Article: 102672 of sci.geo.satellite-nav From: "Pieter Litchfield" Newsgroups: sci.geo.satellite-nav References: <395A4CA0.CE701A05@wa4phy.dyndns.org> Subject: Re: More on GBR-21 Date: Wed, 28 Jun 2000 20:36:21 -0400 Lines: 80 X-Priority: 3 X-MSMail-Priority: Normal X-Newsreader: Microsoft Outlook Express 5.00.2615.200 X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2615.200 NNTP-Posting-Host: 32.101.8.91 Message-ID: <395a9ad3_3@news1.prserv.net> X-Trace: 29 Jun 2000 00:39:47 GMT, 32.101.8.91 Organization: Global Network Services - Remote Access Mail & News Services X-Complaints-To: abuse@prserv.net Path: b4.mv.meer.net!nntp1.ba.best.com!news2.best.com!news.maxwell.syr.edu!news.he.net!cyclone-transit.snfc21.pbi.net!165.87.194.248!newsfeed.us.ibm.net!ibm.net!news1.prserv.net!32.101.8.91 Xref: b4.mv.meer.net sci.geo.satellite-nav:102672 I have used DGPS for some time, both before and after SA was removed, and with both Garmin and Magellan GPSs attached to my Magellan DGPS beacon receiver. I'm going to assume that your GBR-21 is as capable as my old Magellan DBR, since both basically are just receivers and demodulators - performance with any DGPS receiver near a USGS beacon should be pretty binary - either they work or they don't. An old Trimble publication had a chart called the "error budget" which DGPS helps correct, and while SA was clearly the largest part of that error budget, there are other errors that DGPS can still correct (and some it cannot). Correctable error sources are satellite clocks, orbit errors, and the now deceased SA. Partial corrections for Ionosphere and troposphere conditions are possible. DGPS cannot correct for multipath or receiver noise since those errors are unique to the location and performance of the GPS, not the satellite system. The Trimble estimate (excluding SA) was that DGPS could still improve accuracy by about 8.9 meters. This benefit would decrease somwhat with increases in the distance between the GPS and the DGPS beacon station. For the recors, this is from a 1993 publication called " Differential GPS Explained". Of course the worth of this benefit depends on what you do with your GPS. Much of my use is in gathering data for natural resource and trail mapping. When SA was in effect, I had to use DGPS to get reasonable track data from a retail GPS to map trails. Having a trail wrong by 100 meters is significant. I think for my purposes, even gaining about 9 meters of accuracy is still worthwhile. If, however, my only use was to navigate to some feature (like for example a cabin) on a map, it is likely that I could find it without the 9 meter benefit. You must assess the worth of additional accuracy to your specific purpose. Simply put, the beacon receiver you have is not junk - I'm quite sure it will produce a benefit. I suspect the EPE numbers are a function of a mathematical error in display, and in fact the GPS is more accurate with DGPS than without it. If its any consolation, my Magellan just replaces the EPE number with "DGPS" any time its using DGPS correction data in its calculation, thereby relieving me of knowing what the error might be! I assume that it is mathematically possible for a GPS receiver to calculate the EPE if the only errors left are those unique to the GPS receiver, namely multipath and receiver noise performance. I think the advice you got was sound. Try the GPS both ways (in its intended position). With a Garmin, you can clear the track memory, and then set the track record to time, and set the interval to several minutes. Leave it for a day or two. I have read UTM data from my GPS to OZIExplorer, then produced a text fiel of the track data, then used Excel to graph the x,y UTM coordiantes or the change from the average of x and y. Do it again for DGPS position data. That should give you a good idea of how much error is being improved by DGPS at your location with your equipment. S.W. Drinkard wrote in message news:395A4CA0.CE701A05@wa4phy.dyndns.org... > Hi Dale, > > Talked to Garmin Tech Support this morning, and the discrepancy in > the DOP/EPE problem with the beacon enabled is apparently a software > problem. Garmin is aware of the fact that the dop/epe increases with > beacon use, and is, from what I gathered, working on a fix. No > timeframe indicated however. According to the gentleman I spoke with, > use of the GBR-21 while SA was on would only increase the position > accuracy something like 2-3 feet if I understood him correctly. Not > much! Pretty much left up to the individual user to do his own testing > for accuracy while using DGPS correction. Somehow, I read into his > comments something about "buyer beware" :-) Guess I have for the time > being, a $300 worthless black box! > > Guess I'll be watching the Garmin site for any updates to the 180 on > software above 4.03ML. > > > Regards, > > Sam > > > Mailed and posted > >