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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 72 (it was then set back to 100 meters, 2 drms), because of the lack of PPS equipment fielded by the U.S. military at those times. PPS receivers are able to completely remove the effects of both SA dither and epsilon from their observations through the use of a security module. SPS receivers can eliminate the effects of SA through the use of local or wide-area DGPS broadcasts of differential corrections. DGPS reference stations typically broadcast observed range and range-rate errors. The level of SA-induced range acceleration determines the rate at which the corrections must be updated to keep the user error within acceptable bounds. Satellite position errors produced by the epsilon technique will decorrelate as the separation between the reference station and the user increases. Wide-area DGPS will provide orbit corrections for each satellite to compensate for this effect. Military Utility of SA The DOD has stated that SA is an important security feature because it prevents a potential enemy from directly obtaining positioning and navigation accuracy of approximately 12.5 meters CEP (30 meters, 2 drms) from the C/A-code. Since the military has access to a specified accuracy of 8 meters CEP (21 meters, 2 drms), they believe U.S. forces have a distinct strategic and tactical advantage. With SA at its current level, a potential enemy has access only to the 42-meter CEP (100 meters, 2 drms) accuracy available from the SPS. The DOD believes that obtaining accuracies better than 42 meters CEP requires a substantial amount of effort. DOD representatives have expressed their belief that our adversaries are much more likely to exploit the GPS C/A- code, rather than DGPS, because its use requires less effort and technical sophistication than is required to use DGPS. In addition, some DOD representatives contend that local-area DGPS broadcasts do not diminish the military advantage of SA because they could be rendered inoperative, if warranted, through detection and destruction or by jamming. It is the opinion of the NRC committee however, that meter-level accuracies are readily obtainable, even in the presence of SA set at its current level or even at higher levels. As shown in Figure 3-1, several DGPS systems, operated by both commercial and government entities, routinely provide position accuracies approaching 1 meter (2 drms) in the United States and in most of the populated areas of the world. Further information on commercially available systems is provided in Appendix C. Even within the U.S. government, civilian agencies such as the Federal Aviation Administration, the Coast Guard, and the Army Corp of Engineers are planning to operate systems that will, in combination, cover the entire United States and beyond, as shown in Figure 3-2. Furthermore, if the full GLONASS constellation is completed in 1995 as currently planned, this system also will provide properly equipped users with an additional source of highly accurate positioning data, as shown in Figure 3-3.5 5 Unlike GPS, GLONASS does not deny accuracy to some users through the use of SA or a similar technique.
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION Figure 3-1 DGPS coverage provided by commercially available systems, including Skyfix and Sercel. (Courtesy o National Air Intelligence Center) 73
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION Figure 3-2 DGPS coverage provided by the planned FAA WAAS (Wide-Area Augmentation System). Source: Innovative Solutions International, Inc., presentation at the National Technical Meeting of the Institute of Navigation Meeting, California, January 1995. 74
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 75 Figure 3-3 Position estimates from GPS and GLONASS obtained from measurement snapshots taken 1 minute apart over an entire day. Position from (a) GPS with SA off, (b) GPS with SA on, (c) GLONASS, and (d) GPS plus GLONASS. (Courtesy of MIT Lincoln Laboratory)
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 76 Even if potential adversaries are not taking advantage of DGPS at this time, the NRC committee believes that it would be prudent for the DOD to recognize the potential capability that currently exists. In addition, the establishment of a low-cost, militarily controlled local-area DGPS network for use by an adversary in a theater of conflict is an even more likely possibility. Local-area differential systems are easy to build or buy and are inexpensive. Furthermore, the NRC committee believes that the detection and elimination of these military local- area DGPS stations, either in wartime or peacetime, would be difficult. Local-area DGPS reference stations are small and can be installed in less than an hour. Signals from such systems are difficult to detect because they can be broadcast at low power and at spread-spectrum frequencies or in rapid on/off cycles, with very short transmission times. Therefore, they are not easy to detect electronically or visually. The NRC committee expects that any enemy of the United States sophisticated enough to operate GPS- guided weapons will be sophisticated enough to acquire and install local-area differential system or take advantage of an existing commercial system. These systems can have the capability to provide velocity and position corrections to cruise and ballistic missiles with accuracies that are equal to or superior to those available from an undegraded C/A-code. Even if the level of SA is increased, DGPS methods could still be used to provide an enemy with accurate signals. Further, as previously mentioned, if the full GLONASS constellation is completed in 1995 as currently planned, this system also will provide properly equipped users with an additional source of highly accurate positioning data. The unencrypted C/A-code, which is degraded with SA, still provides our adversaries with an accuracy of 100 meters, 2 drms (42 meter CEP), which would still be more than adequate to deliver chemical, biological, or even explosive weapons, if creating terror in a city is the enemy's objective (see Figure 3-4). Further, any enemy encountered is not likely to share the U.S. military's interest in limiting collateral damage. With SA set at zero, the stand-alone accuracy improves to 30 meters, 2 drms (approximately 13 meters CEP) or better, depending on the solar cycle and user equipment capabilities. While this improvement enhances the ability of an adversary to successfully attack high-value point targets, significant damage also can be inflicted with accuracies of 100 meters, 2 drms. Therefore, in either case (30-meter or 100-meter accuracy, 2 drms) the NRC committee believes that the risk is sufficiently high to justify denial of the L1 signal by jamming. The jamming strategy has the additional benefit of denying an adversary all radionavigation capability including the even more accurate DGPS threat.
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 77 Figure 3-4 Horizontal scatter plot of 42 meters CEP (100 meters, 2 drms) with SA at its current level and horizontal scatter plot of approximately 10 meters CEP (24 meters, 2 drms) without SA (Figure Courtesy of Mr. Jules McNeff, Office of the Assistant Secretary of Defense, C3I)
