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Re: Strangeness with speed

From: Gary E. Miller
Subject: Re: Strangeness with speed
Date: Mon, 14 Feb 2022 11:15:50 -0800

Yo Владимир!

On Mon, 14 Feb 2022 17:51:47 +0300
Владимир Калачихин <> wrote:

> Hi all and Gary.


> I have an underground knock.

Sorry, lost me.  Can you translate?

> I have the NMEA file (included).

Got it.  And I can read it with gpsdecode just fine.

> I created this file on my app by filtering RMC and GGA sentences and 
> changing the time.

And you got the checksum correct as well.  But you did not share the
original.  Or the version og gpsd that you are running.

> I play this file in gpsfeed, and gpsd understands him perfectly.

What is gpsfeed?

> As 
> result, gpsd return the following:
> {"class":"TPV","device":"tcp://localhost:2222","mode":3,"time":"2022-02-14T11:46:18.000Z","ept":0.005,"lat":61.079016667,"lon":28.256983333,"altHAE":36.440,"altMSL":49.870,"alt":49.870,"track":283.8300,"magtrack":10.7967,"magvar":10.8,"speed":2.212,"geoidSep":0.000,"eph":0.000}

Looks about right.

> The same result is if I play this file in my app without any changes
> in data.

Not much I can sasy about your app without seeing it.

> But if I during playback changed the time in sentences to now - gpsd 
> returns two differents TPV:
> {"class":"TPV","device":"tcp://localhost:2222","mode":3,"time":"2022-02-14T11:46:18.000Z","ept":0.005,"lat":61.079583333,"lon":28.254500000,"track":296.0900,"magtrack":306.8855,"magvar":10.8,"speed":2.531,"geoidSep":16.832,"eph":0.000}
> {"class":"TPV","device":"tcp://localhost:2222","mode":3,"time":"2022-02-14T13:48:13.000Z","ept":0.005,"lat":61.079583333,"lon":28.254500000,"altHAE":54.300,"altMSL":54.300,"alt":54.300,"magvar":10.8,"speed":0.000,"geoidSep":0.000,"eph":0.000}

Looks normal to me.

> one of these has speed == 0
> Why is it that?

Look at your file, you set ALL the times to 114618.00.  Speed is change
in distance over change in time, but you have no change in time.

> Is it the time value something special for gpsg?

Yes, it is called TPV for a reason.  Time Position and Velocity.  Intertwined.

> Why speed == 0 instead null?

Since the time never changed, gpsd never was asked to compute speed.  gpsd
uses the time steps to internally trigger state changes.

Or, put another way: Garbage In, Gorbage Out.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703  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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