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The Telecom Digest for February 8, 2013
Volume 32 : Issue 35 : "text" Format
Messages in this Issue:
Re: Closer than the satellites that guide your GPS (Neal McLain)

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Date: Thu, 07 Feb 2013 18:18:00 -0600 From: Neal McLain <nmclain@annsgarden.com> To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org. Subject: Re: Closer than the satellites that guide your GPS Message-ID: <51144438.1090504@annsgarden.com> Barry Margolin <barmar@alum.mit.edu wrote: > More likely, they assumed GPS satellites were in > geosynchronous orbits (22,000 miles), like most > communication satellites. I was actually surprised > to see that they aren't. Most communication satellites are indeed in geosynchronous orbits. Furthermore, most of them are in the geostationary orbit; i.e., the Clarke Belt. The term "geosynchronous" means only that the satellite orbit equals one sidereal day. It says nothing about the inclination of the orbital plane or the eccentricity of the orbit. To be geostationary, an orbit must meet three criteria: - It must be geosynchronous. - The inclination of the orbital plane must be zero (i.e., it must lie in the earth's equatorial plane). - The eccentricity of the orbit must be zero (i.e., the orbit must be a circle). There's an infinite number of geosynchronous orbits but only one of them is geostationary. While most communications satellites move in geostationary orbits, a notable exception is the Sirius Satellite Radio fleet. These satellites lie in Tundra orbits, a highly-eccentric geosynchronous orbit lying at an orbital inclination of 63.4 degrees. At this angle, certain "secular perturbations" cancel out, resulting in a stable orbit. XM Satellite Radio's satellites move in geostationary orbits. The merged company, now known as SiriusXM, maintains both fleets in order to continue service to legacy customers. References: - Tundra orbit. Wikipedia http://en.wikipedia.org/w/index.php?title=Tundra_orbit - Orbital perturbation analysis (spacecraft). Wikipedia. http://en.wikipedia.org/wiki/Orbital_perturbation_analysis_%28spacecraft%29 - Tundra. Castor. http://www.castor2.ca/02_Basics/03_Orbits/05_Tundra/index.html - Sirius Tundra Orbit Animation. Verizon. http://tinyurl.com/siriusorbits Neal McLain ***** Moderator's Note ***** What's the third criteria? Bill Horne Moderator
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