Originally Posted by
underfire
I thought that the ADSB showed 0 altitude when the ac was on final (per earlier posts)
No,
FR24 showed zero, which isn't quite the same thing.
I suspect that where a non-corrected ADS-B altitude becomes negative (i.e. in this case at the points where the actual height was less than 150'), FR24 dumbly substitutes null/0 values, so those can't be trusted.
So you know, ADSB data reports altitude in 25 foot increments, and has latency factors in the system, to which there is about an 8 second delay.
It is notoriously inaccurate in reporting altitude on final approach due to the descent rate and kalman reporting latencies, especially if there is rapid descent, a turn or crosswinds.
Other than the 25' granularity, altitude reported by ADS-B should be as accurate (subject to adjustment for QNH) and timely as your altimeter, given that both are coming from the same source and subject to the same errors.
Where did you get the idea that there is a delay of anything like 8 seconds (not that we're particularly interested in absolute time values in this instance) ? Where is the data being buffered in the meantime ?