Business & GA, Commercial

The Real Reason Selective Availability Was Turned Off

By William Reynish | July 1, 2000
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It isn’t too often that the President of the United States announces a change in the signal format of a public navigation aid. Especially when, as he pointed out, his announcement was first reviewed by top government bureaucrats, including the director of the Central Intelligence Agency.

And the whirl of Internet message exchanges following the President’s statement was equally unusual, each citing a different reason for his action and his timing. It was, said one message, aimed at "pulling the rug out from under those upstart Europeans."

Not so, declared another. It was designed to confuse and divide top international officials meeting in Istanbul at the World Radio-communication Conference (WRC).

Wrong again, claimed a third. It is all to do with helping Vice President Al Gore’s bid for the presidency.

And others believed it was all these, along with additional reasons too arcane to be mentioned.

President Bill Clinton’s May 1 announcement was, of course, that the accuracy-degrading selective availability (SA) "jitter" superimposed on the navigation signals of the Global Positioning System (GPS) was to be removed that day, at midnight. SA was a basic design feature of GPS, aimed at preventing U.S. adversaries from using the system’s high accuracy. The intended discordance was to cause an enemy’s calculated position to move erratically and unpredictably, in random directions, sometimes quickly and sometimes slowly, out to 328 feet (100 meters)–and occasionally further–from their true position, and thereby foiling their plans.

Originally, SA was called accuracy denial by the military. But with the onset of civil use in the late 1980s, its name was changed to selective availability, although the technique remained unchanged.

Unfortunately for the Department of Defense (DoD), civil use brought with it a deluge of innovative technologies. One was the development of the "differential" technique of precisely and continuously measuring SA’s movements at an accurately surveyed point and immediately broadcasting them to users as receiver corrections. These corrections would neatly cancel out the SA-induced errors. From then on, SA became more and more irrelevant to users such as surveyors–or pilots landing in low visibility–who require very high GPS precision.

But today, the removal of SA has moved GPS from a +/- 100-meter navigation system to one providing all users with +/- 10 to 20 meters (32.8 to 65.6 feet) position accuracy, with the residual errors now due to ionospheric interference, satellite clock errors, and other minor effects. The signals are very stable, and again demonstrate what an extraordinary achievement the GPS development has been, conceived as it was in the mid-1960s.

However, FAA officials caution that the improved accuracy still does not meet the criteria for a Cat I precision approach, although it will certainly add a healthy margin of safety to GPS-guided non-precision approaches. In next generation receivers–but not in today’s C-129 sets–it also will provide much greater availability of receiver autonomous integrity monitoring (RAIM), and in more advanced units, better satellite failure detection and exclusion (FDE) performance. But neither of these will obviate the need for the FAA’s Wide Area Augmentation System (WAAS), which will become the primary source of integrity messages in the National Airspace System (NAS).

No Real Difference

So, in terms of current day-to-day operations, most users will not notice a great deal of difference in GPS performance. But they will in the future.

For example, cargo airline pilots flying ADS-B tests in 1999 (when GPS with SA gave 100-meter accuracy) found it impossible to determine, while on their landing approaches, whether another ADS-B equipped test aircraft or vehicle was actually on the runway or holding short of it. With SA gone, this should no longer be a problem, and the specter of runway-incursion accidents is on its way to being eliminated. There are a number of other applications where the new accuracy enhancement will pay off in increased safety.

Yet the timing of President Clinton’s announcement was intriguing. While there was general agreement in government and industry circles that SA would eventually be removed, DoD had been charged with conducting an annual study of the need for its continuance through 2006, after which it was no longer expected to be required.

Many observers felt that SA might go off earlier in, say, 2003 or 2004. But no one expected it to happen in May of 2000. Not surprisingly, therefore, this triggered speculation that the timing was keyed to other factors, and possibly aimed at impacting the Europeans, or a meeting in Istanbul, or even the vice president’s electoral campaign, to name the three least fanciful theories.

Theories, Up Close

For those who love conspiracies, here is how the theories play out:

  • The European theory was that the announcement would cause senior European Union (EU) officials to reassess the need to proceed with their own Galileo satellite system, which many in the U.S. GPS industry regard as a competitor.

  • The Istanbul theory concerned the meeting of the International Telecommunications Union in that city in May, at which the United States and several of its allies expected to be fighting hard to retain aviation control of the satellite navigation and communications frequency bands. In other words, eliminating SA would thwart in-roads by the very powerful telecommunications industry, which sees satnav and satcom bands as a greatly underutilized resource and natural expansion area for cellular services.

  • And in the case of the vice president, it was postulated that, as a promoter of new technology, the removal of SA would gain for Al Gore domestic kudos, thus boosting his presidential campaign.

Unfortunately, however, we are not saying goodbye to SA, but only au revoir, the French expression for "until we meet again." This is because, while not explicitly stated in the president’s announcement, it is understood that DoD has reserved the right to reinstate SA if it perceives it to be in the national interest. Industry groups are strongly opposed to this position, pointing out that it will perpetuate the need to incorporate extensive and complex SA correction software programs in such systems as WAAS and the Local Area Augmentation System (LAAS). This, in turn, could impact on user costs and long-term reliability and performance.

A Complete Turnoff

WAAS and LAAS avionics equipment would be similarly affected. And compounding this problem will be the manufacturers’ dilemma of how to adequately test, demonstrate and obtain certification of those parts of their systems that handle SA corrections, when there are no SA signals being broadcast. Industry observers expect that DoD will now come under strong pressure to relinquish SA completely, since there is no civil need–and scarcely any military need–for it.

Indeed, returning to the theories for turning SA off, one hypothesis for the action curiously has not been promoted on the Internet: The military’s study simply had shown SA was no longer needed.

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