-navigation time and frequency applica- it is a very complementary system to GPS. The LORAN-C transmitters emit pulses tions. Required changes to the current sys- Its signals are much less susceptible to the in- of radio frequency (RF) energy at precise in- tem include modern solid-state transmitters, terference that can impair GPS, and its broad- stances in time. Position determination is a new time and frequency equipment suite, cast frequency in the very-low-frequency based on the measurement of the difference modified monitor and control equipment, (VLF) range of the radio spectrum has min- in time of arrival of these RF energy pulses. and revised operational procedures that new imal line-of-sight issues. eLORAN has re- Each master-secondary pair enables deter- receiver technology can exploit. cently been shown to be an adequate backup mination of one line of position, measured system to GPS for many applications. by the difference in arrival time of the two Integrated Tracking System Legacy LORAN-C. LORAN-C (LOng signals; a minimum of two lines of posi- The FAA, with Volpe Center support, es- RAnge Navigation-C) is a low frequency, ter- tion is required to determine a location or tablished a project to assess the perform- restrial radionavigation system operating in “fix” on the Earth's surface. ance of integrated GPS/LORAN tracking the 90 to 110 kHz frequency band and pro- Precise timing and synchronization of the systems in the urban environment, where viding coverage of U.S. coastal waters and LORAN-C system are keys to system per- GPS signal quality varies greatly. Marine the conterminous 48 states, part of Cana- formance, and the LORAN-C transmit- mobile transponder units were re-config- dian coastal waters, the Great Lakes, most of ters incorporate highly accurate cesium clocks ured to utilize land-based digital data links mainland Alaska south of the Brooks Range, as standard equipment. The transmitters need and to process eLORAN positioning and the Gulf of Alaska, the Aleutians, and into to be synchronized with standard time ref- timing signals. State-of-the-art LORAN re- the Bering Sea. Coverage also exists in other erences. The U.S. Naval Observatory pro- ceivers were used in the evaluations, along regions of the world, including northwest vides the time synchronization to Coordi- with receivers providing both un-augmented Europe and the Far East. The LORAN-C nated Universal Time (UTC). and augmented GPS (the FAA Wide Area system comprises transmitters, control sta- eLORAN. The modernized U.S. Augmentation System — WAAS — and tions, and system area monitors. The legacy LORAN-C system also operates in the same Nationwide Differential GPS). The receivers LORAN-C chain is a basic system element. frequency band as legacy LORAN-C and is were used in a mobile system that can It consists of between three and six transmit- synchronized to UTC. However, this mod- broadcast information to a command cen- ting stations. Each chain has a designated ernized system, now almost 85 percent com- ter or other mobile units in real time. master station and several secondary stations. plete, has a recapitalized infrastructure and a The Volpe Center has in the meantime www.gpsworld.com July 2006 | GPS World 41 INNOVATION | Navigation & Tracking

