Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hpda!hpcuhb!hpcilzb!johnson
From: johnson@hpcilzb.HP.COM (Phil Johnson)
Newsgroups: sci.space.shuttle
Subject: Re: Re: Shuttle-ground communication and payloadprices
Message-ID: <2070005@hpcilzb.HP.COM>
Date: 24 Mar 89 22:08:28 GMT
References: <2102@botter.cs.vu.nl>
Organization: HP Design Tech Center - Santa Clara, CA
Lines: 22

> I still don't understand why you need 3 satelites to cover all the 
> communication with the Shuttle.
> Aren't there enough tracking stations on the ground? 

The shuttle flies in a relatively low orbit--just a few hundred miles,
compared to 22,300 miles for geosynchronous communications satellites.
At this low altitude, the shuttle is only "visible" over small patch of
the Earth--a radius of a thousand miles, or something like that.  Most
of the orbital path is not within the coverage area of any (friendly)
ground station.

If you've ever looked closely at the orbital map displayed at Mission 
Control, you may have seen odd-shaped "circles" at various places.  These
represent the coverage areas of particular ground stations (I suppose 
they're not round due to topographical or atmospheric effects on the 
tracking dish at low angles).  From this map it's obvious how little of
the earth's surface is within these coverage areas.

On the other hand, communications satellites in high orbit can cover 
the better part of a hemisphere.

Phil (then again, maybe I don't know what I'm talking about at all) Johnson