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how to forward RTCM data to gpsd?


From: Kristoff
Subject: how to forward RTCM data to gpsd?
Date: Wed, 9 Dec 2020 02:05:09 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

Hi all,



I am new to the mailing-list, with just a quick question.
Can somebody provide a quick overview of what gpsd expects RTCM messages should look like?


As part of an exercise to learn Software Defined Radio and GNU Radio, I "build" a receiver in GNU Radio for the marine DGPS radio-stations in the mediumwave band, including some python-code to decode the RTCM messages. (not yep posted online .. first code-clean-up :-) )


So, as I now have that, I guess it would be fun to actually use this RTCM data for something useful, e.g. feed it into gpsd so it gets forward to my ublox GNSS-receiver.
My plan is to write a small TCP-server in python and let dgps connect to it.


Now, the question is, what is the format for RTCM messages should look like?


I have done a quick scan of the source-code of gpsd, so can somebody confirm these three assumptions?

1/ RTCM messages are 30 bits (i.e. they do not match the byte frames of TCP/IP). Am I correct to assume that I can just stuff the RTCM databits into a non-structured stream -ignoring byte-boundaries- and that gpsd is able to take care of frame-synchronisation?


2/ The marine DGPS radio-service uses bit-inversion (i.e. all databits of the RTCM word are inverted if the bit proceeding that word is 1).

I do not know if this so for all RTCM messages. or if this is something specific for the marine DGPS service.

But I did find a reference to bit-inversion in the source-code so I guess that gpsd does expect the RTCM bits to be as received from the radio (i.e. not bit-inversion corrected).
Is this correct?


3/  is the data expected to be MSB or LSB first?




On a more general point, what would be the best strategy when relaying DGPS/RTCM packets from a radio to gpsd?
Just stuff the bits as I receive them from the radio to gpsd?
Or first do some filtering / error-checking first myself, and only send "validated" data to gpsd?





Thanks for any advice.


Cheerio! Kr. Bonne (on1arf)






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