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'[EE] GPS receiver maximum acceleration'
2009\01\05@073853 by Sean Breheny

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Hi all,

I have a project where I am considering using a small GPS module to
determine the maximum altitude of a model rocket. Looking at the
typical limitations of consumer-grade GPS modules, they often are
limited to 2 or 4G acceleration. I have not been able to find out what
happens if you exceed that acceleration. I would guess that they lose
lock on the GPS satellites and will reacquire lock after the
acceleration goes below the limit. However, how long does it take for
them to recover from the excessive acceleration?

For example, in the model rocket, there might be 20 or 30G
acceleration for 2 seconds and then a further 5 seconds before apogee.
My project will work if the GPS unit reacquires a fix within 5 seconds
of the rocket motor burn ending.

Any thoughts?

Thanks,

Sean

2009\01\05@075728 by Wouter van Ooijen

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> I have a project where I am considering using a small GPS module to
> determine the maximum altitude of a model rocket.
> Any thoughts?

If there is a crystal on the GPS that might be the G-limiting factor,
and if it malfunctions I don't think it will recover. The physical
construction might also be a problem (maybe not with the modern, very
small, GPS modules).

--

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu

2009\01\05@081823 by Adam Field

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> I have a project where I am considering using a small GPS module to
> determine the maximum altitude of a model rocket. Looking at the

This months Nuts and Volts magazine has a project where they fitted a
model rocket with some instrumentation. No GPS though. What I thought
was really interesting: They used a cut vertical slit that allowed
sunlight to shine on an LED and used that as a roll detector.

www.nutsvolts.com/index.php?/magazine/article/instrumentation_for_model_rocketry

2009\01\05@130124 by Brooke Clarke

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Hi Sean:

The limitations you mentioned are the result of munitions regulations, not the
capability of the receiver.

You can get around them by getting a receiver that's approved for your
application.

You might just try it and see what happens.  I seem to remember that there may
be an "AND" in the restrictions so if you meet some of them the receiver may
still work.

Have Fun,

Brooke Clarke
http://www.prc68.com

2009\01\05@150752 by Matthew Mucker

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My first thought is to just try it and see what happens. Worst case, you'll
end up with a GPS module to use in your next project, right?


> I have a project where I am considering using a small GPS module to
> determine the maximum altitude of a model rocket. Looking at the
> typical limitations of consumer-grade GPS modules, they often are
> limited to 2 or 4G acceleration. I have not been able to find out what
> happens if you exceed that acceleration. I would guess that they lose
> lock on the GPS satellites and will reacquire lock after the
> acceleration goes below the limit. However, how long does it take for
> them to recover from the excessive acceleration?


2009\01\05@201426 by Ariel Rocholl

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Acceleration will be just one of your problems. Consider time acquisition, 5
seconds window for commercial GPS is very narrow, it won't really get
accurate data in a short period of time when the GPS is moving fast. I don't
think a commercial GPS will fit your needs.

2009/1/4 Sean Breheny <spam_OUTshb7TakeThisOuTspamcornell.edu>

{Quote hidden}

> -

2009\01\05@215827 by Chris McSweeny

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The other thing to bear in mind is that GPS altitude is pretty inaccurate
anyway.

On Mon, Jan 5, 2009 at 1:24 PM, Ariel Rocholl <.....forosKILLspamspam@spam@arocholl.com> wrote:

{Quote hidden}

2009\01\05@230028 by John Coppens

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On Sun, 4 Jan 2009 16:18:07 -0500
"Sean Breheny" <.....shb7KILLspamspam.....cornell.edu> wrote:

> Any thoughts?

And, just to make the list of observations longer, GPS algorithms are
optimized for the application intended for. So a cheap GPS module is
software optimized for such things as max 3 G, maybe 300 km/h
max., and a certain max altitude, too. This is mainly done to enhance
precision, and getting processing speed into the range of the GPS
processor capacity.

I don't think you'll have destructive problems over 4G (till at least
20G), but the results will be trailing the actual result because of the
slow digital filters. Results may go either way, depending on the filter
types.

John

2009\01\05@233805 by Sean Breheny

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Hi Wouter,

Hmmm, well, I doubt that the 4G limit comes from any hardware problem
because it is listed under the "dynamics" specs, along with max
velocity and max altitude. These factors have to do with the filters
the GPS processor uses to obtain a fix from the satellite signals.

Also, 4G seems like a very low acceleration to damage a crystal. I
have used crystals in rockets before at well over 20G and they seemed
to work fine during and after flight.

Sean


On Mon, Jan 5, 2009 at 7:57 AM, Wouter van Ooijen <EraseMEwouterspam_OUTspamTakeThisOuTvoti.nl> wrote:
{Quote hidden}

> -

2009\01\06@014706 by apptech

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COTS GPS receivers have upper limits on altitude and velocity and possbly
acceleration.

This is to stop them being used for "other purposes [tm]" (eg in Southern
Israel at present)
You can get GPS's with these limitations removed but they are probably ITAR
items and you probably need to have suitable clearances.
The Arocket comminity will know the limits but Gargoyle probably does too.

AFAIK the receivers generally stay in lock if they are able and start
outputting data again when they next exit the forbidden zone.

Here's some possibly useful commentary [:-)]

   http://showcase.netins.net/web/wallio/GPSrcvrsvs60kft.htm

See "Dynamic condition" here These may all be artificially imposed limits as
the 18,000 metres is one of the set limits.
showcase.netins.net/web/wallio/GPSrcvrsvs60kft.htm
Max alt 18000m
Max V 515 m/s
Max acc 4g
Max jerk 20 m/s^3

         R



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