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From: (Henry Spencer)
Subject: Re: Navigating without GPS
Date: Sun, 11 Jun 2000 17:27:41 GMT

In article <>, gcash  <> wrote:
>> It's a combination of inertial navigation, star trackers, and radio
>> tracking by ground stations and TDRS.  (Remember, the T in TDRS is
>> "tracking".)
>So exactly how does TDRS assist in tracking the shuttle?

Actually, I've never seen details... although the obvious method is to do
the usual two-way-Doppler radio tracking via the TDRS.

>Does it use propagation delay timing methods similar to GPS?
>If so, does it have on-board atomic clocks like the GPS satellites?

There are no on-board smarts on a TDRS; it's strictly a "bent pipe" relay
system, receiving signals coming up from a spacecraft and retransmitting
them to the ground, and vice versa.

>Does the tracking accuracy depend on the accuracy of the knowledge of
>the TDRS orbit?

That wouldn't be much of an issue, since satellites in geostationary orbit
can be (and are) tracked very precisely indeed.  The TDRS orbits are
routinely known within tens of meters, with tracking runs done several
times a week.

>Is the tracking method constrained by line-of-sight like the
>communications relaying and does that mean there's a tracking zone of
>exclusion too?

It's absolutely routine in any sort of precision tracking work nowadays to
use fancy filtering techniques which consider past history as well as
current data.  That is to say, a brief interruption in data flow won't
cause the system to completely forget where the target is -- it will
merely result in slowly-growing error estimates as extrapolation from past
data becomes more and more uncertain.
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

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