the use of an independent backup radionav- pending on the test objectives. igation system, and LORAN is a strong can- Data Extraction. Each record of the out- didate to back up GPS. puts is a separate message or data log that The tracking and display application can be produced by a given receiver. While described in this article represents a new the NMEA standard is fairly precise, it is extension of the Volpe-developed technol- flexible enough to allow minor format vari- ogy because it now supports: ations that can help place a message with Ⅲ Operation in a land environment, in- its source. The Transview software, TV32, cluding the “urban canyon,” so-called be- also allocates data-port identifications to cause of the disruptive effects of tall build- avoid confusion. In addition, because most ᮡ FIGURE 2 Tracking with GPS/LORAN: ings on radionavigation; and messages do not have a time stamp, TV32 The Volpe Center mobile test bed Ⅲ Integration of GPS, LORAN, and dead will add one if necessary. reckoning. Another aspect of the data extraction ef- been developing and extending to diverse GPS/LORAN System. An overview of fort involves the nature of much of the applications a GPS-based tracking and situ- the integrated GPS/LORAN tracking sys- LORAN NMEA data. LORAN perform- ation display technology that can be used not tem is shown in FIGURE 2. Note that the sys- ance metrics can be station-specific, unlike only operationally with the core tracking system also uses dead reckoning, which was in- the ensemble nature of most of the GPS met- tem, but also can support the performance stalled specifically for testing in the urban rics (that is, these are not satellite-specific in assessment of candidate tracking system ar- environment. general). This means that, since between five chitectures. The system is called Transview. The test-bed vehicle is a Volpe Center and nine LORAN transmitter signals can be Volpe has recently installed customized minivan, already available for this type of use. tracked in New York City at any one time, versions of Transview in several waterways The baseline equipment also includes pro- this number times the six or so performance worldwide. This not only includes systems vision for a data link to a fixed facility with metrics of interest add up to a rather large that provide important navigation and track- the proper transmit-receive interface. For ma- accounting challenge — large numbers of ing information for commercial and personal rine applications, marine band very-high- variables per update cycle. One New York users, but also systems that enhance maritime frequency radios are used, and for land and City analysis generated 93 distinct output security by providing increased situational urban operations, data services from both variables of interest, including the various awareness. A new generation of transponder Nextel and Verizon cellular telephone net- time variables. equipment is being developed for operational works may be used as dictated by their re- Examples of LORAN station-specific use by harbor protection forces in both do- spective network coverage. metrics (one for each GRI from a particu- mestic and foreign ports. The equipment generates text-formatted lar transmitter being “read” by the receiver) The surveillance and tracking technology output that adheres to the National Ma- are: horizontal dilution of precision, sig- embodied in Transview is based on two rine Electronics Association (NMEA) 0183 nal quality, time of arrival, time difference, important ideas: (1) availability of naviga- standard. The original purpose of this sys- signal-to-noise ratio, and envelope-to-cycle tion signals in space, and (2) a two-way com- tem was tracking of multiple water-borne difference. Update cycles range from 1 to munications link between each mobile unit targets. In this adaptation, which compares about 5 seconds. and similarly equipped fixed or mobile units performance of multiple systems, there are Finally, the NMEA 0183 standard al- in the coverage area. The Volpe Center sys- many positioning outputs of interest, each lows for proprietary data logs. Reelektron- tem was designed for surface applications in- with its own “idea” of where the test van is ika, for example, has provided a proprietary volving several mobile units on either land located. Each of the outputs thus becomes a header “$PRLK” followed in the second or water. Positioning and timing signals are target. The heart of the system is the inte- field by specific internal sub-headers and provided primarily by GPS, but tracking ex- grated GPS/LORAN system, supplemented receiver-specific data. The Loradd receiver perience in the urban environment dictates by DR and consisting of a pair of LORAN uses this header, for example, to log its in- receivers including one with an integrated ternally generated GPS-conditioned TABLE 1 Transponder Components GPS receiver and a pair of GPS receivers in- LORAN fix. Depending on system config- Starlink GPS cluding one with DR output. The Reelek- uration, from 10 to 20 data logs per second Reelektronika Loradd integrated tronika Loradd and Locus Satmate 1030 re- can be recorded. This produces large GPS/WAAS/LORAN ceivers each provide multiple NMEA records amounts of data over, say, a 5-hour data Locus Satmate 1030, with rate gyro for both GPS and LORAN, and the DR sys- gathering session. It is not uncommon for and H-field antenna tem provides two GPS NMEA records. a 4- or 5-hour data run to generate a 50 u-blox SBR-LS dead reckoning TABLE 1 summarizes the components used megabyte text file. The NMEA messages and GPS output in the tests. There is considerable flexibility were parsed into Matlab scripts for the Nextel or Verizon data link in configuring the outputs of interest, de- analysis presented in this article.

