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

Without SA, the committee believes that the number of GPS and DGPS users in North America would increase substantially.⁹

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.