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Re: Decode RTCM3 messages

From: Xavier Ruiz
Subject: Re: Decode RTCM3 messages
Date: Mon, 4 Oct 2021 21:59:02 +0200

Thanks for your quick reply Gary. 

We have a Base Station that is sending observations in RTCM3 format through 2 streams: radio and ntrip. This is really important since our system strongly relies on RTK fix quality.

The Base Station sends the observation messages 1006,1008, 1033, 1230 and MSM5 (1075, 1085, 1095 and 1125). 

However, our GNSS receiver module can only handle one stream of data. The radio looses packages but is faster while the ntrip is slower but more robust. When the receiver module receives an observation with a new tow it will ignore any incoming obs with an older tow. This is problematic since we will loose a lot of information if a new radio package arrives before completing all the ntrip obs from the previous tow.

In order to address that we are trying to receive the observations in an onboard computer (We use rtklib's str2str serial for the radio and str2str as well as the ntrip client).  We want to arbitrate the two channels and group both ntrip and radio obs by their time of week. Once we receive all msgs from specific tow, we send them to the module and then wait for the next tow and repeat.

For that we need to decide the rtcm3 messages to group them by their tow. And then once grouped, decode them to send them to the receiver module.

If you have any suggestions to solve this differently, I am open.

Also, what MSM number gpsd is able to decode? We can configure the base station to send the legacy, MSM4 or MSM5. Not sure if there is any advantage to use any of them compared to the others.

Thanks do much for the help.

Xavier Ruiz

On Mon, Oct 4, 2021, 9:09 PM Gary E. Miller <> wrote:
Yo Xavier!

On Mon, 4 Oct 2021 14:35:06 +0200
Xavier Ruiz <> wrote:

> I am using MSM5.

So many choices...

> Yes it seems like msgs 1006, 1008, 1033 and 1230 are properly
> decoded. But type 1075, 1085, 1125 and 1095 that are MSM5 type are
> not fully decoded.

Because they are new, and no on cared yet.  Why do you care?  Do you have
a use case for the decodes?

> Is there any other open source rtcm3 decoder that
> I can try for full decoding of MSM5?

Not that I know of, would not be too hard to extend gpsd to do that.  If
anyone had a real reason to.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can't measure it, you can't improve it." - Lord Kelvin

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