Article: 94471 of sci.geo.satellite-nav From: "Lawrence R Glickman" Newsgroups: sci.geo.satellite-nav References: <7bpav4$oov$1@remarQ.com> Subject: Re: How is a satellite Almanac generated? Lines: 80 X-Newsreader: Microsoft Outlook Express 4.72.3155.0 X-MimeOLE: Produced By Microsoft MimeOLE V4.72.3155.0 Message-ID: <1GWD2.5202$oA1.301429@nntp0.chicago.il.ameritech.net> Date: Fri, 5 Mar 1999 13:49:37 -0600 NNTP-Posting-Host: 199.179.164.89 X-Trace: nntp0.chicago.il.ameritech.net 920663805 199.179.164.89 (Fri, 05 Mar 1999 13:56:45 CDT) NNTP-Posting-Date: Fri, 05 Mar 1999 13:56:45 CDT Organization: Ameritech.Net www.ameritech.net Complaints: abuse@ameritech.net Path: news1.meer.net!news3.best.com!news2.best.com!news.maxwell.syr.edu!ameritech.net!nntp0.chicago.il.ameritech.net.POSTED!not-for-mail Xref: news1.meer.net sci.geo.satellite-nav:94471 Ed, Now _that's_ what I call an answer. Thank You kindly. I was wondering how the *perturbations* were accommodated. Your last paragraph answers the question nicely. Now I will go to the Mehaffey website and *compare* their idea with your remarkably succinct explanation. Thanks again. p.s. I save this explanation to my *gps folder* for future reference ! -- Larry lglickman@ameritech.net Waiting for Nemesis, the Death Star, to reappear. 15 million years to go and counting. Ed Weston wrote in message <7bpav4$oov$1@remarQ.com>... >How is the almanac generated? > >The Master Control Station (MCS) at Falcon AFB calculates the satellite >orbits. It uploads to the satellites this orbital information (and satellite >clock data) in the form of almanac and subframe 1, 2 and 3 (AKA ephemeris >data) about once a day. The almanac is a less accurate representation of the >orbit because it is a "fit" to the predicted orbit over a period of about a >week, whereas the subframe 1, 2 and 3 data is fit to a four hour period >(also there are more parameters used to represent the orbit, allowing a >higher fidelity "fit"). The satellites are provided many subframe 1, 2 and 3 >data sets, and begin transmitting a new one about every one or two hours. >The MCS uses GPS code and carrier measurements made at monitor stations >located around the world to calculate the satellite orbit and clock offset. >They have a large state vector Kalman Filter to do the computations. >Gravitational effects of the sun, moon and other bodies, perturbations in >the earth's rotation, sun light pressure on the satellites and numerous >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. > >Ed Weston > >Lawrence R Glickman wrote in message ... >>Question for someone: >> >>I would like to know how the sats know exactly where they are in relation >to >>one-another. Do they have the capability to make this determination *on >>their own,* or is it computed by Ground Control? >> >>In the first instance ( autonomic ), what is the mechanism for >>determination? >> >>In the second instance ( Ground Control ), what is the mechanism for >>determination? That is, _how_ does Earth determine with exact precision >the >>altitude of the sats and their positions? I know DoD has programs for >>prediction, but what about absolute correction? >> >>I take it that the satellites don't know much about _anything_ without >input >>from Earth, except, perhaps, the time. >> >>-- >>Larry >>lglickman@ameritech.net >>Waiting for Nemesis, the Death Star, to reappear. 15 million years to go >>and counting. >> >> >> >> > >