Selective Availability

[Guest jonpaul]

My GPS12 coordinates have started drifting. Has SA been turned back on? I first noticed this around 9:35 AM Central today. The drift is less than when SA was on, but is more than I have ever seen with SA off. I have seen the EPE jump from the 20's to the 50's and back in a minute or two. My position will vary by .003 minute over 2 minutes time, but the position will remain stable for a minute or two in between shifts.

[Guest martinp13]

I wonder if they turned it back on in response to the terrorist attacks?

 

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> Martin

Magellan 330 (1.56/WAAS enabled!)

Don't have time to program and record your shows while geocaching? Get a TiVo !

[Guest Lance303]

I noticed the accuracy on my gps was horrible lastnight on my way home from work at 8:00pm EST.

 

This morning was also really bad.

[Guest Ttepee]

Living in the NYC vacinity I'm kind of shaken right now but came across this article... haven't read it all yet but might have something explanatory in it.

http://www.nytimes.com/2001/09/11/national/11NAVI.html

[Guest redd]

quote:

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Originally posted by martinp13:

I wonder if they turned it back on in response to the terrorist attacks?

 

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After the initial shock of this horrible event, I, too, was curious about this. My eTrex is at home, and I almost brought it with me this morning. I phoned my wife and asked her to get a single reading, but the results were inconclusive.

 

Imagine the possibilities if GPS users noticed SA being turned on before this attack.

 

Has anyone else noticed any errors in accuracy, either before/during/after the attack?

 

Living in Omaha, we are used to seeing contrails crossing our skies, mostly east-to-west, as people shuttle across the nation. It was weird to see no trails this morning, but instead, several low flying jets, as if they were coming in for a landing. I also saw a couple of fighter jets flying over the city. The USAF Strategic Air Command is nearby.

 

Scott

redd@interbug.com

http://interbug.com/pigeon

[Guest Lance303]

I checked again around 12:30pm EST and accuracy was back to normal.

[Guest Geoffrey]

Today at 8:30am Eastern US time,I had excellent GPS accuracy of about 15 foot error on my Garmin 3plus.

[Guest Kerry]

http://forums.Groundspeak.com/ubb/Forum2/HTML/000995.html

[Guest Iron Chef]

There is no reason for people to get the idea that SA will be turned back on because of the events of last Tuesday. If it were an ICBM or other guided missile using the GPS signal that was used to attack the targets hit last Tuesday then you can bet that they'd give serious thought to turning SA back on, but that was not even close to being the case. The WTC Towers appear on aviation maps of the area because of their height and are obviously a very visible landmark. Even though the airplanes most likely were equipped with GPSR technology that fact has very little bearing on the situation.

 

Just because there has been a great tragedy doesn't mean that we need to lose our heads over every little thing.

 

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-Iron Chef

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agefive.com/geocache/ ~ Fe-26

Lets Drive Fast and Eat Cheese!

[Guest Kerry]

ize it's a similar effect as being turned off but the reality is there is quite some difference between being turned OFF/shutdown and still on line but set to zero (or close enough to it?).

 

Cheers, Kerry.

[Guest Hawk-eye]

I don't know how many of you were using GPS back during the gulf war ... but during that period ... SA was on of course ... but during the conflict ... all the birds were repositioned for maximum effect over the areas in question. Coverage that was available in the US dropped like a rock. I thought my unit was really screwed up until I read an article about what was happening. It was bad enough with only 8 channels and no parallel reception ... This is very possible if things get bad in the future. I don't think reception will be as bad with the newer units ... but who knows. Anyone remember the problems ... or am I the only old fart from the Stone Age?

[Guest Kerry]

Hawk-eye, back in the early 90's the system wasn't fully operational with only something like 16 satellites up at the time for a system that required 24 & 3 spares. Even given that number of Sats and the available planes/slots to put them (they still all must do 2 laps of the globe per day) there were still apparently some restrictions even in the Gulf War (and even after satellites launched during the conflict were apparently placed to maximize Middle-east coverage). There was something like only 19-20 hours of 2 dimensional coverage in the Gulf with even less 3D coverage for specific operations.

 

The system didn't become fully operational until 1995 and right now those stone age issues don't exist. There exists now an over redundant system (at least 28 maybe 29? from a max of 32 possible satellites) and technology has come some way since the early 90's.

 

Cheers, Kerry.

[Guest bob_renner]

I remember reading in sci.geo.satellite-nav during the Gulf War that SA was turned OFF. There were several people who took measurements and plots of the reduced wanderings of a stationary receiver at the time. The reason given then was that the military didn't have enough military version receivers and many were using civilian receivers.

 

Bob

[Guest Iron Chef]

quote:

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Originally posted by bob_renner:

I remember reading in sci.geo.satellite-nav during the Gulf War that SA was turned OFF. There were several people who took measurements and plots of the reduced wanderings of a stationary receiver at the time. The reason given then was that the military didn't have enough military version receivers and many were using civilian receivers.

 

Bob

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It's very possible. I've used one of the military units before that they are very big/heavy tools despite the accuracy that they get.

 

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-Iron Chef

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agefive.com/geocache/ ~ Fe-26

Lets Drive Fast and Eat Cheese!

[Guest Prime Suspect]

The whole concept of turning off SA in a case such as this is absurd. All that had to be accomplished was to get the hijackers within visual range - SA is certainly not required for that.

 

The question is whether or not the GPS system (at least over North America) should have been completely shut down. That, in fact, is what protocol called for. But it wasn't carried out, probably because it took too long to determine that the attack was actually an attack, and not a horrible accident.

 

For more information on the SCATANA (Security Control of Air Traffic and Air Navigation Aids) protocol, check out this link:

 

http://www.theregister.co.uk/content/57/21709.html

 

[This message has been edited by Prime Suspect (edited 18 September 2001).]