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3.3.3 Technology Description
3.3.3.1 Introduction
Secondary Surveillance Radars consist of a ground interrogator and an airborne
transceiver (which detects the interrogation and produces a reply that is synchronous with
the interrogation). All SSR installations operate on 1030MHz for the ground-air
interrogations and 1090MHz for the air-ground reply. The use of different frequencies for
uplink and downlink ensures that any reflections or echoes to the interrogation signal will
not interfere with the aircraft’s reply. This overcomes one of the main technical limitations
of PSR, and explains why SSR has a much better performance than PSR. However, note
that unlike primary radar, SSR is a dependent system (reliant on airborne equipage of
transponders), and therefore cannot ‘see’ unequipped airborne traffic.
The interrogation and control transmissions (on 1030MHz) must have tolerances within
0.2MHz of the carrier frequency for Mode A/C and ± 0.1MHz for Mode S. The frequency
tolerance for the reply transmission shall be ±3MHz for Mode A/C and ±1MHz for Mode S.
SSR is used as a stand-alone system, or co-located and synchronised with primary radar
(particularly in en-route areas, where all the UK’s primary en-route radars are co-located
and frequency paired with SSR). It is used in en-route, TMA and on the airport surface
(normally as part of a multilateration solution).
3.3.3.2 Technical Description
The ground interrogator transmits a signal, which is responded to by the aircraft’s
transponder. The ground then measures the round trip delay of the received signal, and
extracts data from the aircraft transponder reply. Azimuth is measured from the position of
the rotating radar antennae.
Figure 3-6 below shows a basic schematic for the operation of SSR.
Figure 3-6: Secondary Surveillance Radar, showing the pulse data transfer
The interrogations for Mode A/C can be seen in Fig 3-7 – P1 and P3 are the interrogation
pulses, with P2 being the control pulse. The P2 pulse is used for sidelobe suppression –
the P2 pulse is constant, and thus other pulses can be measured against it to ensure that
the target will only reply when in the main P1/P3 beam. If the P2 pulse returns a higher
signal level than the P1 or P3 pulse, the target is outside the main beam and will not
reply.
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