Q. I've heard about Selective Availability, the government's intentional degradation of GPS accuracy to worse than 100 meters. Doesn't this make GPS useless for most applications that require fairly high accuracy? -G.B. Cookeville, Tenn.

A. John C. Bohlke, Sokkia Corp.: Selective Availability (SA) reduces the number of potential GPS applications that require fairly high accuracy but only when using one receiver. That is one of the reasons why high accuracy applications require the use of at least two GPS receivers. Relative positioning techniques use the data from two or more receivers to solve for the error introduced by SA. Incidentally, SA is not the only limiting factor in determining highly accurate positions. Other sources of error inherent with GPS such as inaccurate clocks, atmospheric delays and ephemeris errors must be reduced through relative positioning in order to achieve survey-grade accuracy.

Charles Branch, Ashtech Inc.: No. Selective Availability, or "SA" as those in the know lovingly refer to it, degrades GPS accuracy from about 10 meters to some 40 meters on average. Horizontal accuracy remains within 100 meters 95 percent of the time with SA on. If you are in a small airplane flying 100 mph, you're moving over the ground at about 45 meters per second. Do you really care if your accuracy is degraded to 40 meters on average? If you're in a small boat trying to get back to the dock and 40 meters of accuracy isn't good enough, you should consider consulting an optician. If you're locating yourself on a typical topographic map (scale = 1:24,000), a 40 meter error circle is about as big as the letter O on this page. Basically, for the majority of GPS users, is the U.S. DoD really such as bully when they activate SA?
      On the other hand, 40 meters of accuracy for surveying or field asset management applications is unacceptable. However, with SA turned off, the user would still only be able to get about 10 meters of accuracy. Surveyors typically demand one centimeter of accuracy and asset managers try to get less than one meter. These GPS customers, and others such as public safety officials, don't really care about SA because they need to apply differential correction techniques on their data anyway. These techniques remove all sources of GPS error that are common to all receivers within about 500 kms, and SA is one such common error. So to say that "SA makes GPS useless for most applications requiring high accuracy" is false. It is more correct to say that SA is irrelevant for most applications requiring high accuracy.

Arthur Lange, Trimble Navigation: Under the present implementation of Selective Availability (SA) the use of non-differential GPS, sometimes called autonomous GPS, is not useful for many GIS mapping applications. However for many navigation functions, GPS, even with Selective Availability, is still extremely useful and more than accurate enough. For instance, the Korean airliner shot down over the former Soviet Union, would not have gotten lost if GPS had been used, even with SA.
      Differential GPS is used for GIS mapping applications to overcome the effects of SA. depending on the technique and the kind of GPS receivers used, differential GPS can provide an accuracy of about 10 meters to better than one centimeter. Differential GPS corrections are available for both real-time and post-processed applications from a variety of public and commercial sources which will meet the requirements of many mapping programs.

Jim White, Magellan Systems: No, SA does not affect high-accuracy applications. For applications such as survey, mapping, GIS data collection, and verification of remote sensing data, differential GPS techniques are required, regardless of the use of SA.

Q. What's the point of SA? The U.S. Air Force is degrading the accuracy of GPS while another branch of the U.S. government (the U.S. Coast Guard) is openly transmitting the corrections to anybody who wants them. Corrections are even being sold on the open market by several companies. If the corrections are so easy to obtain, why even bother with SA? -T.H. Baton Rouge, La.

A. Bohlke: SA was introduced to deny accurate real-time positioning to U.S. adversaries thereby providing an advantage to the U.S. military. Since real-time differential corrections have become so readily available, the effectiveness of SA has been greatly diminished. Although national security is of paramount importance, most civilian users and agencies such as the National Academy of Public Administration (NAPA) and the National Research Council (NRC) support the removal of SA. However the Department of Defense (DoD) believes that eliminating SA will risk national security by making GPS too convenient for the enemies. If differential broadcasts were being used against the U.S., the military might be able to jam the signal or eliminate the source of the signal if it became necessary to regain their advantage of using SA. It is also thought that SA could be increased to such a level that differential broadcasts would be rendered useless. Nevertheless, the DoD's reasoning has been contested and alternative methods for sustaining national security are being investigated.

Branch: The point of SA is to protect the United States, its citizens, and its interests in the face of military threats from hostile forces. GPS was conceived as a military tool that can, among other things, augment weapons systems. One example is a cruise missile guided by GPS. SA degrades the accuracy to 40 meters (on average) of weapon systems not supported by the U.S. DoD. So that's the point of SA. It was a pretty good idea, actually.
      However, the world now is different from how it was when SA was conceived. To protect ships in U.S. coastal waters, the U.S. Coast Guard broadcasts differential corrections to improve accuracy to a couple meters. They would probably be doing this today even had SA never been implemented. Commercial companies such as DCI and ACCQPOINT also broadcast differential corrections to subscribers. They offer accuracy far better than the 10 meters of SA-free GPS.
      So why is SA still turned on? Because it's a matter of national security, only those with top secret clearances know for sure. However, it is doubtful that the DoD plans on fighting any wars on Lake Superior (the U.S. being graced by the world's best neighbors) or in any location where there are lots of commercial subscribers to correction services (mostly the U.S. and Western Europe). So leaving SA on in the face of the Coast Guard's and the commercial providers' broadcasts isn't totally unreasonable.
      Also, it is quite possible that the U.S. DoD fully intends to remove SA - but only when it is ready to do so. The DoD generally does not enact policy changes frivolously, frequently, or quickly. So while they may very well remove SA, it will take time. Some influential people (such as former Secretary of Defense James Schlesinger) are arguing strongly to remove it. As these arguments build in intensity, the desire to do away with SA will increase. Many pundits feel it will be gone within two years. But you will still have to differentially correct your data to beat 10 meters.

Lange: One of the aims of SA was to deny full accuracy of GPS to hostile forces. With the demise of the Soviet Union, the current implementation of SA is considered by some people to be a failure, principally because of the relative ease of overcoming SA's effects with the use of differential techniques.
      Because of these and other considerations, the U.S. Senate Armed Services Committee in the defense authorization bill, has asked DoD to provide a report on how it plans to deny the use of GPS to hostile forces, as well as prevent enemy jamming of GPS. If such a plan is not presented to Congress by May 1, 1996, then the committee recommended that the Secretary of Defense be required to turn off SA.

White: Differential corrections are used not only to negate the effects of SA, they improve the basic accuracy of GPS, permitting high-accuracy applications such as survey and mapping. Currently, the U.S. government is studying the SA issue and its continued usefulness, given the existence of commercial differential broadcast systems.

About the participants:
John C. Bohlke serves as GPS support manager for Sokkia Corp. in Overland Park, Kan. He may be reached at 913-492-4900 or 800-4-SOKKIA in the U.S. (phone) or 913-492-0188 (fax). Charles Branch is the GIS marketing manager for Ashtech Inc. in Sunnyvale, Calif. He may be reached at 408-524-1400 (phone), 408-524-1500 (fax) or e-mail: [email protected] Arthur Lange is the GIS product manger for Trimble Navigation in Sunnyvale, Calif. He may be reached at 408-481-2994 (phone), 408-481-6074 (fax) or e-mail: [email protected] Jim White serves as public relations manager for Magellan Systems in San Dimas, Calif. He may be reached at 909-394-5000 (phone) or 909-394-7050 (fax).

Back