Transition to Mode S Interrogation

[NATS General Aviation Lead]

Following some discussion on another forum I thought it may be useful to share some technical issues surrounding the transition from Mode A/C to Mode S interrogation. We have compiled the following notes about the changes to the technical requirements for our radars and how this effects our ability to detect some faulty, unmodified transponders.

The National IFF/SSR Committee requires that all UK operators of secondary radars cease interrogating Mode S equipped aircraft in Mode A/C by the 31 December 2011. NATS, in common with other radar operators across the UK and Europe, are therefore in the process of transitioning its systems to accommodate this change. This change benefits air traffic controllers by reducing the numbers of false targets and radar reflections presented on their screens and reducing the probability of corruption of displayed height and identity. This transition together with a large amount of technical detail is formally notified to airspace users in a yellow Aeronautical Information Circular to be published in the next few weeks.

Following the transition to this new configuration both modern Mode S and legacy Mode A/C transponders will continue to reply to interrogations, as Mode S is backward compatible - a Mode A/C transponder will respond A/C to a Mode S interrogation.

What’s the problem?

There a number of unmodified transponders within the GA fleet which are known to be faulty and these will not respond to Mode S interrogations. Once the transition has completed within an area, aircraft with such transponders will not be visible on secondary radars. This problem is starting to manifest itself as NATS and other ANSPs complete the required technical changes at various radar sites.

But I don’t fly in airspace that requires a transponder why should I care?

Even if you are not required to carry a transponder there are still advantages to the GA pilot in having their box working correctly:

• Controlled airspace transit and ATSOCAs - Having a correctly functioning transponder makes it easier for a controller to accommodate a transit request of controlled airspace. It also easier for a controller providing ATSOCAs to identify and retain the identity of a transponder equipped aircraft particularly in a busy radar environment. Mode S gives the added benefit that the flight identity (callsign) of the aircraft is emitted by the transponder. This further enhances the controller’s ability to easily identify a transponding aircraft and retain that identity.

• TMZ - Having a correctly functioning transponder with Mode C switched ON is a requirement for entry into Transponder Mandatory Zones.

• TCAS - Traffic Collision Avoidance System is carried by most large aircraft flying IFR in UK airspace. The system provides guidance to the equipped aircraft to enable them to avoid other transponder equipped aircraft in close proximity. In some cases a transponder fault which prevents the transponder working with the Mode S radar also prevents the transponder interacting correctly with TCAS equipped aircraft. Thus no protection is afforded to you or the TCAS equipped aircraft.

• CAIT and STCA - Having a correctly functioning transponder allows electronic ‘safety nets’ such as NATS’ Controlled Airspace Intrusion Tool and Short Term Conflict Alert to warn controllers should you inadvertently infringe or come into conflict with another transponder equipped aircaft. The tool allows the controller to take prompt action and mitigate quickly the effects of an infringing aircraft or a confliction. This protects you and commercial traffic. The additional functionality provided by a Mode S equipped aircraft transmitting flight identity, enhance these tools.

• Touring -Touring in a GA aircraft throughout Northern Europe is becoming more restrictive without a Mode S transponder. Other ANSPs in adjacent states are also affected by the technical changes and in some States the requirement for Mode S carriage is more widespread. Known faults with unmodified transponders are already routinely detected by other ANSP’s radar systems and these problems are becoming more apparent as legislation requires the reduction in Mode A/C interrogation.

How do I know if I’m affected?

The vast majority of such transponder faults are known issues and the subject of Airworthiness Directives and manufacturer’s Service Bulletins. Just check your transponder against those listed below:

Transponder - Filser (Funkwerks) TRT-800
Applicability - All units NOT marked with MOD-Index 10
Airworthiness Directive/ Service Bulletin - “EASA AD 2008-0183” and “SERVICE BULLETIN SB TRT800-A-H-1”
Action to Take - Units should be upgraded to MOD-Index 10 status. This is a factory modification, contact Funkwerks to arrange shipment to the factory at [email protected] Tel: +49 (0) 8246 9699-0

Transponder - Filser (Funkwerks) TRT-800
Applicability - All versions and serial numbers
Airworthiness Directive/ Service Bulletin - “Service Letter SL-1/2009” and “Service Letter SL-1/2010”
Action to Take - ICAO 24-bit address should be correctly programmed to the address allocated by aircrafts registration authority. Address should not be set as 000000. This can be configured using the transponder control panel – see the manual. Those units with Firmware Version 101 or Software Version 4.5 are also advised to have this upgraded at the factory – contact Funkwerks at [email protected] Tel: +49 (0) 8246 9699-0

Transponder - Filser (Funkwerks) TRT-600
Applicability - All versions and serial numbers
Airworthiness Directive/ Service Bulletin - “EASA AD 2008-0158R2” and “Service Letter SL-1/2008”
Action to Take - TRT-600 should be upgraded to become a TRT-800. This is a factory modification, contact Funkwerks to arrange shipment to the factory. [email protected] Tel: +49 (0) 8246 9699-0

Transponder - Terra TRT-250 / TRT-250D
Applicability - Part number (P/N) 0900-0250-00, with serial numbers (S/N) 4194 and below, Modification Level (Mod Level) 4 and below; or P/N 0900-0250-20, with S/N 5324 and below, Mod Level 4 and below; or P/N 0900-0250-30, with S/N 1155 and below, Mod Level 1 and below
Airworthiness Directive/ Service Bulletin - “FAA AD 95-01-01”
Action to take - Support for the TRT-250 has been discontinued by Terra’s successor due to a shortage of spare parts.
The AD suggests returning the unit to Terra, who are no longer trading, or replace the unit.

Transponder - NARCO Avionics AT-150
Applicability - All part numbers, all serial numbers
Airworthiness Directive/ Service Bulletin - “EASA AD No.: 2009-0200” and “NARCO Service Bulletins #1 and #6”
Action to Take - Due to its age support for the NARCO AT-150 has been withdrawn.

EASA Airworthiness Directives can be found at http://ad.easa.europa.eu
Filser/Funkwerks Service Letters can be found at http://www.funkwerk-avionics.com/
FAA Airworthiness Directives can be found at http://rgl.faa.gov/AD
NARCO Avionics Service Bulletins can be found at http://www.narco-avionics.com/