GPS Reliability

[markys]

I have heard reports of the US relaxing GPS accuracy and putting in a few funnys due to the war. Is this true and how will it affect GPS based systems.

[agent006]

This point was raised a few weeks back, and the general consensus was that it won't affect us at all. If anyhting the GPS system will be beefed up a bit, because the military depend on it so much.

[Podie]

Our GPS toys use civilian band, not military ones... so we are unlikely to be affected...

[zumbruk]

There is no such thing as "civilian band" GPS. What you are talking about is called "selective availability" and works by dithering the position signal in a deterministic manner, such that the error can be removed by authorised users. With SA active, position errors of the order of 100m are introduced. SA was switched off on May 1, 2000 and the US has stated it will not be turend back on. Whether that is true or not remains to be seen. Read the FAQ here;

www.ngs.noaa.gov/FGCS/info/sans_SA/docs/GPS_SA_Event_QAs.pdf

[pbrettle]

zumbruk said: There is no such thing as "civilian band" GPS. What you are talking about is called "selective availability" and works by dithering the position signal in a deterministic manner, such that the error can be removed by authorised users. With SA active, position errors of the order of 100m are introduced. SA was switched off on May 1, 2000 and the US has stated it will not be turend back on. Whether that is true or not remains to be seen. Read the FAQ here;

www.ngs.noaa.gov/FGCS/info/sans_SA/docs/GPS_SA_Event_QAs.pdf

Do note that some of the higher end civian GPS units actually work around the SA by some strange and weird mathematics... in fact it should make little difference other than maybe 20 or so metres... So in reality it wont matter too much.

Cheers,

Paul

[Podie]

zumbruk said: There is no such thing as "civilian band" GPS. What you are talking about is called "selective availability" and works by dithering the position signal in a deterministic manner, such that the error can be removed by authorised users. With SA active, position errors of the order of 100m are introduced. SA was switched off on May 1, 2000 and the US has stated it will not be turend back on. Whether that is true or not remains to be seen. Read the FAQ here;

www.ngs.noaa.gov/FGCS/info/sans_SA/docs/GPS_SA_Event_QAs.pdf

Apologies... I wrote it in English, rather than geek speak.

Sa was indeed turned off, which therefore negated the need for D-GPS (which is still active) and the US did reserve the right to switch SA back on, if the need ever arose.

[Niggle]

Podie said:... the US did reserve the right to switch SA back on, if the need ever arose.

IIRC They said at the time they switched SA off that they can now switch it back on for specific regions (i.e. just the gulf) so we shouldn't see any change in the service here.

[Mr2Mike]

zumbruk said: There is no such thing as "civilian band" GPS. What you are talking about is called "selective availability"

Perhaps "bands" was an innapropriate descrption, but I don't think the OP was talking about SA at all. The GPS has two services, the SPS (standard positioning service) and the PPS(Precise positioning service). The "toys" that civilians can buy use the SPS exclusively. Only the military has access to the PPS. The fact that SA (when it was switch on) was only applied to the SPS service just confuses the matter slightly.

[Igg]

May have nothing to do with it, but used the Lexus sat nav today, and for the first time it hadn't got a clue where it was (Corby!). Up to now it's been accurate to within 1 or 2m.
Igg

[MoJocvh]

deterministic manner

I like the sound of that, right I'm off to try it on the wife !!