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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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