Appendix B – Background information on Mode S ELS

(a)     General

This appendix provides background information on Elementary Surveillance (ELS) useful to understand ELS airborne surveillance system defined in the CS ACNS.D.ELS and its associated AMCs.

(b)     Related material

(1)     EASA

ETSO-C112d, Minimum Operational Performance Specification for SSR Mode S Transponders. (Based on EUROCAE ED-73E).

(2)     ICAO

(i)      ICAO Annex 10, Volume IV, Amd. 85, Aeronautical Communications (Surveillance Radar and Collision Avoidance Systems;)

(ii)     ICAO Document 9871 Edition 2 (transponder register formats);

(iii)     ICAO Document 8168-OPS/611 Volume I (Procedures for Air Navigation Services); and

(iv)     ICAO Document Doc 4444-RAC/501 Procedures for Air Navigation Service, Air Traffic Management.

(3)     EUROCAE

(i)      ED-73E Minimum Operational Performance Specification for Secondary Surveillance Radar Mode S Transponders; and

(ii)     ED-26 Minimum Performance Specification for Airborne Altitude Measurement and Coding Systems.

(4)     RTCA

RTCA DO-181E.Minimum Operational Performance Specification for Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne Equipment

(c)      Background information

Airborne surveillance system description

This section describes the ELS system including transponder, interfaces, and antenna.

The following diagram represents the Mode S Transponder and its main functional interfaces.

Figure 3: Mode S ELS transponder interfaces

(1)     Acquisition of aircraft position by Mode S ELS radar

         Aircraft entering the coverage of a Mode S radar is first acquired by All Call interrogations to which the transponder will reply if it is not on the ground. Therefore, it is important to test that the airborne surveillance system correctly takes into account the on-the-ground information. The on-the-ground status is also used by the ACAS systems to select aircraft which will be tracked.

         During this acquisition phase the radar will acquire the Horizontal position and the 24-bit aircraft address corresponding to the aircraft technical address on the RF network.

         The position and the aircraft address will be subsequently used to selectively interrogate the aircraft during the rest of its trajectory through the radar coverage.

         Selective interrogations will be used:

(a)     to update the horizontal position of the aircraft;

(b)     to request the aircraft to not reply to the All Call interrogations specifically transmitted by the radar. This is known as lockout command;

(c)      to request additional information such as Mode A code and altitude and

(d)     to request further information to be downlinked from specific aircraft transponder registers such as the Aircraft Identification.

(2)     Determination of the aircraft surveillance system capability

         Ground surveillance system will need to establish the capabilities of the aircraft surveillance system to extract information only if it is available in the aircraft surveillance system. If this is not done, it could result in a situation where the aircraft would no longer reply to the interrogations used by the radar, and, therefore, the position of the aircraft could be lost. Hence, there is a need to have correct reporting of the aircraft surveillance system capability.

         This process starts by determining whether the transponder is level 2 or above by checking the CA field of the Mode S All Call replies. The CA field is encoded with either 4,5,6,7 to indicate that the transponder is a level 2.

         If the transponder is a level 2 or above transponder, the second step of the process is the verification of the data-link capability provided in register 10₁₆, the ‘Data link capability report’. It contains different information about the data link capability of the airborne surveillance system

         Elementary Surveillance System will use important information from this register, including:

(i)      Aircraft Identification capability (bit 33 of register 10₁₆) to determine the availability of the register containing the Aircraft Identification;

(ii)     Surveillance Identifier code (bit 35 of register 10₁₆) which indicates if SI protocol can be used to lockout the transponder; and

(iii)     the Mode S Specific Services capability (bit 25 of register 10₁₆) which indicates that Mode S specific services; including additional registers used for enhanced surveillance; are supported; and that the particular capability reports should be checked.

If the ‘Mode S Specific Services’ bit is set in register 10_16, the availability of other registers will be checked by extracting register 17₁₆.

(3)     Extraction of Aircraft Identification using Mode S protocol

         Aircraft equipped with Mode S having an aircraft identification feature transmits its Aircraft Identification as specified in Item 7 of the ICAO flight plan, or when no flight plan has been filed, the aircraft registration.

         Aircraft Identification information will be obtained by Mode S radar by extracting the transponder register 20₁₆ at the track initialisation.

         The Aircraft Identification is variable when it changes from one flight to another flight. It is, therefore, possible that input errors may occur. Whenever it is observed on the ground situation display that the Aircraft Identification transmitted by a Mode S-equipped aircraft is different from that expected from the aircraft, the flight crew will be requested to confirm and, if necessary, re-enter the correct Aircraft Identification.

         When Aircraft Identification is modified, the transponder will indicate this change for 18s in its selective replies. This is done using the Mode S Comm-B Broadcast protocol (ICAO Annex 10 Volume IV 3.1.2.6.11.4). The Mode S ground station will extract the Comm–B Broadcast message to obtain the new value of the Aircraft Identification.

(4)     Extraction of Mode A code using Mode S protocol

         Ground Mode S surveillance system will extract Mode A code at track initialisation.

         If the Mode A code is modified, the transponder will indicate this change for 18s in its selective replies. This is done by raising an alert bit which is set for 18s after the change. Once this alert is detected, the Mode S ground stations will extract the new Mode A code.

         It is, therefore important, that the change of the Mode A code happens on the active transponder which is announcing the change for 18s.

         Note: ED-73E contains additional requirement requiring the announcement of a Mode A code change when a transponder becomes active. This is not necessarily available on older Mode S transponders in which it may be necessary to follow a specified procedure on installations with no common control interface. In some instances, a ground system workaround, consisting of periodically extracting the Mode A code, has also been implemented.

(5)     ACAS Resolution Advisory (RA) report extraction

         When a resolution advisory has been produced, the transponder announces the presence of a ‘RA report’ for the time that the RA is active until 18s after it has ceased. The Mode S ground stations will extract the register 30₁₆ to obtain the information

(6)     Summary of registers used for ELS

         Register 10₁₆ to obtain information on data link capability of the airborne surveillance system.

         Register 17₁₆ to obtain information on additional services available. For ELS, it is possible that register 17₁₆ is empty (=0).

         Register 20₁₆ to obtain the Aircraft Identification.

         Register 30₁₆ to obtain the RA Report

(7)     Information on Mode S replies used to support ELS

         The following Mode S reply types are used to track the aircraft and obtain additional data:

         DF11: Mode S All Call replies containing the 24-bit Aircraft Address and the CA field indicating whether the transponder is level 2 or greater and whether the aircraft is on the ground or airborne. DF11 can also be spontaneously transmitted as acquisition squitters. These replies are used for aircraft acquisition.

         DF4: Short Mode S reply containing Altitude information.

         DF5: Short Mode S reply containing the selected Mode A code.

         DF20: Long Mode S reply containing the Altitude information and the content of the transponder register requested.

         DF21: Long Mode S reply containing the Mode A code and the content of the transponder register requested.

[Issue: CS-ACNS/4]