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June 2000

Good News for GPS Users
by Alberto de Capitani

Those who use the GPS regularly are occasionally puzzled by erratic readings. Latitude and/or longitude would suddenly change when you are still moored to your pier, the speed-over-ground indicator would show drastic accelerations or decelerations while you are peacefully chugging
along at five knots, the anchor alarm would go off in the middle of the night when in fact the boat has not moved at all. If you have erratic readings, do not necessarily blame your GPS.
Chances are that the US military is sending you data that are slightly off - what is known as “selective availability” (SA). Having confounded the potential  enemy, your GPS would then go back to its usual reliability.

President Clinton has now decided that, effective May 1 of this year, this
is no longer going to happen - not in normal circumstances at least, or in
trouble-free areas.
What does it mean for all of us?
For one thing, it looks as if the last obstacle has been removed for the Global Positioning System (GPS) to become the primary civilian radio navigation system all over the world. This is not what the GPS was originally intended for. The original plan was that the US Air Force would put in orbit a sufficient number of satellites, so that the US military could accurately estimate the position of their ships, aircraft or missiles at any point on earth at any given time.
As it turned out, 21 satellites have been positioned in such a way that in virtually every location on earth there are now at least four of them above the horizon all the time - often many more than that. Each satellite transmits a radio signal and the GPS receiver calculates the position based on the
differences in the amount of time it takes for signals to reach the receiver. .
Three satellites are sufficient for a fix. As more satellites are in view the accuracy of the fix increases.
For a number of reasons that it would be too long to discuss here, it was decided from the beginning that the GPS should be made available “ in some form” to civilian users as well. The US military was never comfortable with this decision. And the reasons are obvious: the GPS can
be so accurate that terrorists or other potential enemies could program a missile to hit any US target with a margin of error of a few meters -courtesy of US technology and US taxpayers...
For these reasons, the GPS was designed to provide two levels of accuracy:
- the Precise Position Service (PPS) with an accuracy of 20 m or better 95% of the time
- and the Standard Position Service (SPS) with an accuracy of 100 m or better.
The PPS was to be transmitted encrypted, to be available only to the US military. For use outside the military, “selective availability” would be applied to the PPS signal - meaning in practice that the accuracy of PPS would be deliberately degraded to SPS standards.
A margin of accuracy of 100 m 95% of the time means that if you draw a circle with a radius of 100 m around your current position, the GPS should give you estimates of your location within that circle 95 % of the time. Whereas this is perfectly adequate for most navigational needs, it may not be sufficient to navigate a narrow harbor or to land a plane safely on a runway.

For these reasons, the US Coast Guard has been busy establishing Differential Global Positioning System (DGPS) stations near the most important ports of the country - in practice, to
counteract the SA and thus provide the civilian users with signals having the degree of accuracy that the military was trying to deny to them. At the same time, the Federal Aviation Administration (FAA) is about to embark on a similar effort (costing about $ 1 billion) at the major airports of the nation. Talk about different branches of Government working at cross purposes!
Already in 1997 President Clinton tried (and failed ) to persuade the Joint Chiefs of Staff to give up the Selective Availability. Now the decision has been made and is effective immediately.
The implication for all of us is that from now on our position as estimated by the GPS will be within 20 m from the real position 95 % of the time ( and often much better than that if there are more than 3 satellites over the horizon in our location). Twenty meters means that one can use the GPS estimate to the second decimal place (one hundredth of a minute of latitude is in fact equal to about 18 m ). This is more than can be drawn on most coastal charts and is quite adequate even for
harbor charts.
Does it mean that the GPS will render obsolete all other navigation systems?
Not at all.
- First, Selective Availability is not totally dead. The US military can still apply it over specific regions where there is trouble and can even jam or turn off GPS signals that could be received by civilians: they still control the switch.
- Second, whereas GPS accuracy is guaranteed 95% of the time, GPS radio signals can be distorted in special conditions.
- Thirdly, both GPS satellites and GPS receivers can malfunction.
The old lesson therefore still applies:
"whenever possible, do not rely on any single navigation aid (even one as
good as the GPS), but use all information available to you".

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