On Mon, Apr 14, 2008 at 8:38 AM, LeeE wrote:
> I've been doing some experimentation using the gps instrument for
> navigation functions but I've hit a minor problem due to the gps
> instrument update rate.
>
> I'm running a nasal loop at 1/(frame-rate/2), which typically works
> out to between 10-20 Hz, but because the gps update rate is much
> slower (0.45 sec if I'm interpreting the code correctly) the
> results aren't very smooth - the effect is that the results follow
> a sawtooth pattern i.e. they ramp up while the gps output is
> unchanged but then drop back down when the gps output is updated,
> and then start to ramp up again.
>
> Increasing the gps update rate to 0.1 sec in instrument_mgr helps to
> smooth the output because each 'tooth' is smaller, with the result
> that each ramp-up and drop-back is similarly reduced in size.
>
> What I was wondering though, is what is the max update rate for
> NAVSTAR gps receivers?
>
> I dug out my Garmin e-trex manual, because I knew that had a battery
> save mode that reduces the update rate but it only says that the
> unit will update once per second or 'continuously'.
>
> After a bit of digging around on the web I found a discussion where
> it was stated that "The theoretical limit is down to the
> integration time for the receiver, typically 1-10ms" but the
> practical limit turns out to be more down to the receiver's
> processing power.
>
> Does anyone else here know anything about this?
>
> Ideally, I'd like to make the gps update rate configurable - can
> anyone foresee any problems in doing this?
I don't know specific update rates for every device, but my understanding is
that a typical consumer handheld device will update every 1-2 seconds.
I have a u-blox gps on my UAS and that updates at 4hz. I've seen other gps
units that update at 5hz, but I've heard it's almost, but not quite 5, so
you end up closer to 4hz anyway.
I have seen some trimble differential units that update at 10hz, but those
are starting to get really expensive.
I wouldn't be surprised to find an inertially augmented system that could
report approximate locations at much higher rates. My UAS flight computer
computes position estimates at 10hz based on a 4hz gps + gyro &
accelerometer data. I could probably bump that up to a higher rate if I
wanted to.
So a true 0.1sec update interval is plausible, but expensive and probably
not characteristic of the typical gps you would see on an aircraft. But I
could be way off on that ... (?)
Regards,
Curt.
--
Curtis Olson: http://baron.flightgear.org/~curt/
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