Posted by Pekka Ahola on October 17, 2009 at 3:11am
Just came across a "new" module from Sparkfun, that promises 10Hz update rate and has a binary protocol. Does anyone in the community have any experiences working with one of these?
I use the same chipset in an other (much cheaper; 25 USD) GPS receiver with antenna. See here: http://www.mr-lee-catcam.de/pe_cc_o2_en.htm
Update rate can be scaled from 1 to 10 Hz, lock quiet fast and the current draw is really pretty low.
Tom.
GPS altitude accuracy is very different to the horizonatal accuracy - it's the nature of the beast. A barometric altitude, whilst not having great absolute accuracy does not have as wide an envelope of error as GPS - in my experience at least...
Pekka: You mentioned something about more accurate altitude, do you often see altitudes that are less accurate? I always though that GPS math was completely 3D, and I guess I wouldn't expect one of the dimensions to be different from the others. Have others seen this?
We have tried the Venus GPS Modules from Sparkfun and initially had a bad experience with cold start issues. It was taking anywhere from 5 to 15 minutes to coldstart maybe even longer in some instances. The problem is the sparkfun board ties the vcc to the vbat pin keeping you from having a backup battery to keep the ephemeris data backed up for the next cold start. I emailed the mfg about the problem and they reponded the next day with new firmware that solves the problem by storing the ephemeris data in onboard flash. It also fixed some other performance and accuracy issues. They did not however claim any altitude or airspeed accuracy improvements. I have not yet determined the accuracy of either of those yet. I should be able to do that soon because I am adding presure sensors to my setup to do some comparisons. I did find the Sparkfun embedded antennas do not work well at all with these GPS units. I am using one simular to this one only smaller.
Thanks for the detailed explanation. Though I'm planning on using an old extremely slow motorized glider as a test platform - I think I'll manage for now with my old 1Hz GPS that I have lying around.
Ps. Are there any new reasonably priced air optimized and over 1Hz GPS units available or coming to market?
@Pekka: With uBlox you can change the receiver's Kalman filter parameters to "expect" greater accelerations and thus you ought to get better performance. With normal GPS units the problem is that they are not set to expect sudden accelerations and they filter small movement completely out which is good for a car GPS navigator system, for example, but will cause some problems in UAV use. I don't think that uBlox has much greater absolute accuracy, however (with similar antennas anyways).
Ok, good to know. Does "optimized for air use" mean, that uBox has better and more accurate altitude positioning than other gps-units? Something to do with the location approximation calculations inside the unit?
They've had it since April, but I haven't tried one. uBlox is optimized for air use and 5Hz is plenty for us, so we haven't been drawn to this. The comments suggest that long cold-starts are a problem, too.
Comments
Would you be able to post that firmware update?
Update rate can be scaled from 1 to 10 Hz, lock quiet fast and the current draw is really pretty low.
GPS altitude accuracy is very different to the horizonatal accuracy - it's the nature of the beast. A barometric altitude, whilst not having great absolute accuracy does not have as wide an envelope of error as GPS - in my experience at least...
Mike
Ps. Are there any new reasonably priced air optimized and over 1Hz GPS units available or coming to market?