Originally Posted by
scard08
Calculating the speed based on adjacent ADS-B data points will not work. The data is just not precise enough to do this. Suppose that second data point was actually received at 23:11:00.999. If it had been received a thousandth of a second later then it would be timestamped a second later and the first delta t would have been 4 seconds instead of 3 and the second would be 3 seconds instead of 4. That would change the caclulated speeds by about 1/3, which is a couple of hundred knots.
A similar but smaller problem comes from the precision in the coordinates. If the delta in the latitude is .004 in one 3 second interval and .005 in the next, that does not mean the speed changed by 25%.
ADS-B data carries the time at which the aircraft generated that GPS position. So it does not matter when that was
received as each ADS-B report is perfectly valid. This is to ensure that delays in transmission do
not cause the errors you describe.
The problem the receiving system has is that these reports do not come in with the same delay each time, especially if the receiving system is using several remote antennae or even several different receiving stations. This can result in position reports coming in at random intervals and in extreme cases out of order. This is why it is important to use the original data from the aircraft which will carry the timestamp of the GPS position. It is not clear that FR24 and other services do this.