Quoting Nathan Bergey <nat...@psas.pdx.edu>:

Yes, I think the main goal is to get people to think about GPS on rockets.
Even if we don't make an attempt this should be fun to watch.

More (selfishly) importantly I've been invited on the Evadot podcast
tomorrow morning to be an "expert" pundit of sorts about the contest and
what is hard about it, what's easy, etc.

Anyone have thoughts about it? Is it a good idea? Is it easy?

It's not trivial, but it can be done. The hard part is tracking the *rate* that the Doppler-affected satellite signal frequency changes. If you build a GPS that can track (the frequency change) that fast, it becomes susceptible to noise. This can be greatly alleviated by tightly coupling the GPS and INS sensors (+ details).

How long will
it take for someone to win?

GPS units capable of high dynamics already exist. If someone that has one knows a rocket guy, problem solved.

Hopefully I have my facts straight about GPS
(yes I know about the ITAR or vs and in the cutoff
conditions). Anything else relevant? Why not just fly an OEM board?

OEM's have a tracking loop (google Costas loop) typically with a 1Hz bandwidth (source: a GPS INS book I have. It's at home at the moment). Some OEM units can change the loop bandwidth to something higher, but with poorer accuracy and greater likelihood of loosing the satellite lock. The result is this: if the GPS accelerates (relative to the satellite) above a certain rate (some GPS OEMs advertise 4 g's) the correlator cannot change the carrier tracking frequency fast enough to keep up, so the signal lock is lost.

Another approach is to adapt the rocket to work with the GPS. This would get you the tracking, but not necessarily the altitude unless you can unlock the GPS (ITAR restriction). The OS GPS or GPL/GPS could really shine here.

I'd say one could do this with a 2 stage rocket using a slow first stage, a slow second stage, and long launch rail. The key is to keep the OEM GPS acceleration under 4 G's the whole way. Hybrids might have an advantage here with long slow burns.



I'm getting on the aRocket list now. Super annoying that it's closed.

-Nathan



On Mon, Feb 28, 2011 at 12:05 PM, Jamey Sharp <ja...@minilop.net> wrote:

On Mon, Feb 28, 2011 at 9:57 AM, Nathan Bergey <nat...@psas.pdx.edu>
wrote:
>
http://www.rocketryplanet.com/index.php?option=com_content&task=view&id=3542&Itemid=29
> 100,000 ft is not even space, only a mere 30 km. Note the requirement of
> having GPS. Something we should already know how to do.

Note the requirement amounts to "still getting data from the GPS
receiver above 100k feet." ITAR limits export of GPS receivers
"designed for producing navigation results above 60,000 feet altitude
and at 1,000 knots velocity or greater", but as I recall, PSAS found
that 1) American GPS makers don't want to build commercial receivers
that they can't export, and 2) they treat the restriction as an "or".
So maybe the real point of this challenge is to demonstrate an amateur
GPS receiver that works well above 60k feet?

I wish the "arocket" list archives weren't closed. I'd like to see the
original challenge without having to subscribe.

Also note the prize is now almost meme-compliant, at $9000. Somebody
should throw in another dollar.

Jamey







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