Brandon Fosdick wrote:
>
> Matt Bennett wrote:
> > case).  This device was part of an GPS artillery fuze, the GPS signal
> > was sent back to us to track the bullet in flight.  Pretty neat, and it
> > actually worked.
>
> I once looked into using GPS to guide a sub-orbital rocket and now I'm
> using it to guide R/C airplanes. One of the difficulties I've found is
> the 1Hz update rate is just too slow for high speed flight. How did you
> deal with that on something as fast as an artillery shell?
>

It wasn't used for guidance, it was used for tracking the path.  There's
nothing magic about 1 s update rates that's a feature of the GPS board-
the C/A code repeats every 1ms, and if you look at code phase, you could
update much, much, faster.  But if the rocket is moving very fast, that
can get very hard.  But another thing to worry about is that if you use
a commercial GPS, there are altitude and speed restrictions on
commercially available GPS boards to discourage their use in (offensive
weapon) missles.  Even with a 1 s update rate- you can use it as an
input to a Kalman filter, with other inputs like an INS (good short
term, relative accuracy), compass, horizon sensors and the like.

Our GPS device was just a translator- the real reciever was done in post
processing- we would sample the raw data, archive it, and process it
later.  It wasn't an incredible amount of data- since a howitzer bullet
is in the air usually a lot less than a minute.  When I left that job,
we were working on doing it in real time with P(Y) code, but that was a
lot harder to do.

Matt

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