While a Mode S transponder does emit some stuff by itself all the time (we had a thread on this recently, and this surprised me) I don’t think it is visible to FR24 or even ATC unless pinged by ATC SSR radar.
So that’s another factor why a lot of planes may not be visible on tracking sites: they fly in mountainous areas and/or at low level and/or in the back of beyond.
OTOH, ADS-B OUT is emitting all the various data all the time regardless of ATC radar.
Airliner TCAS systems are pinging transponders all the time. Even if an ATC SSR radar cannot reach you.
Stampe,
Mode S alone won’t usually do it, in order to get regular pick up and reliable tracking you would need ADSB-Out.
ADSB-Out however requires a WAAS GPS as data source. Older GPS’s like the GNS430 non Waas can still feed some ADSB-Out capable transpoders, but it is illegal to connect them.
Maybe that is what your installer referred to.
Trigg sells two GPS sources for ADSB which work with their transponder range. TN70 and 72. I am not sure if they work with other brands, probably not.
It is true that some tracking sites use MLAT for planes which don’t have ADSB-Out, but these tracks are very unsatisfactory and often outright wrong.
Even with ADSB out, tracks are often lost at low altitude or in areas of bad coverage. It is not a good idea for instance to tell relatives or let alone loved ones who are waiting for you at a destination or want to follow your flight to rely on any of those sites, they might loose your signal in some places and freak out if they don’t know. Don’t ask how I know….
Do all interrogations get the aircraft identifier in the response?
Only in the response to Mode S interrogations (there are several types of them)
Would airliner TCAS cause a Mode S transponder to radiate the Mode S data which is picked up by FR24 etc? I don’t think so. TCAS is based purely on Mode C and its existence predates Mode S by many years. GA TAS systems do OTOH do some Mode S stuff… not sure what. Some previous threads on N-reg tail numbers being displayed with some equipment combinations.
It is true that some tracking sites use MLAT for planes which don’t have ADSB-Out, but these tracks are very unsatisfactory and often outright wrong.
It depends on where and how high you fly. At say 5000ft+ in Europe you are very continuously visible. One exception is the Albania / Greek border (this was FL100-120)
It probably also depends on whether your transponder is working well; judging from how some targets aren’t visible even from very close I am sure many aren’t.
This was flown around 4000ft 
Mooney_Driver wrote:
ADSB-Out however requires a WAAS GPS as data source. Older GPS’s like the GNS430 non Waas can still feed some ADSB-Out capable transpoders, but it is illegal to connect them.
It’s a bit more complex. It really depends which installation data you use to connect the GPS and XPDR.
But in most cases it will be either CS-STAN or a STC for EASA aircraft.
CS-STAN says :
This SC does not satisfy requirements set by CS-ACNS Subpart D Section 4 1090 MHz Extended Squitter (ES) ADS-B Out installations compliant to Section 4 of CS-ACNS or nor AMC 20-24.
However, the voluntary transmission of additional ADS-B data (e.g. GPS position and velocity) can be accepted when the position and velocity quality indicators report the lowest quality, the equipment manufacturer has stated compatibility with the directly connected GNSS source, and the transponder is not authorised in accordance with ETSO-C166b or equivalent.
Very helpful thread so it appears there is no reason my mode S should not be appearing on FR24.No sign so far but I made sure yesterday to fly at 3000 feet for a protracted time whilst squawking several allocated codes.Maybe FR24 picked me by multilateration and will eventually recognise my squawk but nothing so far.Thank you for you replies we have some very knowledgeable members and the discussions remain very friendly compared to the horror stories of other forums! Regards Stampe
3000ft may be marginal. This is my flight of yesterday (just a little local, testing some kit like the ADL150 with new firmware, a mike adaptor kit for a QC25 headset, and some avionics) which was done mostly at 4700/5300ft. You can see, relative to where Shoreham is, and climbing at +1000fpm, that 3000ft may not be showing.
Another factor is that lots of transponders are not wired with the best quality cable, and that’s before you get onto antenna ground planes etc. I see huge variations in distances (even on a given bearing) at which other GA traffic shows up on my TAS605 active traffic box.
Here is the vertical profile. The speed BTW is unreliable, often so on FR24
Actually I have just done a playback on FR24 and the fix was picked up at 2758ft and dropped at 1500ft.
I suggest @stampe that you re-test at ~5000ft, and not too far from “civilisation”
Update :
one of my club’s aircraft just appeared on the FR24 database, because of this :
So every aircraft with a Trig or other Mode S should appear one day or other on FR24.
It is still very strange that our 5000 ft cruise to Friedrishafen and back never got tracked by MLAT.
