Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC), that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude.

Unlike primary radar systems that measure only the range and bearing of targets by detecting reflected radio signals, SSR relies on targets equipped with a radar transponder, which replies to each interrogation signal by transmitting a response containing encoded data.

SSR is based on the military identification friend or foe (IFF) technology originally developed during World War II and therefore the two systems are still compatible.

• The secondary Source radar identifies a flying objects in the air space, with SSR code
• A civil air traffic control operators then identifies the unknown flying object, and associates the SSR code with the appropriate civil flight number
• The association then has to be served for military air surveillance as soon as possible. This means in real-time.

 The flight associations are available in the civil air control system in form of:

• Online Data Interchange (OLDI) transit messages from other air traffic control systems
• New flight associations created by the operator of the air traffic controller workstation

 The custom made information system developed by R-Systems Ltd

• extracts the necessary data (1-2% of all of the data)
• composes the messages according to the Online Data Interchange (OLDI) format, a standard by European Organization for the Safety of Air Navigation.
• combines OLDI messages with the Aeronautical Fixed Telecommunication Network (AFTN) messages from flight plans
• transmits everything to the military air surveillance system

• The FlightWatch Server
• The link to Air Traffic Control center
• The Airport Dispatcher Workstation
• The Radar and Flight Data Monitors
• Noise Monitoring System

For linking the flights with radar data the FlightWatch system exploits the associations between the radar code and the flight plan created by air traffic controllers. The flights are determined according to the flight CallSign. The active flights table is created, where the flight plan data is collected and relevant changes are made. The active flights table is supplemented with the secondary source radar (SSR) code after the air traffic controller has made the appropriate association.

The result is displayed int workstations on cartographic background.

The Radar Filter reads signals from radar, analyses and filters them according to the pre-determined conditions – limited areas and height ranges.

The transmission of the radar data (both reading and forwarding) is based on the ASTERIX (All Purpose Structured Eurocontrol Standard for Radar Information Exchange) format -, which is defined by the standard document Eurocontrol Standard for Radar Data Exchange. 

The filtering conditions of the radar data are determined by a filtering algorithm and its changeable configuration.

The Radar Filter processes and releases only ASTERIX CAT 001 radar plot messages described in the previously mentioned standard sub-document Part 2a - Transmission of Monoradar Target Reports. All other messages, though received, are not released on any conditions.

The filtering of radar messages is carried out on the basis of:

• 
  geographically predefined areas
• 
  height ranges
• 
  message types
• 
  data field selection

There may be several areas and height ranges and the filtering conditions can be combined. Both the input and the output of the Radar Filter are unidirectional. No data is sent to the radar and no data is received from the output port. In addition to the software level the uni directionality of the data flow can be supported on hardware level (cabling).