In response to a request from Congress, a joint study on the Department of Defense's Global Positioning System (GPS) was conducted by the National Academy of Sciences and the National Academy of Public Administration. The National Academy of Sciences was asked to recommend technical improvements and augmentations that could enhance military, civilian, and commercial use of the system. The National Academy of Public Administration was asked to address GPS management and funding issues, including commercialization, governance, and international participation. To conduct its part of the study, the National Academy of Sciences established an expert committee, through the National Research Council (NRC), the operating arm of the National Academy of Sciences and the National Academy of Engineering.
Specifically, the National Academy of Sciences was asked to address the following three technical questions:
(1) Based on presentations by the Department of Defense (DOD) and the intelligence community on threats, countermeasures, and safeguards, what are the implications of such security-related safeguards and countermeasures for the various classes of civilian GPS users and for future management of GPS? In addition, are the Selective Availability and Anti-Spoofing capabilities of the GPS system meeting their intended purpose?
(2) What augmentations and technical improvements to the GPS itself are feasible and could enhance military, civilian, and commercial use of the system?
(3) In order to preserve and promote U.S. industry leadership in this field, how can communication, navigation, and computing technology be integrated to support and enhance the utility of GPS in all transportation sectors, in scientific and engineering applications beyond transportation, and in other civilian applications identified by the study in the context of national security considerations?
In its interpretation of Task 1, the NRC committee decided not only to determine whether Selective Availability (SA) and Anti-Spoofing (A-S) were meeting their intended
purpose, but also to determine the broad ramifications of the use of these techniques and to make specific recommendations for each. In response to Task 2, the committee made recommendations for technical improvements because it believed that only identification of technical improvements would be of little value without an accompanying recommendation. In response to Task 3, the NRC committee considered ''U.S. industry leadership" to mean technical preeminence focused on meeting the demands of a growing number of user applications, while maintaining a technical advantage for the DOD.
Based on presentations by the DOD and the intelligence community on threats, countermeasures, and safeguards, what are the implications of such security-related safeguards and countermeasures for the various classes of civilian GPS users and for future management of GPS? In addition, are the Selective Availability and Anti-Spoofing capabilities of the GPS system meeting their intended purpose?
The DOD has stated that SA1 is an important security feature because it prevents a potential enemy from directly obtaining positioning and navigation accuracy of 30 meters (95 percent probability) or better from the C/A-code.2 Since the military has access to a specified accuracy of 21 meters (95 percent probability), 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 C/A-code signal with a degraded accuracy of only 100 meters (95 percent probability). The DOD believes that obtaining accuracies better than 100 meters (95 percent probability) requires a substantial amount of effort on the part of an unauthorized user. Further, DOD representatives have expressed their belief that our adversaries are much more likely to exploit the GPS C/A-code rather than differential GPS (DGPS), because its use requires less effort and technical sophistication than is required to use DGPS.3 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 opinion of the NRC committee, however, that any enemy of the United States sophisticated enough to operate GPS-guided weapons will be sophisticated enough to acquire and operate differential systems. Enemies could potentially take advantage either of the existing, commercial systems available worldwide or install a local DGPS system, which could be designed and operated in a manner that would be difficult to detect. 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. It should be noted that with both GPS- and DGPS-guided weapons, accurate knowledge of the target location is a prerequisite for weapon accuracy. Even if the level of SA is increased, DGPS methods could still be used to provide an enemy with accurate signals. Thus, the NRC committee concluded that the existence and widespread proliferation of DGPS augmentations have significantly undermined the effectiveness of SA in denying accurate radionavigation signals to our adversaries. In addition, the Russian GLONASS system broadcasts unencrypted signals with an accuracy comparable to an undegraded GPS C/A-code, which further erodes the effectiveness of SA.4
The unencrypted C/A-code, which is degraded by SA, still provides our adversaries with an accuracy of 100 meters (95 percent probability). With SA set at zero, the standalone accuracy improves to around 30 meters (95 percent probability).5 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, (95 percent probability). Therefore, in either case (30-meter or 100-meter accuracy) 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.
