2
GPS Applications and Requirements

Introduction

GPS specifications were originally developed by the DOD (Department of Defense) in the late 1960s with the primary objective of satisfying military navigation requirements. A secondary objective was to provide a separate, less accurate signal for both military and civilian use. This signal, described in Appendix C, and known as the Standard Positioning Service (SPS), was intentionally degraded in accuracy (100 meters, 2 drms) to avoid its exploitation by potentially unfriendly users.

As the GPS satellite constellation expanded and was eventually completed in 1993, the use of the freely available SPS signal for civil applications also continuously expanded. GPS is now used for positioning, navigation, and timing applications in a number of civil and commercial activities related to aviation; maritime commerce and recreation; land transportation; mapping, surveying, and geodesy; scientific research; timing and telecommunications; and spacecraft. Each of these broadly defined civilian user categories, along with military applications, is discussed in this chapter.

Many of the innovative civilian applications that this chapter will address were not foreseen by the original designers and developers of GPS and cannot be accomplished without augmenting and/or enhancing the stand-alone capabilities of the system as currently configured. As a result, differential correction methods and user equipment integrated with other positioning technologies, as described in Appendix C, have been utilized to meet the requirements of many of these applications. Within this context, there have been no requirements imposed on the basic GPS by civilian users to date beyond the assurance that the basic SPS signal-in-space will remain freely available at its currently defined accuracy level.1 Users have taken this signal and adapted it to their applications. The basic GPS has therefore become a ''dual-use" system, which is still designed to meet the requirements of only a single user, the Department of Defense.2

1  

This official U.S. government policy is currently reiterated every 2 years in the Federal Radionavigation Plan.

2  

The term "dual-use" usually refers to use by both the military and civilians.



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--> 2 GPS Applications and Requirements Introduction GPS specifications were originally developed by the DOD (Department of Defense) in the late 1960s with the primary objective of satisfying military navigation requirements. A secondary objective was to provide a separate, less accurate signal for both military and civilian use. This signal, described in Appendix C, and known as the Standard Positioning Service (SPS), was intentionally degraded in accuracy (100 meters, 2 drms) to avoid its exploitation by potentially unfriendly users. As the GPS satellite constellation expanded and was eventually completed in 1993, the use of the freely available SPS signal for civil applications also continuously expanded. GPS is now used for positioning, navigation, and timing applications in a number of civil and commercial activities related to aviation; maritime commerce and recreation; land transportation; mapping, surveying, and geodesy; scientific research; timing and telecommunications; and spacecraft. Each of these broadly defined civilian user categories, along with military applications, is discussed in this chapter. Many of the innovative civilian applications that this chapter will address were not foreseen by the original designers and developers of GPS and cannot be accomplished without augmenting and/or enhancing the stand-alone capabilities of the system as currently configured. As a result, differential correction methods and user equipment integrated with other positioning technologies, as described in Appendix C, have been utilized to meet the requirements of many of these applications. Within this context, there have been no requirements imposed on the basic GPS by civilian users to date beyond the assurance that the basic SPS signal-in-space will remain freely available at its currently defined accuracy level.1 Users have taken this signal and adapted it to their applications. The basic GPS has therefore become a ''dual-use" system, which is still designed to meet the requirements of only a single user, the Department of Defense.2 1   This official U.S. government policy is currently reiterated every 2 years in the Federal Radionavigation Plan. 2   The term "dual-use" usually refers to use by both the military and civilians.

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--> Although the continued existence of GPS as a dual-use system clearly requires some trade-offs between civilian utility and national security, the NRC committee has concluded from its deliberations that because GPS provides tremendous benefits to both civilian and military users, as the remainder of this chapter will clearly illustrate, it should firmly remain a dual-use system. From the committee's perspective, recognition that GPS is truly a dual-use system brings with it the responsibility of meeting the requirements of all users to the highest degree possible. This implies that the system must be designed to the specifications of both civilian and military requirements. Many nonmilitary users of GPS have requirements that have been validated by standard-setting bodies and federal agencies that can now only be achieved through the additional cost of differential GPS (DGPS). Because human safety is an important consideration for many of these applications, a specified level of accuracy is not the only requirement. Integrity, availability, and resistance to RF (radio frequency) interference (both intentional and unintentional), as defined in Appendix C, are of significant importance as well. The sections that immediately follow discuss these requirements for each user category. The task given to the NRC committee by Congress also recognized the dual-use nature of GPS and the trade-offs that exist between civil and military utility when it asked the following questions: "What augmentations and technical improvements to the GPS itself are feasible and could enhance military, civilian, and commercial use of the system?"; and, "What are the implications of security-related safeguards and countermeasures for the various classes of civilian GPS users?" These questions are examined in the remainder of the chapter by determining the challenges that currently exist for full utilization of GPS in each user community, including challenges that are related to Selective Availability (SA) and Anti-Spoofing (A-S). Although some of these challenges relate to the limitations of associated technologies and technology policies, findings in this chapter reveal that the biggest challenge for most users is meeting the requirements of a given application through augmentation of the GPS SPS. It stands to reason, therefore, that improving the basic capabilities of GPS and the freely available SPS signal will enhance the ability of civilian users to meet their requirements more easily, more cost-effectively, and in some cases, without augmentation or enhancement from DGPS or other positioning technologies. Improvements to the basic GPS can be made that will improve the military's ability to meet its requirements as well. Specific technical recommendations that would achieve this goal and address the tasks assigned to the NRC committee are discussed in detail in the next chapter. GPS Military Applications Although the overall use of GPS in the civilian sector has grown much faster than military usage, the system was designed with military requirements in mind, and the importance of the system to national security has not diminished. GPS is more accurate than any other radionavigation or positioning technology developed by the DOD, and is beginning

