Mode S Technical Overview

SSR Mode S relies on a unique ICAO 24-bit aircraft address for selective interrogation of an individual aircraft. 16,777,214 aircraft addresses are allocated in blocks by ICAO to the state of registry, or common mark registering authority, for assignment to aircraft according to their country of registration.

Mode S transponder equipped aircraft must also incorporate an Aircraft Identification feature to permit flight crew to set the Aircraft Identification, commonly referred to as Flight ID, for transmission by the transponder. The Aircraft Identification transmission must correspond with the aircraft identification specified in item 7 of the ICAO flight plan, or, when no flight plan has been filed, the aircraft registration.

In addition to the downlinking of Aircraft Identification, which is a prerequisite for Mode S Elementary Surveillance (ELS), other specified downlink parameters (DAPs) may be acquired by the ground system to meet the requirements of Mode S Enhanced Surveillance (EHS).

The Mode S system requires each interrogator to have an Identifier Code (IC), which can be carried within the uplink and downlink transmissions (1030/1090 MHz). Responding aircraft transponder identification is achieved by acquiring the unique ICAO 24-bit aircraft address. Figure 1, below, provides an illustrative guide as to how Mode S works.


Backwards Compatibility with Mode 3/A/C

When compliant with ICAO SARPs, a Mode S transponder will always reply to SSR Mode 3/A interrogators and thus only one type of transponder needs to be carried by an aircraft. This is pertinent to aircraft, mandated to equip with Mode S transponders, which still need to operate in non-Mode S airspace.

Transponder Interrogator Code Supportability

In accordance with ICAO SARPs, Mode S transponders are to be capable of supporting both Interrogator Identifier (II) and Surveillance Identifier (SI) codes.

Originally ICAO SARPs provided for a 4-bit Interrogator Identifier (II) Code. This permitted only 15 II Codes to be available for operational use. However, amendment change 73 to ICAO Annex 10 resulted in an additional 63 codes being made available in the form of Surveillance Identifier (SI) Codes. It is essential in Europe, particularly for the maintenance of civil/military interoperability, for aircraft to be equipped with Mode S transponders that support both II and SI code functionality.

A scheme for the central allocation of Interrogator Codes is administered by EUROCONTROL on behalf of the ICAO European and North Atlantic Office. The process is incorporated within the ICAO EUR-Region Air Navigation Plan.

Vertical View display

Use of the unique 24-bit aircraft address overcomes miss-association problems for aircraft tracks that overlap in the horizontal (azimuth) plane and allows them to be presented separately in terms of their height. Adaptation of HMIs to provide this additional vertical resolution and display enables the effective monitoring of aircraft vertical separation in situations where this could not previously be performed using horizontal (azimuth only) presentations (e.g. with operation of stacks where aircraft tracks overlap horizontally). This is depicted in the illustration below.

Vertical view display (provided courtesy of NATS, UK)

Interrogator Methods

A Mode S sensor has two methods of interrogation: All-Call and Selective. All-call interrogations are transmitted regularly at a steady rate in a similar way to conventional SSR. Any Mode S transponder that is not ‘locked out’ will reply to an all-call interrogation, transmitting its unique 24-bit aircraft address. In this way, the interrogator acquires targets not previously detected.

Once a transponder is known to the interrogator and its track has been established, it can be ‘locked out’. This prevents the transponder from replying to any more all-call interrogations from that or any other Mode S sensor with the same identifier code, and it will then only respond to Selective interrogations. However, it will continue to respond to interrogations from other Mode S sensors with a dissimilar Identifier Code and also to Mode A/C sensors.

Lockout is a new concept for SSR and is one of the major factors in radically reducing reply rates and, thus, reducing interference. To prevent the potential for undesirable, uncontrolled lockout of targets, a number of safeguards have been built into the international standards for both interrogators and transponders to ensure that lockout is handled in a fail-safe manner.

Selective interrogations make use of the unique 24-bit aircraft address and can be sent out close to the azimuth where the aircraft is expected to be. No other aircraft that happens to be in the radar beam at that time will reply. The aircraft addressed will reply with its Mode A code (assuming one has been assigned), aircraft identification and altitude. The type of reply is controlled by the interrogator, but in either case only a single reply is required because there is no ambiguity as to which aircraft the reply belongs. Extra interrogations can be made to ensure that at least one reply is received and that the azimuth performance is maintained.

Mode S selective principle

Aircraft EHS Capability

Fixed wing aircraft that can provide the list of 8 Downlink Aircraft Parameters (DAPs) displayed in the following table are considered to be Mode S EHS capable. [Note: Where the parameter ‘Track Angle Rate’ cannot be provided, ‘True Air Speed’ should be used instead.]

Aircraft parameters