Re: Strangeness with speed

[Gary E. Miller]

Yo Владимир!

On Thu, 17 Feb 2022 17:20:37 +0300
Владимир Калачихин <v.kalachikhin@gmail.com> wrote:

> In attachment - two NMEA files and two JSON outputs.

I'm ignoring your JSON, I have gpsd to compute my own.

> I don't see any differences between sentences1.nmea and
> sentences2.nmea, except time. Both were created from one real NMEA,
> but sentences1.nmea with changed time.

Well, time is imortant!

> If this is about time - what's wrong with times in senteces1.gpsd?

A lot, hard to know where to start:

$ gpsdecode -D 5 < /tmp/sentences1.nmea 

[...]
gpsdecode:IO: <= GPS: 
$GPRMC,102405.11,A,6104.592,N,02811.957,E,3.67,227.65,170222,,*3F
gpsdecode:PROG: NMEA0183: GPRMC starts a reporting cycle. lasttag 23
gpsdecode:PROG: NMEA0183: GPRMC ends a reporting cycle.
$GPRMC,102405.11,A,6104.592,N,02811.957,E,3.67,227.65,170222,,*3F
{"class":"TPV","device":"stdin","mode":3,"time":"2022-02-17T10:24:05.110Z","ept":0.005,"lat":61.076533333,"lon":28.199283333,"track":227.6500,"magtrack":238.4233,"magvar":10.8,"speed":1.888,"geoidSep":16.846}
gpsdecode:IO: <= GPS: 
$GPGGA,102405.31,6104.592,N,02811.957,E,1,06,0.0,56.940,M,0.0,M,,*68
gpsdecode:PROG: NMEA0183: GPGGA starts a reporting cycle. lasttag 66
gpsdecode:PROG: NMEA0183: GPGGA ends a reporting cycle.
$GPGGA,102405.31,6104.592,N,02811.957,E,1,06,0.0,56.940,M,0.0,M,,*68
{"class":"TPV","device":"stdin","mode":3,"time":"2022-02-17T10:24:05.310Z","ept":0.005,"lat":61.076533333,"lon":28.199283333,"altHAE":56.9400,"altMSL":56.9400,"alt":56.9400,"magvar":10.8,"speed":0.000,"geoidSep":0.000,"eph":0.000}
{"class":"SKY","device":"stdin","hdop":0.00}
gpsdecode:IO: <= GPS: 
$GPRMC,102405.51,A,6104.583,N,02811.936,E,3.74,227.01,170222,,*3C
gpsdecode:PROG: NMEA0183: GPRMC starts a reporting cycle. lasttag 23
gpsdecode:PROG: NMEA0183: GPRMC ends a reporting cycle.
$GPRMC,102405.51,A,6104.583,N,02811.936,E,3.74,227.01,170222,,*3C
{"class":"TPV","device":"stdin","mode":3,"time":"2022-02-17T10:24:05.510Z","ept":0.005,"lat":61.076383333,"lon":28.198933333,"track":227.0100,"magtrack":237.7831,"magvar":10.8,"speed":1.924,"geoidSep":16.846}
gpsdecode:IO: <= GPS: 
$GPGGA,102405.71,6104.583,N,02811.936,E,1,06,0.0,57.010,M,0.0,M,,*66
gpsdecode:PROG: NMEA0183: GPGGA starts a reporting cycle. lasttag 66

[...]


$GPGGA and $GMRMC do not have the same time.  So gpsd know that the $GPGGA
is in one epoch, and the $GPRMC is in the next epoch.

I'm gonna guess that there should be pairs of $GPGGA and $GPRMC.  The
flip-flopping confuses things.  Note that the 5.110Z epoch reports a
speed of 1.888 and the 5.310 epoch ($GPGGA) reports a speed of o.  Since the
lat/lon did not change between epochs, but the time changed, that is dP = 0
and dT = 0.200.  So, speed Dp/Dt, is 0/0.200, is zero.  

Then in the next epoch 5.510, the RMC reports a speed, and that is reported.

gpsd is doing exactly as it should.

Fix your NMEAA!

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

pgpKJ_YwO4MCS.pgp
Description: OpenPGP digital signature