A SHORT HISTORY First, a Little Background The idea of space-based technology as an The World of WAAS: Part 1 aid to air navigation was hardly new by the time the WAAS program appeared on Editor’s Note: One of the largest and most important collateral developments in the rise of the scene. Studies about the use of satel- global navigation satellite systems (GNSS) is the creation of satellite-based augmentation lites to support air traffic control (ATC) systems (SBAS). The first of these was the U.S. Wide Area Augmentation System (WAAS). operations began back in the 1960s. Designed originally to support air navigation in North America, WAAS has also come to But serious interest in making this be used for a variety of ground- and sea-based operations and inspired in large part the idea a reality only arose when the GPS emergence of five other SBAS programs in Europe, Russia, China, , and . program began in the early 1970s. A The Institute of Navigation and many of its members have long played an important role in the progress of WAAS. A paper on Wide Area Differential GPS, a cornerstone of WAAS, variety of concerns, however, dampened authored by Dr. Changdon Kee, Dr. Brad Parkinson, and Dr. Penina Axelrad, received early FAA enthusiasm about the potential the Outstanding Paper Award at the third international technical meeting of the Satellite use of GPS to support civil aviation in the Division, ION GPS 1990. Since then scores, if not hundreds, of papers presented at ION National Airspace System, concerns that conferences and articles in the ION's NAVIGATION journal have addressed technical and persisted into the 1980s. These included operational aspects of the WAAS program. Moreover, WAAS is cited in the professional funding limits affecting the GPS program activities of at least 21 ION special award recipients and named ION Fellows. (which led to reduction of the planned On the 25th anniversary of the first flight demonstrations of SBAS technology by constellation to fewer than 24 satellites), the WAAS program, this two-part series describes the background of the program, the establishment of a testbed to support its development, the technical challenges encountered accuracy limitations of the GPS L1-C/A and overcome, the declaration of WAAS initial operational capability in 2003 and the signal (due to the policy of Selective ongoing expansion of the system since then. Availability, or SA, as well as ionosphere- The article was written by Dr. Bryant Elrod, former project manager at Stanford induced errors), and the need to develop Telecom/STel for the WAAS National Satellite Testbed (NSTB); Frank Persello, a a reliable method of integrity warnings, a member of the Federal Aviation Administration (FAA) Technical Center’s NSTB system for independent monitoring and Dr. Todd Walter engineering team; and , a past president of the ION and director of the notification to users when GPS should wide-area differential GNSS laboratory at Stanford University where ground-breaking work on WAAS has long taken place. not be employed for aircraft operations. That situation had turned around and become more encouraging by the end n December 1993, the first demonstra- through en route to Category I (Cat-I) of the 1980s, however. By then, funding Ition in the world of using GPS plus precision approach operations. As of of a full (24-satellite) GPS constellation space-based augmentation system (SBAS) January 3, 2019, FAA has approved 3,969 had been approved. In addition, studies technology took place. An FAA aircraft WAAS Localizer Performance with Vertical in academia and industry had identified performed approach and landing trials guidance (LPV) approach procedures serv- techniques for countering SA’s limitations in New Jersey and, after a cross-country ing 1,930 U.S. airports, including 1,164 on civil GPS signal accuracy and provid- flight, again in California. Throughout the that lack instrument landing systems. ing a GPS integrity warning to users. The operations, the aircraft received GPS cor- Since September 2009, a navaid using former concern benefitted from pioneer- rection and integrity messages broadcast GPS together with ground-based aug- ing work at Stanford University's GPS at L-band via an Inmarsat-2 geostationary mentation system (GBAS) technology for Laboratory to define and evaluate local satellite (GEO). A few months later, in Cat-I operations has been approved with and wide area differential GPS (DGPS) concert with counterparts in Canada, the validation activity progressing toward techniques that improved positioning FAA completed more extensive flight trials Cat-II and III approval. Both of these accuracy. Additional studies by RTCA, using three aircraft. augmentation technologies also enable The MITRE Corporation, and others The success of these and later flights support of FAA surveillance needs for all addressed the need for integrity alerts by helped convince the FAA to initiate an aircraft equipped to transmit automatic means of the broadcast of GPS integrity SBAS procurement and development dependent surveillance–broadcast out- data using a geostationary satellite. called the Wide Area Augmentation bound messages (ADS-B Out). By 1990, the FAA’s Satellite Program System (WAAS). Its initial operational An important contributor to pursuing Office (SPO) had begun exploring the capability (WAAS IOC) began in July the WAAS program in the 1990s and to wide-area DGPS (WDGPS) concept 2003. With improvements along the way, the promotion of SBAS technology inter- coupled with a wideband integrity broad- GPS+WAAS now serves as a primary nationally was the FAA’s National Satellite cast (WIB) from GEOs. The SPO had navigation aid (navaid) in the National Test Bed (NSTB), whose development and in mind not only support for en route, Airspace System (NAS) from take-off applications are described in this article. terminal area, and non-precision approach

