Re: [gpsd-users] Processing RTCM messages

[Greg Troxel]

Michele Pangrazzi <address@hidden> writes:

> 1. First step

> Android device (with GPS) —————> NMEA over HTTP POST ——————> Webserver

> The Android device sends a NMEA string over HTTP POST API to web server, for 
> example:
> $GPGGA,150505.00,4608.072256,N,01105.977973,E,1,10,1.2,247.7,M,47.0,M,,*60”

What's the point of the webserver?   Why can't the device itself just
talk to the caster?

> 2. Second step
>
> Webserver —————> NMEA over NTRIP ——————> Local NTRIP caster 
> (194.105.50.232:2101)
>
> The webserver sends the NMEA string obtained at first step using NTRIP
> protocol to a local caster. I’ve implemented a simple NTRIP protocol
> communication using Node.js.

This may well be what's required, but sending your position to the
Caster is irregular from an OPSEC* viewpoint.

* https://en.wikipedia.org/wiki/Operations_security

In all seriousness, you may want to truncate the coordinates to a tenth
of a degree or less.  But your caster may be interpolating.

> 3. Third step
>
> Local caster (194.105.50.232:2101)  —————> RTCM over NTRIP ——————> Webserver
>
> Basically, I receive first a 1013 message (system parameters), then a
> lot of 1004 / 1012 messages (GPS / GLONASS observations). The NTRIP
> caster is a Leica
> SpiderWeb<http://leica-geosystems.com/products/gnss-reference-networks/software/leica-spiderweb>.

That all sounds ok (modulo OPSEC concerns), and the unclear rationale
for the intermediate server.

> I was wondering if there is a way to actually compute the correction
> on server-side starting from those messages (and the NMEA string
> obtained at first step). If yes, the webserver could send to the
> Android device a more precise position, maybe in lat/lon format (it
> would be great).

Basically, no.  One needs to apply the calculations to the raw
pseudoranges, and those stay internal to the smartphone receiver.  This
first page is much too high level to lead to understanding, but they
provide a starting point:

  https://en.wikipedia.org/wiki/Differential_GPS
  https://en.wikipedia.org/wiki/Pseudorange

However, you may be able to carry RTCM to the device and inject it to
the receiver on the phone.  Traditional GPS receivers with serial ports
like the Garmin GPS V, could accept RTCM over the serial port, such as
one might receive from a LF DGPS transmitter.  I am unclear on whether
the GPS chip in phones is wired to do that, and whether there are
interfaces in the OS.

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