Re: gps etc

fairwater@xxxxxxxxx (Derek Lyons) wrote in news:451a2e55.932635078
@news.supernews.com:

"Brian Gaff" <Briang1@xxxxxxxxxxxxxxxx> wrote:

I noted when Atlantis came back, they included gps data into the mix, I
understand this is going to happen more and more soon.What I have never
really grasped about gps is how height information is obtained though.

The same way position is determined - by spherical triangulation.

GPS height calculations are however notoriously flawed. The system was
originally designed to support SSBN's, and when it was adapted to
wider [military] use this was not seen as a flaw. (I.E. no military
user particularly cares for precise height.) It wasn't until they
started using for weapons and proposing it [civil] aircraft guidance
that it became a problem.

I don't know offhand how the shuttle copes with this flaw.

Mostly by not trying to use it all the way to the ground. Pre-GPS, the
shuttle's entry navaids were incorporated in this order: DRAG H, then
TACAN, then air data, then MLS. GPS is mainly intended to replace TACAN,
since the USAF is retiring it. Since air data provides altitude, it can be
used to cross-check the GPS altitude after the probes are deployed. And
once MLS locks on, it is automatically incorporated in place of GPS (or
anything else).

On STS-115, the GPS data looked very good, so the ground never called for
the crew to take air data to NAV. They did call for air data to G&C, and
continued to monitor the air data residuals to make sure GPS was behaving.


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