Traffic not shown

[a.alexeev.p]

Hello :-) I recently was departing with PAW onboard and couldn't see some traffic on SkyDemon.

 GBWNB was in front of me for power checks but not shown on SkyDemon or red out in audio- that was confirmed when I played back the track at home. However, that plane could be viewed on FligtRadar24 with contact on May 03 at 0920 UTC. I was in GWARU and Flightradar shows contact at 0923 UTC so pretty sure I got the other aircraft right.

See installation of my antennas on photo below which I think is as good as it can get without permanent installation. So basically now I have few options what happened:

1) Not all traffic shown on flightradar is seen by PAW, but I always thought that is not the case
2) Some PAW fault
3) GBWNB forgot to put transponder on and only did it later in flight
4) Something else

Would appreciate any help/advice

[Admin]

Does this aircraft show on fr24 as
Adsb or mlat ?

If mlat, interrogation is required which will
Only hapoen in the air and when the transponder
Is not in ground mode

Thx
Lee

[a.alexeev.p]

I am not sure I understood the terminology, but it says Mode S - see below. I just thought FR24 only can see ADSB

[Paul_Sengupta]

FR24 also works through multilateration, or MLAT, which is a method of triangulation by measuring the time it takes for the transponder transmissions to reach three or more receivers. It can give a rough position for an otherwise bearingless target. But it's not that accurate so you'll see things like C172s having a ground speed of 300 knots.

If a transponder hasn't got ES then it will need a radar interrogation before sending anything out. It'll then just be bearingless and show up as a ring, but it may not show up on the ground as it's not being illuminated by a radar down there.

Best option is to persuade the other person to get a PilotAware...

[a.alexeev.p]

Thank you! Understood!

[AlanG]

Hi

Having just replayed a flight by the said aircraft G-BWNB I can confirm that it is indeed Mode S only and therefore is represented on FR24 by MLAT and will not show on the ground or even in the air below radar coverage.  Also worth remembering that some of the ADS-B larger traffic also have Squat switches on the undercarriage that switches the Xpdr to Ground Mode until lift off.

Regards
Alan

[a.alexeev.p]

Thank you, Alan!:)

[exfirepro]

Alan / Alex,

IIRC from my early Mode S testing, CAT ADSB Aircraft show in ‘Ground Mode’ (effectively Mode S) until fairly well after takeoff.

Regards

Peter

[PaulSS]

I don't know at what point our ADSB starts doing its thing but 99.9% of the time we're using MLAT on the ground.

[AlanG]

Info from the Trigg TT21/22 installation manual.

5.6.7 Squat Switch Input
The Squat switch input allows the transponder to automatically switch
between Airborne and Ground modes, and affects both the Mode S reply
behaviour and the ADS-B reporting behaviour. The input will be asserted
when the voltage to ground is pulled below approximately 4 Volts. The
logical sense of the input can be programmed to be either active low or active
high.
On an aircraft with no appropriate squat switch circuit this input should be left
unconnected, and the transponder programmed to ignore the input.

Regards
Alan