GPS Consumer Series: The End of S/A As We Know It?
By Chuck Gilbert

Introduction
In Washington D.C. on March 29, 1996, President Clinton made an announcement that captured the attention of both the GPS industry and the user community. The topic of the presidential directive was Selective Availability (S/A), the intentional degradation of the GPS signal by the U.S. Department of Defense. In question, was the future of Selective Availability. Will S/A continue to be implemented or will S/A be discontinued? More importantly, how will any changes impact the typical GPS user?
      According to the Presidential Directive, S/A will be turned off within the decade, four years at the earliest and 10 years at the latest. Therefore, we will be living with GPS for a while. This essay will discuss the impact of S/A on the typical GPS user, and since it will be turned off eventually I will discuss the impact that the elimination of S/A could have on the GPS user community.

What did he say?
The announcement was published by the Office of Science and Technology Policy on behalf of the White House. In summary, the following conclusions may be drawn from the announcement:
1. The White House recognizes that GPS is gaining wide acceptance internationally and is an important tool to a wide variety of industries. As such, GPS is likely to remain available to all users worldwide free of direct user charges.
2. Selective Availability will be discontinued within a decade.
3. Beginning in the year 2000, the president will make an annual determination on continued use of Selective Availability. (See the sidebar on the following page for excerpts of the original announcement.)

What does this mean?
The bottom line is that the U.S. government is committed to keeping GPS usable and available for the peaceful use of all nations. It is also very clear that S/A will, eventually, be discontinued. However I would be careful about putting much stock in the specified time frame for the change in S/A policy. The government could be significantly earlier or significantly later. I suspect earlier, due to the conservative language required by the Washington budgetary process.

How will this change impact users?
The more interesting question is, "How will the eventual discontinuation of S/A affect users of GPS for GIS data capture?" If S/A were to be discontinued today, the typical accuracy of a single GPS position would improve from approximately 50 meters to approximately 12 meters.
      Therefore any person with only one GPS receiver, and with no access to real-time or post-processed differential GPS corrections, could achieve 10-15 meter accuracy on a second-by-second basis. If the user occupied one location for a longer time (perhaps 15 minutes to an hour) and averaged the position data, it is likely that the user could achieve accuracy on the order of about 5 meters. This time estimate is based on the analysis of the residual tropospheric and ionosphere errors that are slowly changing and don't average out very quickly.
      Note that the specification for S/A is that the position data will be within 100 meters of truth 95 percent of the time. This specification is conservative. If one observes many hours of typical position data that is under the influence of S/A, they will find that most of the time the position data is actually within 50 meters of truth. This is comfortably within the accuracy specification.
      On the other hand, most users today are using differential GPS to remove both S/A and other systematic errors. The least accurate differential GPS systems on the market today have accuracy in the 5-10 meter range. The most accurate systems that utilize differential techniques can compute positions on a second-by-second basis that are accurate to less than 1 centimeter.

Is this a good thing?
I expect that the discontinuation of S/A will probably be a big boon to the consumer, navigation, and vehicle tracking industries. Improving the accuracy from 50 meters to 10 meters will make it easier to pinpoint a specific landing site or building, or to more easily determine which road you occupy on your digital map. Casual users such as boaters, campers, or trekkers will find the GPS utility a little more useful with the increased navigational accuracy.
      However, already I have heard from scores of people who hope to use GPS for GIS data capture and other survey and mapping applications. A common comment is that they "are excited about the end of S/A so that they will no longer require differential data links or post-processing." This is a dangerous belief.

Summary
If your application can tolerate an accuracy of only 10-15 meters, then the end of S/A will simplify your life. However, if your GIS or mapping application requires higher accuracy, such as 2 meter, sub-meter, or centimeter(s); your need for differential correction will not change.
      It is possible that future developments in either GPS receivers, in data processing techniques, or in the GPS system itself could improve these "non-S/A accuracy figures. However, that day is still probably many months (if not years) in the future. So if you are going to be using GPS for populating your GIS data base with reasonably accurate coordinates, then you must be prepared to use differential GPS, both now and in the future.

About the Author:
Chuck Gilbert has over a decade of experience as a GPS user. He has been employed as an applications engineer for Trimble Navigation since 1989. If you have a suggestion or request for a future article, please drop a line to Chuck care of EOM, 13741 E. Rice Place, Suite 200, Aurora, CO 80015, fax to 303-690-2522, or send via e-mail: [email protected]

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