Re: [gpsd-dev] [PATCH 1/1] Initial draft; how to estimate time1 offset

[Sanjeev Gupta]

On Wed, Nov 6, 2013 at 8:20 PM, Greg Troxel <address@hidden> wrote:

There are two separate things to estimate:

  delay for the PPS siganl

  delay for the timecode-based signal

As I read your instructions, they seem to be about measuring the
*difference* between the pps signal and the timecode signal.

Yes, please.  I believe we have to assume the 1PPS is on-time.  We need to measure the offset from _somewhere_.  I am not clear how we would measure, absent scopes, the delay in the 1PPS. 

There are some corrections we can do for the length of the cable, but this will be on the nS scale.

 

Also, I would avoid the word "GPS time".   GPS time refers to a
different timescale, and that's not what we are talking about.   I would
describe and use terms "pps-based offset" and "timecode-based offset" to
refer to gpsd obtaining offsets from the system clock to a pps signal
and a timecode.

I started off using the term "message-decoded time" from the gpsd web page (see: http://gpsd.berlios.de/gpsd.html ).  How would I refer to the NMEA or similarly reported GPS solution? 

 

  +If, for example, your estimate of the offset is 0.32s, your time1 fudge
  +value will be '-0.32'.  Note the change of sign.

My impression is that fudge1 is added to the offset, and that for
typical timecodes (e.g. ublox binary), a fudge of around 0.110s is
normal, indicating that the timecode arrives 110 ms late, which would
produce an offset of -0.110s.

On my system, I see, using Andy's script, a value close to the 0.32 I used.  But in my case, there is a rather long cable between the GPS (at the window), and the server (in the Data Centre).  I estimate about 9m.

If we include typical values, people (like me) would start using them, irrespectiove of the fact that my firmware might be different to yours, etc. 

Different question, after the last month's activity on the list: Is there a use case for gpsd apart from NTP?

<ducks and runs>

--
Sanjeev Gupta
+65 98551208     http://www.linkedin.com/in/ghane