I was watching Future Weapons on Discovery and they had a missile that was guided by GPS that could detonate above, below or at ground level.
During the show they commented that there is a more precise GPS system that is exclusively for military use.....is that true? I thought that there is only one GPS system that was used by both civilian and military equipment and there is no mention about a military specific system in the GPS Wikipedia article.
Sorry if this is the wrong newsgroup, I was going to ask about the technology inside the missile but worked it out half way through.
Yes but it claimed that it wasn't currently enabled: "Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Selective Availability is still a system capability of GPS, and error could, in theory, be reintroduced at any time. In practice, in view of the hazards and costs this would induce for US and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA, have stated that it is not intended to be reintroduced."
Civilian GPS (standard positioning service, or SPS) uses the C/A (coarse/acquisition) code on one of the (currently) two GPS frequencies. Military receivers (precise positioning service or PPS) use encrypted pseudo-random codes at a higher chirp rate on each of the frequencies. The encryption, higher data rate, and the use of two frequencies permit a somewhat better position/time accuracy as well as contributing to anti-spoofing/anti-jamming.
In actual use, PPS is not necessarily always more accurate than SPS when DGPS and WAAS are taken into account. However, PPS is more reliable, particularly in scenarios where active denial is likely.
There is differential GPS which, as I understand it, requires a number of supplementary transmitters in known, fixed locations. By comparing the information from the satelites with that from the fixed transmitters, additional corrections are possible that can produce very high positional accuracy. Check out the relevant section here:
This is hardly military only however. Even my little handheld can make use of differential GPS.
Differential GPS is used in Land Survey applications (one of trimble's specialties).
The other kind of GPS you hear about is "kinetic" GPS. As I understand this a true GPS receiver is placed on a known (coordinated) point and stays there. This device then generates a wireless signal that broadcasts the error between it's GPS data and it's known true value. A remote collector is then moved around to the unknown point and the collector sees the error codes from the transmitter and adjusts it's GPS data accordingly. I suppose it's a kind of differential GPS but has resolutions of about 0.5 cm. Differential can get to sub mm resolutions (if you let the receivers sit long enough).
There is another variable in GPS observations that is often overlooked. Time! A stationary GPS receiver will close in on it's true position as time passes. Depending on the number and orbits of satellites in view and with post processing of the data the true position can be determined within 0.1 seconds of arc in as little as 20 min. As a surveyor I worked on a few jobs where this was done. This was also well before selective availability was disabled. Of course having to sit still limits your applications but this does have it's uses.
There is one final way to increase your precision if you are willing to wait. In normal GPS the satellites broadcast their position based on where the satellite believes it is. Variations in earths gravity density, wobble in the earths axis, etc, etc introduce errors in the satellites reported position. These variations are of course tracked by ground stations. (I really have no idea if the satellites "course" is corrected of the satellite is given new information to broadcast). There is a service available from the U.S. Gov't (either NIST or NOAA) that will process your collected data to eliminate the errors in the satellite ephemeris data based on the ground observations. This is really only necessary for determining "absolute" coordinates to high precision over long lines. (Think applications like the border between the U.S. and Canada)
Most applications really require relative data as the usual goal is a course. In this case the errors of simple GPS reception are well within the bounds of tolerance and other methods are not required.