Communications, Radar & Electronic Warfare (201.. - Index of

126 Communications, Radar and Electronic Warfare7.3. 11 Secondary RadarBefore completing the section on radar systems, it is necessary to consider aslightly different form of radar that is in use throughout the world. This is known assecondary radar and it works on quite different plinciples to that of the other radars wehave so far considered. Secondary radar is used with primary radar for air traffic control.However, instead of receiving a reflection of the airbome target, the purpose of the radartransmission is to tJigger a pre-coded response (known as a 'squawk' ). In the case ofcivil air traffic control, the secondary radar pulse leads to a retum message consisting ofthe aircraft identity and some flight parameters. In military terms, the system can beused in an IFF (Identify, Fliend or Foe) system. In the military case, the aircrafttransponder responds with a code identifying it as friendly. If no response, or a falseresponse, is made, then the illuminated aircraft is considered an enemy.The basic process is shown in Figure 7.26. The ground-based radar (in this case)sends out the intenogative pul se. When this is received by the transponder on the- -- - TransponderactivatedSecondary radar transmit phase[1'- « . .... , .. .f- .......... ..der return ............. .lra!.'??!.' . .... . . . . . · ··.... . . ·(j.TargetSecondary radar receive phaseFigure 7.26 Illmtration of secondary radar operation. The radar transmits on a given frequency. Theaircraft receives this transmission and it triggers the onboard transponder to transmit. This transmission isreceived and proce%ed by the receiving radar antenna.