Selective Availability?

[Macgyver]

Hey guys,

I was reading a blurb on GPS and WAAS avionics, and there was a statement about something called "selective availability," which deliberately degrades the GPS accuracy. It also mentionned that SA has been discontinued because of WAAS-capable receivers.

I just dont understand what the purpose of SA was? How could something that deliberately degraded the GPS accuracy be beneficial?

Thanks in advance.

[Max Angle]

How could something that deliberately degraded the GPS accuracy be beneficial?Well if you are at war with someone who has a GPS receiver and is trying to drop a bomb on you it would seem pretty beneficial to turn off their signal whilst leaving the specially encoded signal, that only you can receive, switched on. That's all SA was, just a way for the US military to deny GPS to anyone who did not have a special GPS unit.

[Tinstaafl]

GPS was designed as a military aid, not civil. SA is a deliberate feature that allowed the military to degrade the accuracy of the GPS system in an area of operation to prevent unfriendlies from using the system against its owners. Receivers of the GPS system's owners are immune to the SA limitations.

[lionco]

The US DoD control GPS satelites and could, if and when it suited them, cause said satilites to transmit deliberate errors. USAAF aircraft and other US assets would of course have all the correction info via the military only L2 signals. So no good what so ever to commercial aviation. The US DoD have however stated that they will not use this option.

[Alex Whittingham]

The mechanism used was to dither the timing of the satellite clocks to produce a deliberate position error.

[BEagle]

Didn't it also include false vehicle ephemeris information?

[Avtrician]

During GW1 SA was used, and caused a lot of cray boats here in the west of OZ to lose their pots as everything changed overnight.:mad:

[punkalouver]

During GW1 SA was used, and caused a lot of cray boats here in the west of OZ to lose their pots as everything changed overnight.:mad:

I thought SA was something that was always on throughout the world for civil users. Perhaps not.

[411A]

SA has been switched off for quite some time...:rolleyes:

[PA-28-180]

Believe it was a continuance of SCATANA procedures (do they still teach this for PPL???). In time of war, DOD could/would take over all radio navigation facilities for the reason stated above...don't want to make the 'other' guys job any easier! All started back in the bad old days of the cold war.

[punkalouver]

SA has been switched off for quite some time...:rolleyes:

Well aware of that. However, before they discontinued it, as I asked before, wasn't it on at all times for civilian users?

[Jetstream Rider]

SA was proved to be almost useless - which some conspiracists think is the "real" reason that is was switched off (I am not one of them!). If you had a very accurately surveyed point and a GPS on it, you could tell what the error was and apply that to your (second) GPS reading of wherever you were. Not so useful a method for aviation, but for artillery or similar, SA proved to be a pain, rather than a hinderance.

SA was always on before it was abandoned, but they could vary the amount of error.

[green granite]

Not long after GPS became operational with the 24 sat cluster, the DOD added SA to downgrade civil use to about 100mts. Some clever chappie realised that if you placed a receiver at an accuratley surveyed location, you could work out the psuedo range errors and transmit them to a sat nav receiver which would then correct its readings and the accuracy then improved to abt 6mtrs. This was known (and still is) as differential sat nav. Intitially developed for marine use
the corrections were overlade on the marine beacon frequencies (200 to 400 Kcs). Nowadays surveying equipment can get down to centimeter accuracy using this system. That basically why SA was removed as it no-longer degraded anything. (The accuracy of an ordinary, non mil, unit is about 20 mtrs)

Hope this helps