High Accuracy RTK Positions with the LEA-6 using RTCM via NTRIP

I was just thinking...

Has anyone looking into real-time correcting the LEA-6 (or other DIY Drones GPS) using RTCM via Internet Protocol (NTRIP). I'm assuming you would just need to have a cellular modem (or phone) on the copter connected to the internet, receiving the RTCM data and passing it along to the GPS unit. I checked the U-Blox website and it says the LEA-6 has an interface for the RTCM protocol (http://www.u-blox.com/images/downloads/Product_Docs/LEA-6N_ProductSummary_%28GPS.G6-HW-11038%29.pdf). Many state's now offer a real-time GNSS network via NTRIP, some are free subscriptions while others are paid subscriptions.

I'm not a programmer so I'm unsure what it would require on that end to make it work.

Here is some example code I found for .NET (http://www.sharpgis.net/post/2005/01/21/Differential-GPS-using-RTCM-from-NTRIP.aspx)

Any thoughts?

-Steve

A few notes on cm-level GPS.

There are basically two levels of accuracy achievable with GPS. Meter-level and centimeter-level. Meter-level accuracy relies on C/A code data and centimeter-level relies on carrier phase data. Any properly designed GPS receiver can provide meter-level accuracy under good receiving conditions by itself using the C/A code data. For centimeter accuracy you need a receiver which will process local carrier phase data in conjunction with carrier phase data from a base station.

The cm-level receiver market is much smaller than the meter-level one so these type of receivers are much more expensive. You also need a data link for the base station data so that's more hardware and on a small UAV even a little extra electronic gear can be a problem. This all assumes you have a carrier phase base station source.

Now, as pointed out, there are more and more sources of "carrier phase" (NTRIP) base station data becoming available all the time. If one is available for your area and the radio modem is small enough you are good to go. That is assuming your GPS receiver can process carrier phase data which the Ublox LEA6 does not.

Yes, it can process RTCM data and NTRIP servers may provide data in RTCM format, but it only processes CODE RTCM data, not CARRIER data. Back in the days of Selective Availability (DoD purposely degrading civilian GPS accuracy) code RTCM corrections were important as they would get the accuracy down to meter-level from a typical 60 meters or so. Now C/A code corrections do essentially nothing to improve GPS accuracy. At least for the cheap chipsets.

So if you really want cm-level accuracy you still have to pay for it one way or another. The most straightforward way is to buy a compatible GPS receiver if you have an NTRIP or similar server available. The other way which costs less in dollars, but much more in time is to spend a little more money on GPS boards with raw carrier phase data output and use software like RTKLIB to do the processing. Of course you need data link(s) and a computer also.

A couple more things to think about.

You may want cm accuracy, but do you really need it? Typically professional applications like hydrographic surveying and photogrammetry navigate with meter-level GPS and later post-process GPS data for cm-level accuracy. Small UAVs can do the same. While the potential is there to navigate much more precisely your not going to be able to fly real close to objects - except from above - because the view of the satellites will be compromised. Also in the case of flying very close to the ground or a roof of a building you will need to know the ground elevation or roof height to a high degree of accuracy. And do you really want to trust the position hold abilities of the software algorithm in the varying wind currents near an object?

If you want to use the DIYdrones software does it even currently handle cm-level resolution? I haven't been following the development lately, but for the best results some code modification will probably be necessary.

One last practical note. Code GPS processing is much more robust than carrier phase processing. When you get to the tight confines of a small UAV the electrical noise environment may be such that code processing works well enough, but carrier phase processing fails. Your exact situation can make all the difference, but in the real world this is a common problem especially on small rotorcraft. Now you're back to spending more time designing your own noise filters or spending more on the GPS receiver (anti-jamming technology).

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Thiago Gabriel April 3, 2014 at 3:20pm

Must use arudupilot 2.6 and GPS uBlox LEA 6 H with NTRIP network, but I have no idea. What can I do?
Thiago Gabriel December 3, 2013 at 10:20am

Good afternoon.

