I've read through a few discussions on this forum about poor man's DGPS and getting relative accuracy down to the ~1m range. It seems no one's having much luck even with identical GPS units. I'm wondering if anyone has manged this and what are your thoughts on what I'm currently building.
I'm laying out a PCB for a ublox LEA-6T (that's the one that outputs raw data and carrier phase) to interface to a MCU and a 433MHz radio. That's the mobile unit. My base station is then a single board computer with it's own LEA-6T, radio, and software running GPSTk post-processing software. The mobile board will then radio the raw data (subframes 1 and 2 and psuedo range and carrier info) to my base station. Base station code then chooses a set of sattellites that work well for both units and calculates position of both units using the same satallites and same ephemeris data. (calculated position could then be radioed back to mobile unit)
The units are genearlly pretty close to each other (spatially) so the only error here should be that inherent to the ublox measuring psuedorange--no constellation, algorithm, iono/troposphere, ephemeris differences.
Has anybody tried this? Do you think I can get a relative accuracy of ~1m?
Thanks for any thoughts guys and gals!
Simon, I am interested in using RTKLIB to survey GCPs for use in SfM for mapping and modelling. I am happy to post-process and expect that this will be a cheaper option, not requiring live telemetry or necessarily on-board computing.
My main practical issue will be what data I need to store on the rover and how do I log waypoints? Is there an existing solution for this or do I need to do some coding?
Thanks to you and everyone else on this thread for your contributions - often overlooked these are the subjects that give serious UAVs purpose.
I'm not sure what GCP/SFM stand for... but in order to perform a live RTK or post process you need to record pseudorange/phase data on the either end (rover and base station). If you are lucky enough to have a public base station close enough (say up to 10 or 15km) then these can be used and will often publish daily archives.
The messages you need record can vary by receiver, and depending on your application you may want to record at a higher/lower rate (ie. 1Hz or 0.1Hz). What type of receiver are you using?
Personally I also record the 'GPX' data at the same time as this ensures I have a fall back, even if it's accuracy would be some what less.
Regarding actually collecting the data; if you have a serial based receiver you could look at the OpenLog project, which is a small micro with Mirco-SD socket. Power it up and it records everything it sees.Simon.
SfM is a method for constructing 3D models from 2D images taken from different viewpoints. Microsoft's Photosynth is an example.
I haven't bought any receivers yet but am looking at options - any recommendations?
Presumably with OpenLog I could set it off recording and leave the GPS on the GCP for a minute or so and that would be more than enough. The base station would be recording for continuously whilst I move from GCP to GPP. That's the plan anyway.
Interesting... would you be forming distance measurements with theodolite or just working from angular measure off the optic/photo's? You may have to carefully calibrate the optics of you system to account for distortion/etc.
Once RTK (or post processing) is 'fixed' the instaneous readings should give you a good position. If you are taking 'snapshot' pictures you may want to use a 'mark' input or record waypoints.
I believe that the only choices in 'consumer grade' are Ublox5/6-T, SkyTraq (Raw) and DIY.
Stepping up to commerical look at Novatel, Topcon, Leica, etc... though lots of money. The quality of you antennas will probably be the largest factor in accuracy.