dave497

Interesting. The GPS I use has a SiRF Star III chipset which, I've read, is very sensitive. In fact, one reviewer (a GPS-phile, not a professional) suggested that it may actually be too sensitive for people who use it strictly in urban areas because it may pick up low amplitude bounced signals so well that the bounced signals pollute the readings. The test might be to record the stationary drift in the city, by buildings and compare it to drift out in an open field. If your explanation is right, drift should be much less away from anything that might reflect the signals. I'm going to give that a try, and record the GPS track data under both conditions.

I've searched the forum and the internet trying to find a technical explanation for a phenemenon I noticed just recently when I looked at track logs from BeelineGPS: drifting GPS position when stationary, which clears up when I start moving. It seems that everyone who has looked at raw GPS information (as opposed to software filtered information displayed by many software packages) has noticed this. It also seems like a phenomenon inherent to GPS, not due to poorly engineered receivers. The GPS setup I've been using is Globalsat BT359 with a Pocket PC using BeelineGPS and OCN6 mainly. When I upload a track file from BeelineGPS to Google Earth, the accuracy while moving in an automobile is good enough that I can zoom in and see two parallel tracks on either side of the road if I do a U-turn and reverse my direction. But when I come to a stop my position, based on the track data, starts to drift, sometimes erratically, until I begin moving again. It doesn't seem to be related to signal quality. I've seen position drift when stationary with 11 satellites tracked and WAAS signal acquired. I have read various explanations, including GPS signal bounce from buildings, signal attenuation from trees, satellite based corrections (WAAS) vs. ground based (DGPS), software filtering algorithms, and others. The explanations I've read are inconsistent with each other and none of them really explain what causes the drift. What are the reasons you've heard for GPS position drift when the receiver is stationary as opposed to moving? An explanation won't make my GPS drift go away, but I'd feel better knowing anyway