42 GPS World | July 2006 www.gpsworld.com Navigation & Tracking | INNOVATION

Performance Assessment GPS performance in the urban canyon is well known to be very poor. When combined with a dead-reckoning system, however, most of the location issues are effectively mitigated. The Wall Street area of New York City is ex- ceptionally difficult due to the skyscrapers and narrow streets that characterize the area. Poor geometry results when only a small por- tion of the sky is visible to the GPS antenna. Multipath, the reflection of radionavigation ranging signals, is also a major detriment to acceptable performance. LORAN operates in the VLF band, mak- ing the LORAN signal less subject to blockage than GPS. LORAN also has a ground surface conductivity property that enables its signal to follow terrain and reach areas blocked to GPS. However, LORAN also is plagued by multipath. The advance in ᮡ FIGURE 3 Heading south on 11th Avenue LORAN technology in recent years raised again the possibility of using LORAN as a backup to GPS within a large city. Using only severe urban canyon anywhere. A DR capa- the others by a few hundred yards (meters). LORAN to locate reliably a vehicle on a par- bility is necessary for reliable tracking near This is typical of observed LORAN perform- ticular corner of any New York block remains Wall Street. ance in the great majority of test locations. an unrealistic goal. However, detailed test- Potential users of integrated tracking sys- It is indicative of a LORAN solution unaided ing may yield data that can point to a valid tems in New York City have differing require- by ASF corrections. use for LORAN in this environment. ments. Reliable location of the nearest inter- FIGURE 4a shows a Matlab plot of the DR- Initial integrated system tests in Boston's section to the vehicle is likely a foremost derived position in an area just north of the financial district confirmed that LORAN requirement, and it will be the implied stan- area shown in Figure 3. will not reliably outperform GPS. There are dard in the subsequent analysis. Loradd Receiver. The Loradd integrated areas where LORAN signals can persist West-Side Results. FIGURE 3, a computer receiver produces its own GPS output, which longer than GPS, but this probably won't display using TV32, shows a very good per- matches the DR result very well (FIGURE 4b). justify LORAN as a primary backup to GPS formance area both for GPS and LORAN. The Loradd receiver also generates in a large city. A DR system was acquired and The van is just north of the Holland Tunnel LORAN outputs, including basic LORAN added to the integrated tracking system, so entrance. Data “bubbles” in the upper left and a GPS-conditioned LORAN. The ad- that a “truth” reference could be established corner depict the simultaneous tracking of dition of these outputs to the Loradd GPS for quantifying GPS and LORAN perform- seven “targets.” These include two LORAN, output seen in Figure 4b is shown in FIGURE ance more exactly. one DR, one integrated GPS/LORAN, and 5. The GPS output is blue, as before, and also Data were gathered in New York City in three GPS outputs. more granular, to match slower fix times for April 2005. The tests covered not only the One of the two LORAN outputs (Locus the LORAN receivers (5-second updates for Wall Street area, but more benign locations Satmate) is tracking right over the GPS so- LORAN outputs, 1 second for GPS). in Manhattan, the Bronx, and Brooklyn. lutions in Figure 3, indicating some possibil- The red plot, which matches the reference Based on test data examined to date, two ities that need further examination. So-called GPS plot very well, is the Loradd GPS-con- areas of interest will be discussed in this arti- additional secondary (phase) factors (ASFs), ditioned LORAN solution. The basic, or cle: the west side of Manhattan and Wall which account for the effect of ground con- pure, LORAN output (the green plot) from Street. The west-side data are more typical ductivity, are being applied here, but they the Loradd receiver does not have real-time of an “average” performance zone found in may be supplemented in the Satmate receiver ASF updates like the Satmate in Figure 3. Its many cities. There aren't enough tall build- with an integrated GPS/LORAN solution. offset with respect to GPS is normal to good, ings to significantly disrupt DR, GPS, or A fortuitous combination of LORAN self- except where the loop is. A longitude error LORAN performance. DR can provide the canceling errors is possible but unlikely. is prominent in the latitude band 40.770 to “truth” reference for analysis of the west-side The second LORAN track is very evident. 40.785 degrees. data. The Wall Street area is possibly the most It is the maroon track shifted mostly east of The number of LORAN transmitters www.gpsworld.com July 2006 | GPS World 43

INNOVATION | Navigation & Tracking

4a 4b

ᮡ FIGURE 4a and 4b Dead reckoning (a) and Loradd GPS (b). Latitude vs. longitude, West Side (degrees).

