GPS and Military Use

did you read the part on "Selective Availability"?

Basically an encryption to reduce accuracy that's known to the US
Military and allies...

regards,
Colin

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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,[14] have stated that it is not intended to be
reintroduced."



Michael

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:

http://www.trimble.com/gps/index.shtml

This is hardly military only however. Even my little handheld can make
use of differential GPS.

Pete Harrison

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.

Kevin

Hi, Michael,

Also, please, keep in mind, that GPS is just most popular system for
geo navigation, but it is not the only one.

For example, there is Russian for global navigation - GLONASS.

See some info:
http://en.wikipedia.org/wiki/GLONASS

Also some other countries and blocks may have their own systems.

Sincerely, Oleg

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