A TAS box which doesn’t show Mode A targets would be worth a lot less. One can’t tell their relative altitude, but there are times (e.g. in the UK, when flying 1000-2000ft, or at 2300ft under the 2500ft LTMA and other bottlenecks) when the depicted traffic is quite likely to be at your level, or close, so lateral avoidance is a good idea.
Next time I do a local low level flight on a weekend (which I do rarely) I will take some pics and count them.
Peter wrote:
A TAS box which doesn’t show Mode A targets would be worth a lot less.
I think you might be misunderstanding the point. You may see many targets without altitude indication, but their transponders were interrogated using a Mode C interrogation and the Mode C response said that no altitude indication was available.
That’s very interesting.
Would a Mode A transponder (I mean an old Mode A – only one) return anything to a Mode C interrogation?
Peter wrote:
IIRC, they would then use “unverified”. I am sure an ATCO will clarify this soon… not that we have many UK ATCOs writing here because of the Official Secrets Act
They can talk about things that aren’t secret. I’ve signed the Official Secrets Act for my job (we have an equivalent act here) but if I couldn’t talk about work it would make gaining customers really hard. Virtually nothing I do is actually covered by the act.
I’m sure ATC procedures aren’t secret.
bookworm wrote:
Mode C Surveillance (p17)
TCAS uses a modified Mode C interrogation known as the Mode C Only All Call to interrogate nearby Mode A/C transponders. The nominal interrogation rate for these
transponders is once per second. Since TCAS does not use Mode A interrogations, the Mode A transponder codes of nearby aircraft are not known to TCAS.
I suspect they are referring to true TCAS II as fitted to grown up airplanes which issue Resolution Advisories and the like…which necessarily required relative altitude information….so no point in interrogating mode A….
Having seen this on UK forums for over a decade, it’s clear that NATS is incredibly nervous about its employees participating on forums.
There have been some comical situations… and the forums are monitored. I recall one case where an ATCO described what happens to radar tapes. Immediately, a poster calling himself Menwith Hill reminded him of the OSA 
One senior NATS guy used to post under a nickname, and at the same time using his real name when on “official business”.
It is a pity because ATCOs could contribute a great deal, of the practical country-specific stuff which is obviously not secret. But the UK ones don’t. Try finding out the rules which e.g. London Control use to accept or reject IFR flight plans from GA. Top secret, MATS PT 2, according to one ATCO post on a paid-members-only site. We have several non UK ATCOs posting here, however.
I don’t think this “Mode A” stuff exists outside the UK. There, low level traffic is either totally invisible (probably the majority of the UL/homebuilt community) or is Mode C/S. A lot of people do turn off Mode S transponders completely however.
Peter wrote:
I don’t think this “Mode A” stuff exists outside the UK.
I still do not understand why those guys are transponding at all if they won’t be seen. One should either be transponding altitude or nothing at all.
I think weekend VFR flyers are so scared about being ‘done’ for vertical infringements due to the serious amounts of Class A airspace in the UK, so they choose to hide their altitude instead.
This will NEVER be resolved until a TMZ is put in place.
Peter wrote:
Would a Mode A transponder (I mean an old Mode A – only one) return anything to a Mode C interrogation?
I think you would have to look back a long way to find one. Most of the transponders that you may think of as “an old Mode A transponder” are actually capable of Mode C but have no altitude encoder connected. Can you for example find a picture of a transponder front panel without an “alt” position? Here’s an antique.
OK; you are right, bookworm. I had a trawl through the US Ebay antique avionics section and failed to find one. Mode C must be at least 40 years old.
But then what you seem to be saying is that practically all the Mode A Only returns must be due to
As regards TMZs, there would be war if that was tried all over the UK. And GA would get exemptions which would render it useless.
