Paul wrote:

I don't think this is done with the lat long data from the GPS. I think
this is done by measuring  the GPS RF signal phase difference from two or
more different antennas on the same vehicle. How this could improve lat/long
position accuracy (if it can) is not clear to me.

Well, I didn't really want to get into the actual "how" of it because it gets really dense, really quickly.  Plus it's getting pretty close to the edge of my knowledge.

You're correct in that the phase of the signal, in addition to the signal content, is used for high-precision measurements.   This is why it is important for the reference GPS and rover GPS to be tracking the same satellites.  It also explains why the reference and rover units have to be initialized together before beginning.  If you don't
initialize, then all you can report is high-precision measurements of relative velocity.  Phase tracking also limits the maximum velocity between the reference and rover GPS (you cannot go faster than the GPS can measure cycle changes; sort of a Nyquist problem).

Phase angle differences only work when you have two units that move separately.  So you could use 2 units (far apart) on a helicopter to get attitude information, but it wouldn't help you get any better position precision unless you had another, off-ship unit to compare against.  And then only if you could initialize the on-ship GPS position to
the off-ship GPS position; otherwise you've merely re-invented DGPS.

It can measure the roll and pitch and the bending of the wings if antennas
are mounted on the wing tips and the fuselage. I know the GPS receiver
knows the azimuth and elevation of the satellites it is receiving data
from so in theory it can calculate the angle to those satellites.

I'm pretty sure that it back-calculates the azimuth and elevation from the satellite's own position information (in the satellite datastream).  It doesn't actually measure the signal reception angle.  It calculates the asimuth and elevation relative to the horizontal plane passing through the receiver.  Easy test: hold your GPS upside-down.
Does the azimuth or elevation change?  Nope.  If you change the antenna height by a few feet, then the change in elevation angle is so small as to be buried in the noise.  You're not going to get roll, pitch, or bending information by using only the lat/lon/alt/azimuth/elevation data.  You must use the phase angle change.

Russell Kent

The Aircraft Rotary Engine Newsletter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org