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From: dietz%USC-CSE%ECLA@sri-unix.UUCP
Newsgroups: net.space
Subject: Re: How Does Geostar Work?
Message-ID: <644@sri-arpa.UUCP>
Date: Thu, 2-Aug-84 11:49:00 EDT
Article-I.D.: sri-arpa.644
Posted: Thu Aug  2 11:49:00 1984
Date-Received: Sun, 5-Aug-84 01:03:56 EDT
Lines: 20

Geostar works by triangulation.  A signal addressed to a portable
tranceiver is sent from the central ground station via one of the
satellites.  The ground unit then emits a very short omnidirectional
microwave pulse (with ID information included).  All four satellites
receive this pulse, and transmit it back to the ground station.  Time
delays are used to compute position.  The position is then transmitted
back to the ground unit.

The Geostar ground unit is simply a microwave tranceiver capable of
emitting high power (500 watts, I believe) short duration pulses, along
with some fast control logic to detect when the unit is being polled
and to receive and display position information.  Average power
consumption is low, because the pulses are so short (a microsecond?).

The Navstar system uses passive ground units, since they must operate
in combat conditions where radio silence is critical.  These passive
units must do the triangulation themselves, so they are expensive.

Paul Dietz
dietz%usc-cse@usc-ecl