Originally Posted by
Dont Hang Up
ADS-B from the SSR Transponder is not time stamped by the aircraft. The receiving station adds the time stamp. There is a mechanism for allowing the receiving station to compensate for various latencies in the aircraft system (known as the T-bit). However this is seldom implemented in current installations. Consequently velocity calculations from the ADS-B position reports alone should not be considered reliable. HOWEVER ADS-B does transmit the aircraft's own calculated velocity (usually ground speed) at the same rate that it transmits position (twice per second). This velocity should be considered perfectly reliable but I am not sure if Flight Radar carries the velocity component.
You are being a little pedantic
To save space as they are trying to crunch a lot of information into a very small space, a 'time of applicability' is sent that refers to even and odd numbered periods (called epochs) of 0.2secs within a longer time frame. This encoding of the UTC time allows the ground station to recreate the 'time of applicability' of the geographic (surface) position data - the latitude and longitude. This is possibly not a worthwhile exercise for FR24 and others but the information is there in the ADS-B message. (See RTCA DO-260B Appendix A A.1.4.2.3.1)