On Monday 14 April 2008 15:09, Curtis Olson wrote: > 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.
In the discussion I found (on www.dsprelated.com) one person said they'd seen 20Hz units, but they cost US$2/3K, and they knew of 50Hz units which were much more expensive. Another poster reckoned that he'd been using an automotive unit that seemed to be limited by it's 60MHz 32bit MIPS cpu and maxed out at around 10Hz. I also found an automotive racing company that did 10Hz & 20Hz units - no prices but I'd guess they'd be in the several thousand US$ range too. For a pro racing car or military aircraft I'd guess a few thousand US$ is ok but it might be a bit steep for GA. Perhaps another possibility is to use a noise spike filter with a 0.45 resolution time to smooth the output between the 0.45 sec updates but this would mean that the data is always going to be 0.45 sec late. This might be less of a problem than the roughness in the output though - I'll have to play with it. LeeE ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. Use priority code J8TL2D2. http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