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--> to replace all other operational systems.3 The coalition military forces demonstrated the effective use of GPS for many of these proposed applications during the Persian Gulf War, despite the fact that the GPS constellation consisted of only 16 satellites at the time. This limited three-dimensional coverage of the Persian Gulf region to 18 hours per day. Another limiting factor was the small number of P-code military receivers in the DOD inventory at the time of the conflict. This prompted a National Command Authority decision to turn SA to zero during the war and led to the DOD's purchase of thousands of civilian GPS receivers, which became known as "sluggers".4 In addition to this official procurement, many units and individuals deployed to the Persian Gulf ordered their own GPS receivers directly from vendors and manufacturers.5 Current and Future Applications and Requirements The most common use of GPS during the Persian Gulf War, and perhaps the most critical, was for land navigation. U.S. Army tanks and infantry relied heavily on GPS to avoid getting lost during movements to various destinations in the featureless desert. GPS also was used by coalition forces for en route navigation by aircraft, helicopter search and rescue, marine navigation, and even munitions guidance in the case of the U.S. Navy's Standoff Land Attack Missile (SLAM).6 The use of GPS for precision-guided munitions such as the SLAM will increase in the future. The U.S. military currently has, or is developing, eight additional types of precision land attack weapons that utilize GPS integrated with inertial navigation systems for mid-course guidance.7 Another important GPS application under consideration is the 3   The DOD plans to phase out use of Loran-C and Omega in 1994, Transit in 1996, and land-based navigation aids by 2000, depending on the progress of GPS installation and integration. Civilian use of these systems, however, may continue. Source: Radionavigation System Users Conference held in Washington D.C. on November 9-10, 1993, (unpublished). 4   The "slugger" or Small Lightweight GPS Receiver is a Trimble Navigation TRIMPACK, three-channel receiver that utilizes the L1, C/A-code to provide three-dimensional navigation capability. More than 10,000 receivers were purchased by the DOD from Trimble Navigation and other receiver manufacturers during the Persian Gulf War. 5   Bruce D. Nordwall, "Imagination Only Limit to Military, Commercial Applications for GPS," Aviation Week & Space Technology, 14 October 1991, p. 60. 6   Joseph Wysocki, "GPS and Selective Availability—The Military Perspective," GPS World, July/August 1991, pp. 38-43. 7   These eight weapons include: the Tomahawk Block III and IV cruise missile; the Tri-Service Stand-Off Attack Missile; the Joint Direct Attack Munition; the Joint Stand-Off Weapon; the GBU-15 precision glide bomb; the AGM-130, a powered version of the GBU-15; and finally, the ATACMS ballistic missile. Source: J.G. Roos, "A Pair of Achilles Heels: How Vulnerable to Jamming are U.S. Precision-Strike Weapons?" Armed Forces Journal International, November 1994, p. 22.

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--> use of GPS for the precision delivery of cargo by parachute or paraglider. For this application, GPS must be capable of providing steering commands to a reefing system to steer the parachute or paraglider to the desired landing point. Combat search and rescue is another important function for which the use of GPS is increasing. Although GPS is already used for navigation by helicopters and other aircraft involved in combat search and rescue, it also will be used in the future to determine the exact location of downed aircrew members. By combining GPS with space-based communications capabilities, individuals can be found quickly, saving lives, time, and money. Communications capabilities would allow the location of aircraft, helicopters, and tanks to be monitored in real time, reducing casualties by friendly fire. Further, if GPS and communications capabilities are combined with guidance systems, unmanned aerial vehicles could be used for surveillance of target areas. Tables 2-1 through 2-3 represent an extensive list of the military's positioning and navigation applications and their requirements. Challenges to Full GPS Utilization Accuracy and Integrity The shaded cells in Tables 2-1 through 2-3 point out positioning and navigation requirements that cannot be met with the current 16 meter (SEP) specified accuracy8, or 8-meter (CEP) derived accuracy9, of the GPS PPS (Precise Positioning Service). Presumably, many of these requirements are currently being met by other guidance systems, such as highly accurate inertial navigation systems and terminal seekers, and other radionavigation systems, such as the microwave landing system. If these applications were to rely on GPS alone in the future, their accuracy requirements could only be met with some form of DGPS or a significantly improved PPS. Some of the aviation applications listed in Table 2-1 also have specified integrity requirements. These requirements cannot be met with the PPS as currently configured. 8   SEP, or spherical error probable, represents an accuracy that is achievable 50 percent of the time in all three dimensions (latitude, longitude, and altitude). PPS accuracy is normally represented in this manner. The 2 drms PPS specified accuracy value is 21 meters SEP, as shown in Figure C-7 in Appendix C. 9   CEP, or circular error probable, represents an accuracy that is achievable 50 percent of the time in two dimensions (latitude and longitude). Most military accuracy requirements are defined in this manner. CEP, and other positioning accuracy definitions are discussed in greater detail in Appendix D.