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The master station determined GPS integrity and differential correction data and generated messages with this information for relay to user equipment, either at a surveyed location for static testing or onboard an FAA aircraft for in-flight testing while in view of a laser tracker. The SBAS messages were transmitted using signals generated and communicated to an Inmarsat-2 GEO by a testbed uplink station at a COMSAT ground terminal in Con- necticut. The GEO, in orbit at 54°W, broadcast these messages on L-band, although at 1533 MHz rather than at the GPS L1 frequency (centered at 1575.42 MHz). When the GEO was not available, the SBAS messages were Table 1. NSTB Participants & Responsibilities transmitted via a testbed VHF station at FAA’s tech center or from another test site. User platforms (i.e., equipment on board aircraft or at ground test sites) received GPS L1-C/A and GEO (or VHF) signals and computed a WIB-plus-WADGPS position solution for post-test analysis. The data also drove a course devia- tion indicator display in the test aircraft. A separate testbed reference station at the test site collected GPS L1/L2 signal measurements and data to provide an NSTB reference time base and to support post-test comparisons of local area (LDGPS-) and WDGPS-based accuracy performance. Dial- up telephone lines and modem technology provided commu- nication interfaces among all testbed elements. Algorithms and prototype software for each element were implemented on PCs and verified by means of simulation.

Flight Trials The FAA initiated flight trials in 1992 in order to test and validate fielded testbed hardware along with communication interfaces, Figure 1. NSTB Configuration (1992–94): TUS, testbed prototype software, and data reduction operations. A Convair 580 uplink station; TRS, testbed reference station; TMS, testbed equipped with a WAAS user platform was flown over several race- master station; TVS, testbed VHS station; TRfS, secondary testbed reference station; FAATC, FAA Technical Center; STel, track/approach profiles at the tech center in New Jersey. Post-test Stanford Telecom; SU, Stanford University analysis indicated WDGPS-based horizontal and vertical position- ing errors to be less than three and seven meters (95%), respective- operations, but potentially for Cat-I precision approaches as well. ly, using just the three previously mentioned reference stations. FAA upper management approved the launch of a test and Although preliminary, these results demonstrated that it could evaluation effort to formally assess the SBAS concept. The agency be possible to satisfy the most stringent navigation sensor error established a planning group drawing in representatives from gov- (NSE) goal for WAAS, which was 7.6 meters (95%) for horizon- ernment and industry to define the resources needed for an effort tal and vertical components. In October 1993, a testbed uplink that ultimately became the National Satellite Testbed (NSTB). station was implemented to enable data communications to an Numerous organizations participated in its development and Inmarsat-2 GEO. The SPO then began planning for trials to as- operations and are listed in Table 1. sess the SBAS (WIB+WDGPS) concept over a larger geographical area. NSTB Development First SBAS-Aided Transcontinental Flight. In December The satellite testbed, as initially designed in 1991–92 and 1993 a first-of-its-kind trial saw an FAA Boeing 727 equipped implemented during the following two years (see Figure 1), sup- to receive GPS and GEO signals flown from Atlantic City, New ported testing at the FAA’s William J. Hughes Technical Center Jersey, to Crows Landing, California, and back again. Figure 2 (FAATC) near Atlantic City, New Jersey. It included three testbed shows the routes flown by the aircraft. To ensure that the aircraft reference stations (TRSs) located about 500 miles away in Maine, was always within 500 miles of at least one reference station over Ohio, and South Carolina. These reference stations collected and all flight legs, five additional TRSs had been added to the network preprocessed GPS L1/L2 code and carrier measurements and data beforehand (two in California and one each in Nevada, Colorado, and then delivered them to a testbed master station. and Oklahoma). The stations in California and Nevada were in-

