Originally Posted by
ManaAdaSystem
How many enthusiast feeds is available off the coast of Java? This is why I have a problem with FR24. It is treated as the truth even if is «recording» events in the middle of an ocean with no feeds available anywhere. People simply have no idea how it works, or what it takes to get reliable information from it.
That's a fair point about events in the middle of the ocean, but this flight went down less than 20 miles from a city of 10 million people, so we shouldn't be surprised if it was being tracked by several FR24 feeds.
Originally Posted by
waito
Why is the question. GS surely is calculated. From INS? What happened to the sources for INS? Was 735 retrofitted with GPS anyway?
While it's true that some B735s are still flying around with inertially-sourced ADS-B, it's pretty clear from looking at previous tracks for the aircraft in question that it had GPS.
Groundspeed is indeed calculated by the receiving station from N-S and E-W velocity components, which are what the aircraft transmits. Those, of course, are also what is used to derive true track. As a general rule, if the track timeseries looks consistent (which it does in this case once the time jitters are corrected) then the groundspeeds are also likely to be correct.