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--> Table 2-1 Military Aviation and Precision-Guided Munitions Applications and Requirementsa   Application Accuracy Integrity   Resistance to RF Interference       1 minus PHE times PMDb Time to Alarm   Aviationb Low-level Navigation and Air Drop 50.0 m (2 drms) 0.999 10 sec High   Non-precision Sea App/Landings 12.0m (2 drms) 0.999 10 sec High   Precision App/Landings Unprepared Surface 12.5m (2 drms) 0.999 6 sec High   Precision Sea App/Landings 0.6m (2 drms) 0.999 6 sec High   Amphibious and Anti-submarine Warfare 50.0 m CEP Not specified Not specified High   Anti-air Warfare 18.1 m CEP Not specified Not specified High   Conventional Bombing 37.5 m CEP Not specified Not specified High   Nuclear Bombing 75.0 m CEP Not specified Not specified High   Close Air Support/Interdiction 9.0 m CEP Not specified Not specified High   Electronic Warfare 22.5 m CEP Not specified Not specified High   Command, Control & Communications 37.5 m CEP Not specified Not specified High   Air Refueling 370.0 m CEP Not specified Not specified High   Mine Warfare 16.0 m CEP Not specified Not specified High   Reconnaissance 18.1 m CEP Not specified Not specified High   Magnetic and Gravity Survey 20.0 m CEP Not specified Not specified High   Search & Rescue and Medical Evacuation 125.0 m CEP Not specified Not specified High   Mapping 50.0 m CEP Not specified Not specified High Precision-guided Munitions Precision-guided Munitions 3.0 m CEP Not specified Not specified High a. Availability and continuity of service requirements are not specified for military aviation and precision-guided munitions applications. b. This measure relates the probability that a hazardously misleading error will occur (PHE) and the probability that this error will go undetected (PMD). c. Peacetime requirements for the en route through Category I approach and landing phases of flight are identical to FAA requirements.

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--> Table 2-2 Naval Applications and Requirementsa   Application Accuracy Resistance to RF Interference En route Navigation Pilotage & Coastal Waters 72.0 m CEP High   Inland Waters 25.0 m CEP High   Open Waters 2400.0 m CEP High   Rendezvous 380.0 m CEP High   Harbor 8.0 m CEP High Mine Warfare Swept Channel Navigation & Defensive Mining 16.0 m CEP High   Offensive Mining 50.0 m CEP High   Anti-mine Countermeasures <5.0 m CEP High   Geodetic Reference Guide (WGS-84) 128.0 m CEP High Special Warfare Airdrop 20.0 m CEP High   Small Craft 50.0 m CEP High   Combat Swimming 1.0 m CEP High   Land Warfare & Insertion/Extraction 1.0 m CEP High   Task Group Operations 72.0 m CEP High Amphibious Beach Surveys 185.0 m CEP High Warfare Landing Craft 50.0 m CEP High   Artillery & Reconnaissance <6.0 m CEP High Surveying Hydrographic <5.0 m (2 drms)b High   Ocean & Geophysical Deep Ocean 90.0 m (2 drms) High   Oceanographic 100.0 m (2 drms) High a. Availability, integrity, and continuity of service requirements are not specified for naval applications. b. This requirement can currently be met with data post-processing.

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--> Table 2-3 Military Land Applications and Requirementsa   Application Accuracy Resistance to RF Interference   Chemical Warfare 100.0 m CEP High Engineer Mine Neutralization 100.0 m CEP High   Mine Dispensing & Gap Crossing 50.0m CEP High Field Artillery MLRS 20.0 m CEP High   Howitzer 175 m CEP High   Mortars 50.0 m CEP High   Fist-V & Forward Observer 30.0 m CEP High   Artillery and Mortar Radar 10.0 m CEP High   Infantry & Armorb 100.0 m CEP High   Missile Munitions 93.0 m CEP High   Signal 15.0 m CEP High   Special Operations Forces 30.0 m CEP High   Intelligence Electronic Warfare 20.0 m CEP High   Ordnance 84.0 m CEP High Air Defense Artillery Patriot 10.0 m CEP High   Hawk 40.0 m CEP High a. Availability, integrity, and continuity of service requirements are not specified for military land transportation applications. b. The Infantry & Armor category also includes transportation, soldier support, military police, and quartermaster. Anti-Jam and Anti-Spoof Capability Although the "Resistance to RF Interference" column in Tables 2-1 through 2-3 does not include quantitative values, a high level of resistance to RF interference is a critical requirement for most military applications.10 For the military, the primary interference concerns are deliberate jamming and spoofing by an adversary or by our own forces. In future conflicts, a potential enemy also will be utilizing the capabilities of GPS and DGPS against U.S. and allied military forces. In order to deny this use, friendly forces must have the ability to eliminate an adversary's use of GPS signals without impacting the effectiveness of their own user equipment. This dictates that military GPS receivers also must be capable of continued operation in an environment populated with both U.S. and enemy jammers. Therefore, GPS-based navigation systems used on aircraft, ships, land vehicles, and precision-guided munitions must possess one or more of the following capabilities: 10   Quantifiable values for resistance to RF interference are given in decibels (dB), and relate to the ratio of jammer power to signal power (J/S). These values are very specific to a given mission and operational environment, making a generic J/S requirement for a given application difficult to determine.

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--> have sufficient jamming-to-signal ratio strength to navigate through the jamming environment successfully; be able to null out the jamming signal; and/or have an alternative to GPS for navigating through the jamming environment. The military currently relies on its SA and A-S (Anti-spoofing) security procedures to deny full GPS accuracy to the enemy while maintaining the use of a highly accurate spoof resistant signal. Anti-jam antennas and antenna electronics also are deployed on many weapons systems to provide increased jam resistance, and integrated GPS/inertial navigation systems provide a means of navigating to a target in spite of successful jamming. None of these procedures and technical measures, however, can be considered the final solution to the military's requirement to simultaneously use GPS and deny its use to the enemy. A number of candidate improvements in this regard are presented in the next chapter. Findings The GPS PPS meets most of the military's positioning and navigation accuracy requirements, although some applications require accuracy and integrity that is beyond the capability of the PPS as currently configured. The anti-jamming and anti-spoofing capabilities of military GPS user equipment are critical to successful mission completion in a battlefield environment characterized by both U.S. and enemy spoofers and jammers. GPS Aviation Applications Despite the fact that investigations into the use of satellites for civil aviation applications have been conducted for over 20 years, the concept was not considered financially or technically feasible until the development of GPS.11 Instead, a large number of ground-based radionavigation systems have been relied upon around the world for air navigation services, and ground-based air traffic controllers have utilized radar, voice position reporting, and visual sightings for aircraft surveillance.12 It now appears that a 11   Federal Aviation Administration (FAA), FAA Satellite Navigation Program Master Plan. FAA Research and Development Service, Satellite Program Office (ARD-70), 15 February 1993, p. 2. 12   Existing ground-based radionavigation systems include NDBs (Non-Directional Beacon), VORs (VHF Omni-directional Range), VOR/DMEs (VORs with Distance Measuring Equipment), TACANs (Tactical Air Navigation), and VORTACs (combined VORs and TACANs). Other systems include the Instrument Landing System (ILS), used for precision approach and landing, and Loran-C and Omega, both of which are used for en route navigation. Each of these systems is described in detail in the 1992 Federal Radionavigation Plan.