ION Newsletter 22 Winter 2019 THE WORLD OF WAAS stalled by Stanford University to also support their ongoing SBAS 55 R&D work under an FAA grant. 50 Unfortunately, at Crows Landing, inclement weather and a tracking laser error hampered proper assessment of WDGPS-based 45 positioning accuracy during approaches. However, the flight trial 40 gathered useful data and provided insight into operations near the edge of GEO coverage. Specifically, the message data gathered on 35 all flight segments verified the GEO link and the successful broad- 30 cast and reception of correction and integrity data. Latitude (degrees) Additional NSTB Flight Trials. In May and June 1994, 25 in concert with Transport Canada Aviation (TCA, now Nav 20 Canada), the FAA performed an extensive series of en route and 15 approach flights over four days using three aircraft (two FAA- −130 −120 −110 −100 −90 −80 −70 −60 owned airplanes and a Challenger 601 from TCA). East Longitude (degrees) Once again, two airplanes flew cross-country routes to Crows Figure 2. NSTB & Flight Paths for Cross-Country Test in Landing and back. Altogether, the three aircraft performed a Dec. 1993 total of 140 precision approaches at three different airports (Crows Landing, Atlantic City, and Hamilton, Ontario), some the accuracy, integrity, availability and continuity of service to simultaneously. support civil aviation in all phases of flight. At the time, many an- To support testing in Canada, TCA expanded TRS coverage by ticipated that GPS, thus augmented, would become a sole-source adding three more reference stations — at Ottawa, Gander, and navaid that would enable the phase-out of existing ground-based Winnipeg. Subsequently, testing continued at the FAA Techni- systems and associated costs. cal Center in New Jersey. Flight trials conducted by Stanford In June 1994, just five months after the FAA administrator’s University also began at the Palo Alto, California, airport. Those announcement, the FAA issued an RFP with the goal of having trials used a private plane, the testbed master station, software a WAAS signal-in-space (SIS) by late 1997. However, evaluation developed independently at SU, and three reference stations in of proposals from five bidders and contract negotiations with the the NSTB network (two in California and one in Nevada). winning team (Wilcox Electric Corporation) took until August At this point, the flight trials and test results completed the 1995 to complete. As things turned out, the FAA ended up can- FAA’s early NSTB test and evaluation program. Those efforts had celing that contract within eight months and awarding another successfully verified the technical feasibility of satisfying two key to Hughes Aircraft Company, now part of Raytheon Systems WAAS performance goals over a large geographical area: Corporation, in May 1996. • Required Navigation Performance (RNP) for Cat-I precision The FAA’s goal was to complete WAAS in three phases. Phase I approaches (vertical and horizontal navigation sensor errors of called for the fielding and testing of 25 reference stations, 2 mas- less than 7.6 meters, 95%) ter stations, 4 ground Earth stations, and the leasing of transpon- • Integrity time-to-alarm requirements for Cat-I (6 seconds), ders on 2 Inmarsat-3 geostationary satellites. Phase II would add nonprecision approach (10 seconds), terminal area (15 sec- more reference stations (including some outside the continental onds), and en route (5 meters) operations ) to increase overall WAAS coverage. More reference Post-test analysis also indicated that the GEO broadcast data’s stations and GEOs came online in Phase 3. bit/message error rates without forward error correction — a Originally scheduled for late 1998, WAAS initial operations ca- future NSTB upgrade — were in the expected range. pability (IOC) would be delayed due to technology challenges, risk management measures, cost increases and funding shortfalls, and WAAS Procurement Begins conditions imposed on program management by Congress. On Jan 5, 1994 — even before the second set of flight trials began — the FAA administrator had authorized the Satellite Part II in the history of WAAS will appear in the next issue of Program Office to expedite implementation of WAAS for GPS. the ION Newsletter. It will describe the upgrading of the NSTB A WAAS specification was developed with major contributions and the results of new tests and international demonstrations, based on the R&D work performed using the National Satellite how the program met and overcame integrity requirements and Testbed. In May 1994, the U.S. Department of Transportation other technical issues, faced skepticism about the program as a approved the SPO’s request to proceed and authorized release of a whole, and accomplished the eventual declaration of WAAS initial operational capability in 2003. Part II concludes with a brief request for proposals. overview of development and evolution of WAAS operations in the The combination of GPS and SBAS (WAAS) and eventually 15 years since IOC. its terrestrial complement (GBAS) would be required to provide