The NRC committee strongly believes that preservation of our military advantage with regard to radionavigation systems should focus on electronic denial of all useful signals to an opponent, for example, by jamming and spoofing, while improving the ability of civil and friendly military users to employ GPS in a jamming and spoofing environment. Continued effort to deny the accuracy of GPS to all users except the U.S. military via SA appears to be a strategy that ultimately will fail. Thus, the NRC committee recommends that the military employ denial techniques in a theater of conflict to prevent enemy use of GPS or other radionavigation systems.
The NRC committee believes that the principal shortcoming in a denial strategy, regardless of the level of SA, is the difficulty that military GPS receivers currently have in acquiring the Y-code during periods when the C/A-code is unavailable due to jamming of the L1 signal. The implementation of direct Y-code acquisition capability, as recommended in Chapter 3, would provide the optimal solution to this problem. The technology for developing direct Y-code receivers is available today. The committee believes that a focused, high-priority effort by the DOD to develop and deploy direct Y-code user equipment, backed by forceful political will from both the legislative and executive branches, can bring about the desired result in a relatively short period of time. In the interim before direct Y-code receivers can be fielded by the military, various operating disciplines, also discussed in Chapter 3, can be used to minimize the impact of L1 C/A-code jamming on the ability to acquire the Y-code directly.
From the onset of the study, the NRC committee agreed that national security was of paramount importance and, without exception, the U.S. military advantage should be maintained. As outlined above, the committee determined that the military effectiveness of SA is greatly diminished because of the widespread proliferation of DGPS and existence of GLONASS. In addition, the NRC committee compiled the following findings related to the effects of SA and A-S6 on the various classes of civilian users:
- The presence of SA and A-S increases the cost and complexity of Federal Aviation Administration's Wide Area Augmentation System (WAAS)7 and limits the effectiveness of Receiver Autonomous Integrity Monitoring (RAIM).8
- The presence of SA affects the acceptance of GPS by some commercial users and limits the ability of the Coast Guard's DGPS service to provide important safety-related information to its users.
- GPS-based automobile navigation systems, which require accuracies in the 5- to 20-meter range, would no longer require DGPS if SA was
- eliminated and further improvements were made to the basic GPS as suggested in Chapter 3. The elimination of SA would also improve the performance of those DGPS systems required for higher-accuracy applications, such as collision avoidance, that are important to the future Intelligent Transportation System.
- Most mapping, surveying, and geodetic applications would be enhanced by cost savings from quicker acquisition of data. The elimination of SA and the ability to track code on two frequencies can improve acquisition time.
- Post-processing can eliminate the effects of SA for most Earth science applications, but the presence of A-S increases the cost and limits the performance of many techniques.
- Although 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.
- SA has little or no effect on the ability to use GPS for spacecraft orbit or attitude determination, but A-S limits the performance of orbit determination for spacecraft that rely on dual-frequency codeless measurements. A-S may also contribute to limitations on achievable attitude determination accuracy.
The six most important findings of the NRC committee regarding the impact of SA on the various classes of civilian users and on meeting its intended purpose are
The military effectiveness of SA is significantly undermined by the existence and widespread proliferation of DGPS augmentations as well as the potential availability of GLONASS signals.
Turning SA to zero would have an immediate positive impact on civil GPS users. Without SA, the use of DGPS would no longer be necessary for many applications. System modifications that would further improve civilian accuracy also would be possible without SA.
Deactivation of SA would likely be viewed as a good faith gesture by the civil community and could substantially improve international acceptance and potentially forestall the development of rival satellite navigation systems.
Without SA, the committee believes that the number of GPS and DGPS users in North America would increase substantially.9
It is the opinion of the committee that the military should be able to develop doctrine, establish procedures, and train troops to operate in an L1 jamming environment in less than three years.
The technology for developing direct Y-code receivers is currently available and the development and initial deployment of these receivers could be accomplished in a short period of time if adequately funded.
The FAA's WAAS, the Coast Guard's differential system, and GLONASS are expected to be fully operational in the next 1 to 3 years. The Coast Guard's DGPS network and the WAAS will provide accuracies greater than that available from GPS with SA turned to zero and GLONASS provides accuracies that are comparable to GPS without SA. At the same time, other local DGPS capabilities are likely to continue to proliferate.
