[Paul_Sengupta]

I can't remember if this has been brought up before, but I would suggest that Mode C should be allowed in a Transponder Mandatory Zone. There is no logical reason why Mode S should be required, Mode C gives all the information a controller requires to know the position and altitude of an aircraft.

I suggest that the regulation is changed accordingly.

[chevvron]

I can't remember if this has been brought up before, but I would suggest that Mode C should be allowed in a Transponder Mandatory Zone. There is no logical reason why Mode S should be required, Mode C gives all the information a controller requires to know the position and altitude of an aircraft.

Apart from speed, registration and callsign

[Jodelman]

Apart from speed, registration and callsign

But that doesn't answer the basic question - why isn't mode C acceptable?

We were sold the idea of a TMZ on the basis that the controller wanted a known environment - mode C does that and was acceptable when the Stansted TMZ was first enforced.

[Cookie]

http://flyontrack.co.uk/wp-content/uplo ... /TMZv4.pdf

The Stansted Transponder Mandatory Zones were established in 2009 as a result of the large number and severity of controlled airspace infringements in the vicinity of London Stansted Airport.

https://www.caa.co.uk/docs/33/Informati ... 012151.pdf

Following the end of the temporary exemptions put in place for the Olympics and Paralympics that permitted aircraft equipped with Mode A/C to continue to operate within the Stansted TMZ and associated Olympic Restricted Airspace, this Information Notice is a reminder of the carriage requirements for Secondary Surveillance Radar (SSR) Transponder equipment from 13 September 2012 onwards.

AIP GEN 1.5 states that Mode S Elementary Surveillance is required for GA aircraft:

5.3.2.2

Notified ‘Transponder Mandatory Zone’ Airspace. For the purposes of Article 39(2) and Schedule 5 paragraph 3(6)(b) of the Air Navigation Order 2009, the following airspace is notified as a 'Transponder Mandatory Zone':

* The vertical and lateral bounds of the Stansted TMZ as established under Statutory Instrument 2009 No. 2020 and detailed in AIP AD2-EGSS.

I agree that these requirements are excessive and that additional information provided by Mode S (ELS) is a NICE to know, not a NEED to know for the purpose of collision avoidance under/around/within the Stansted stubs.

One of the advantages of TCAS II is that it co-ordinates a TCAS event providing both aircraft are fitted with TCAS II/Mode S (very few GA aircraft). Another benefit Mode S is the transmitted data (often very limited from GA aircraft and of limited value from Mode S ELS).

TMZs in the UK are akin to the Mode C 'veil' used in the USA around busy airfields which require...Mode C. Whilst Mode S may be desirable for the above reasons, this should not be a requirement for entry without clearance into a TMZ. Mode C should be sufficient.

Cookie

[bookworm]

But that doesn't answer the basic question - why isn't mode C acceptable?

Because Mode C (and Mode A) are incredibly heavy on bandwidth compared to Mode S. In the age when there were few transponders around, it was fine to have every transponder respond to every interrogation. These days, the transmissions have to be selective. Compare a VHF radio communication system, where anyone can talk and everyone hears, with a mobile phone network.

The extra data carried by Mode S is useful but only a minor part of the reason why Mode S has replaced Mode A/C.

[Jodelman]

Because Mode C (and Mode A) are incredibly heavy on bandwidth compared to Mode S.

In that case, perhaps I should leave my transponder permanently switched off!

[Cookie]

Because Mode C (and Mode A) are incredibly heavy on bandwidth compared to Mode S.

For the same reasons as in the 8.33kHz thread, all aircraft within Class A, B, C under IFR will already be Mode S equipped. Mode S for Class G airspace (albeit under the Stansted stubs) whilst it may be desirable is not necessary.

https://www.eurocontrol.int/articles/mo ... l-overview

Cookie

[bookworm]

Mode S for Class G airspace (albeit under the Stansted stubs) whilst it may be desirable is not necessary.

It's difficult to be sure about that without more information on the radars involved. Mode A/C can't be supported forever, though there may be an argument that they can continue to be supported in environments like that for some time still. But to support Mode A/C doesn't it require the interrogations to be sent, which then means that all transponders will reply?

Ten years ago I remember making a very similar argument in the consultation response to the mandate for ELS in the London TMA. "98% of the aircraft have to have EHS, so why does it matter if the remaining 2% of us are flying around with Mode A/C rather than ELS?" It fell on deaf ears at the time.

[Jodelman]

I find it very difficult to understand why I should be better flying with no transponder rather than mode C.

Is that really what I am expected to do?

[Dave W]

Bandwidth: Why is that not, then, a problem in the USA?

[bookworm]

It is. Hence the 2020 mandate for ADS B where Mode C is now required.

[Dave W]

Aha! Thanks. At least there'd be potentially something in it for us if the demand was for ADS-B instead of Mode S. (And I speak as someone with Mode S in my aircraft already)

[bookworm]

At least there'd be potentially something in it for us if the demand was for ADS-B instead of Mode S. (And I speak as someone with Mode S in my aircraft already)

Well, there is and there isn't something in it for us. In the US, there's a complex dual system of ADS-B so that GA aircraft can use a UAT datalink for it, offering a high bandwidth (by 1990s standards!) datalink for wx etc. At the same time airliners will use 1090ES for ADS-B. I'm still not sure how that's going to work.

In the EU, all ADS-B will be over 1090ES, probably because of the installed base of Mode S transponders, many of which are capable of ADS-B out -- if I were buying a transponder now, I'd want to ensure at least an upgrade path to ADS-B. ADS-B out doesn't give you anything directly, but if most aircraft are equipped with ADS-B out and you have an ADS-B in receiver (Skydemon has an ADS-B in add on under development, the Garmin GDL-39 does ADS-B in, etc), you do get traffic.

[bookworm]

I find it very difficult to understand why I should be better flying with no transponder rather than mode C.

Is that really what I am expected to do?

No, I don't think so. But at some point when a technology switch over is required, the regulator has to set a forward fit date at which all new devices fitted have to use the new technology, and need to encourage migration by offering benefits to those who have retrofitted.

To be clear, I'm not saying that the requirement for the Stansted stubs is justified (nor the opposite) -- the case is quite a technical one. The intention of my post was to point out that the migration to Mode S is not primarily about the extra information that a Mode S datalink can carry, but rather the necessity to expand the capacity of the system as more and more aircraft become transponder equipped.

[421C]

the necessity to expand the capacity of the system as more and more aircraft become transponder equipped.

I have a problem with ever believing what the "authorities" say about the "necessity" for aircraft operators to incur new equipment cost for capacity reasons, because I believe the arguments in respect of 8.33 and Mode S were false. They just represented a path of least resistance (getting aircraft operators to spend zillions) compared to the more efficient but politically more difficult paths of organising radio freqs and ATC and ATC equipment more effectively.