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 Colin Bouriquet on February 13, 2012 at 12:52pm

ionut > yes it does, any uBlox or SkyTraq that can output pseudo-range and carrier phase will do the job... now the big difference will be on the antenna used and the baseline length... with a cheap antenna and short baseline you can achieve something really good over a short time period... of course with a survey geodetic grade antenna much better in much less time...

Comment by Colin Bouriquet on February 13, 2012 at 12:56pm
Comment by MarcS on February 13, 2012 at 1:20pm

Did anybody try rtklib on a plane/Copter? Whats the accuracy? Does it work in high dynamics?

I think all solutions apart from "real RTK" using L1/L2 will provide resolution in the meter domain, especially in altitude... Which means for the part of flight where high accuracy is needed, the landing (or other stunts close to the ground :-), it´s still not good enough.

So for me there are two options: I got the money and buy RTK or I look for alternatives like optical, sonar, radar etc...

Comment by ionut on February 13, 2012 at 1:20pm

Colin-Great, but how are the base and rover going to communicate to each other.Rtklib is a Linux or Windows library, therefor you will need at least 2 computers and 2 usb connected uBlox receivers to have a minimal test platform.Also you will need some kind of wireless communication between these 2 computers.Right?

Comment by wolfgang.90 on February 13, 2012 at 1:35pm

I did this with a pair of Hemisphere Crescent / Eclipse II receivers, and a Trimble ProXRT with all of the subscriptions at work. The 2cm is over a 5 minute time period, assuming you are in the middle of no where, and have an antenna from god, and there is no sun storms. Typical resolution we are seeing is 10cm, assuming there is no building south of the antenna. 

I never got RTKLib to work.

If I had the money I would buy personally the Eclipse II  from Hemisphere GPS. Great to work with, and has an integrated base station mode.

GPS Pathfinder ProXRT receiver

Comment by Simon Wood on February 13, 2012 at 1:50pm

For RTK to work you will need (at least) 1 way data channel for corrections, and a processor (PC?) to perform calculations. The library is straight C, so could run directly on a processor of your choice (given enough MIPS).

Correction data can be sent as RTCM which is low bandwidth/compressed. RTK can work OK with a reasonable latency (say 5s).

All uBlox receivers support serial (RS232) as well as USB, so this can be hooked directly to a wireless modem/cell/zigbee or other radio (ie. without the need for a PC at that part of the system).

Will this work for you? Depends on what you are trying to achieve....

Comment by MarcS on February 13, 2012 at 2:50pm

Simon, you should separate RTK and RTKlib, otherwise we will end in confusion here :-) RTK receivers work in pairs without additional PC for the processing... (Thats what Wolfgang called base station mode..)

Wolfgang, are you talking about the RTK L1/L2 mode of the Eclipse II? I would expect better results from high grade components like these...

My personal experience with commercial RTK is much better, 2cm at 20Hz with high dynamic rover receiver (Plane)...

Comment by Colin Bouriquet on February 13, 2012 at 3:15pm

ionut > Simon and MarcS just answered for me... However, if you don't need real time you can also have only one receiver (uBlog + computer, arduino board, etc) record raw measurements and post process the data with RTKLIB combining it with data recorded by permanent station at the same time. Some national agencies provide data from their GNSS bases for free. Of course this doesn't work in real time.

Comment by wolfgang.90 on February 13, 2012 at 3:42pm

If your lucky, some of the states offer access to the CORS network operated by the UN / NOAA / some states. the website for it is I've got code somewhere that connects to it. It can work in real time if you have permissions, and an Internet connection (>9600bps is required)..

Comment by Mike Trieu on February 17, 2014 at 6:48pm

wolfgang.90 Seems like even the CORS servers would need to be fused with an IMU because the quickest sampling time they've got is 1 second. A lot can happen to a fast moving drone in 1000ms ;)


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