Selective Availability should be turned to zero immediately and deactivated after three years. In the interim, the prerogative to reintroduce SA at its current level should be retained by the National Command Authority.
Although many civil users could benefit if A-S is turned off as noted above, the NRC committee found that A-S remains critically important to the military because it forces potential adversaries to use the C/A-code on L1, which can be jammed if necessary without inhibiting the U.S. military's use of the encrypted Y-code on L2. Further, encryption provides resistance to spoofing of the military code. The NRC committee determined, however, that the current method of manual distribution of Y-code decryption keys is laborious and time consuming. The DOD has recognized this problem and has ongoing efforts to distribute keys electronically. The NRC committee believes that an electronic key distribution capability would greatly enhance the use of the encrypted L2 Y-code. The committee also believes that technology is available to upgrade the current encryption method and suggests that the Air Force should explore the necessity of utilizing this technology. Modifications to the Block IIR satellites and the Block IIF request for proposal may be required if upgraded encryption methods are necessary. Changes to military receivers also will be required.
A-S should remain on and the electronic distribution of keys should be implemented at the earliest possible date. In addition, the Air Force should explore the necessity of upgrading the current encryption method. Required receiver enhancements should be incorporated in future planned upgrades.
The analysis by Michael Dyment, Booz·Allen & Hamilton, 1 May 1995, is shown in Appendix E.
What augmentations and technical improvements to the GPS itself are feasible and could enhance military, civilian, and commercial use of the system?
Today GPS is a true dual-use system. Although it was originally designed to provide a military advantage for U.S. forces, the number of civilian users now exceeds the number of military users. During the course of the study, the NRC committee examined various technologies and augmentations applicable to GPS. It determined that several improvements could be made to the system that would enhance its use for civilian, commercial, and military users without compromising national security. Some of the improvements could be made immediately; others could be incorporated on some of the Block IIR spacecraft that are currently being built and included in the specification requirements for the next generation Block IIF spacecraft. The committee's recommendations are listed below and a detailed discussion of each is provided in Chapter 3. Although the approximate cost of each improvement is given when available, potential funding mechanisms for each improvement are not discussed. In general, the issue of GPS funding is addressed by the National Academy of Public Administration.
Recommendations that Enhance GPS Performance for Civil and Commercial Users
The NRC committee found that the most prominent need for commercial and civil users is greater stand-alone accuracy, availability, and integrity. With improved performance of the basic GPS signal, many users would no longer require augmentations to obtain the data they require. Any additional system enhancements and modifications to improve standalone positioning accuracy for civilian users are relatively ineffective in the presence of SA. However, if the recommendation to deactivate SA is implemented, the committee has identified several enhancements that could provide significant improvement for both civilian and military users. With SA removed, the major enhancement that would greatly increase accuracy for civilian users is the addition of a new, unencrypted signal that allows for corrections of errors introduced by the ionosphere.10 While very important for civil users, this feature will provide minimal additional capability to military users because they already have this capability through use of their encrypted signals.
Immediate steps should be taken to obtain authorization to use an L-band frequency for an additional GPS signal, and the new signal should be added to GPS Block IIR satellites at the earliest opportunity.
A preliminary analysis of the L-band spectrum allocation that was conducted by Mr. Melvin Barmat, Jansky/Barmat Telecommunications Inc., Washington D.C., January 1994, is shown in Appendix I.
Recommendations that Enhance GPS Performance for Military Users
As stated above, GPS was originally designed to provide our forces with a military advantage. In the past, DOD has depended on a strategy of global signal degradation, through SA, to reduce the GPS signal accuracy to civilian and unauthorized users, while providing a more accurate, encrypted signal to authorized users. However, as stated above, the committee believes that the military usefulness of SA is severely diminished and that it is urgent that the DOD focus its attention on denial of all useful signals to an opponent, for example, through jamming and spoofing techniques, including jamming of the unencrypted C/A-code, rather than relying on SA The NRC committee therefore recommends several military receiver enhancements that would support such a strategy.