Winter 2019 23 ION Newsletter A CONTINUING HISTORY evaluate candidate WAAS algorithm per- formance, including during its own flight The World of WAAS: Part 2 tests, initially using data from West Coast testbed reference stations and later the full Editor’s Note: This is the second of a three-part series describing the history of the NSTB TRS network. Wide Area Augmentation System (WAAS) on the 25th anniversary of the first flight demonstrations of satellite-based augmentation to improve the performance of the Global NSTB Testing & International Demos Positioning System (GPS). (1996-2007) The articles were written by Dr. Bryant Elrod, former project manager at Stanford Telecom/STel for the WAAS National Satellite Testbed (NSTB); Frank Persello, a The upgraded NSTB provided a valuable member of the Federal Aviation Administration (FAA) Technical Center’s NSTB resource, not only for testing and validat- engineering team; and Dr. Todd Walter, a past president of the ION and director of the ing WAAS technology but also to support wide-area differential GNSS laboratory at Stanford University where ground-breaking the many demonstrations conducted for work on WAAS has long taken place. both domestic and international observ- The first demonstration in the world of using GPS plus space-based augmentation ers. These also served the FAA’s interest in system (SBAS) technology took place in December 1993. (Note: In Part 1, an editorial promoting development of satellite-based error mistakenly listed December 1998 as the first GPS/SBAS flight demonstration.) The first part of this article, published in the Winter 2019 issue of the ION Newsletter, augmentation systems in other regions took readers from the earliest interest in wide-area differential GPS (WADGPS) through of the world, with the eventual goal of announcement of the initial WAAS procurement in 1994. We pick up the story in 1995 as a seamless global GNSS augmentation the FAA undertakes improvement in the NTSB to support the WAAS program. system. The success of the WAAS National Sat- uccessful flight trials and initial ment Certification,” DO-178B. ellite Testbed stimulated interest among Ssteps in procurement of a Wide Area As part of its ongoing R&D efforts, nations’ aviation authorities to develop Augmentation System (WAAS), described Stanford University developed its own similar systems, including in Europe, in Part 1 of this article, represented just testbed master station (TMS) and user Australia, Latin America, and Asia Pacific, the beginning, or perhaps, in Churchillian platform (UP) software with which to often with help from the U.S. FAA. terms, the end of the beginning. A long and winding road of technologi- cal and regulatory evolution lay ahead before the United States could arrive at practical implementation of the WAAS. Even as the WAAS development effort continued, the FAA decided (for risk mitigation) to upgrade the National Satel- lite Test Bed (NTSB), not only to support technical feasibility demonstrations but also to pursue broader goals: to serve as a WAAS prototype, a test & evaluation facility for WAAS technical alternatives, and a data collection resource. This led to changes and additions to various NSTB elements (See summary in Table 1). Expansion of the testbed reference stations (TRSs) enabled the NSTB to support testing under conditions closer to an operational environment and to provide more extensive support for future demonstrations. Upgrading the software on NSTB platforms followed the proce- dures for developing certifiable software in accordance with the Level A requirements of the FAA document “Software Consid- erations in Airborne Systems and Equip- Table 1. Summary of National Satellite Testbed upgrades (1995-96)

