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GPS Selective Availability lifted 10 years ago
Article by: Darren Griffin
Date: 2 May 2010
 Many of you may not have heard of Selective Availability (SA). But for those of us who were fortunate enough to own a GPS receiver prior to 2000, SA was a very big deal indeed!
SA was a feature of the military GPS network (NAVSTAR is a US military funded and operated system) that added an intentional error to the GPS signal made available to non-military users.
This error was added so as to degrade the accuracy of the GPS signal and was intended to preclude the use of civilian GPS receivers by enemy forces.
It was the removal of SA that made mapping based GPS a viable service and initiated the explosion in the mapping based navigation devices that we all benefit from today.
SA was a simple design with a very damaging effect to those who were forced to endure the degraded signal it produced.
By introducing a pseudorandom code the signal caused an error of about 50m horizontally and up to 100m vertically. Authorised users had access to special military GPS receivers (used by the US and their allies) which when input with the tightly controlled daily 'key' could unlock the code and access the more accurate GPS signal.
To circumvent this intentional degradation of the signal for civilian users, Differential GPS (DGPS) was invented and became commonplace. This used fixed stations which given an accurate location can correct for the SA errors and broadcast a corrected position.
Given the wide availability of DGPS solutions it became clear that SA was no longer a viable countermeasure and so it was eventually turned off by order of the then US President, Bill Clinton on May 2 2000.
SA is still available as an option to those who operate the service and it could be switched back on should it be needed. This is partly the reason that other systems such as the EU's Galileo GPS satellite network have been funded. That is, to give users in the EU some measure of control over their own network.
But the costs of design, launch, and operation of such a network are huge and ongoing and we all owe a measure of thanks to NAVSTAR. A service that offers so much, and does so free of charge, a very rare thing indeed!
| Comments
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 Posted by MaFt on Mon May 03, 2010 9:28 am |
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interesting. i only 'got' gps in september 2005 when the i3 was launched. as such it was already 5years in to this period.
i knew they could tamper with the signal but had always assumed this was only done in iraq, afghanistan etc etc hadn't realised it was on such a big scale.
i feel slightly more priveleged now but still slightly paranoid that the americans could change their mind any time they like!
MaFt
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| Posted by Darren on Mon May 03, 2010 9:35 am |
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| MaFt Wrote: | | i knew they could tamper with the signal but had always assumed this was only done in iraq, afghanistan etc etc hadn't realised it was on such a big scale. |
They can introduce SA in quite tightly confined regions and conversely they can, as they did during the first Gulf War, lift it in controlled areas.
An interesting anecdote concerns the first Gulf War back in 1991 IIRC. This was the first conflict where handheld GPS was being used.
But thanks to a shortage of the Military GPS units (spot a trend here in kit shortages?) many soldiers were buying handheld Garmin and Magellan units to use.
But with SA still being enforced, these civilian units were causing commanders major issues with inaccurate positions being reported and so SA was lifted in the Gulf region for the duration of the conflict.
The results of this temporary suspension of SA may well have added to the weight of subsequent calls for SA to be lifted permanently.
As an ex-soldier the real shame is that 15years later, squaddies are still having to buy their own GPS as defence procurement still has insufficient units for even one per patrol let alone one per soldier 
Darren Griffin |
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| Posted by MaFt on Mon May 03, 2010 1:01 pm |
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i know a fella who was out there in 1991. he now swears by garmin owing to the number of times his handheld saved their backsides!
50m in some of those conditions is a long way to err...!
MaFt
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| Posted by SlimboyFat on Fri May 07, 2010 6:50 pm |
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I was told by a mates Dad that he was on a boat on September 11th 2001 and the Sat Nav had a message stating "Accuracy within 5 miles".
I would now assume this would have been SA due to the terrorist attacks.
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| Posted by Darren on Fri May 07, 2010 7:42 pm |
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| SlimboyFat Wrote: | I was told by a mates Dad that he was on a boat on September 11th 2001 and the Sat Nav had a message stating "Accuracy within 5 miles".
I would now assume this would have been SA due to the terrorist attacks. |
Highly unlikely, were it SA it would not cause the great an error. Also it cannot be turned on that quickly nor would it have been turned on globally without many more of us knowing.
More likely just a temporary reception issue.
Darren Griffin |
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