ANY installed transponder must be turned ON

FR24 is not the only outfit doing this. There is this and this and any half competent radio amateur with web development skills could set up a 1090ES receiver and put the data on a website. The cat is out of the bag…

My understanding (right or wrong) is that a mode S transponder will squit the reg number (or whatever is coded into it) every second as a default.

Again, NO.

The time a “Mode S transponder” will emit stuff by itself, without an incoming SSR ping, is when it is a part of an ADS-B installation, which is how most (all in GA?) ADS-B installations are done: they use the TXP to radiate the packets.

The end result is the same however because in “European civilisation” a TXP is always getting pinged.

Peter wrote:

No, it doesn’t. Only when pinged by SSR.

SSR Mode S transponders transmit acquisition squitters (unsolicited downlink transmissions) to permit passive acquisition by interrogators with broad antenna beams, where active acquisition may be hindered by all-call synchronous garble. Examples of such interrogators are an airborne collision avoidance system and an airport surface surveillance system.

Acquisition squitter transmissions shall be emitted at random intervals that are uniformly distributed over the range from 0.8 to 1.2 seconds using a time quantization of no greater than 15 milliseconds relative to the previous acquisition squitter…

Then there is also Extended Squitter (ES) which is a different thing (used for ADS-B) and which not all mode S transponders provide. Extended squitter is described in section 3.1.2.8.6 of Annex 10, volume IV.

After that, you get into the Elementary Mode S versus Enhanced Mode S business, which is a purely Eurocontrol invention and there is no config for it in the transponder(s) which were designed in the USA.

This is completely unrelated to squitter or extended squitter.

Silvaire wrote:

Understanding that you were IFR and that I would not have been, I don’t think I’d ever want ATC to ‘reach out to me’ (in the latest sugar coated vernacular ) when outside of their zone of authority – because typically the reason I’m staying outside of their zone is so I don’t have to talk to them. Assuming you’re not already in their controlled airspace and not subject to their ATC authority, what function does this serve? I think listening in may beneficial in that you can hear traffic coming your way, but what benefit is there in ATC making unsolicited contact if they have no authority over you? Would they do that just to give you some advisory info that you may or may not find relevant?

Wow, just caught up on this thread… I see what you mean but I hadn’t really considered it your way Silvaire…

In my case I departed a field in Class G and needed to contact either London Info or Birmingham Approach to pick up my IFR clearance to enter their airspace on my way North back to Scotland. if I had contacted London, as probably most people would have done, they would have got on the phone to Birmingham, got the necessary details and then called me back, giving me the clearance and instructing me to call Birmingham….In this case the Birmingham controller saw the listening squawk and my tail number and short-circuited the process….which I thought was quite cool…

Peter wrote:

No, it doesn’t. Only when pinged by SSR.

It does, all mode S transponder do squitter 1/second without interrogation. Airborne_Again is right.

Rwy20 wrote:

My experience having lived in Sweden for a year is that the concept of privacy is basically non-existent there. You just don’t have anything to hide.

Wow….

Since fitting Mode S, returning to Inverness after over an hour squawking 7000 and not talking to any ATC unit, but using 135.475 to keep awareness with military traffic, I’ve been greeted “G-WF are you on frequency yet?” while well away from normal contact area.
Our Trigger 8.33 allowing the standby frequency to be monitored, and Mode S have been big improvements for me.

Rwy20 wrote:

They do, it’s called Datainspektionen (English).

My point exactly. They do have something to hide afterall, just like the Norwegians

And although it was a decade and a half ago, I did live in Scandinavia for a couple of decades… And I do not think that much has changed in terms of data privacy for the last decades. I have not found any reference, but my impression is that the Scandinavians were forerunners in terms of data protection legislation in Europe.

all mode S transponder do squitter 1/second without interrogation

Interesting. So many people in the business have told me otherwise for so many years.

Does FR24 use this 1/sec data packet?

So what’s the point of ADS-B if a bog standard year-2005 GTX330 does this?

Peter wrote:

So what’s the point of ADS-B if a bog standard year-2005 GTX330 does this?

ADS-B transmits your GPS location. Mode S doesn’t.

Peter wrote:

So what’s the point of ADS-B if a bog standard year-2005 GTX330 does this?

As I wrote above, there is a difference between squitter and extended squitter. Squitter only transmits the aircraft id. Extended squitter transmits the ADS-B data. A mode S transponder may not support ES, but it has to support (simple) squitter.