Was Mode S really necessary?

Are you sure that isn’t a case of European “well it might work in practice but will never work in theory” approach?

The US didn’t have a problem, despite vastly more traffic. They didn’t have a need for 8.33 either

Can anyone work out what that EASA survey is trying to establish, relative to anything realistically possible?

I’m not an expert on the chronology but it goes something like this: Mode S has never been mandatory for GA in the US, and prior to ADS-B it was never widely used by grass roots aviation. Despite FAA ambitions it was the subject of a substantial political debate years ago, after which it was largely put aside until ADS-B. In the interim there were a number of case studies that focused on the FAA’s waste of resources in creating an unsuccessful technology and program.

ADS-B was then developed as FAA’s ‘give it another try’ replacement for Mode S, but structured to reflect the lessons learned in the lost Mode S political battle. It is required only in specific airspace and introduced UAT so that Mode S remained optional and anonymity remained possible in most airspace. It also provided traffic and weather more readily inside the cockpit to make it more interesting to the buyer. I installed Mode S in my plane in 2019 because my Mode C transponder was dying and barely compliant, but most of my flying cohorts here use UAT instead of Mode S, with requirements for the equipment and installation also being lower for many of them due to the plane being E-AB.

I’m sure the only reason Mode S was interesting to European regulators was that unlike Mode C it supplied them with the tail number of airborne aircraft, as well as tracking their location, providing potential support for ATC billing and airspace enforcement. I’m equally sure the same was a motivation for FAA, with that potential FAA usage of ADS-B data now mitigated to a degree by UAT providing an anonymous mode (unlike Mode S). On the other hand ADS-B data around US airports is now being used by local government to track where planes are based and collect property tax. Rust never sleeps.

In the US, Mode S offered pilots the ability to display TIS traffic within 6 NM centric on the ownship aircraft position, so it was an early poor mans means of getting traffic in the cockpit well before ADS-B was available. Although TIS sounds like TIS-B, they are not the same, TIS is some times called TIS-A where the A only is used to distinguish it from TIS-B. Only certain terminal radar mode S systems have the ability to use the Mode S data link to transmit target information up to the Mode S transponder. Up to 6 targets could be transmitted to the aircraft when in radar contact and were all based on bearings and distances from the aircraft receiving the service. I had my GTX-330 installed in 2005. It worked with a GNS430/530 non WAAS GPS. Most TRACONs provided the service within 60 NM of the radar site. All in all there were about 110 sites around the country and in many areas provided nearly uninterrupted traffic service between adjacent TRACON facilities. I could fly most of the east coast and remain in TIS service areas for the vast majority of the flight. I updated my aircraft to include the ADS-B Out/In system in 2013 with a Garmin GDL88 and was able to expand the traffic coverage using mostly TIS-B until around 2020 when the mandate kicked in. By that time most targets were equipped with ADS-B Out, so TIS-B that was dependent on ground stations was replace with aircraft to aircraft ADS-B targets and some TIS-B. I never upgraded my GTX330 to ES. If I had it to do over in the current environment, I would have just installed a GTX-345 in lieu of the GDL-88 and GTX-330.

If the US is going away from Mode S, the technical justification for it can’t be applicable anymore, anywhere.

I’ve only fitted Mode S transponders when one of my Mode C units has failed. I simply can’t afford to replace a perfectly working unit with one that as far as I can tell gives no added benefits.

If I was based in or near to a TMZ then I might be forced to as a TMZ in the UK requires a Mode S transponder to enter. But I don’t think this is the case for the rest of Europe.

You can set up the Trig units for ADSB out and I think under CS-Stan you can use a none approved source e.g. Gamin Aera 660. But that still doesn’t give you ADSB in so there isn’t really enough benefit for me to justify the expense.

a TMZ in the UK requires a Mode S transponder

That surprises me.

I agree it hurts financially but a few points to consider:

Bathman wrote:

I simply can’t afford to replace a perfectly working unit with one that as far as I can tell gives no added benefits.

The added benefit is on the ATC side. A Mode S transponder generates a lot less load on the 1090 MHz radio frequency which else could be overloaded with all kind of bad consequences for ATC. Thus it depends on the country but operation of Mode A/C can be forbidden to protect the radio channel like any other form of unauthorized radio transmission can be forbidden.

Bathman wrote:

ADSB out … so there isn’t really enough benefit for me to justify the expense.

Please reconsider. You make yourself visible to everyone else. After all at night on the street the little lights of a bicycle usually are of limited use to see the road but they improve your safety a lot because everybody else can see you much better! The same applies to ADS-B out. Please put in that effort. It might protect you from beeing run over by somebody else. If you also want ADS-B in get a cheap portable device.

Peter wrote:

a TMZ in the UK requires a Mode S transponder

That surprises me.

It is is either or, from my last discussion with a CAA person – you:
either have a Mode-S and under VFR just fly through a TMZ in the UK without any radio conact/listenning, or
you contact the ATC responsible and ask for a transit.
In some areas they insist on Mode-S, just like they do sometimes for some other CAS transit.
And I’ve been told that legally you need to have Mode-S if you want to fly under IFR in any CAS (in the AIP and ANO).

As an IT guy I understand why they want Mode-S or ES (and ADS-B is just broadcasting the same info (pong) without a ping), but I’m still haven’t made my peace with with 8.33 radios…