Extensive field experience and technology advances have combined to bring new levels of sophistication to use of the GPS Precise Positioning Service (PPS). The Selective Availability Anti-Spoofing Module (SAASM), a new GPS receiver design currently being incorporated into military and agency user equipment, promises a more robust operational capability as a result of advances in cryptography, "keying" techniques, and direct signal acquisition of the P(Y) code. These will alleviate the security risks and logistics faced by military users as well as eliminate the dependence on open C/A-code for obtaining initial access to the P(Y) code itself.

This article will discuss some of the key technology innovations behind SAASM and its implications for operational use of the PPS, particularly in its application to time and frequency generation and synchronization for communications systems and command/control terminals.

The Need For SAASM Civil use of the Standard Positioning Service (SPS), based on the coarse-acquisition (C/A) code at the L1 frequency, has produced a huge international market in which millions of low-cost commercial receivers have already been sold for civil navigation and timing applications. This global dispersion of GPS technology, however, means that some of these receivers are also now in the hands of current and potential adversaries of the United States and its allies.

Today, anyone with a C/A-code receiver can navigate with at least 10-meter accuracy most of the time and synchronize to UTC (Universal Coordinated Time) within better than 100 nanoseconds. This proliferation of commercial GPS receivers poses a major dilemma: how to protect U.S. and allied forces from hostile use of the civil signal during critical military operations while sustaining operation of the worldwide infrastructures that have been built on civil GPS, such as communication networks, surveying, positioning, and civil aviation. Essentially, the problem revolves around methods for denying use of the open SPS to adversaries while sustaining authorized user access to the encrypted PPS.

Military planners have done a very good job thinking their way through this scenario and have improvised clever ways to solve the problem. Until recently, using GPS called for making the 'in-the-clear' civil signal available to all users, but not at full capability. During certain military operations, the C/A-code signal could be degraded even more, a technique called Selective Availability or SA.

However, a fundamental change has taken place in the political context in which military leaders and GPS users operate - from global strategic conflict to tactical, localized warfare - and in the accompanying implications for user equipment design.

The Elimination of SA. The U.S. Air Force's satellite controllers can set the navigation and timing accuracy of the broadcast GPS signal to any level desired. Beginning in the early 1990s, the controllers intentionally degraded the civil C/A-code signal in GPS satellite transmissions to a horizontal positioning accuracy of about 100 meters, making the related vertical accuracy about 150 meters.

In May 2000, however, SA was turned off. This availability of the full-capability civil signal is the result of the new philosophy from U.S. military planners-to provide full civil accuracy, even with future enhancements and augmentations, while retaining the ability to locally deny the civil C/A signal in times of conflict.

Figure 1: The new warfare realities..Click to Enlarge

This shift in thinking is actually quite profound. Previously, U.S. military planners relied on SA to make the civil signal unusable in case of a conflict, but the effect of SA cannot be applied regionally and thus is worldwide in scope. Of course, during the Cold War, with its potential for strategic and global conflict, this seem-ed a reasonable approach. Today, however, with GPS integrated into almost every corner of our lives, the sweeping effects of SA would prove undesirable because they would be felt around the world. Consequently, the new scenario to deny the civil signal locally is very desirable, because it affects only a targeted region. The issues with strategic and tactical warfare scenarios are illustrated in Figure 1.

The Role of SAASM. Local denial of C/A instead of implementing SA has a drawback, however, and this is where one of the SAASM functionalities comes in-direct-P(Y) signal acquisition. The new war fighting environment requires authorized users to be equipped with receivers that allow them to continue using GPS signals while the "bad guys" can't.

Traditionally, P(Y) code receivers need to initially access the C/A-code transmitted by satellites, which repeats its pseudorandom noise (PRN) sequence every millisecond. This is needed to help acquire the encrypted military signal, which has a significantly longer PRN sequence. Use of C/A-code enables the PPS receivers to obtain an accurate "time-tick" and with the transmitted hand-over word (HOW), enables alignment with the encrypted P(Y)-code (assuming the receiver has been crypto-keyed).

In the conventional SA signal-degradation scenario, the civil signal is still present, distorted as it may be and so, the PPS receiver can still use it to initialize. In a theater of operations in which C/A-code is denied, however, a SAASM receiver with direct P(Y)-code acquisition is the only practical option for continued use of GPS. A less desirable alternative is to have a precise (atomic) cesium clock in the user equipment that provides the ability to synchronize with the P(Y)-code, an option not practically available to most users for various reasons, including cost, size, and weight disadvantages. We will discuss the other SAASM receiver functionalities next and return to a more detailed discussion of direct-P(Y) later.

Fielding SAASM Most observers acknowledge that the military establishment as a whole lags far behind the commercial industry in the availability of state-of-the-art GPS receivers. Of course, many soldiers have commercial receivers, but these do not substitute for PPS receivers. The fielding of military PPS receivers has improved recently, but equipping of U.S. and allied military forces with the new SAASM hardware needs to be expedited for reasons that will become obvious in the course of this article.

The logistically intensive nature of current-technology PPS hardware has slowed the distribution of equipment. Classification considerations surrounding distribution, use, and disposal of "crypto keys" as well as issues involving hardware, whether keyed or not, have contributed to this situation. If a current-technology PPS receiver falls into the wrong hands, key security may be compromised for a time, and long-range effects will be felt after the hardware is analyzed by a foe. No doubt, this fact has contributed to the phenomenon of PPS hardware reaching higher ranking military officers first rather than combatants in the field.