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– Contract (ADS-C), Automatic Dependent Surveillance – Broadcast (ADS-B), and Wide Area Multilateration (WAM). These new surveillance methods aim at the same goal: the fulfillment of the aircraft identification and position reporting function in areas where the installation of radar is not cost-effective.
Regardless of the technology, a surveillance method may be either:
- Dependent: The aircraft sends its position to the ground station, or
- Independent: The aircraft does not send any position data to the ground station. The ground station calculates the aircraft position, or
- Cooperative: The method requires an active system onboard the aircraft, or
- Non-cooperative: The method does not require any system onboard the aircraft.
Independent  Dependent 
Non-cooperative  PSR 
Cooperative  SSR – Mode A, C, S WAM  ADS-C, ADS-B 

This chapter reviews all the surveillance methods available for the aircraft identification and position reporting function.

2.1. DESCRIPTION OF TRANSPONDER
The aircraft identification and position reporting function requires a transponder onboard where a
Secondary Surveillance Radar (SSR1) is in operation. The SSR interrogates the transponder on 1030 MHz and waits for a reply from the transponder on 1090 MHz (refer to Figure 2-1). Based on this principle, the SSR operates in three different modes: Mode A, Mode C, and Mode S.
The interrogation mode determines the reply content. For instance, when the ground station interrogates an aircraft for its altitude, the transponder replies in Mode C. Other modes exist (Modes 1, 2, 4 and 5) but they are used in military aviation only.
Surveillance method based on SSR is:
- Cooperative as the SSR interrogates the airborne transponders to identify the aircraft
1 In some documents, the reader may find the acronym ATCRBS for Air Traffic Control Radar Beacon System. It designates the SSR.

- Independent as the SSR calculates the horizontal2aircraft position (i.e. bearing) according to the signal from the transponder reply.
The Primary Surveillance Radar (PSR) is used for military purposes, as it detects any vehicles that reflect the radar signal, and for civilian purposes coupled with an SSR. The surveillance method using a PSR is then non-cooperative and independent.
Most readers would certainly know what are behind Mode A and Mode C, as they have operated these modes for years. But, recently the reader may have heard about the Mode S and various designations (e.g. ELS, EHS, extended squitter, ADS-B). The following paragraphs provide the reader with a clear view of the different modes.
2.1.1. MODE A
When a SSR interrogates a transponder in Mode A, the transponder replies with the aircraft identification (SQWK code – also called Mode A code – on four octal digits from 0000 to 7777). The SQWK code format permits 4096 different codes.
　
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