Must use the NTRIP network with ardupilot and GPS, but I am not knowing how to proceed. Someone would have something like the tutorial (step by step) of how to configure the same?
John Bond August 4, 2012 at 1:17pm

A few notes on cm-level GPS.

There are basically two levels of accuracy achievable with GPS. Meter-level and centimeter-level. Meter-level accuracy relies on C/A code data and centimeter-level relies on carrier phase data. Any properly designed GPS receiver can provide meter-level accuracy under good receiving conditions by itself using the C/A code data. For centimeter accuracy you need a receiver which will process local carrier phase data in conjunction with carrier phase data from a base station.

The cm-level receiver market is much smaller than the meter-level one so these type of receivers are much more expensive. You also need a data link for the base station data so that's more hardware and on a small UAV even a little extra electronic gear can be a problem. This all assumes you have a carrier phase base station source.

Now, as pointed out, there are more and more sources of "carrier phase" (NTRIP) base station data becoming available all the time. If one is available for your area and the radio modem is small enough you are good to go. That is assuming your GPS receiver can process carrier phase data which the Ublox LEA6 does not.

Yes, it can process RTCM data and NTRIP servers may provide data in RTCM format, but it only processes CODE RTCM data, not CARRIER data. Back in the days of Selective Availability (DoD purposely degrading civilian GPS accuracy) code RTCM corrections were important as they would get the accuracy down to meter-level from a typical 60 meters or so. Now C/A code corrections do essentially nothing to improve GPS accuracy. At least for the cheap chipsets.

So if you really want cm-level accuracy you still have to pay for it one way or another. The most straightforward way is to buy a compatible GPS receiver if you have an NTRIP or similar server available. The other way which costs less in dollars, but much more in time is to spend a little more money on GPS boards with raw carrier phase data output and use software like RTKLIB to do the processing. Of course you need data link(s) and a computer also.

A couple more things to think about.

You may want cm accuracy, but do you really need it? Typically professional applications like hydrographic surveying and photogrammetry navigate with meter-level GPS and later post-process GPS data for cm-level accuracy. Small UAVs can do the same. While the potential is there to navigate much more precisely your not going to be able to fly real close to objects - except from above - because the view of the satellites will be compromised. Also in the case of flying very close to the ground or a roof of a building you will need to know the ground elevation or roof height to a high degree of accuracy. And do you really want to trust the position hold abilities of the software algorithm in the varying wind currents near an object?

If you want to use the DIYdrones software does it even currently handle cm-level resolution? I haven't been following the development lately, but for the best results some code modification will probably be necessary.

One last practical note. Code GPS processing is much more robust than carrier phase processing. When you get to the tight confines of a small UAV the electrical noise environment may be such that code processing works well enough, but carrier phase processing fails. Your exact situation can make all the difference, but in the real world this is a common problem especially on small rotorcraft. Now you're back to spending more time designing your own noise filters or spending more on the GPS receiver (anti-jamming technology).
Kevin Schumacher August 2, 2012 at 9:44am

I would love to get real time <10cm accuracy.

Another option would be using some post processing. Is there a way to log the raw GPS observations in the APM2 with LEA-6, and post process (RTKLIB) with base station data. Then update the log file with the corrected information?

I would be happy with either process.
Jake Stew August 1, 2012 at 1:13pm

Why use this? Same data comes from WAAS. Why not make sure that's working first?

I don't think that's even currently enabled in the MT GPS on the APM.
Barrett Ames August 1, 2012 at 10:15am

What I would have given to know about this stuff 3 years ago...

Definitely looks feasible. On my list of things to test out. First I need to buy a plane...

I'd be willing to write the software if someone else furnishes the bird.

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High Accuracy RTK Positions with the LEA-6 using RTCM via NTRIP

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