Re: gpsd Satellite Elev & Azim

[Gary E. Miller]

Yo Nick!

On Mon, 10 Jun 2024 10:57:44 +0100
Nick Taylor <nicktaylor@dataskill.uk> wrote:

> On 07/06/2024 19:53, Gary E. Miller wrote:
> > Interesting.  Got a raw data capture from the two receivers?
> >
> > The decimal fraction i nboth is zero, so maybe a rounding error in
> > the reporting?
> >
> >  
> Attached!

The ubx starts at 09:51:06 and ends at 09:51:34

The NMEA starts at 09:50:32 and ense at 09:51:00

Without an overlap, I can't say for sure that they even disagree.

Looking at the NEO-M8N, at time 09:51:06:

Using: ubxtool -f /tmp/gps-raw-neo8m.log

UBX-NAV-SAT only reports elevation and azimuth to resolution of 1 degree:

UBX-NAV-SAT:
  iTOW 121884000 version 1 numSvs 28 reserved1 x0
   gnssId 0 svid   2 cno  0 elev   0 azim 239 prRes      0 flags x1211
   gnssId 0 svid   7 cno 41 elev  19 azim 306 prRes      3 flags x195f
   gnssId 0 svid   8 cno 41 elev  44 azim 288 prRes     15 flags x195f
   gnssId 0 svid  10 cno 43 elev  41 azim 140 prRes      8 flags x195f
   gnssId 0 svid  13 cno 22 elev   4 azim  14 prRes  -5044 flags x1214
   gnssId 0 svid  15 cno 44 elev   9 azim  42 prRes     -5 flags x195f
   gnssId 0 svid  16 cno 46 elev  58 azim 187 prRes     -9 flags x195f
   gnssId 0 svid  18 cno 44 elev  25 azim  58 prRes     -2 flags x195f
   gnssId 0 svid  21 cno  0 elev  14 azim 239 prRes      0 flags x1911
   gnssId 0 svid  23 cno 45 elev  47 azim  89 prRes     -3 flags x195f
   gnssId 0 svid  26 cno 32 elev  23 azim 168 prRes   -123 flags x195f
   gnssId 0 svid  27 cno 49 elev  81 azim 303 prRes    -30 flags x195f
   gnssId 0 svid  30 cno 37 elev   8 azim 334 prRes    -12 flags x195f


So PRN 7, elevation 19, azimuth 260.

Looking at the MTK-3301 raw NMEA, At time 09:51:00

$GNGGA,095100.000,5136.9263,N,00011.1060,W,2,19,0.61,97.9,M,47.0,M,,*5D
[...]
$GPGSV,4,1,13,27,81,303,49,16,58,187,47,23,47,089,46,08,44,288,41*76
$GPGSV,4,2,13,10,41,140,43,49,30,173,34,18,25,058,45,26,23,168,33*7F
$GPGSV,4,3,13,07,19,306,42,21,14,239,28,15,09,042,44,30,08,334,38*7B
$GPGSV,4,4,13,13,04,014,23*49

A quick look and they match.  To the 1 degree resolutioin they both have.

I'm not going to dig deeper unless you have samplee that actually overlap
and also show you data that does not match.

The interanl lat/lon/alt computations do not use azimuth at all, and only
use elevation to mask low elevation sats.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  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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