Article: 94469 of sci.geo.satellite-nav Message-ID: <36E041BA.2E72@flash.net> From: Charles Wagner Reply-To: cwagner1@flash.net Organization: self X-Mailer: Mozilla 3.01C-FlashNet (Win95; U) MIME-Version: 1.0 Newsgroups: sci.geo.satellite-nav Subject: Re: How is a satellite Almanac generated? References: <7bpav4$oov$1@remarQ.com> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 26 Date: Fri, 05 Mar 1999 20:40:51 GMT NNTP-Posting-Host: 209.30.116.91 X-Complaints-To: abuse@flash.net X-Trace: news.flash.net 920666451 209.30.116.91 (Fri, 05 Mar 1999 14:40:51 CDT) NNTP-Posting-Date: Fri, 05 Mar 1999 14:40:51 CDT Path: news1.meer.net!news3.best.com!news2.best.com!newsfeed.berkeley.edu!nntp.flash.net!news.flash.net!not-for-mail Xref: news1.meer.net sci.geo.satellite-nav:94469 Ed Weston wrote: > > How is the almanac generated? > > other factors are considered in the calculations. Their predicted orbits are > accurate to within 10 meters (or better) and this accuracy is maintained in > the subframe 1, 2 and 3 data. Almanac data is accurate to roughly 1000 > meters. > I agree with Ed's reply and can add that the ephemeris data describes the satellites orbit to an accuracy of about 1.5 meters or less; given no anomolies. These numbers were determined several years, before SA, by comparing broadcast ephemeris for all satellites for 24 hours with the post processed ephemeris produced by the USGS and confirmed with NIMA (was DMA before their name change). Almanac is rather unusual, in my opinion, as there exists a 500 meter bias due to the fewer terms used to describe the orbits. The error in the almanac generates two sine waves daily within a range of not more then +/- 1875 meters within the six day fit interval of the data sets. Error growth differs from satellite to satellite beyond the four hour ephemeris fit interval and six day almanac fit interval. Regards, Chuck Wagner