Aircraft Systems

5Primary and Secondary Radar

5.1 Primary Radar

Primary radar range finding is based on measuring the time between a transmitted pulse in the UHF or SHF band and the reception of the reflected pulse from the target. After each pulse is transmitted, the antenna is switched to receive mode to detect the return signal. In the case of primary radar the return is a direct reflection of the transmitted pulse and so the carrier within the reflected pulse is at the same frequency as the transmitted one. Another mode known as secondary radar is also used where the transmitted pulse triggers a transponder at the target, which then transmits its own pulse or set of pulses in return that are of different frequency. The timing between the transmitted and return pulses gives the distance of a target and the transmitted beam is directional so that the vector to the target is also known. Additional information can be returned from the target in the case of secondary radar, which is described in Section 5.4. In this section, the focus is on primary radar.

The transmitter and receiver share the same directional antenna and as discussed in Section 4.4.8, directional antennas in the SHF band are either parabolic dishes, usually with waveguide feeders (Figure 5.1a), or slotted waveguide arrays (Figure 5.1b) with the latter type commonly installed in the nose of aircraft as the antenna for weather radar. Both types of antenna emit a directional beam whose beam width is defined as the angular range ...