tracked by the receiver falls below five only LORAN (red). ent performance level relative to that in Fig- for an instant, another indicator of very good Wall Street Results. The most signifi- ure 3, shortly after entering the Wall Street performance along Manhattan's west side cant tracking system performance difference area from the south (it isn't possible now to (since LORAN is a two-dimensional posi- between the west side of Manhattan and the actually drive along Wall Street, for security tioning system, fewer transmitters are needed Wall Street area is the regular loss of lock reasons). The GPS and LORAN solutions for a good LORAN fix than are needed by on GPS and, to a lesser degree, LORAN have become fully unusable. One LORAN GPS receivers). ranging signals to enable a reliable fix. In an- solution has gone off the map. Dead reck- Satmate Receiver. As stated earlier, the ticipation of this situation, the DR system oning, the green track just below “DO- Satmate receiver produced excellent LORAN was added to maintain a “truth” location per- HERTY” in Figure 7, is active and work- results using dynamic ASF corrections, and formance reference for both GPS and ing well at this point. As long as a good GPS possibly also some GPS conditioning. The LORAN. The u-blox DR system can op- signal is available, the DR output will be fixed performance was captured on TV32 (Figure erate for a while without GPS, as it has an to GPS. Volpe Center testing in New York 3), and Matlab plots reaffirm this result. odometer sensor for along-track distance and City indicated that, when the DR had no FIGURE 6 shows latitude vs. longitude plots a rate gyro for bearing changes. GPS fix for about 30 to 45 minutes, the unit's for the DR baseline (blue) and the Satmate FIGURE 7 dramatizes the markedly differ- GPS input in fact became destabilizing.

ᮡ FIGURE 5 Loradd GPS (blue, cf., Figure 4b), LORAN (green), and ᮡ FIGURE 6 Dead reckoning (blue) and Satmate LORAN (red). GPS-conditioned LORAN (red). Latitude vs. longitude plot, West Latitude vs. longitude, West Side (degrees). Side (degrees).

44 GPS World | July 2006 www.gpsworld.com

Navigation & Tracking | INNOVATION

Corner of Nassau and Pine

Begin Corner of William and Pine End

ᮡ FIGURE 8 Recorded DR track, without GPS aiding, Wall Street. Data record began at the red circle on the left, and ended on the right-side red circle. The two green asterisks marking the Pine ᮡ FIGURE 7 Onset of blockage and multipath effects near Wall Street intersections with William and Nassau streets are located Street. by averaging the precision DeLorme/Google data.

DR and Truth Reference. DR perform- signal became unusable for navigation, it ap- unit.). The scenario examined here began in ance for the system examined deteriorates in parently was still being used by the DR unit. time at the left-side red circle and continued two ways. The more common way is due to Doing so added significantly to the overall clockwise to the right-side red circle. The ef- drift error in the heading gyro output. This DR system error. fect of gyro drift, to be examined further error grows steadily over time unless the GPS Analysis of Wall Street area performance below, is evident here also. At the end point signal has a good fix often enough to keep in the Matlab environment proves insight- in Figure 8, the DR location is closer to the the gyro error contained. In areas like Wall ful. The test area of interest is bordered wrong intersection. The final leg to the stop Street, where GPS can be unreliable for ex- roughly by Nassau, Liberty, Pearl, William, point should be on top of the adjacent leg, tended periods, there is no check on this error. and Pine Streets. Wall Street is just south of as they both are on Liberty Street. The Volpe Center test plans called for a Pine. Green asterisks in this figure mark inter- few circuits of the Wall Street area, in part to The analysis data shown in FIGURE 8 cov- sections determined within about 5 feet (1.5 add to the difficult environment. ered more than a full circuit in the test area, meters) using DeLorme and Google Earth The second problem may be more spe- albeit not the complete test run. The figure measures. While the precision between the cific to the u-blox DR unit used in the Volpe shows the DR location output without GPS DeLorme and Google measurements is very tracking system. For some time after the GPS aiding (GPS antenna detached from the DR high, the absolute location of the intersec-

ᮡ FIGURE 9 Recorded DR track, with GPS aiding, Wall Street. This ᮡ FIGURE 10 Recorded DR track, without GPS aiding, Wall Street run repeats the run shown in Figure 8. (see Figure 8). The closest offset distance (red line) to the “sur- veyed” DeLorme/Google point is about 130 feet (37 meters). www.gpsworld.com July 2006 | GPS World 45