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--> Global Navigation Satellite System (GNSS), based on GPS and additional satellite augmentations, could eventually replace most of these ground-based systems. Current and Future Applications and Requirements Civilian pilots have been utilizing GPS in uncontrolled airspace for applications such as crop dusting, aerial photography and surveying, search and rescue, and basic point-to-point navigation for some time.13 On June 9, 1993, the Federal Aviation Administration (FAA) approved GPS for supplemental use in the domestic, oceanic, terminal, and non-precision approach phases of flight in controlled airspace as well. This supplemental use required that another navigation source, such as a ground-based radio aid, must still be monitored while using GPS as the primary system. Once initial operating capability was declared for GPS by the DOD and DOT (Department of Transportation) on December 8, 1993, the monitoring of another navigation system for integrity purposes became unnecessary, provided that the GPS receiver utilized meets the FAA's TSO C-129 criteria for Receiver Autonomous Integrity Monitoring (RAIM).14 In addition, traditional navigation sources such as VORs and TACANs must still be operational, and their associated receiver equipment must be on board the aircraft as a backup. Several GPS receivers have already been certified under the FAA's TSO C-129 criteria. The use of GPS as the primary means of navigation for the domestic en route through non-precision approach phases of flight will require better availability and continuity of service (reliability) than is currently available from the stand-alone system. Phase I of the FAA's Wide Area Augmentation System (WAAS), which is scheduled to be in place by 1997, will make this possible. Table 2-4 contains the quantitative performance requirements that the WAAS is being designed to meet.15 In the near future, the FAA hopes that GPS also will be used for Category I precision approaches. Precision approaches are required when the weather conditions at a given airport reduce the ceiling, or height of the base of a cloud layer, and the visibility, or the distance a pilot can see visually, to levels that are below non-precision approach criteria.16 Phase II of the FAA's WAAS implementation, scheduled for completion in 2001, will improve GPS-derived accuracy enough to allow the system to be used for these types of approaches. This increased accuracy requirement, which was also derived from the WAAS request for proposal (RFP), is included in Table 2-4. 13   In uncontrolled airspace, pilots are not in direct communications with air traffic controllers, are responsible for their own navigation, and must be able to avoid terrain and collisions with other aircraft visually. 14   RAIM is discussed in the next chapter, and is further explained in Appendix C. 15   Federal Aviation Administration. Wide Area Augmentation System (WAAS), Request For Proposal, DTFA01-94-R-21474. 16   Category I approaches can be flown when the visibility is no less than 0.81 kilometers (0.5 miles), and the ceiling is no lower than 61 meters (200 feet).

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--> Testing by the FAA and several contractors is currently underway to determine the feasibility of also using GPS to conduct Category II and III approaches and landings, and the results to date have been very promising. These approaches are flown when the weather conditions at an airport are even worse than those described previously for Category I.17 As can be expected, the accuracy, integrity, and continuity of service requirements are stricter than those for Category I landing systems, and therefore, the concepts currently under development utilize local-area differential GPS augmentations, rather than the WAAS. The requirements for Category II and III, which were derived from the Federal Radionavigation Plan and existing International Civil Aviation Organization (ICAO) requirements for instrument landing systems (ILS), are listed in Table 2-4.18 GPS also shows promise for use in Traffic Alert/Collision Avoidance Systems (TCAS) and Automatic Dependent Surveillance (ADS) systems. TCAS is already used by U.S. airlines and by many airlines in Europe.19 Testing of an updated TCAS, which broadcasts an aircraft's position and velocity derived from GPS on the existing Mode-S datalink, has proven to be more accurate than the existing system.20 The requirements for this application are listed in Table 2-4. ADS systems, which are still under study and development, would automatically broadcast an aircraft's GPS-derived position to the air traffic management (ATM) system via geostationary communications satellites in oceanic airspaces, and via terrestrial-based communications links in domestic airspace.21 This would allow for more efficient ocean crossings than are currently possible using the existing ATM reporting system. ADS would also be useful in the domestic en route and terminal phases of flight, where current aircraft separation is primarily the responsibility of air traffic controllers who utilize secondary surveillance radars. ADS systems are also being considered for monitoring the land-based operations of an airport, such as aircraft taxiing, and service-vehicle collision avoidance. The requirements listed for ADS in Table 2-4, which are based on current radar-based surveillance requirements, should be considered preliminary because the FAA is in the early phases of studying how to use GPS in performing the surveillance function. 17   For example, a properly equipped aircraft can fly a Category IIIB approach when the ceiling is below 15 meters (50 feet) and the visibility is between 50 and 200 meters. Source: Federal Aviation Administration, FAA Advisory Circular No. 120-28C: Criteria for Approval of Category III Landing Weather Minima, 9 March 1984. 18   These requirements are currently under review and may be revised due to an emerging concept known as required navigation performance (RNP). See: R. J. Kelley and J. M. Davis, "Required Navigation Performance (RNP) for Precision Approach and Landing with GNSS Application," Navigation: Journal of the Institute of Navigation 41, no. 1 (1994): pp. 1-30. 19   The current TCAS configuration uses a data link known as Mode-S to measure the vertical separation between two aircraft in close proximity to one another. Measurements that are determined to be too close by the TCAS software set off an alarm that warns the flight crew and allows them to take action. 20   "FAA Redirects TCAS-3 Effort," Aviation Week and Space Technology, 27 September 1993, p. 37. 21   The exact method of transmission in U.S. domestic airspace has not yet been determined.

