Firstly, Real Time Kinematic correction involves a stationary base station unit and a moving rover unit for differential correction. The drift in the base station's satellite timings is used to correct the rover's position. A Japanese project to implement RTK in an open source library was created two years ago. I remember reading discussions of RKTLib that trailed off noting its initial lack of support for various models. It looks like development has continued to add features and expand support in the intervening time, and I'm wondering if there's any chance it could see use around here, since they seem to have accrued several semi-affordable models.
http://gpspp.sakura.ne.jp/rtklib/rtklib.htm
Second, as of this year there now exist APM-size boards which will implement dual-frequency correction for centimeter accuracy. Dual frequency receivers use different amounts of interference at the different bands to model atmospheric distortion directly, vastly improving accuracy. I'm looking particularly at the Hemisphere miniEclipse, the Novatel OEMV-1DF, and the Geneq SXBlue III. The unknown variable here is price in batches of 1000, which is not made public. Are any of these boards viable candidates for the DIYDrones store?
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Great info from Pbreed.
RTK takes a bit of processing. Using a NovAtel or similar gives you a separate processor dedicated to the purpose. It costs, but you're paying for both hardware and software capability.
RTK involves phase-locked tracking. I'd be curious whether Pbreed's observation is really about vibration, or whether it could be the irregular ways a multicopter can move. Compare it to agricultural machinery, which may be doing only 10km/h or so when precision matters, and even then in a relatively straight line.
Since phase-locked tracking is required, visibility of satellites has to be pretty constant. Satellites near the horizon are more helpful than those overhead, except for calculating altitude. When a multicopter banks/tilts, does the antenna lose much of a view of the horizon?
In terms of the time to first lock at centimetre precision, the receiver has to decide how many wavelengths it is from each satellite. Once it's sure, it can then keep count as long as it maintains phase lock. Getting the first fix would be quicker: at a known location, with a differential system, and particularly with L1/L2 dual band operation.
I hope to build my first multicopter in the new year, aiming for decimetre accuracy using DGPS.
Take a look at this. The test is showing cm accuracy.
http://gpspp.sakura.ne.jp/rtklib/rtklib.htm
http://gpspp.sakura.ne.jp/rtklib/rtklib_beagleboard.htm
Have fun
All of the dual freq boards are more than $1K in small quantities.
I've purchased and used (>$5K ea) dual frequency boards and flown them on an RC helicopter.
To get sub cm level precision on these you need a serious antenna.
The units I had were not very vibration tolerant, they kept losing the RTK solution on the vibrating helicopter.
The Omnistar high precision correction service actually seemd a bit more robust than the local reference server.
My guess is that it was not as precise in a static situation.
With this working the GPS precision in flight was easily better than +/-10cm. this used a 4lb choke ring antenna.
I tried a smaller lighter antenna and never got good lock. I probably needed more gain. Since I could carry the weight I did as I had other problems to solve at the time.
Before trying the dual frequency system I purchased an L1 20Hz RTK system. It worked fine stand alone.
adding telemetry transmitters, RC transmitters, and vibration each individually degraded the L1 RTK so it would sometimes take 20min to get an RTK lock and then loose it randomly in flight. When it worked it was awesome, when if failed the helicopter results were spectacular, jumps of 50+/-ft in altitude. I was using GPS as my altitude reference so a sudden 50ft change in altitude could be sporty if you were under the helicopter and or were not paying attention with the override transmitter in hand and were on top of engaging the manual override.
High-precision GPS is an enabling technology for future applications, in particular applications which involve autonomous charging, takeoff, and landing. 15 minutes of flight time is very short for a lot of remote sensing and industrial activities, particularly given the 400ft altitude limit in the US that we may / may not have to comply with. A reliable 10cm precision would enable relatively painless low-voltage charging stations, and vastly reduce the amount of labor that goes into collecting data over wide areas. Simple charging stations, in quantity, would enable repeatable coverage of a wide area by a swarm of multirotors.
Furthermore, either of these would reduce the gulf between US/Europe, where we have sparse, but reliable state-run differential base stations that are compatible with these units (WAAS, EGNOS), and the rest of the world's relatively weak capability to use waypoints. This has been a theoretical consideration in promoting our techniques abroad.
Is there an actual problem that you are trying to solve, or is the quest for ultimate GPS accuracy its own goal?
To a a man with only a hammer for a tool, every job looks like a nail.