Strangeness with speed

[Владимир Калачихин]

Hi all and Gary. I have an underground knock.

I have the NMEA file (included).
I created this file on my app by filtering RMC and GGA sentences and 
changing the time.

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

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

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}

one of these has speed == 0

Why is it that?
Is it the time value something special for gpsg?
Why speed == 0 instead null?


--
Владимир Калачихин

sentences.nmea
Description: Text document