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From: karn@mouton.UUCP (Phil Karn)
Newsgroups: net.columbia
Subject: Re: transponder? scramjet?
Message-ID: <17@mouton.UUCP>
Date: Sun, 13-May-84 12:51:32 EDT
Article-I.D.: mouton.17
Posted: Sun May 13 12:51:32 1984
Date-Received: Mon, 14-May-84 01:24:34 EDT
References: <236@ames-lm.UUCP> <1208@proper.UUCP>
Organization: Bell Communications Research, Inc
Lines: 18

Since the Apollo days, NASA manned spacecraft have used something called
the "Unified S-Band Tracking and Telemetry System". This is a coherent
two-way transponder system in which the ground tracking station sends up
a phase-modulated carrier on 2106.4063 mhz. The shuttle carries a
phase-locked-loop synthesizer which multiplies the received carrier
frequency by 240/221 and transmits the resulting 2287.5 mhz signal.
The ground can now lock onto this carrier and regenerate its original
uplink frequency, which will of course be doppler shifted. In fact,
if the system stays locked, the ground can count individual wave fronts
as the shuttle moves, much like a police radar (but much more reliable).
The phase modulation on both the uplink and the downlink can go on
simultaneously with the doppler measurement, but it can also contain
pseudo-random data sequences to provide range information as well as
velocity. Take these numbers and put them into a least-squares nonlinear
curve fitting program with an orbit model, and you get the shuttle's
orbital elements.

Phil