Tuesday, June 21, 2011

The day GPS Selective Availability (SA) was switched off

I was going through some old archived projects recently and came across an interesting GPS track log I recorded on 2 May 2000.

For us GPS early adopters, this was a remarkable day: the day GPS became truly useful for terrestrial applications. I had left my Garmin GPS35 logging to capture the event, and here it is:

It may seem that the GPS was violently oscillating about the place and then stopped half way through the period of time recorded on the chart. Believe it or not it was completely stationary for the entire duration! So what happened?

First a little GPS history. GPS was conceived by the US military as an exclusively military navigation aid in the early 1970s.

In September 1983, Korean Airline Flight 007 was mistaken for a hostile military aircraft by the Soviets when it accidently strayed into their airspace. It was shot down with the loss of 269 lives. Following that disaster it was decided that GPS would be made available for civilian use to prevent such tragedies from happening again.

Naturally the military were concerned that the super accurate navigation provided by GPS could be abused. To address these concerns two signals were transmitted: one for military receivers with the full 10m accuracy, and a separate signal for civilian receivers with about 100m accuracy. 100m was considered adequate for aviation and marine applications at the time (but not accurate enough to allow a missile to hit a target). This degraded signal was known a Selected Availability (SA).

During the 1990s with the end of the cold war and an increasing civilian use of GPS the thinking at the US government was that the economic and safety benefit of removing SA far outweighed the risks.

So on 1 May 2000, the White House issued a press release announcing the end of SA with the following quote attributed to President Bill Clinton:
"The decision to discontinue Selective Availability is the latest measure in an ongoing effort to make GPS more responsive to civil and commercial users worldwide…This increase in accuracy will allow new GPS applications to emerge and continue to enhance the lives of people around the world."
Right on cue, on 2 May 2000 the SA was disabled and that moment was caught here. Only longitude is displayed for clarity, but latitude and altitude would have been oscillating with a similar amplitude. And a big difference it made, opening up a mass market for consumer GPS devices.

[Aside: SA was temporarily removed at some point [can't find a date] during the first Persian Gulf War in 1991 because of a shortage of military GPS receivers. Instead civilian units were used. These were just as accurate with SA switched off. SA was restored again on 1 July 1991.]



27 Dec 2018: I thought I had lost the original data file, but found it a few days ago on an old archive CD-ROM while cleaning for Christmas. I created a better chart showing both east-west deviation and also a north/south deviation as function of time.    Data and Gnuplot source files are available here: https://github.com/jdesbonnet/GPS_SA_switch_off

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