INNOVATION | Navigation & Tracking

11a 11b

ᮡ FIGURE 11a and 11b GPS (a) and LORAN (b) location results vs. the DR reference, Wall Street.

tions was not validated. (green) tracks. Axis limits hide the fact that ing. The DR system, blocked from GPS for Intersection location is an important ca- there were about twice as many “good” several minutes, performed well in this area pability for an urban tracking system, and LORAN points as GPS points. Of course, of Manhattan. DR components need to provide an ade- none of the non-DR points seen in this fig- When the urban canyon and to some ex- quate truth reference when location compo- ure have much value, except the Loradd tent the jamming environments are better nents like GPS or LORAN cannot. track at the bottom of Figure 11a. Other understood, and when eLORAN design de- FIGURE 9 shows a prior test run that is essen- performance metrics were investigated, in- tails are fully implemented in a production tially the same scenario as that shown in Fig- cluding “distance into scenario,” but space receiver, LORAN should become an even ure 8. The Figure 9 test began at least 30 min- considerations preclude their discussion more valuable element in the future radion- utes after the DR equipment was activated here. For details, see the paper cited in “Fur- avigation and timing mix. and using GPS aiding. The de-stabilizing ef- ther Reading.” fect of not disengaging the GPS antenna after Acknowledgments GPS had been lost for several minutes is Conclusions This article is based on the paper “Performance evident. LORAN is no cure-all for the environ- Analysis of an Integrated GPS/Loran-C Track- When the GPS antenna is removed (or ments in which these systems were tested, ing System” presented at The Institute of Nav- perhaps some tuning of the DR system but it continues to offer promise in mitigat- igation National Technical Meeting, held in done), DR without GPS aiding is a reason- ing GPS loss. The two LORAN receiver sys- Monterey, California, January 18-20, 2006. able truth reference until the gyro drift effect tems tested in New York City showed incon- The author wishes to thank Mitch Narins grows too large. sistent but promising performance in some of the FAA for his leadership on this impor- Looking closer at the DR with no GPS sections of the city, including the difficult fi- tant project, and Dr. Ben Peterson of Peter- aiding track (FIGURE 10), note that the DR nancial district. son Integrated Geopositioning, LLC, for his track corner is not the closest geometrical It still appears that LORAN would be technical insight. The statements and opin- point to the reference (red asterisk). of little help in the Wall Street area unless ions in this article are the author's only, and GPS and LORAN Performance. As aided by DR. A real-time optical system they do not necessarily reflect policy or opin- stated above, a reasonable truth reference could calibrate the digital map measurements. ions of the U.S. Government. c for the Wall Street results addressed here is A real advantage to LORAN would be real- JAMES CARROLL has been at the Volpe Center for the track produced by DR without GPS ized when jamming or extended unavailabil- 15 years. He is working on integrated navigation aiding (Figure 9). GPS and LORAN per- ity of GPS occurs. In this case, a systems in the areas of performance analysis formance for this scenario is much worse, LORAN/DR combination could be bene- and integrated system testing. Prior activities at the Volpe Center include directing a successful not surprisingly, than unaided DR (even ficial in most areas of Manhattan, espe- project to install a vessel communications, track- unaided DR suffers from gyro drift effects cially for emergency, rescue, or security op- ing, and navigation system at the Panama Canal. near Wall Street after about 45 minutes). erations. This would set a framework for even He also was a co-author and primary editor of the Volpe Center GPS vulnerability assessment. FIGURE 11a shows the baseline DR reference better performance as LORAN technology He has degrees in aeronautics and astronautics (green) and the Loradd GPS (red) tracks, evolves. Finally, LORAN's edge on GPS in from the Massachusetts Institute of Technology and FIGURE 11b shows the same DR base- signal penetration is evident in areas like the and Stanford University, and is a member of The Institute of Navigation and the Institute of line (now blue) and the Satmate LORAN open tunnel under the United Nations Build- Electrical and Electronics Engineers. 46 GPS World | July 2006 www.gpsworld.com Navigation & Tracking | INNOVATION