View Full Version : Mode S and the alleviation of SSR frequency congestion

28th Mar 2004, 10:18
The cornerstones of the case for mandating Mode S Elementary Surveillance (forget Enhanced for now) appear to rest on two capacity arguments. One surround the limited availability of Mode A codes, the other the congestion of the SSR fequency spectrum (see for example section 2.2.1 of the lastest CAA RIA (http://www.caa.co.uk/docs/7/DAP_SSM_Consultation_ModeS_SSR_ANO_Stage 2.pdf)). Selective interrogartion ostensibly reduces the congestion.

There's one thing I don't get. All current Mode S boxes are also Mode A/C transponders, and Mode A conspicuity codes (or even temporary discrete codes!) will be assigned (http://www.eurocontrol.int/mode_s/FAQ/mode_s_faq.htm#36) and be active.

So until the day that Mode A/C interrogations cease completely in our airspace, how does this reduce SSR frequency congestion?

1st Apr 2004, 10:34
Second call. No one has any input for this? Surely I haven't found such a fundamental flaw in the Eurocontrol Mode S strategy?

1st Apr 2004, 21:11

You're correct, when Mode S started out (at the start of the 70's!) one of the concepts was to replace conventional SSR with selective interrogations. However, there are a number of reasons why Mode S and SSR will co-exist for many years. For example, many display systems will continue to use the Mode 3/A code for code/callsign identification. I also suspect the day when all transponders are Mode S is also probably some way off! Whether a Mode S equipped aircraft is allocated a discrete Mode A code or a conspicuity code (1000) will depend if the complete flight is to be completed in 'Mode S' airspace. (All interrogators and display systems can use Mode S derived aircraft Id)

The introduction of lower prf monopulse SSR radars has probably had more impact on the SSR frequencies than the introduction of Mode S is likely to have, at least initially. In fact congestion is likely to increase as ADS-B 1090 extended squitter starts to be fitted and used. However, by using more sophisticated decoders the newer radars (both SSR and Mode S) are far more resistant to interference.

So, will the introduction of Mode S reduce frequency congestion? No - but the introduction of monopulse SSR and Mode S probably means that it won't increase to a point where it becomes unusable. The principal reason for Mode S finally being introduced lies elsewhere.


CJ Driver
3rd Apr 2004, 16:03
A slightly simplified summary...

Mode S transponders are backwards compatible with Mode A/C in that they will also reply to an old-style interrogation; similarly Mode S interrogators are backwards compatible with Mode A/C transponders in that they are able to send both old-style and new-style interrogations. There is however a special style of interrogation invented to ease the Mode A to Mode S transition. which looks like an old-style interrogation, but then has an extra pulse (P4) transmitted after the end. The theory is simple - old Mode A/C transponders see the interrogation sequence they are familiar with, and reply in the usual manner. Mode S transponders meanwhile wait to see if P4 is present; if they see it (and subject to some other stuff to do with already being in touch with the ground station, and the length of the pulse) they do NOT reply because they know they are being scanned by Mode S.

So, in a mixed Mode A/C and Mode S transponder environment, by using a modern Mode S interrogator, frequency congestion is significantly reduced.

4th Apr 2004, 18:36
Thanks for that, it helps. So while this compatibility mode is in use, interrogations by the newer radars will cause less frequency congestion.

However, it seems that if there are older non-Mode-S ground systems out there, they will interrogate using Mode A/C and all transponders, including the newer Mode S/A/C ones, will respond.

CJ Driver
5th Apr 2004, 09:12
Bookworm: True - in an environment being painted with a Mode A/C interrogator, all the transponders will respond in the old-fashioned manner and all the usual problems of fruit and garble will be present.

Fortunately, I think you will find that most interrogators in the world already support Mode S - because most of the world's interrogators are not on the ground, but are part of an airborne TCAS. TCAS also does Mode C interrogations, but as noted, with P4 these do not interfere with Mode S equipped aircraft.

I think part of the reasoning for mandating Mode S (perhaps not well explained by the authorities), is slightly circular - TCAS is good, so more aircraft should have one; for TCAS to work, everyone needs a transponder; but if all the transponders were Mode A/C the congestion would bring down the whole system. Hence the new transponders should be Mode S.

There are other advantages to Mode S of course, including accurately identifying aircraft (so that two aircraft could carry the same Mode A code and still be uniquely identified), the ability to support datalink (such as the TIS initiative in the US), and so on.