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--> Table 2-4 GPS Performance Requirements for Aviation Applicationsa   Application Accuracy (2 drms) Integrity   Availability Continuity of Service Resistance to RF Interference       1 minus PHE times PMDb Time to Alarm       Navigation En route Oceanicc 23.0 km Not Specified 30.0 s 99.977% Not Available High   En route to Non-Prec. App/Landing 100.0 m 1-1 x10-7 per hourd 8.0 s 99.999% 1-1x 10-8 per hour High   CAT I App/Landing 7.6 m 1-4 x 10-8 per app. 5.2 s 99.9% 1-5.5x10-5 per app. Very High   CAT II App/Landing 1.7 m (vertical) 1-0.5 x 10-9 per app. 2.0 s Not specified 1-2x10-6 per 15 sec. Very High   CAT III App/Landing 0.6-1.2 m(vertical) 1-0.5 x 10-9 per app. 2.0 s Not specified 1-2x10-6 per 15 sec. Very High Survei- llance TCAS 14.4 me Not Specified Not Spec. Several daysf Essential equip.g Installed equipmenth   Oceanic ADS Not specified Not specified Not spec. Not specified Not specified Not specified   Domestic ADS 200.0 mi Not specified Not Spec. 99.999%i Not specified Very High   Surface Surveillance 12.0 m (resol.)j Not specified Not spec. 99.87%j Not specified Very High a. Unless otherwise annotated, GPS aviation requirements were provided by the MITRE Corporation. b. This measure relates the probability that a hazardously misleading error will occur (PHE) and the probability that this error will go undetected (PMD). c. Source of en route oceanic requirements: U.S. Department of Commerce, National Telecommunications and Information Administration, A Technical Report to the Secretary of Transportation on a National Approach to Augmented GPS Services, NTIA Special Publication 94-30, November 1994, p. 12. It is likely that the accuracy requirement will become significantly more stringent in the future to allow tighter spacing between aircraft. d. This number is equivalent to 0.9999999 or 99.99999 percent. e. Based on current TCAS specifications. f. According to airline minimum equipment list (MEL) practice approved by FAA certification. g. Based on reliability certification for essential equipment. h. Must meet installed equipment test. Otherwise unspecified. i. Based on current radar surveillance. j. Based on Airport Surface Detection Equipment-3 specifications, which require the resolution of two targets separated by 12 meters.

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--> The presence of SA, despite the fact that it does not degrade timing accuracy to less than currently acceptable levels, is considered to be another limitation on overall system reliability. The telecommunications industry believes that GPS, being "subject to failures or deliberate denial of signal", cannot and should not be used without being backed up by other technologies able to provide the same information.54 In the future, it is also likely that accuracies in the range of 50 to 100 nanoseconds will be required for some telecommunications applications. It will be difficult for direct GPS timing to meet this requirement, even without the presence of SA. Findings GPS currently meets all accuracy requirements for both GPS time transfer and time synchronization using direct GPS time. Many telecommunications companies are still hesitant to utilize GPS because of concerns about system reliability and the presence of SA. Future accuracy requirements for both time synchronization and time transfer will be difficult to achieve with the current capabilities of GPS. Spacecraft Uses of GPS The application of GPS to spacecraft navigation and control has the potential to provide significant savings in spacecraft costs and mission operations and is being introduced into spacecraft systems today in both government and commercial programs. The feasibility of using GPS for satellite navigation was first demonstrated in 1982 by a receiver placed aboard Landsat 4.55 Since then a number of additional missions and satellites have utilized GPS, including the Topex/Poseidon satellite launched in 1992. Other spacecraft programs have flown GPS, but it has been used primarily in an experimental mode. GPS receivers also have been used experimentally for launch vehicle applications. The experimental Ballistic Missile Defense Organization/McDonnell Douglas Delta Clipper (DC-X) utilized a GPS receiver integrated with an inertial navigation unit and flight control avionics during its flight testing. The system has reportedly cut the rocket's development 54   GPS and Global Telecommunications: A Summary Briefing Prepared for the National Research Council Committee on the Future of the Global Positioning System, p. 8. 55   H. Heuberger and L. Church, "Landsat-4 Global Positioning System Navigation Results," (Presentation to the American Astronautical Society/American Association of Aeronautics and Astronautics (AAS/AIAA) Astrodynamics Conference, AAS 83-363, August 1983).