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Table 2 lists various activities supported Date(s) Location Activity by the NSTB or its heritage from 1996- Jan-Feb’96 FAATC (NJ) Convair-580: 1st Cat-I trials to ACY-13 w/ & w/o GEO ranging 2007. Those in Oct 1997 & 1998 were Feb-Jul’96 Palo Alto (CA) Piper Dakota: SU flight trials w/ west coast TRSs, POR GEO early flight trials that demonstrated SBAS interoperability with message data gener- Mid ‘96 Eastern Canada Challenger-601: TCA approach trials at 13 sites (≥ 10 per site) ated by various testbeds and broadcast Sep’96 FAATC (NJ) Convair-580 & Challenger-601: Live SIS demos for 65 ICAO by different Inmarsat geostationary sat- GNSSP observers incl 24 Cat-I approaches into ACY-13 ellites (GEOs): Jan’97 FAATC (NJ) Convair-580: 49 Cat-I approaches validating NSTB s/w upgrade  Atlantic Ocean Region-West (AOR-W): Sep’97 Tijuana, Mex. Convair-580: Multiple approach demos for SENEAM officials Used for most demonstrations by the Oct’97 Rome, Boeing 727: Trans-Atlantic flight & approach demos into NSTB. Italy Ciampino airport using SBAS GEO signals from two inter- connected testbeds: NSTB (AOR-W) & ENAV’s MTB (IOR).  AOR-East: Employed for the Iceland Mar-Jun’98 Melbourne, Turbo-Commander (G1000): 1st tests using Airservice Austra- flight demo by the NEST Bed (North- Australia lia’s ASTB; flying terminal area patterns & over 40 approaches ern European Satellite Test Bed), a to Point Cook airport with Cat-I level performance. precursor to the European Geosta- Aug’98 Juneau, Queen Air: SU conducted 1st SBAS flight tests in tionary Navigation Overlay Service Alaska AK incl 48 approaches using expanded TRS network (5 in AK) SU’s TMS in CA and the POR GEO. (EGNOS) System Test Bed (ESTB). Oct’98 Reykjavík, FAA’s Boeing 727; UK’s DERA BAC1-11: SBAS interoperability  Indian Ocean Region (IOR): Used to Iceland demo using GEO broadcasts from U.S. (AOR-W) and Europe support FAA demonstration flights in (AOR-E) testbeds for precision approach trials into Keflavik Rome with the Mediterranean Test Bed Dec’98 Santiago, Boeing 727: 1st SBAS flight & approach demos into Santiago (MTB), which was implemented for Chile Int’l airport using 3 loaned TRSs connected to FAATC TMS Italy's Civil Aviation Authority (ENAV) & AOR-W GEO. under contract to Nuovo Telespazio Jan-Mar’01 Mérida, Boeing 727:Static and flight tests in Yucatan peninsula using Mexico upgraded NSTB incl 3 loaned TRSs in Mexico & SU’s TMS (with FAA authorization for STel to software with the latest algorithms for iono error correction. adapt NSTB software to the MTB). In 2001-03 Caribbean / Static & flight trials in and other nations; vertical perfor- 2000, the MTB was merged with the South America mance reqts for instrument approaches were not achieved ESTB which gained access to a second (CAR/SAM) reliably due to iono effects on GPS signals in the region GEO (IOR) via the MTB's testbed 2006-07 SE Asia Static & flight trials in using the APEC testbed with uplink station (TUS) at Fucino, Italy. TRSs in 5 SE Asia countries participating under USTDA grant with tech support by ITT-AES (formerly STel/E)  Pacific Ocean Region (POR): Used in west coast and Alaska flight trials. Table 2: Flight Trials & International Demos (1996-2007) Three other testbeds were also adapta- developed by RTCA SC-159; and a com- analysis of the ionospheric environment tions of the NSTB: mercial development attempt that ended encountered there. A long term solution  Augmentation Satellite TestBed in 2008. Since 2016, a new look at SBAS to mitigating the severe iono effects there (ASTB): Air-services Australia (AA) used applicability is underway with Australia will come as a transition to dual frequency the ASTB with support from STel to and New Zealand collaborating and other usage occurs (e.g. L1, L5). evaluate a hybrid satellite/ground-based sectors involved besides aviation.  APEC GNSS Testbed: Implemented in augmentation system concept. Interest  CAR/SAM TestBed (CSTB): Imple- 2006 under a U.S. Trade and Develop- in the hybrid concept, later renamed the mented in 2001-02 with the aim of estab- ment Agency technical assistance grant Ground-based Regional Augmentation lishing a trial platform for development to Aerothai, Inc. on behalf of the Kingdom System, grew from concerns over the cost of a WAAS type SBAS. Under an ICAO/ of Thailand and other participating Asia effectiveness of using GEOs when FAA memorandum of understanding, Pacific Economic Cooperation co-members Australia's national VHF network might technical support was provided to over 10 in southeast Asia (Indonesia, Malaysia, suffice. participating nations in the region. the , and Vietnam). Partici- With potential interest in a ground- After inconclusive flight trials in 2002 the pants benefited from the hands-on based solution expressed by other project was reoriented to collecting and application of GNSS technology and a nations, AA pursued: inclusion in the processing data from 13 TRSs for more cooperative experience that encouraged future APEC activity. ICAO GNSS SARPs; getting a MOPS

