A few questions about GPS.
I use two RoyalTek REB-21R GPS units together, collecting data from them at the same time. I was expecting the relative position value to indicate the actual relative position of the units quite accurately. Because the two units should be suffering from the same error sources, which should not affect the relative value. Just like the concept of DGPS.
However, looking at the data I've got, the two sets of data are pretty much floating randomly as much as 10 meters between each other.
Somebody please explain this to me!

Observing the UTC time value from the two units, they are about 10-30 ms off. Is there a way to improve that?

From GGA data, for Channel 1, Satellite ID is shifting back and forth between two different value every time. Is there a way to lock the receiver on one set of the constellation. I assume that will improve the relative accuration.

Thanks a lot, whoever would give me a little hint!

Yes you can. But do you have an extra $5k to $10k to purchase that surveyors grade gpsr? Secondly, remember that all of us are hiding stuff using consumer grade gpsr's. You would just prove each time that none of us could place a cache anywhere near the actual coordinates.

I don't know anything about RoyalTek GPS's. But this link "Poor Man's DGPS" will provide some information. But basically even if any two GPS's are close together, there is no way to make sure they are using the same satellites. Also this article doesn't mention it but two GPS's can interfere with each other. This is because all radio receivers (which all GPS's include) are also weak transmitters. So if they are close together they can mess up each other.

quote:

Observing the UTC time value from the two units, they are about 10-30 ms off. Is there a way to improve that?

If I assume that your collection is NMEA via RS232 asynch, then at 9600bps and a 20-byte data stream, you will get 50 readings per second. Since the two CPUs are unsynchronized the variance will average 20ms.

Weird.
I've never really heard of DGPS.
I got a reference to a site about it today (DGPS) and then saw this thread.

Oh...and the link above is from the same site Allen linked too, but this one looks more technical (or too technical based on how much you want).

Kenneth