National Academies Press: OpenBook

The Global Positioning System: A Shared National Asset (1995)

Chapter: Accuracy and Integrity

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Suggested Citation:"Accuracy and Integrity." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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Page 22
Suggested Citation:"Accuracy and Integrity." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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Page 23
Suggested Citation:"Accuracy and Integrity." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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Page 24

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GPS APPLICATIONS AND REQUIREMENTS 22 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.

GPS APPLICATIONS AND REQUIREMENTS 23 Table 2-1 Military Aviation and Precision-Guided Munitions Applications and Requirementsa Application Accuracy Integrity Resistance to RF Interference 1 minus PHE Time to times PMDb Alarm Aviationb Low-level Navigation 50.0 m (2 drms) 0.999 10 sec High and Air Drop Non-precision Sea 12.0m (2 drms) 0.999 10 sec High App/Landings Precision App/ 12.5m (2 drms) 0.999 6 sec High Landings Unprepared Surface Precision Sea App/ 0.6m (2 drms) 0.999 6 sec High Landings Amphibious and Anti- 50.0 m CEP Not specified Not specified High submarine Warfare Anti-air Warfare 18.1 m CEP Not specified Not specified High Conventional 37.5 m CEP Not specified Not specified High Bombing Nuclear Bombing 75.0 m CEP Not specified Not specified High Close Air Support/ 9.0 m CEP Not specified Not specified High Interdiction Electronic Warfare 22.5 m CEP Not specified Not specified High Command, Control & 37.5 m CEP Not specified Not specified High Communications 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 20.0 m CEP Not specified Not specified High Survey Search & Rescue and 125.0 m CEP Not specified Not specified High Medical Evacuation Mapping 50.0 m CEP Not specified Not specified High Precision- Precision-guided 3.0 m CEP Not specified Not specified High guided Munitions Munitions 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.

GPS APPLICATIONS AND REQUIREMENTS 24 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 16.0 m CEP High Mining 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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The Global Positioning System (GPS) is a satellite-based navigation system that was originally designed for the U.S. military. However, the number of civilian GPS users now exceeds the military users, and many commercial markets have emerged. This book identifies technical improvements that would enhance military, civilian, and commercial use of the GPS. Several technical improvements are recommended that could be made to enhance the overall system performance.

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