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WAAS Progress (1997-2003) By June 1998, Raytheon Systems Com- pany (RSC) had fielded Phase I hardware and connected and tested the terrestrial communications supporting the WAAS network. However, about this time con- gressional concerns arose regarding program management, cost growth and schedule delays, and technical risks. This led to skepticism about the prospects for the WAAS program and, ultimately, the imposition of conditions in the congres- sional budget allocations in fiscal years 1998 and 1999 as well as mandated reviews of the program. was highly accurate, the FAA questioned United States. The NSOL category Consequently, at various stages the FAA WAAS reliability and whether it could be included some aviation operations and sought advice and support from indepen- certified for aviation use. Commissioning all non-aviation applications. dent experts to deal with immediate and was postponed to no sooner than With WIPP guidance, by mid-2002 the longer-term issues. During 1997-2001, September 2002. integrity monitoring software had been three separate advisory panels reaffirmed The FAA then established a WAAS redesigned, installed, tested and made the soundness of the WAAS concept with Integrity Performance Panel (WIPP) in operational in both WAAS Master Stations. several recommendations including: February 2000 comprised of experts from In September 2002, a 60-day stability test  allocate more time for problem solving, government, industry, and academia and was successfully completed to conclude including delay of Phase II/III work led by the late Prof. Per Enge of Stanford final integration and testing of the system pending operational experience with University. The WIPP's role was to inde- before FAA acceptance from RSC. Phase I; pendently assess the safety of WAAS, Before declaring initial operational  conduct further analysis/design/vali- determine how best to interpret the capability (IOC) of WAAS, FAA activities dation of algorithms for ionospheric integrity requirement, develop algorithms included certifying user avionics and pub- correction and integrity monitoring and to meet this requirement, validate the lishing WAAS approach procedures for an alerts; algorithms, and recommend system initial set of airports across the United  firm up the GEO requirements (number/ improvements. States. These procedures were mostly for location) to meet coverage needs and The effort led to proving that WAAS Lateral Navigation/Vertical Navigation mitigate loss of service from a single- could satisfy its most stringent require- (LNAV/VNAV) approaches with mini- thread failure or need for replenishment ment: ≤ 1 in 10 million chance of transmit- mums as low as 350 feet and a few that that required lengthy lead times for ting hazardous misleading information to qualified for Localizer Performance with procurement/launch of satellites. a user without notification of its degraded Vertical Guidance (LPV) service at lower In January 1999 the WAAS Phase I status within six seconds or less. minimums. (See Figure 1.) commissioning date was moved to Meanwhile, RSC corrected non- On July 10, 2003 after a lengthy but September 2000 to allow more time to integrity-related errors affecting WAAS rewarding effort, the FAA declared WAAS address any problems arising during stability and conducted a successful part of the NAS, thereby expanding GPS integration and testing. RSC worked to 21-day stability test in March 2000 that use with the safety, reliability, and complete Phase I development including demonstrated a stable and reliable signal accuracy necessary for safety-of-life the Correction and Verification (C&V) operating without interruption. In April (SOL) applications.

module, the most complex in the WAAS 2000, the FAA declared the WAAS SIS Master Station software. immediately available for non-safety- Part III in the history of WAAS will In December 1999, however, tests of-life (NSOL) users with a capability appear in the next issue of the ION revealed the failure of a warning system of delivering an accuracy of one to two Newsletter. It will provide a brief overview designed to identify integrity and other meters (horizontal) and two to three of development and evolution of WAAS issues. Although positioning performance meters (vertical throughout the continental operations in the 15 years since IOC.