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--> time and contributed to its success.56 An integrated GPS/inertial navigation unit is also being test flown on the Orbital Science Corporation's Pegasus launch vehicle. The company hopes that an operational version of the unit will one day improve the vehicle's en route navigation and orbital injection accuracy. Current and Future Applications and Requirements GPS is currently being tested or used for several spacecraft applications, including orbit determination, attitude determination, launch and reentry vehicle positioning and trajectory determination, and time synchronization. Precise time synchronization, which is required by many spacecraft, such as telecommunications satellites, to an accuracy of 100 nanoseconds was discussed in some detail in the previous section, but the remaining applications are discussed below. Orbit Determination The use of GPS for real-time determination of orbital parameters provides an economical means of determining a spacecraft's orbit very accurately. A properly designed, space-qualified GPS receiver can replace several conventional orbital positioning spacecraft sensors, reducing both weight and cost, and in some cases relieving the requirement for worldwide, ground-based stations to track orbital positions. In addition, the orbital parameters determined with GPS can in some cases be input to an on-board control computer and propulsion system to provide autonomous station keeping. This would alleviate or reduce the need for mission operations personnel to control a spacecraft's orbital position from the ground. In general the requirements for real-time orbit determination are not very stringent, ranging from about 50 meters to several kilometers. Although these requirements are quite lax, the same is not true for post-flight or post-processed solution accuracies. Many spacecraft, in particular those used for scientific missions, require very precise knowledge of where the satellite was when scientific data were being collected. The desire to achieve ± 1 centimeter orbit determination accuracy for the Topex/Poseidon spacecraft, as discussed in the Earth Science section of this chapter, provides an excellent example. In order to achieve this level of accuracy, GPS measurements from the spacecraft are processed together with GPS data from a worldwide network of ground stations and an extensive set of dynamic models. Future science missions are likely to push this requirement even further towards the millimeter level. 56   "Delta Clipper Contractors Tout Components' Success," Space News, 27 September - 3 October 1993, p. 17. The DC-X is a one-third-scale sub-orbital, single-stage-to-orbit (SSTO) technology demonstrator developed with funding from the Ballistic Missile Defense Organization (BMDO).

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--> Attitude Determination In the last several years, several manufacturers of GPS receivers have started collaborating with spacecraft developers to design GPS receivers for use as attitude sensors on board spacecraft. On-board attitude determination is a requirement for virtually every modern spacecraft, and most also require an automatic attitude control system. The traditional suite of sensors used for attitude determination range from relatively low-cost magnetometers and horizon sensors to precise gyroscopes, sun sensors, and star trackers. GPS may provide a cost-effective complement or even alternative to many of these existing systems. GPS attitude determination is accomplished by observing the carrier phase of an incoming GPS signal at two or more antennas on board the spacecraft. The difference in phase between the antennas can be related to the vehicle orientation and the rate of change of these phase observations is an indication of the attitude rate of change. The accuracy of GPS for this application is limited by multipath, the phase noise in the receiver, the separation of the antennas, and the stability of the structure supporting the antennas. With the best current technology, accuracies as good as 0.1 degrees (2s) can be expected. The accuracy requirements for satellite attitude determination range from 5 degrees for some simple spacecraft to well below 3 x 10-6 degrees (0.01 arc seconds) for a spacecraft like the Hubble Telescope. At this stage GPS cannot replace the high performance of star trackers for this ultimate precision, but may provide a cost-effective alternative for many mission requirements. Launch and Re-entry Vehicle Guidance GPS also has applications to space launch vehicles as a sensor in the vehicle's navigation system and for providing positioning information to ground controllers for range safety purposes. As previously mentioned, an integrated GPS/inertial navigation system has been tested on the experimental BMDO/McDonnell Douglas Delta Clipper (DC-X), and on Orbital Science Corporation's Pegasus launch vehicle. In addition, an experimental space re-entry vehicle called the Spacewedge, designed for re-entry rather than launch, is demonstrating the ability to make an automatic precision landing using a parafoil and a commercial GPS receiver. A full-scale space vehicle, either piloted or unpiloted, may one day use GPS-based technology for emergency crew return or cargo return from Earth's orbit.57 Accuracy requirements have not been provided for these experimental applications. Most range safety tracking for launch vehicles currently is conducted using a rather elaborate and expensive system consisting of ground tracking radars and associated equipment. According to a previously published NRC study, it is conceivable that pending 57   Spacewedge, known formally as the "spacecraft autoland gliding parachute experiment," has been developed by NASA's Dryden Flight Research Facility, Edwards AFB, California for under $100,000 annually. J. R. Asker, "Space Autoland System Shows GPS' Wide Uses," Aviation Week & Space Technology, 18 October 1993, pp. 54-55.

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--> further study by range safety experts, GPS-derived trajectory data could be used as a more cost-effective alternative.58 The DOD has been considering the use of GPS as the primary time and space position information source for the national ranges ever since the Range Applications Joint Program Office was established approximately 5 years ago, and the Navy has been utilizing GPS trajectory data for Trident missile testing since the early 1980s.59 Accuracy requirements for GPS range safety applications are very mission specific, and have not been generalized. GPS also could be used to improve range safety by sending flight termination commands to missiles and launch vehicles carrying GPS receivers. This could be accomplished using a DGPS datalink or a pseudolite located at the range or, as suggested by one expert in range safety, by using some spare data bits available in the GPS navigation message itself.60 Current flight termination telecommands, which are used to initiate self-destruction, are broadcast in the UHF frequency band. This band is very susceptible to spoofing, jamming, and interference. Integrating a telecommand with other GPS and DGPS equipment and datalinks already under development for time and position range applications could provide a more secure and cost effective means of initiating a flight termination when it is necessary. A consolidated list of available GPS requirements for spacecraft applications is provided in Table 2-10. Table 2-10 Requirements for GPS Spacecraft Applicationsa   Application Accuracy Satellites Orbit Determination (Real Time) 50 m (2 drms)   Orbit Determination (Post-Process) ± 0.001 m (2 drms)   Attitude Determination 5 degrees to 3 x 10-6 degrees     (2σ)b Launch Vehicles Launch Trajectory and Position Mission Specific   Determination   a. Accuracy is currently the only specified requirement for GPS spacecraft applications. The values in this table were derived by the committee from input received during the study. b. Accuracy as good as 3 x 10-6 degrees (2σ) is currently available only from star trackers. GPS is currently capable of 0.1 degree (2σ) attitude determination accuracy, which is suitable for most spacecraft missions. 58   NRC, Technology For Small Spacecraft, Aeronautics and Space Engineering Board, National Research Council (Washington, D.C.: National Academy Press, 1994), p. 16. 59   Source of Information: Personal conversation with Daniel F. Alves, Jr. of Alpha Instrumentation/Information Management, AI2M, Santa Maria, California, 20 February 1994. 60   Daniel F. Alves, Jr., Global Positioning System Telecommand Link, U.S. Patent number 5,153,598, 6 October 1992.

