Many unknown RTCM messages over XBEE

Q&A forumCategory: QuestionsMany unknown RTCM messages over XBEE
eeek asked 4 months ago

Hello,
I have a MR simpleRTK2B kit configured as base and rover. Currently I experience quite a unstable RTK performance. I’m getting RTK fix for some seconds and then I loose fix again until it comes back after some time and so on.
The satellite signals look fine for rover and base in u-center. But I have discovered that there seems to be an issue with receiving RTCM messages. When I connect u-center with the XBEE USB connector of the rover the packet console looks like that:

??:??:?? R -> UNKNOWN ???, Size 26, 'UNKNOWN'
??:??:?? R -> RTCM3 1074, Size 144, 'GPS MSM4'
??:??:?? R -> RTCM3 1084, Size 144, 'GLONASS MSM4'
??:??:?? R -> UNKNOWN ???, Size 133, 'UNKNOWN'
??:??:?? R -> RTCM3 1005, Size 25, 'Stationary RTK reference station ARP'
??:??:?? R -> UNKNOWN ???, Size 223, 'UNKNOWN'
??:??:?? R -> RTCM3 1230, Size 14, 'GLONASS code-phase biases'
??:??:?? R -> RTCM3 1005, Size 25, 'Stationary RTK reference station ARP'
??:??:?? R -> RTCM3 1074, Size 144, 'GPS MSM4'
??:??:?? R -> RTCM3 1084, Size 144, 'GLONASS MSM4'
??:??:?? R -> RTCM3 1094, Size 129, 'Galileo MSM4'
??:??:?? R -> UNKNOWN ???, Size 26, 'UNKNOWN'
??:??:?? R -> RTCM3 1074, Size 144, 'GPS MSM4'
??:??:?? R -> UNKNOWN ???, Size 121, 'UNKNOWN'
??:??:?? R -> RTCM3 1230, Size 14, 'GLONASS code-phase biases'
??:??:?? R -> RTCM3 1005, Size 25, 'Stationary RTK reference station ARP'
??:??:?? R -> UNKNOWN ???, Size 284, 'UNKNOWN'
??:??:?? R -> RTCM3 1074, Size 144, 'GPS MSM4'
??:??:?? R -> RTCM3 1084, Size 144, 'GLONASS MSM4'
??:??:?? R -> UNKNOWN ???, Size 135, 'UNKNOWN'
??:??:?? R -> RTCM3 1230, Size 14, 'GLONASS code-phase biases'
??:??:?? R -> UNKNOWN ???, Size 15, 'UNKNOWN'
??:??:?? R -> RTCM3 1074, Size 144, 'GPS MSM4'
??:??:?? R -> RTCM3 1084, Size 144, 'GLONASS MSM4'
??:??:?? R -> RTCM3 1094, Size 129, 'Galileo MSM4'
??:??:?? R -> UNKNOWN ???, Size 26, 'UNKNOWN'
??:??:?? R -> RTCM3 1074, Size 144, 'GPS MSM4'
??:??:?? R -> UNKNOWN ???, Size 192, 'UNKNOWN'
??:??:?? R -> RTCM3 1074, Size 144, 'GPS MSM4'
??:??:?? R -> RTCM3 1084, Size 144, 'GLONASS MSM4'
??:??:?? R -> RTCM3 1094, Size 129, 'Galileo MSM4'
??:??:?? R -> UNKNOWN ???, Size 26, 'UNKNOWN'
??:??:?? R -> RTCM3 1074, Size 144, 'GPS MSM4'
??:??:?? R -> RTCM3 1084, Size 144, 'GLONASS MSM4'
??:??:?? R -> UNKNOWN ???, Size 152, 'UNKNOWN'
??:??:?? R -> RTCM3 1074, Size 144, 'GPS MSM4'
??:??:?? R -> UNKNOWN ???, Size 168, 'UNKNOWN'

With so many “UNKNOWN” messages, it is probably no surprise that the RTK performance is very poor. When I check the RTCM messaged outputted from the base over USB, they look all fine. So I guess that there is either an issue with the UART2 port of F9P or the radio transmission itself. Range shouldn’t be an issue, as during my test the distance between rover and base was only 5 m.
The amount of “UNKNOWN” messages changes over time, sometimes everything is ok for some seconds and then there is plenty of “UNKNOWN” messages again.
Does anyone have an idea where I should dig deeper to solve the issue?

Ardusimple Staff replied 4 months ago

Hello eeek,

Just a guess, this looks like NMEA messages are enabled on the base, and the rover is configured to only receive RTCM. Did you change the default configuration or loaded a configuration file from one firmware version to another firmware version?

5 Answers
eeek answered 4 months ago

Actually I had some NMEA messages enabled because I wanted to monitor the status of the satellite signals of my base station. Do you think this might be an issue?
Regarding my RTCM problem I was trying out different configurations all day long to track down the problem. I swapped base and rover, tried another pair of XBee S1 and monitored the serial ports of the XBee. In the end I figured out that the RTCM output of the F9P is perfectly fine, but somehow on the way from one XBee to the other one they get messed up. I was suspecting an EMI issue and could achieve some kind of improvement, but it never got really 100 % reliable.
In the end I’ve decided to switch from the XBee’s to some other 433 MHz based radio module. Now the rover is receiving rock solid RTCM messages and I’m getting a reliable RTK fix.

Ponpon answered 1 week ago

Hello eeek,
 
Which 433 Mhz based radio module do you use please ?
 
Thank you very much.
 
Best regards

eeek answered 6 days ago
Ponpon answered 6 days ago

Hi Eeek,
Thank you very much for your answer.
How far are you able to send corrections ?
 
Have a good week. 
 
Best regards,

eeek answered 6 days ago

Actually I never tried the maximum distance, I just know that for my application with a maximum distance of not more than 50 m it works. 
If you are interested in very long range applications, you might want to have a look at those transmitters: http://store.rfdesign.com.au/radio-modem-bundles/
Lately I’ve also been thinking about using LoRa transmitters for the correction data, as they promise very high range at low data rates, but I didn’t manage to try them yet.

clive1 replied 6 days ago

LoRa radios can get several KM at relatively high rates, and very significant distances at lower rates.
Working to get a LoRa module built into an XBee form factor, that is more cost effective than the Multitech xDot.
Currently plugging the ArduSimple boards into STMicro LoRa Discovery boards, for which it is well suited, but far larger than necessary.