It seems there are methods to increase accuracy of the GPS location to cm using a reference station network.

Real Time Kinematic (RTK) satellite navigation  is a technique used in land survey and in hydrographic survey based on the use of carrier phase measurements of the GPSGLONASS and/or Galileo signals where a single reference station provides the real-time corrections, providing up to centimetre-level accuracy. When referring to GPS in particular, the system is also commonly referred to as Carrier-Phase EnhancementCPGPS.

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Comment by Eric Tweet on February 13, 2012 at 12:27am

This certainly would work. As a land surveyor, I can verify this from experience! :)

Experience also shows that a few things are necessary to make this possible, including larger and multi-layered antenna, as well as more processing power and computational electronics. This means some bigger PCB's included. This is also needed for the base station, as well as the need to transmit this signal. All of these things add up to more weight in the air, and much more cost overall, though with some clever designs and probably some slack included (i.e. we are not surveying to the centimeter level in the air!), this is totally doable!

Things like WAAS are already available in small formats which can help in bringing positioning down to the meter-or-less accuracy. I guess it has not yet been a primary focus in projects like ArduPilot, yet.

Comment by Gary Mortimer on February 13, 2012 at 1:33am

The OS in the UK has a countrywide network of transmitters correcting for a fee. Often thought that might be a plan. Can't remember what its called.

Comment by Dave on February 13, 2012 at 1:51am

Gary, is this ones that use Classic FM's RDS carrier wave ?

Comment by Lyn Rees on February 13, 2012 at 5:23am

I think Gary is thinking about OS NET. This is as he said a service offered, at a price of course, by OS in the UK

Comment by Dave on February 13, 2012 at 5:42am

I think so. You could always make your own reference station...

Comment by Dave on February 13, 2012 at 5:44am
Comment by SaadT on February 13, 2012 at 6:32am

The one you've mentioned requires special antenna and GPS receivers (those that are capable of tracking phase as well), which are more expensive (and heavier). 
A better, lower cost option, in my opinion is Differential GPS (known as DGPS). It uses normal antenna, normal GPS receivers...only RTCM corrections are sent from the base to the mobile units over a data link. This gives accuracy of within a meter, which is reasonable in many situations. This is from my knowledge working as a seismic engineer,..we use both RTK and DGPS out here on the field.
I'm myself thinking of implementing a DGPS scheme myself. Basically i'll fix my base station and then generate and send corrections to the mobile unit. One problem here is that either i know my base station position very accurately OR i keep all positions relative. Lets see how it works.

Comment by Dave on February 13, 2012 at 6:56am

Yes, indeed DGPS is really the only practical option and LF beacons like these would be a good idea..

Making a reference station is not that difficult, just record RINEX data over say 24hrs and compare that to a known OSGB reference station data (that is free - well the last time I looked) bingo, you have your site within a few cms.. this can be done with some Garmin GPS, I've done it with a Garmin Vista.
Comment by Simon Wood on February 13, 2012 at 11:01am

It's been mentioned here before several times.....

Just add a couple of (cheapish) uBlox and SkyTraq receivers, or if your budget stretches to it a survey grade receiver such as Trimble, Novatel or TopCon you can get better performance. If you're really cheap some Garmin receivers can output raw data.

Comment by ionut on February 13, 2012 at 12:52pm

Simon, I don't think that library rtklib will work with cheap GPS receivers like uBlox.It will work only with the survey ones.Maybe Dave is closer to a practical DIY solution with DGPS.


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