# GPS Pseudo-range Rate Determination

1. Nov 6, 2009

### chingkui

While GPS pseudorange is measured by the satellite and GPS local clock difference (together with some compensations for atmospheric effects), how does a GPS receiver measure the pseudorange rate? Thanks.

2. Nov 6, 2009

### waht

The gps receiver knows the position of the satellites, and measures how long it took the signal to travel.

3. Nov 6, 2009

### chingkui

Hi waht, I am asking how the rate of change of pseudo-range is determined, which would help to estimate the receiver velocity. Thanks.

4. Nov 6, 2009

### mgb_phys

You just compare the rate of change of the difference in the received clock signal with the local clock signal don't you?

5. Nov 6, 2009

### chingkui

you mean getting the difference between the two most recent pseudo-range from the same satellite divided by delta t? That will give us an average range rate over the period, but since GPS operates at 1 Hz, waiting for 1 second would seem too long for an accurate measurement. Is there any way of doing better than this?

6. Nov 6, 2009

### mgb_phys

No, cheap hiking type receivers do speed from difference in position/time - but this is very inacurate.
Aviation/Marine GPS do it from the doppler on the clock signal. using a kind of vernier scheme where you compare newly arrived gps clock edges to the local clock an count how many cycles until they line up again - that and knowing the clock rate gives you a velocity.

7. Nov 7, 2009

### chingkui

Hi mgb_phys, thanks for the info., do you know of any book or online resources that explain in details how this is done, including the equations involved?
Also, I don't know why I have this idea that GPS must updates at 1 Hz, but it has been deeply rooted in me for a long time. When I google last night it seems there are GPS receivers running at much higher rate, is there an intrinsic limit on how fast the GPS can update because of the way the messages are structured? Thanks again.

8. Nov 7, 2009

9. Nov 7, 2009

### mgb_phys

High end GPS put out a 1 Hz pulse to allow you sync the exact position with any other sensors. Ideally this should also be the time that the output position message string was calculated.
There isn't really a rate as such, the unit is constantly refining it's solution as new data arrives the speed of this depends on the hardware and the number of satelites in vew.

There is also limit from the output data rate, the NMEA strings are rather long and take a sgnificant fraction of a second to send over a serial link.