Re: [gpsd-dev] PRN numbers

[Gerry Creager - NOAA Affiliate]

This is a really good question. The IS-GPS-200 is very specific and is extremely GPS-centric. Of course, at the time, there were no competitors. I'm uncertain of how *I'd* proceed, but at this instant, I think detecting the true PRNs is a *good* idea, regardless of the output scheme. I'm also leaning toward outputting the NMEA PRNs. 

There needs to be some method to identify the real constellation (and hence associated error budgets) to a given SVN. 

gerry

On Tue, Apr 7, 2015 at 8:55 AM, Sanjeev Gupta <address@hidden> wrote:

Folks,

I am confused on what GPSD expects as PRN, and what it outputs.

PRNs are NOT standardised accross systems.  For GLONASS, as it uses FDMA, not CDMA, there is no PRN number at all, just a pseudo PRN :-)

In a recent commit, Eric says:
+ * According to IS-GPS-200 Revision H paragraph 6.3.6, and earlier revisions
+ * at least back to E, the upper bound of U.S. GPS PRNs is actually 64. However,
+ * NMEA0183 only allocates 1-32 for U.S. GPS IDs; it uses 33-64 for IDs ub the
+ * SBAS range.

I see an issue here.  Within the GPSD ecosystem ( and only for this, ignore everything else), do we follow IS-GPS-200 (which is not canonical for GLONASS, BeiDou, etc), or NMEA?

We seem to be mixing up the two.

And this is _detection_ .  It may be quite possible that we output, while pretending to be a NMEA talker, the NMEA PRNs.

Example:  IS-GPS-200 is quite clear that the range 1-64 is reserved for them.  1-32 is for our current constellation, 33 and 37 are currently in use by supplementary systems, and the rest will be used eventually.

NMEA 0183 (I believe because the field is limited to 6 bits) needs PRNs to be 1-64.  Hence its mapping of GLONASS into that area.  How they plan to handle the other 182 national GNSS systems soon to be announced is left as an exercise for the reader.

My patches earlier were trying to update documentation on one issue I saw, but I see from Eric's following mails that the issue may be deeper, and requires some design understanding.

--
Sanjeev Gupta
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Gerry Creager

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