Article 62507 of sci.geo.satellite-nav: From: Wolfgang Rupprecht Newsgroups: sci.geo.satellite-nav Subject: Re: Post Processing (0x29 command) Date: 21 Apr 1998 21:39:46 -0700 Organization: W S Rupprecht Computer Consulting, Fremont CA Lines: 32 Message-ID: References: <6h2utc$ga9@bgtnsc02.worldnet.att.net> <353526F4.296F@mehaffey.com> <6h59p5$63o@bgtnsc03.worldnet.att.net> <35399F70.5058@telusplanet.net> <353A3CA6.C1F4B869@mehaffey.com> NNTP-Posting-Host: c460058-a.frmt1.sfba.home.com Mime-Version: 1.0 (generated by tm-edit 7.106) Content-Type: text/plain; charset=US-ASCII X-Newsreader: Gnus v5.6.3/Emacs 19.34 Path: news1.meer.net!nntp2.ba.best.com!news1.best.com!newshub.northeast.verio.net!howland.erols.net!news-peer.gip.net!news.gsl.net!gip.net!newshub1.home.com!news.home.com!news.rdc1.sfba.home.net!not-for-mail Joe Mehaffey writes: > I think you are thinking of GPS units which store the pseudo ranges and then > allow you to adjust the pseudo ranges by post processing and thus correct pseudo > range errors. The common consumer GPS receivers do not have that capability. It > is demonstrable that when you do not know WHICH SVs your GPS is looking at when > it takes a measurement, you cannot do a post processing evaluation to correct > for SA and other errors. > > Am I missing something? Actually it looks like some of the garmins do put out pseudo-ranges, but due to secret squirrel issues it isn't documented very well. For an interesting read grab the pdf specs for the 25 oem sensor at http://www.garmin.com/db?MItab=garmin&MIval=oemgps25LPSpec and flip to to PDF page 38 (p. 34 marked on the printed page). You will notice that the elusive 0x28 and 0x29 garmin protocol command is documented. Also documented is 0x2a the ephemeris record. 0x28 gives the alt, epe, lat, lon, lat velocity, lon velocity, alt velocity. 0x29 gives cycles, pseudo range, phase signal strength, SVID, weeknumber, timeofweek. Notice that 0x29 will spit out the pseudo-range, which should be enough to do all the interesting differential hacks that couldn't be done before. happy hacking. -wolfgang