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--> Challenges to Full GPS Utilization Orbit Determination and Orbital Positioning The GPS SPS currently delivers sufficient accuracy for real-time orbit determination requirements. The well-known orbital dynamics of most spacecraft allow filtering of the data, which helps to mitigate the effects of SA. A-S does not have a significant effect on real-time orbit determination because most satellites are above the densest part of the ionosphere and can probably ignore the ionospheric delay contribution. For precise post-flight requirements SA does not pose a problem, but the presence of A-S reduces the orbit determination accuracy of spacecraft missions which rely on codeless L2 measurements. Topex/Poseidon, for example, relies on the use of dual-frequency data, but does not carry a receiver capable of tracking the Y-code. The processing of raw data from a large network of stations will still be required, however, even if the basic GPS accuracy is improved. Nevertheless, all improvements to the basic system will aid in the search for the last centimeter or millimeter of precision. Attitude Determination Because GPS attitude determination techniques use differential carrier-phase measurements, SA has little or no effect on the accuracy achievable. A-S may have some effect in that it prevents the use of differential P-Code measurements for coarse attitude determination and makes the use of dual frequency differential carrier-phase measurements more difficult. As mentioned previously, however, the accuracy of GPS for this application is limited primarily by design parameters related to receiver electronics and antenna structure. Signal Visibility Satellites in orbit near or above the GPS constellation are only able to track GPS signals that pass beyond the limb of the Earth. On the current Block II/IIA satellites there is sufficient antenna beamwidth to allow orbit determination to be performed at geosynchronous altitudes using GPS and a significant amount of dynamic modeling.61 The Block IIR and IIF satellites, however, may not have the same antenna beamwidth, and the L-band signals broadcast from these antennas may no longer pass beyond the limb of the Earth. This could eliminate the ability of a geosynchronous satellite to receive GPS signals, precluding a potentially important GPS application. 61   S. C. Wu et al., "GPS-Based Precise Tracking of Earth Satellites from Very Low to Geosynchronous Orbits," in Proceedings of the National Telesystems Conference (Ashburn, Virginia, May 1992), pp. 4-1 to 4-8.

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--> Findings The presence of SA has little or no effect on the ability to use GPS for orbit determination, but A-S limits the performance of orbit determination for spacecraft that rely on dual-frequency measurements, such as Topex/Poseidon. SA has no effect on the accuracy of GPS attitude determination methods for spacecraft. A-S may place some limitations on achievable accuracy, but so do design parameters related to receiver electronics and antenna structure. The ability to use GPS for orbit determination on board geosynchronous satellites will be lost if the Block IIR and IIF spacecraft are built with narrower beamwidth antennas than the Block II/IIA satellites. Summary Although this chapter does not represent a complete list of all GPS applications and their requirements, it should be clear from its content that the Global Positioning System has become an integral part of our nation's technical infrastructure, which contributes to our security, economy, and overall quality of life. Indeed, a fully exhaustive list of GPS applications may be impossible to compile, for as soon as it was completed, dozens of new and innovative applications, such as navigation systems for the visually impaired, would be developed that exploit GPS to the limits of its technological capability. Although requirements for currently undiscovered applications such as this one cannot be quantified, a strong case can be made for not only maintaining the basic system's operational capability but also for continuously improving it in order to meet the increasingly demanding requirements of a multitude of military and civilian users who rely on GPS on a routine basis. The tables included in this summary represent a compilation of the GPS applications that have been discussed in this chapter. Military applications with accuracy requirements currently unmet by the PPS are included in Table 2-11, and civil applications are grouped according to their accuracy requirements in tables 2-12 though 2-16. As these tables and the preceding discussions in this chapter clearly illustrate, the civilian applications that currently require augmentation or enhancement of the GPS SPS far outweigh those that do not. Most integrity and availability requirements for civilian applications are also unmet by the GPS SPS and are highlighted in the tables through the use of grey shading. Candidate technical improvements and modifications to the basic GPS that would enhance its functionality and make it more capable of meeting the requirements of both civilian and military users are discussed in the next two chapters.

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--> Table 2-11 Summary of Military Applications with Accuracy Requirements Unmet by the GPS PPS as Currently Specifieda   Application Accuracy Integrity         1 minus PHE times PMD Time to Alarm Aviation Non-precision Sea Approach/Landings 12.0 m (2 drms) 0.999 10s   Precision Approach/Landings Unprepared Surface 125 m (2 drms) 0.999 6 s   Precision Sea Approach/Landings 0.6 m (2 drms) 0.999 6 Mine Warfare Anti-mine Countermeasures < 5.0 m CEP Not specified Not specified Special Warfare Combat Swimming 1.0 m CEP Not specified Not specified   Land Warfare & Insertion/Extraction 1.0 m CEP Not specified Not specified Amphibious Warfare Artillery & Reconnaissance < 6.0 m CEP Not specified Not specified   Precision-guided Munitions 3.0 m CEP Not specified Not specified a. References and/or additional notes for each of the requirements listed in this table can be found by referring to previous tables (2-1 through 2-10) included in this chapter. Table 2-12 Summary of Civilian Applications with Accuracy Requirements of 100 Meters or Greater (currently achievable with the basic GPS SPS)a   Application Accuracy (2 drms) Integrity   Availability       1 minus PHE times PMD Time to Alarm   Aviation En route Oceanic 23 km Not specified 30 s 99.977%   En route through Non-precision Approach/Landings 100 m 1-1x10-7 per hour 8 s 99.999%   Domestic Automatic Dependent Surveillance (ADS) 200 m Not specified Not specified 99.999% Maritime Oceanic Navigation 1800 to 3700 m Not specified Not specified 99.0%   Coastal Navigation 460 m Not specified Not specified 99.7% a. References and/or additional notes for each of the requirements listed in this table can be found by referring to previous tables (2-1 through 2-10) included in this chapter.