The development of receivers that can rapidly lock onto the Y-coded signals in the absence of the C/A-code should be completed. The deployment of direct Y-code receivers should be given high priority by the DOD.
Nulling antennas and antenna electronics should be employed whenever feasible and cost effective. Research and development focused on reducing the size and cost of this hardware should actively be supported.
The development of low-cost, solid-state, tightly-coupled integrated inertial navigation system/GPS receivers to improve immunity to jamming and spoofing should be accelerated.
The development and operational use of GPS receivers with improved integration of signal processing and navigation functions for enhanced performance in jamming and spoofing should be accelerated.
Military receivers should be developed that compensate for ionospheric errors when L1 is jammed, by improved software modeling and use of local-area ionospheric corrections.
In the interim time before such enhancements can be fielded by the military, various operating disciplines, which are discussed in Chapter 3, can be used to minimize the impact of C/A-code jamming on the ability to acquire the Y-code directly.
Recommendations that Enhance GPS Performance for All Users (Civil, Commercial, and Military)
In view of the rapidly expanding use of GPS, the NRC committee believes that GPS must be capable of continuous operation in all foreseeable contingencies. This capability is critical. The one area where the NRC committee found limited redundancy was in the operational control segment (OCS). Although the NRC committee determined that the Air
Force has several experiments planned to improve the system, it believes there are some additional improvements that can be made to the OCS that would increase stand-alone accuracy, availability, and integrity; improve the overall reliability of the system; or simplify day-to-day operations. Recommendations that would result in greater stand-alone GPS accuracy and integrity include uploading more current clock and orbit information to all satellites, increasing the number of monitor sites, reducing the clock and ephemeris errors, and improving Block IIR and Block IIF integrity monitoring capability. In addition, the NRC committee found a need for (1) a simulator to test software and train personnel, (2) modern receivers at the monitor stations, and (3) a permanent, backup master control station. Specifically, the NRC committee recommends:
Additional GPS monitoring stations should be added to the existing operational control segment. Comparison studies between cost and location should be completed to determine if Defense Mapping Agency or Air Force sites should be used.
The operational control segment Kalman Filter should be improved to solve for all GPS satellites' clock and ephemeris errors simultaneously through the elimination of partitioning, and the inclusion of more accurate dynamic models. These changes should be implemented in the 1995 OCS upgrade request for proposal.
Procurements for the replacement of the monitor station receivers, computers, and software should be carefully coordinated. The new receivers should be capable of tracking all satellites in view and providing C/A-code, Y-code, and L1, and L2 carrier observables to the OCS. Upgradability to track a new L4 signal also should be considered. OCS software also should be made capable of processing this additional data.
Firm plans should be made to ensure the continuous availability of a backup master control station.
A simulator for the space and ground segment should be provided as soon as possible to test software and train personnel.
The operational control segment software should be updated using modern software engineering methods in order to permit easy and cost-effective updating of the system and to enhance system integrity. This should be specified in the 1995 OCS upgrade request for proposal.
The planned Block IIR operation should be reexamined and compared to the accuracy advantages gained by incorporating inter-satellite ranging data in the ground-based Kalman Filter and uploading data at some optimal time interval, such as every hour, to all GPS satellites.
Block IIR satellite communication crosslinks should be used to the extent possible with the existing crosslink data rate to support on-board satellite health monitoring for improved reliability and availability and in order to permit a more rapid response time by the operational control segment.
The Block IIR inter-satellite communication crosslinks should be used to relay integrity information determined through ground-based monitoring.
The DOD's more frequent satellite navigation correction update strategy should be fully implemented as soon as possible following the successful test demonstration of its effectiveness. In addition, the current security classification policy should be examined to determine the feasibility of relaxing the 48-hour embargo on the clock and ephemeris parameters to civilian users.
If the above recommendations are implemented, the NRC committee believes that the overall GPS performance and reliability will be greatly enhanced and that a stand-alone horizontal accuracy of the basic GPS signal approaching 5 meters (95 percent probability) could be achieved for both civilian and military users.
