[MAVLINK] Is there a message for DGPS RTCM data?

[Igor Campos]

Hello,

I am working on an university project that might require precise positioning and was thinking if it is possible to send DGPS messages over mavlink through the APM/PIXHawk flight controllers to RTCM enabled receivers (like the LEA-6T). Have anyone done that?

Thanks in advance for your attention!

Sincerely,

Igor Campos

[Jesus Alvarez]

That would be definitely great.

Event with a raspi on board and 3G, could we get those corrections and feed them into APM?

How do you plan to do it Igor?

[Igor Campos]

There isn't the need of an on-board computer.

The GPS is connected only to the APM/PIXHawk. The FC need to receive RTCM messages packed into MAVLink messages and provide them to the GPS receiver through the TTL-UART interface.

What I want to know if there are such messages in the MAVLink protocol, if not, I propose such a message is created and added to the common dialect.

Sincerely,

Igor Campos

[Meier Lorenz]

While the MAVLink docs server is migrated, you can find the machine-readable message specs here:
https://github.com/mavlink/mavlink/blob/master/message_definitions/v1.0/common.xml

A similar use case has been presented for this message:
https://github.com/mavlink/mavlink/blob/master/message_definitions/v1.0/common.xml#L2338

and this one:
https://github.com/mavlink/mavlink/blob/master/message_definitions/v1.0/common.xml#L2375

Cheers,
Lorenz

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[Ben Nizette]

Hi Igor,

Yep, though I haven't got the APM side of it upstream (I should try
and get around to that..). If you load the firmware from [1] then the
APM will forward the payload of a GPS_INJECT_DATA message to the GPS.

If you 'module load DGPS' inside MAVProxy then it will listen on a UDP
port (I forget which one), package any data on that port inside the
right message and send it to the rover (that component is upstream).

Finally, pyUblox [2] has two scripts: local_to_udp.py and
ntrip_to_udp.py, both of which publish RTCMv2.3 messages on the right
UDP port to plug in to the above. In the first case, you should plug a
raw-capable ublox module with known position in to the ground station
and it will generate DGPS corrections from that. In the second case,
it will unpack RTCMv3 messages from an NTRIP stream and convert them
to RTCMv2.3 suitable for input to the GPS modules.

If you have your own source, just publish them on UDP using the above
as a reference.

A quick note:

> to RTCM enabled receivers (like the LEA-6T).

In my testing, older versions of the 6T module cannot accept RTCM,
even though the documentation says they can. Newer versions can accept
RTCM, but only if the output is 1Hz. I don't know whether there's a
working version in the middle somewhere, but pretty much you're better
off just using the normal LEA-6H, 6S etc modules for the roving side
as their RTCM support seems to actually work reliably.

Second note: Don't expect the result to be that good. If you're trying
to generate your own DGPS corrections (local_to_udp.py) then you
actually end up with /heaps/ more random position error, even if the
position is on /average/ moved closer to truth. This is for two
reasons:
1) you're adding all the non-correlated noise from the DGPS base to
the roving position. This is true of every DGPS system, but using a
rubbish LEA-6T and tiny antenna means that this effect is pretty big
2) The uBlox modules seem to turn off a large amount of internal
filtering as soon as they start receiving RTCM.

The second point is true even for 'good' sources of corrections, such
as a proper expensive base station or your local geospatial reference
service. In my tests you /might/ get sub-metre performance in 2
dimensions with an excellent source of corrections. Altitude is the
same or worse, as the common-mode noise couples more strongly in to
that.

For more info, see [3], [4].

HTH.

Ben.



[1] https://github.com/benizl/ardupilot/dgps-wip
[2] https://github.com/benizl/pyUblox
[3] http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6878299&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D6878299
[4] https://www.youtube.com/watch?v=sl8aJlG52PE

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[Vishu Gupta]

Hello Igor,

I am trying to implement DGPS on Pixhawk, but facing some problems. Are you able to do it ?
Your Help is greatly appreciated..
Thanks in advance

Vishu Gupta

[Tom Pittenger]

Hi Vishu,

Take a look at SwiftNav Piksi and the Emlid NavIO.

-TomP

--

[Craig Elder]

Are you trying to get one of the RTK GPS units working or are you trying to add RTK functionality to the Pixhawk?

If it is the former then please post user support questions to the support forums.  If it is the latter then please tell us more.

The RTK setup is described here 

https://www.youtube.com/watch?v=HWJnG3tu9iM

[Michael du Breuil]

DGPS injection is meant to be done with https://pixhawk.ethz.ch/mavlink/#GPS_INJECT_DATA I know tridge has used it with a ublox at one point, but I don't think its been used in a long time. I can't state if it still works within APM correctly or not, there is a GPS_INJECT_TO param meant for routing but I haven't used it before.

@Tom: DGPS via RTCM is different then RTK, it will provide a better fix then straight GPS, but not nearly as good as RTK.

[Michael Oborne]

Just fyi,

 

MP is using GPS_INJECT_DATA for rtk, the video craig posted uses just that, and it allows transport of any data to the gps. This technically could include config packets as well as rtcm/sbp/cmr, etc.

 

Michael

[Vishu Gupta]

Thank you for your reply sir

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[Vishu Gupta]

My exact aim is to bring the accuracy of my 3DR gps within 1m accuracy.

I have Pixhawk and i want to do this by using DGPS..
But i am not understanding how to do it...

:(

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