Was Mode S really necessary?

LeSving wrote:

if you install ADS-B instead of Modes S

That is not possible. To avoid confusion. ADS-B in Europe is always 1090 MHz ES which means it is Mode S ES (extended squitter). So every ADS-B out installation in Europe is going to be a Mode S transponder with some “extra” broadcasting the GPS position.

The only exceptions I know is first the SkyEcho2 which seems to be really just 1090 MHz ADS-B out without Mode S. But that is illegal outside the UK. And then second there is the US 978 MHz UAT also not legal in Europe.

Airborne_Again wrote:

Mode S is mandatory for GA under IFR, per EU regulation 1207/2011 (with amendments).

Not according to AIP. There are two requirements:

AIP GEN 1.5 section 2.1 c :
Aircraft flying IFR or VFR within Norwegian flight information regions shall in airspace class A, C and D, carry and use SSR-transponder:
for transmissions in Mode A with 4096 codes
for transmissions in Mode C (automatic altitude reporting).

AIP ENR 1.6 section 2.1.1 and 2.1.3:
When an aircraft carries a serviceable transponder, the pilot shall operate the transponder at all times, regardless of whether the aircraft is within or outside airspace where surveillance is used for ATS purposes.

When the aircraft carries serviceable mode C equipment, the pilot shall continuously operate this mode, unless otherwise directed by ATS.

LeSving, should this be in some hard law? AIP is usually just an extraction from it (and not the law).

Sebastian_G wrote:

That is not possible. To avoid confusion. ADS-B in Europe is always 1090 MHz ES which means it is Mode S ES (extended squitter). So every ADS-B out installation in Europe is going to be a Mode S transponder with some “extra” broadcasting the GPS position.

I don’t think it’s possible to avoid confusing regarding this

Again, according to AIP, the requirements for ADS-B is (ENR 1.6 section 3.3) :

3.3 ADS-B system requirements
3.3.1 For aircraft required to equip with ADS-B OUT per European Commission Implementing Regulation (EU) No 1207/2011 and subsequent amendments the applicable requirements are European Aviation Safety Agency (EASA) CS ACNS Subpart D,1090 MHZ EXTENDED SQUITTER ADS-B.

3.3.2 For aircraft not required to equip with ADS-B OUT per European Commission Implementing Regulation (EU) No 1207/2011 and subsequent amendments the ADS-B OUT systems shall comply with one of the standards below:
- European Aviation Safety Agency (EASA) CS ACNS Subpart D,1090 MHZ EXTENDED SQUITTER ADS-B ; or
- Federal Aviation Administration (FAA) Title 14 Code of Federal Regulations (14 CFR) section 91.227 or AC No. 20-165B (or replacement) – Airworthiness Approval of ADS-B; or
- European Aviation Safety Agency (EASA) CS-STAN

3.3.3 ADS-B Out systems that are unable to meet the requirements above, must
- ensure that the aircraft always transmits a value of 0 (zero) for one or more of the following position quality indicators: NUCp (only for ADS-B version 0 units), NIC and/or SIL, or
- disable ADS-B transmission.

For Norway, since Mode S is not required, and since the normal GA aircraft is not required to be equipped with ADS-B, then I can install either a EASA version ADS-B or a US Version ADS-B. (Don’t know what the CS-STAN is).

Clearly a US GA aircraft can fly in Europe with a US ADS-B, no? It would be insane if it couldn’t. Therefore I can also fly a Norwegian aircraft with a US ADS-B out, and it will be as good as a EASA Modes S. To me this is evident, but perhaps not ?

Peter wrote:

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

The US allows both Mode A/C and Mode S transponders. There is no indication that this will change. Most new transponders are probably mode S because of ADS-B. Transponders with mode A/C are not required except in specific airspace, A, B, C, Mode C veil, E above 10,000 MSL. Below 10,000 MSL, the vast majority of E airspace does not require a transponder at all, even for IFR. Since A airspace includes the airspace above 18000 MSL, a mode S transponder is required in this airspace because of ADS-B Out.

LeSving wrote:

Clearly a US GA aircraft can fly in Europe with a US ADS-B, no? It would be insane if it couldn’t. Therefore I can also fly a Norwegian aircraft with a US ADS-B out, and it will be as good as a EASA Modes S. To me this is evident, but perhaps not ?

No, not correct – US ADS-B is UAT, so that is not used here. 978MHz conflicts with some military frequencies (that is the current excuse, apparently).

lionel wrote:

Mode S ES is great for safety, if/when ATC upgrades their radars to make use of it, it adds a supplemental slice of cheese with respect to the Swiss cheese model. I once set my altitude bug incorrectly, ATC saw it and “reminded” me to check (before I even got to the said altitude, so Mode S ES is the only way they could have seen anything; and it was literally the week after they got their new radar…). I also once forgot to switch to QNE and levelled at the wrong level, again ATC challenged me on it after some time, but this one they could have seen just through plain old Mode C. Also it gives them your FL in 25 ft increments.

AFAIK, Mode S Radars don’t process the ES data. ADS-B ground stations processes ES. Mode A/C transponders only provide pressure altitude to the nearest 100 feet. Mode S is capable of providing pressure altitude to the nearest 25 feet, but that is dependent on the altitude encoder that is installed.

arj1 wrote:

978MHz conflicts with some military frequencies (that is the current excuse, apparently).

978 MHz is the reply frequency of DME channel 17X paired to 108.00 Mhz (it is acutally crazy how many valuable frequencies are taken by DME but that is another issue). To avoid trouble it is not used often (at all) in Europe but flying with a US style UAT transponder is definitely not ok in Europe. UAT ADS-B out is really a US thing only. All aircraft likely to be flown outside the US should buy a 1090 MHz Mode S ES ADS-B out solution.
In reality there is most likely no problem because UAT aircraft in the US are usually not suitable for transatlantic trips anyway and the others will have Mode S ES.

LeSving wrote:

Clearly a US GA aircraft can fly in Europe with a US ADS-B, no? It would be insane if it couldn’t. Therefore I can also fly a Norwegian aircraft with a US ADS-B out, and it will be as good as a EASA Modes S. To me this is evident, but perhaps not ?

It depends on the type of ADS-B Out installed in a US aircraft. If it is 1090ES like a GTX 330ES or GTX 345, those would be compatible with most international requirements. Only the US supports ADS-B Out using UAT, so if that is what is installed, it won’t satisfy an ADS-B Out requirement outside of the US. The UAT frequency of 978 MHz has the potential of interfering with DME near the same frequency, but analysis has shown it does not. I would guess that such an aircraft operating in Europe might be required to disable the UAT ADS-B Out function, but it would probably go undetected if left on and other aircraft with dual frequency ADS-B In systems (which are effectively the defacto standard for these systems) would still see such aircraft as traffic even if ATC could not.