GPS for the Rest of Us
September 1, 2003 By: John W. Lavrakas GPS WorldMonitoring the Civil Signals
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On Sunday, May 25, 2003, the atomic frequency standard on GPS satellite number 27 (SVN/PRN 27) became erratic. By Monday the range errors caused by the faulty clock had compelled the GPS satellite operators at Schriever Air Force Base to perform two contingency uploads in an attempt to correct the condition. Still, the atomic clock did not stabilize. Its effect on GPS users grew as signal ranging errors grew.
Finally, after satellite operators saw the user range error (URE) spike to approximately 38 meters at 1733 hours universal coordinated time (UTC) on May 26, they commanded the signal to "unhealthy," which effectively removed the satellite from service. (Information based on a presentation by Karl Kovach of ARINC Inc., "Two Recent GPS Integrity Anomalies," presentation to Interagency GPS Executive Board's Integrity Failure Modes and Effects Analysis team, July 29, 2003.)
Fortunately these incidents are rare, but when they happen, many user groups are affected, and the repercussions can range from annoying to severe. As the GPS constellation ages, many users are asking tough questions of the service provider. How often do these events occur? How are users affected by such events? What is the service provider doing to more rapidly detect and correct such anomalies?
Background
The first GPS satellite was launched February 22, 1978, and at that time, GPS's purpose was clear - an inexpensive, reliable source of position, velocity, and time for the U.S. military. The idea of civil use of GPS was only lightly addressed by the system's founders and never formally incorporated into the planning process.
In an article in the September 1994 issue of GPS World, Bradford Parkinson, the first executive director of the Department of Defense (DoD) NAVSTAR GPS Joint Program Office, describes the early DoD view of civil use of GPS. "From the beginning of GPS, it was recognized that the proposed GPS system would provide utility for many more users, other than the U.S. military. The system's designers arranged the code structure to have a precise, protected modulation (the P-code) that could be encrypted and a clear acquisition modulation (the C/A code) that could be exploited by civil users. The earliest presentations always included descriptions of the system's usefulness to the worldwide civil community."
Yet recognizing the utility of civil GPS is not the same as planning for it and incorporating it into a system design. In fact, the system designers viewed any benefits to civil users as secondary to the need for specific constraints to limit the use of GPS against U.S. forces. They incorporated into the system two distinct security features to prevent or limit its use by adversaries: selective availability (SA), which intentionally skewed the accuracy of the unencrypted Standard Positioning Service (SPS) signal, and anti-spoofing, which encrypted the Precise Positioning Service (PPS) signals. (See article by Bradford Parkinson in "Further Reading" for additional details.)
Speaking to this issue of adverse use of GPS, Parkinson notes that "in the beginning, the system's designers did not seriously consider this a threat, because they believed that the military P-code's projected accuracies, 10 times better than the civil C/A code, would create much less accurate civil receivers." He concludes dryly, "Technology, however, has invalidated this assumption." History has indeed shown that civil GPS accuracies are every bit as good as the military accuracies.
Thus, although civilian users had access to the unencrypted signals, the U.S. government's position toward this class of users remained undefined. In 1983, however, all that changed. During the month of September that year, the Soviets shot down a Korean airliner that had allegedly drifted off course into the air space of the former USSR. In the aftermath of that event, the Reagan administration announced that GPS would be made available for civilian users worldwide as well as for military operations to preclude such incidents from recurring.
During the next 10 years, civil use grew by leaps and bounds, and by the mid-1990s civil GPS sets outnumbered military sets by 10 to 1. In 1996, President Clinton further solidified civil GPS policy in a presidential decision directive (PDD) stating the U.S. government would provide the SPS for peaceful civil, commercial, and scientific use on a continuous, worldwide basis, free of direct user fees. Congress codified the PDD in 1998 with the passage of Title 10, Section 2281 of the United States Code.
 (Click on chart for larger view.) Figure 1: "The veil is lifted." SA is discontinued in May 2000. (Source: GPS Support Center, Schriever AFB, CO) |
At the time of the PDD, GPS still employed SA, essentially "guaranteeing" civil users a real-time positioning accuracy of no worse than 100 meters horizontal 95 percent of the time. Actual GPS performance, however, was masked by this 100-meter random error. In May 2000, however, the veil was lifted when President Clinton directed the cessation of SA. Finally civil users had access to the full capability of GPS and experienced an immediate and dramatic improvement in positioning and timing accuracy. With the improvement came increased visibility into GPS performance. Variances in accuracy that had been masked by SA were suddenly visible, and GPS users could see the effect ephemeris and clock behaviors, along with satellite geometries, had on signal-in-space performance. Figure 1 illustrates the effect of the removal of selective availability.
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