I found this link on the topic. It might be of interest: http://gpsinformation.net/main/poordgps.htm
I'm no expert, but I think one of the reasons this doesn't work is that recreation-grade receivers do not store the actual measurements to the satellites. Instead they store only the calculated positions resulting from the measurements. This means both receivers would have to be observing the same set of satellites to provide an accurate error correction. This rarely happens. This isn't a problem in higher grade receivers because the measurements to each satellite are stored, and the measurements from all the satellites observed by both receivers can be identified, corrected, and the resulting position recalculated. I almost suspect you could bet better results from having both receivers with you at the measurement point and averaging the observations from both receivers. The Sunburned Surveyor -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Simon Garton Sent: Sunday, August 05, 2007 11:01 PM To: [email protected] Subject: RE: [Geowanking] Cheap post-process DGPS - why not? Hi all, I have tried that approach with a pair of cheap GPSs and wrote the software to automatically do the add/subtract. Got absolutely rubbish results - even with two of the same model of GPS. I guess it is something to do with them not choosing the same set of satellites to use - certainly on inspection they were tracking different sets. At one point I had three GPSs on the table, reasonably clear location, and all three were drifting in different directions from the true location. A mate also found this quote : "Q. Can I post-process the data collected with my Garmin GPS unit to obtain greater accuracy? A. Unfortunately not. Garmin GPS units and other handheld consumer-grade units do not internally store the raw pseudorange data from the satellites required to post-process differential corrections. This type of capability is only found in survey-grade GPS equipment." Would be very interested in at least an algorithm for doing this correction ... Simon -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Ben Discoe Sent: Monday, 6 August 2007 1:44 p.m. To: [email protected] Subject: [Geowanking] Cheap post-process DGPS - why not? Hi list, Like a great many people, i can afford consumer GPS units (~$100) but not a 'professional' unit (~$4000-8000). Of course, the well-known lack of accuracy in consumer units (~10m) is nowhere near usable for many applications. The solution that springs to mind would be a cheap differential: 1. Buy a second consumer GPS unit. 2. Tie it to a post or other fixed object. 3. Walk around, gathering data with the first GPS. 4. Download data from both units. 5. Using the timecode to correlate, subtract the second unit's drift from the first unit's coordinates. >From everything i've read, it seems to me that would bring the 5-10m error down to 1-2m. However, i didn't find any software to do this simple operation. There is plenty of information out there about fancier DGPS using WAAS or other things which are not widespread and/or not reliable (http://www.gpsinformation.org/dale/dgps.htm) There is high-end proprietary software like GrafNav (http://www.novatel.com/products/waypoint_grafnav.htm) which apparently costs thousands of dollars. But it should be a really simple operation to subtract one track's offset from another. Is there some reason this simple approach wouldn't work? Is there some FOSS which will do it? Thanks, Ben http://vterrain.org/ _______________________________________________ Geowanking mailing list [email protected] http://lists.burri.to/mailman/listinfo/geowanking Warning: Information provided via electronic media is not guaranteed against defects including translation and transmission errors. If the reader is not the intended recipient, you are hereby notified that any dissemination, distribution or copying of this communication is strictly prohibited. If you have received this information in error, please notify the sender immediately. _______________________________________________ Geowanking mailing list [email protected] http://lists.burri.to/mailman/listinfo/geowanking
