- Hook a GPS to GCS.
- GCS calculates GPS difference.
- GCS transmits GPS correction over xbee MAVLINK connection
- ArduCopter Stabilize mode gets very fine resolution.
Details here: http://eastbay-rc.blogspot.com/2011/05/idea-ardupilot-differential-gps.html
DGPS background: http://www.ngs.noaa.gov/CORS
Comments
What about WAAS ? Mediatek is supposed to support WAAS so no need for DGPS it think.
Basic concept:
Setup fixed base station with GPS and wireless connection to drone. Allow the base station to average it's position over some amount of time and/or use CORS to help locate your base station. At this point let's say you have an absolute accuracy of the base station to 5m. That is fine. Define the base station's location based on the best information you have. You will see where this comes into play later.
Now using the fixed location you can calculate your GPS offsets due to ionospheric and tropospheric distortions.
Relay these offsets to your drone which will take it's GPS position obtained from it's GPS unit and apply the offsets from the base station.
Now you have sub 1m relative positioning to the base station. Accuracy is reduce the further you move from the base station.
Limitations: Say you have a four waypoints forming a square that are directly above poles that have a known absolute position. Your drone will be able to fly the square correct leg lengths and angles very accuratly. But it may be offset sligtly, by the error in base station position. In other word, plot the path of the drone. Over lay this with an actual map. Your path should be very close to correct you may just have to shift the path overlay for it to match up with the map.
I'm sorry guys, this is a hard concept to explain in just words. If there is any interest I can do a more complete write up at some point.
If the goal is to get a steady position hold/loiter on the cheap (not using commercial GCS equipment), this could work if we do some assumptions. But it would not be GCS since the corrected GPS coordinates would not necessary be "correct" ones. They would only be corrected in reference to a fixed stationary position with an unknown GPS error.
For this to work we would have to assume that GPS coordinates reported by a unit are correct, and that the only limiting factor is the random signal error generated by the GPS satellites.
We also have to assume that two identical GPS units (A and B) at the same location, will produce identical GPS coordinates.
If this is the case, then by placing GPS A in a fixed stationary position. You would know that any variation in the GPS position are the result of a GPS signal random error. So if you transmit the GPS A (stationary unit) error variation to GPS B (mobile unit) and subtract it, GPS B should reflect a true position without random error.
Will it work? Frankly I have no clue, since the are a lot of assumptions that has to be true.
If not, the alternative (true GCS) will require specialized/expensive hardware.
This is what is used in our AutoFarm tractor guidance systems. Another unique feature is they use 2 GPS receiver antenas on the tractor mounted about 3 feet apart. By using 2 antenas the system knows how much of a roll there is left to right and what dirrection the tractor is facing without having any movement. This provides us with RTK sub inch accuracy.
The board inside the tractor has 3 "GPS clocks" one for each tractor antenna and one for the base station packets. I am guessing, but to do something similar would require several more componants added to this system.
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