eleven bravo wrote:
I know the gps I used in the army had a +/- of 10 meters I think it was
Two signals coming from a GPS sat; C/A and P(y) signal. C/A is the unencrypted 'less accurate' civilian data stream. P(y) or P stream is the encryptable stream, depending on whether or not the US military wants to allow it to be used by the rest of the world, to improve the accuracy of the C/A stream.
If the secondary P(y) stream is encrypted, you need a GPS receiver loaded with the encryption fill. i.e., a military GPS.
The other large part of the accuracy of a GPS lies within the GPS receiver itself.
To make an accurte GPS receiver, you need;
- a precision timing circuit - to time the differences between the incoming satellite signals. Not a clock, a timing circuit. The signal coming from the GPS is the current time, as known by the atomic clock onboard that GPS satellite.
- a decent math processing circuit - to do Trilateration math (intersection of spheres), a little 'best fit' statistics, and a few calculus equations... oh, and a fair pile of relativistic physics (yes, 'Einstein math'. really) Think: TI-89/TI-92 calculator circuitry.
- proper programming of the above circuitry - to take the time differences, do all of the math properly, and in a timely manner.Doing 'analog' math on digital circuitry (relativistic physics, calculus, etc), in a real-time coding manner, on digital circuitry.. not easy.
Even if you have both the C/A and P data streams available, a cheap GPS receiver won't do the full equation sets to get a precise answer.
Both data streams
and a good Receiver, and you can get down to 1m accuracy.
Both data streams, a good receiver, an onboard inertial reference system, and a precision clock with a current time-hack from the atomic clock ground station (naval observatory, as i recall) used to periodically update the GPS sats... and you can get
really serious accuracy.
Fly cruise missiles down Baghdad streets, take a left turn on Saddam street, fly through a window - kinda accurate
Note: mostly from memory, so I may be a little off on a few areas. Used wiki to remind me what the two signals were named.