Improvement Implementation Strategy
Because of the relatively long life time of GPS satellites (5 to 10 years) and the length of time required to replace the total constellation of 24 satellites, opportunities for introducing enhancements and technology improvements to the system are limited.
Figure 1 shows the current plan for satellite replacements. According to the GPS Joint Program Office, current plans for the Block IIF contract include 6 short-term and 45 long-term "sustainment" satellites. As currently planned, the Block IIF satellites will be designed to essentially the same specifications as the Block IIR satellites. The current program and schedule make it possible for another country to put up a technically superior system that uses currently available technology before the United States can do so. Under the current planning and in the absence of a preplanned product improvement (P3I) program, the earliest opportunity for an infusion of new technology in the GPS space segment would be after Block IIF, probably sometime after the year 2020.
The NRC committee believes that there are significant improvements that could be made to the system much earlier than post-Block IIF that would not only enhance its performance for civilian and military use but also make it more acceptable and competitive internationally. One method to incorporate technology in an efficient and timely manner is through a P3I program beginning as early as possible in Block IIR. With this type of approach, planned changes and improvements could be intentionally designed into the production of the satellites at specific time intervals.
Assuming that the first improvements suggested in this report are incorporated in the later half of the Block IIR satellites, additional funding might be required to incorporate changes for the already completed Block IIR satellites. However, the NRC committee
believes that the timely improvement in system performance is adequate justification for the additional cost.
In addition to the specific recommendations given in this report, the NRC committee also discussed several enhancements that it believes have particular merit and should be seriously considered for future incorporation. These items are discussed in Chapter 4. Although a few enhancements could be included on the Block IIR spacecraft, especially if a P3I program were implemented, most of the enhancements would have to be incorporated in the Block IIF spacecraft design.
In order to preserve and promote U.S. industry leadership in this field, how can communication, navigation, and computing technology be integrated to support and enhance the utility of GPS in all transportation sectors, in scientific and engineering applications beyond transportation, and in other civilian applications identified by the study in the context of national security considerations?
As described in Chapter 2 and Appendix C, the NRC committee found that civil, commercial, and military GPS users are making rapid progress in developing and utilizing systems that integrate GPS with other technologies. For many navigation and position location applications, GPS is being combined with one or more of the following: radar; inertial navigation systems; dead reckoning systems; aircraft avionics and flight management systems; digital maps; computers and computer databases; and communication datalinks. For timing applications, GPS can be combined with reference clocks and digital communication networks. Surveying and mapping users have combined GPS with computer databases, inertial navigation systems, digital imaging systems, and laser measuring systems. Earth science users have integrated GPS with radar altimeters, precision accelerometers, synthetic aperture radar, computer databases and workstations, and communications datalinks.
By integrating GPS with other technologies, highly accurate positioning and timing information can be obtained at a very modest cost, which provides a large incentive to system designers to develop integrated GPS products. For example, with the large market potential for ground vehicle position location and guidance systems, there is considerable motivation for the vigorous commercially funded research and development activity that is underway. The NRC committee believes that the U.S. user equipment industry's intensive focus on research and development is sufficient to ensure that its technical competitiveness will be maintained.
During its deliberations, the committee found that some user communities had a limited number of very specific issues related to the integrated use of GPS with other technologies that may require government action. Examples include the need to modernize the air traffic management system to take advantage of the full capabilities of GPS-based navigation and surveillance and the need to speed up the process of providing up-to-date digital hydrographic data for use in Electronic Chart Display Information Systems (ECDIS). These findings and others have been reported in Chapter 2. In general, however, the GPS industry is meeting most user demands by continuously improving integrated user equipment and services and is limited only by the need to augment and enhance the characteristics of the basic GPS constellation. Therefore, it is the opinion of the NRC committee that the most important government action required is to improve the performance of the basic GPS satellite system to provide the highest levels of position accuracy, signal integrity, and signal availability that can be technologically achieved at reasonable cost without negatively impacting national security. The committee believes that the performance improvements summarized in response to Task 2 above and further discussed in Chapter 3 meet these criteria.