ION Newsletter 23 Spring 2019 A SHORT HISTORY Although nominal WAAS perfor- mance was quite good, occasions arose The World of WAAS: Part 3 when the ionosphere was moderately disturbed and vertical guidance would briefly be unavailable. In late 2011 Editor’s Note: This is the final installment of a three-part series describing the history of the Wide Area Augmentation System (WAAS) on the 25th anniversary of the first flight another improvement to the ionospheric demonstrations of satellite-based augmentation to improve the performance of the Global algorithm, called kriging, was imple- Positioning System. mented to better handle these moderate The articles were written by Dr. Bryant Elrod, former project manager at Stanford disturbances, increasing the amount of Telecom/STel for the WAAS National Satellite Testbed (NSTB); Frank Persello, a time that vertical guidance would be member of the Federal Aviation Administration (FAA) Technical Center’s NSTB available. Kriging also further improved engineering team; and Dr. Todd Walter, a past president of the ION and director of the coverage over Alaska. wide-area differential GNSS laboratory at Stanford University where ground-breaking Meanwhile, in 2007 the FAA began work on WAAS has long taken place. The first demonstration in the world of using GPS plus space-based augmentation expanding use of LPV-200, which re- system (SBAS) technology took place in December 1993. (Note: In Part 1, an editorial quired tighter bounds on WAAS perfor- error mistakenly listed December 1998 as the first GPS/SBAS flight demonstration.) mance that were not supported on the The first parts of this article, published in the Winter and Spring 2019 issues of the ION West Coast. Once again, the ionospheric Newsletter, took readers from the earliest interest in wide-area differential GPS (WADGPS) algorithms were modified to exploit the through declaration of WAAS as part of the National Air Space (NAS) in 2003. We typically quiet behavior of the ionosphere conclude the story with a brief description of the evolution of WAAS over the past 15 and remove more conservatism. In 2016 years. a Moderate Storm Detector (MSD) was introduced that allowed for nearly full ince 2003, the FAA has continued provide a denser grid of corrections. This coverage of LPV-200 service throughout Sbuilding on the WAAS IOC capabil- update significantly increased vertical CONUS and most of Alaska. ity. This includes several steps for im- guidance coverage to include nearly all of Figure 1b illustrates the regional proving performance and enhancing ro- CONUS as well as the southern part of expansion of LPV-200 availability result- bustness to enable support of approaches Alaska. ing from the various improvements since down to a vertical goal of 200 feet above In 2008, two more WRSs were added WAAS IOC. A more detailed exposition threshold (also known as LPV-200). The to both Canada and Mexico bringing the of WAAS improvements and important vertical guidance service provided by total number of reference stations up to steps along the way can be found in the WAAS IOC covered much of the conter- the current level of 38. The WIPP was article “WAAS at 15” published in the minous United States (CONUS), but it also able to use the historical ionospheric Winter 2018 issue (volume 65, number didn’t fully extend to all coasts nor did it observations (including severe storms 4) of the ION journal NAVIGATION. cover Alaska. (See Figure 1a.) observed in October and November of More recent FAA activities involve pre- The WIPP continued to refine the 2003) to more accurately model dis- paring for dual-frequency operations as integrity algorithms while the FAA turbed ionospheric error as well as to tune the new GPS L5 signal becomes available. expanded the network of WAAS reference the uncertainty bounding equations and This will allow for direct estimation and stations (WRSs) from the original set of remove excessively conservative margins removal of ionospheric effects and better 25. In 2006 four additional WRSs were of error that had been incorporated into WAAS service over broader areas. added to Alaska, and in 2007 two were the earlier iterations of the model. This fielded in Canada and three in Mexico. round of improvements now allowed ver- International SBAS Developments Also in 2007, a significant upgrade was tical guidance to be available throughout Through its sharing of information made to the ionospheric algorithms used virtually all of CONUS and Alaska. gained from NSTB experience, numer- to both estimate the amount of iono- At the same time, the two original ous flight trials, and WAAS lessons- spheric delay affecting the GPS ranging GEOs located over the Atlantic and learned, the FAA has contributed sig- measurements and the uncertainty re- Pacific Oceans were replaced with two nificantly to the international expansion maining after applying those corrections. GEOs that were much more centrally po- of satellite-based augmentation systems. Moreover, because the original iono- sitioned over CONUS. These new GEOs These include SBASs currently certified spheric grid definition above 55° N was provided overlapping coverage at much for safety-of-life operations (Japan/MSAS very sparse, having more WRSs in the far higher elevation angles. In addition, they in 2007, Europe/EGNOS in 2011, and north allowed WAAS to better resolve the also provided ranging measurements that India/GAGAN in 2015) or in develop- ionospheric behavior over Alaska and to help to fill in gaps in GPS coverage. ment (Russia/System for Differential