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--> Table 2-13 Summary of Civilian Accuracy Requirements Between 25 and 100 Metersa   Application Accuracy (2 drms) Integrity   Availability       1 minus PHE times PMD Time to Alarm   ITS and Vehicle Navigation/Position Location Fleet Management (AVL/AVI) 25 to 1500 m Not specified 1 to 15 s 99.7%   Emergency Response 75 to 100 m Not specified 1 to 15 s 99.7%   Vehicle Command and Control 30 to 50 m Not specified 1 to 15 s 99.7%   Accident Data Collection 30 m Not specified 1 to 15 s 99.7% Spacecraft (Satellites) Orbit Determination (real time) 50 m Not specified Not specified Not specified a. References and/or additional notes for each of the requirements listed in this table can be found by referring to previous tables (2-1 through 2-10) included in this chapter. Table 2-14 Summary of Civilian Accuracy Requirements Between 10 and 25 Metersa   Application Accuracy (2 drms) Integrity   Availability       1 minus PHE times PMD Time to Alarm   Aviation TCAS 14.4 m Not specified Not specified Several Days   Surface Surveillance 24.0 m Not specified Not specified 99.87% Maritime Recreational Boating 10.0 m Not specified Not specified 99.9%   Vessel Traffic Services 10.0 m Not specified Not specified 99.9% ITS Infrastructure Management 10.0 m Not specified 1 to 15 s 99.7% Search & rescue Location Determination 10.0 m Not specified minutes 99.0% Oceanography Real-time Navigation and Positioning 10.0 to 30.0 m Not specified Not Not specified a. References and/or additional notes for each of the requirements listed in this table can be found by referring to previous tables (2-1 through 2-10) included in this chapter.

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--> Table 2-15 Summary of Civilian Accuracy Requirements Between 1 and 10 Metersa   Application Accuracy (2 drms) Integrity   Availability       1 minus PHE times PMD Time to Alarm   Aviation CAT I Approach/Landing 7.6 m 1-4 x 10-8 per approach 5.2s 99.9%   CAT II Approach/Landing 1.7 m (vertical) 1 -0.5 x 10-9 per approach 2.0s Not specified Maritime Harbor/Harbor Approach Navigation 8.0 to 20.0 m Not specified 6 to 10 s 99.7%   Inland Waterway Navigation 3.0 m Not specified 6 to 10 s Not specified Railroad Train Control 1.0 m Not specified 5 s 99.7% ITS and Vehicle Navigation/Position-Location Highway Navigation and Guidance 5.0 to 20.0 m Not specified 1 to 15 s 99.7%   Mayday/Incident Alert 5.0 to 30.0 m Not specified 1 to 15 s 99.7%   Automated Bus/Rail-Stop Annunciation 5.0 to 30.0 m Not specified 1 to 15 s 99.7%   Collision Avoidance, Control 1.0 m Not specified 1 to 15 s 99.7%   Collision Avoidance, Hazardous Situation 5.0 m Not specified 1 to 15 s 99.7% Hazmat Transport Vehicle or Cargo Location 5.0 m Not specified 1 s 99.7% Land Recreation Off-road Vehicles, Hikers, Back-country Skiers, etc. 5.0 m Not specified Minutes 99.0% Earth Science Airborne Geophysics 3.0 m (vertical) Not specified Minutes Not specified Mapping/ Surveying Geographic Information Systems (GIS) 1.0 to 10.0 m Moderate Not specified 98% a. References and/or additional notes for each of the requirements listed in this table can be found by referring to previous tables (2-1 through 2-10) included in this chapter.

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--> Table 2—16 Summary of Submeter Civilian Accuracy Requirementsa   Application Accuracy (2 drms) Integrity   Availability       1 minus PHE times PMD Time to Alarm   Aviation CAT III Approach/Landing 0.6 to 1.2 m (vertical) 1 -0.5 X 10-9 per approach 2.0 s Not specified Precision Farming Automatic Vehicle Guidance 0.05 m Not specified 5.0 s 99.7% Mapping/ Surveying/ Geodesy Photogrammetry 0.02 to 0.05 m Not specified Minutes 98.0%   Remote Sensing 0.1 to 20.0 m Not specified Not specified 98.0%   Geodesy 0.01 to 0.05 m Not specified Hours 98.0%   Mapping 0.1 to 10.0 m Not specified Hours 98.0%   Surveying 0.01 to 10.0 m Not specified Hours 98.0% Earth Science Oceanography (ocean circulation determination) 0.01 m Not specified Hours Not specified   Geodynamics 0.001 m + 109 x baseline length Not specified Hours Not specified Spacecraft (satellites) Orbit Determination (post-process) ± 0.001 m Not specified Not specified Not specified   Attitude Determination 3 x 10-6 degrees (0.01 arc second), 2σ Not specified Not specified Not specified a. References and/or additional notes for each of the requirements listed in this table can be found by referring to previous tables (2-1 through 2-10) included in this chapter.

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