Re: gpsd_json(5) and cgps seem to differ

[Greg Troxel]

Bryan Christianson <bryan@whatroute.net> writes:

> I have gpsd: 3.25 (revision 3.25) installed and running on macOS 13.4 Ventura.

good, that's an up to date release.

> I have noticed that values I am seeing in the S and PRN columns of
> cgps seem to be at odds with the descriptions of the SKY/satellite
> fields 'svid' and 'PRN' as reported at gpsd_json(5)
> https://gpsd.gitlab.io/gpsd/gpsd_json.html.

cgps output should be as documented by cgpsd, but consistency is good.
Explain what you think is happening and what the mismatch is.

> cgps is accurately reporting the same values as I see when looking at
> the json provided by gpspipe, so if there is a problem it is in the
> generation of the json stream and would be in gpsd itself.

ok.  Still, please explain "I see X and for reason Y I think I should
see Z".

> Also - I think the numbers in the S column should be right
> justified. They're all squished up against the GNSS column. Not sure
> if this a macOS artefact or something easily fixed in cgps.

This is trying to squeeze things in, but you may be right.

> My receiver is a very cheap no-brand USB ubox-7 device and maybe this is the 
> cause of the problem.

Unlikely.  If it works it's just a box for the chip.

> | Time         2023-06-09T03:56:03.000Z (18)││GNSS  S PRN  Elev  Azim   SNR 
> Use│
> │ Latitude          xx.xxxxxxxx S           ││GP  4     4  41.0   9.0  22.0  
> Y │
> │ Longitude        yyy.yyyyyyyy E           ││GP  7     7  59.0 214.0  19.0  
> Y │
> │ Alt (HAE, MSL)      81.591,     52.742  m ││GP  8     8  62.0  70.0  17.0  
> Y │
> │ Speed              0.25              km/h ││GP  9     9  63.0 323.0  26.0  
> Y │
> │ Track (true, var)     132.2,  19.9    deg ││GP 14    14  15.0 292.0  25.0  
> Y │
> │ Climb            -20.82             m/min ││GP 16    16  21.0 127.0  29.0  
> Y │
> │ Status          3D FIX (340 secs)         ││GP 27    27  42.0 122.0  35.0  
> Y │
> │ Long Err  (XDOP, EPX)   0.58, +/-  8.7 m  ││GP 30    30  29.0 236.0  37.0  
> Y │
> │ Lat Err   (YDOP, EPY)   0.67, +/- 10.0 m  ││GP  2     2   3.0  32.0   0.0  
> N │
> │ Alt Err   (VDOP, EPV)   1.62, +/- 37.3 m  ││GP 20    20   2.0 238.0  23.0  
> N │
> │ Speed Err (EPS)            +/- 72.0 km/h  ││GP 21    21   6.0  41.0  19.0  
> N │
> │ Track Err (EPD)         n/a               ││GP 22    22   n/a   0.0   0.0  
> N │
> │ Time offset             0.119176000     s ││SB122    35  28.0  57.0   0.0  
> N │
> │ Grid Square             RFxxifyy          ││SB129    42  34.0 310.0   0.0  
> N │
> │ ECEF X, VX   -5092288.190  m    0.300  m/s││SB134    47  47.0   5.0   0.0  
> N │
> │ ECEF Y, VY     471007.460  m   -0.080  m/s││                                
>  │
> │ ECEF Z, VZ   -3798837.690  m    0.170  m/s││                  


Note that there are namespaces

  PRN - per constellation

  SVID - also per constellation

  NMEA id - sort of like PRN, but in ranges per constellation

  ublox id - sort of like NMEA id, but different, but I'm really unclear on this