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a. IOC LPV-200 Availability - July 10, 2003 b. MSD LPV-200 Availability - August 27, 2016

70 70

60 60

50 50

40 40 Latitude (deg) Latitude (deg) 30 30

20 20

–160 –140 –120 –100 –80 –60 –160 –140 –120 –100 –80 –60 Longitude (deg) Longitude (deg)

<50% >50% >75% >85% >90% >95% >99% >99.5% >99.9% Availability with VAL = 35, HAL = 40 Figure 1. LPV-200 Coverage at the time of WAAS IOC and by 2016

Region SBASs 1st (12/15/93) NSTB SBAS flight trials & demos

USA WAAS Phase: I II | III | IV (GPS) N. America FLP MSAS 7/10/03 200 Japan MSAS (GPS) 9/27/07 QZSS GEO ESTB EGNOS MTB Europe (GPS & GLONASS) 3/2/11 9/29/15 200 GAGAN Test only India TDS / IES OPN’L SYS Studies / Reqts Def. (GPS & NSOL INRSS) 5/19/15 Legend: Development SOL: NPA* SDCM SIS SOL: LNAV / VNAV* Russia (GLONASS Operational SOL: LPV* & GPS) SOL: LPV-200* BSSBAS Project Approval 200 China (Beidou * See Figure 1 & GPS) 1990 2000 2010 2020 Figure 2. SBAS Historical Timeline (1990–Present)Figure 2 - SBAS Historical Timeline (1990 – Present)

Corrections and Monitoring, China/ will evolve to use higher power, dual-fre- pated in the FAA’s successful program to BeiDou SBAS). (See Figure 2.) quency signals (e.g., GPS L1C, L5 and/ develop SBAS technology that became SBASs in others regions are still under or Galileo E1/E5a) from more satellites. WAAS. Special recognition goes to study or initial testing (e.g., Australia, This will lessen interference vulnerabili- the succession of program leaders: J.J. South Korea). While the primary focus ties, enable self-canceling of ionosphere- Fee, who in 1990 convinced upper for such systems to date has been aug- induced ranging errors, and reduce the management of the need to establish menting GPS for L1-C/A signal users size and complexity of the ground system. an R&D program and began planning to meet civil aviation requirements, the the NSTB; J. Dorfler, who in 1991–96 approaching phase for SBASs will involve At the time of those first SBAS flight secured funding, established the NSTB SatNav augmentation in a multi-national demos, 25 years ago, who would have team, and got the WAAS procurement constellation and multi-signal/frequency thought of such possibilities? underway; J.C. Johns, D. Peterson, D. environment. Hanlon, L. Eldredge, D. Lawrence, D. Support for legacy GPS users will Acknowledgment Bunce, and D. Wilkerson, who led the continue during what will probably be We wish to applaud the many contribu- WAAS development through IOC and a lengthy transition period. Eventually, tions made by members of government, its subsequent evolution. however, satellite-based augmentation industry, and academia who partici-

Summer 2019 21 ION Newsletter