First few positions of a flight are discarded?

FlightAware ADS-B Flight Tracking
1

I noticed tracklogs will often not show the first few ADSB positions after takeoff or before landing… about a minute on each end. Are those discarded on purpose?

For example flightaware.com/live/flight/JBU5 … S/tracklog
The last airborne position is 1100ft, then a ground position 71 seconds later.
Usually the tracklog interval is around 30 seconds for ADSB, so I’m just curious why there’s a gap.

My receiver sent many positions to FA during that time; I would have expected the position around say 750ft to be used:


17:04:58 clock 1467583498 hexid A98445 ident JBU511   squawk 1441 alt 1150 speed 129 airGround A lat 30.24179 lon -97.68014 heading 177
17:05:04 clock 1467583504 hexid A98445 ident JBU511   squawk 1441 alt 1075 speed 128 airGround A lat 30.23886 lon -97.68004 heading 178
17:05:09 clock 1467583509 hexid A98445 ident JBU511   squawk 1441 alt 1025 speed 128 airGround A lat 30.23557 lon -97.67998 heading 179
17:05:14 clock 1467583514 hexid A98445 ident JBU511   squawk 1441 alt 975 speed 128 airGround A lat 30.23242 lon -97.67994 heading 179
17:05:19 clock 1467583519 hexid A98445 ident JBU511   squawk 1441 alt 925 speed 127 airGround A lat 30.22939 lon -97.67987 heading 178
17:05:25 clock 1467583525 hexid A98445 ident JBU511   squawk 1441 alt 850 speed 128 airGround A lat 30.22623 lon -97.67977 heading 178
17:05:30 clock 1467583530 hexid A98445 ident JBU511   squawk 1441 alt 800 speed 128 airGround A lat 30.22292 lon -97.67966 heading 178
17:05:35 clock 1467583535 hexid A98445 ident JBU511   squawk 1441 alt 750 speed 126 airGround A lat 30.21996 lon -97.67955 heading 178
17:05:41 clock 1467583541 hexid A98445 ident JBU511   squawk 1441 alt 700 speed 127 airGround A lat 30.21643 lon -97.67944 heading 178
17:05:46 clock 1467583546 hexid A98445 ident JBU511   squawk 1441 alt 625 speed 126 airGround A lat 30.21341 lon -97.67939 heading 178
17:05:51 clock 1467583551 hexid A98445 ident JBU511   squawk 1441 alt 575 speed 127 airGround A lat 30.21115 lon -97.67933 heading 179
17:05:57 clock 1467583557 hexid A98445 ident JBU511   squawk 1441 alt 0 speed 125 airGround G lat 30.20706 lon -97.67919 heading 180
17:06:06 clock 1467583566 hexid A98445 ident JBU511   squawk 1441 alt 0 speed 105 airGround G lat 30.20457 lon -97.67914 heading 180
17:06:13 clock 1467583573 hexid A98445 ident JBU511   squawk 1441 alt 0 speed 78 airGround G lat 30.20097 lon -97.67905 heading 180

Another example flightaware.com/live/flight/N319 … S/tracklog
The first point is at 1200ft but I sent positions for about a minute before that:


15:37:44 clock 1467578264 hexid A36B8C ident N31969   alt 600 speed 69 airGround A lat 30.19443 lon -97.65769 heading 180
15:37:49 clock 1467578269 hexid A36B8C ident N31969   squawk 7324 alt 725 speed 64 airGround A lat 30.19243 lon -97.65769 heading 180
15:37:55 clock 1467578275 hexid A36B8C ident N31969   squawk 7324 alt 775 speed 65 airGround A lat 30.19103 lon -97.65769 heading 179
15:38:00 clock 1467578280 hexid A36B8C ident N31969   squawk 7324 alt 800 speed 67 airGround A lat 30.18922 lon -97.65769 heading 181
15:38:06 clock 1467578286 hexid A36B8C ident N31969   squawk 7324 alt 850 speed 67 airGround A lat 30.18754 lon -97.65775 heading 188
15:38:09 clock 1467578289 hexid A36B8C ident N31969   squawk 7324 alt 875 speed 67 airGround A lat 30.18643 lon -97.65802 heading 199
15:38:17 clock 1467578297 hexid A36B8C ident N31969   squawk 7324 alt 950 speed 72 airGround A lat 30.18428 lon -97.65950 heading 227
15:38:22 clock 1467578302 hexid A36B8C ident N31969   squawk 7324 alt 1000 speed 76 airGround A lat 30.18317 lon -97.66126 heading 240
15:38:28 clock 1467578308 hexid A36B8C ident N31969   squawk 7324 alt 1100 speed 78 airGround A lat 30.18224 lon -97.66329 heading 249
15:38:32 clock 1467578312 hexid A36B8C ident N31969   squawk 7324 alt 1150 speed 81 airGround A lat 30.18182 lon -97.66522 heading 265
15:38:38 clock 1467578318 hexid A36B8C ident N31969   squawk 7324 alt 1200 speed 80 airGround A lat 30.18182 lon -97.66725 heading 273
15:38:43 clock 1467578323 hexid A36B8C ident N31969   squawk 7324 alt 1325 speed 78 airGround A lat 30.18191 lon -97.66961 heading 276
15:38:48 clock 1467578328 hexid A36B8C ident N31969   squawk 7324 alt 1450 speed 75 airGround A lat 30.18210 lon -97.67164 heading 275
2

It looks like N31969 is mostly tracked via MLAT? It appears to have 978UAT-out, so only one station got that direct data. But, in the case of taking off, that shouldn’t negatively affect the situation.

3

Interesting - when I opened this thread, I didn’t expect the first flight example to include data from my receiver. :slight_smile:

I’m curious to know the answer to this question as well. I know FA uses multiple sources for their data, and I suspect what you’re seeing may be a transition from ADS-B sourced data to/from airport ground-based data. I have nothing to back up this assertion. This is pure speculation on my part.