Interesting problem arose with a client recently: The client was getting complaints from certain ATC services that they were not “receiving the ac’s Mode S code” .
The transponder, a Garmin GTX 330, was positively double checked for the correct reg number & hex code in the set-up config file.
Yesterday the problem arose on IFR departure from Strasbourg . I called the ATC after the flight and spoke with the Controller. He confirmed that when the ac was given a “1000” squawk, the “system” was not showing the ac’s id data.
Further digging and here’s what we learned : The 1000 squawk invokes “selective interrogation” where the hex code in the ac’s xponder is then linked to the ATC Flight Plan data system where the ac’s pertinent details are stored .
The problem in this case was the client was using Rocket Route and he did not fill in the details of the AC’s Mode S hex codes , hence the ATC system could not make the connect with the FP as filed !
What I learned :
- The “FULL” Mode S system does NOT require a discrete squawk for the AC, hence the use of the generic “1000” .
- To date, very few ATC services are actually using the FULL MODE S capabilities, so despite the proliferation and obligation to have on-board Modes S, ATC is not actually USING it in every sector.
I went to bed just a bit less dumb last night 
More info on this subject:
EUROPEAN SECONDARY SURVEILLANCE RADAR
Mode S Conspicuity Code:
In order to maximise SSR code savings through Mode S Elementary
Surveillance (ELS), all aircraft identified via the downlinked ACID use
the same SSR code, the Mode S Conspicuity Code A1000
Michael wrote:
The problem in this case was the client was using Rocket Route and he did not fill in the details of the AC’s Mode S hex codes , hence the ATC system could not make the connect with the FP as filed !
I’ve never heard that the mode S hex code should be entered into the flight plan. The ATC system gets the hex code from the aircraft itself when the transponder squitters.
On the other hand, the transponder should squawk the call sign of the flight and that connects the aircraft with the ATC flight plan data. The call sign is either set permanently to the aircraft registration when the transponder is configured or it can be set by the pilot. (Obviously always pilot-settable in airliners.)
Airborne_Again wrote:
I’ve never heard that the mode S hex code should be entered into the flight plan. The ATC system gets the hex code from the aircraft itself when the transponder squitters.
P.S. I know for a fact that with a mode S transponder ATC will see the call sign even if the aircraft is squawking 7000 and no flight plan has been filed. Our local ATC has tracked aircraft straying into CAS without a clearance in this way.
I have not entered any data pertaining to my xpdr 24 bit identifier into Autorouter and I have never had a problem with ATC identifying my aircraft by Paris, Strasbourg or Langen (or anyone else for that matter). I have not seen any such field in the Autorouter aircraft setup. Nor in EuroFPL.
When you fly through sectors that do not have mode S capabilities, like Brest, I believe you get a discrete squawk.
This document from Eurocontrol does not say anything about the flightplan having to include the 24 bit HEX number. It is targeted at military personnel but I think a lot of the contents also applies to civilian aircraft except that the 24 bit code is randomly allocated to military aircraft AFAIU.
Maybe @achimha can shed some more light on this?
I know for a fact that with a mode S transponder ATC will see the call sign even if the aircraft is squawking 7000 and no flight plan has been filed. Our local ATC has tracked aircraft straying into CAS without a clearance in this way.
Yes; the tail number is entered directly into the transponder. Also the 24-bit code is entered and, for an N-reg and I believe no others, this can be translated directly into the tail number, so for an N-reg ATC have two ways to get the tail number. Others regs need a database lookup which I don’t believe any ATC unit has, so ATC has to use the configured tail number.
If you bust CAS, they can get you from the 24-bit code too.
I got 1000 only once, near Nice in France.
Aviathor wrote:
I have not entered any data pertaining to my xpdr 24 bit identifier into Autorouter and I have never had a problem with ATC identifying my aircraft by Paris, Strasbourg or Langen (or anyone else for that matter). I have not seen any such field in the Autorouter aircraft setup.
autorouter automatically transmits the CODE/ field if the aircraft’s registration starts with N because there is a defined correlation for N-regs. You can add it manually when changing the flight plan for other registrations. We have not made it an official field in the aircraft setup because European ATC are supposed to be able to read Mode S IDs (why did they make us buy new transponders?) and if we add it, people will probably get it wrong and cause more harm than good.
I don’t think we should encourage users to enter their transponder hex code…
PS: the best way to lookup your 24-bit hex code is http://www.airframes.org
Airborne_Again wrote:
I’ve never heard that the mode S hex code should be entered into the flight plan. The ATC system gets the hex code from the aircraft itself when the transponder squitters.On the other hand, the transponder should squawk the call sign of the flight and that connects the aircraft with the ATC flight plan data. The call sign is either set permanently to the aircraft registration when the transponder is configured or it can be set by the pilot. (Obviously always pilot-settable in airliners.)
I don’t think you guys actually get what I explained, Achima excepted.
Primo : There is NO REQUIREMENT to enter the tail number in the transponder itself, just the hex number.
Secondo : Whilst the BASIC ATC system can indeed DIRECTLY read the hex and/or tail number from the broadcasting xponder, when the Mode S Conspicuity Code is used , the ATC system AUTOMATICALLY looks up the CODE field that is transmitted wit the FP by the FP filing system. Thus, if the CODE field that was transmitted with the FP is in error, the ATC system is incapabale of positively IDing the AC, unless a DISCRETE squawk CODE is assigned to the AC .
Michael wrote:
Primo : There is NO REQUIREMENT to enter the tail number in the transponder itself, just the hex number.
That is not right. In Europe, Mode S is a legal requirement and the callsign has to be configured in the transponder and is sent as part of the Mode S response. Maybe France isn’t equipped to read it but try crossing the German border without a Mode S flight ID or a wrong ID and you will get an immediate reaction.
Secondo : Whilst the BASIC ATC system can indeed DIRECTLY read the hex and/or tail number from the broadcasting xponder, when the Mode S Conspicuity Code is used , the ATC system AUTOMATICALLY looks up the CODE field that is transmitted wit the FP by the FP filing system.
Maybe for (some) French ATC with old technology but the way this was designed is that the Mode S flight ID is used for the correlation, not the CODE/ field in the flight plan which is not typically found in a flight plan.
As Mode S is prescribed in all of Europe, CODE/ in the flight plan doesn’t add much value. ATC units will have to update all their systems to receive Mode S information. Germany has done that many years ago. It can be disturbing, once in a DA42 with G1000 I got an altitude assigned and then ATC asked me why I haven’t put it in the autopilot’s altitude preselect. I was looking for a camera in the cabin and couldn’t find it. Then I realized it’s Mode S…
achimha wrote:
Maybe for (some) French ATC with old technology but the way this was designed is that the Mode S flight ID is used for the correlation, not the CODE/ field in the flight plan which is not typically found in a flight plan.
Airborne_Again wrote:
I’ve never heard that the mode S hex code should be entered into the flight plan. The ATC system gets the hex code from the aircraft itself when the transponder squitters.
So basically, even if the HEX code entered into the xpdr is wrong, ATC will be able to read the flight ID from the extended squitter and associate it with the HEX code sent by the xpdr, and use that HEX code to interrogate the transponder.
If ATC is not able to identify the aircraft, it must be because the flight ID transmitted by the xpdr does not match that in the FPL?
