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3.2.2.6 Receivers
The receiver system is responsible for the amplification of the weak signals received from
the antenna. Virtually all radar receivers operate on the double superhetrodyne principle.
Most of the gain and selectivity of the receiver is implemented in the intermediate
frequency amplifier. The low frequency of the IF amplifier makes it easier to achieve good
pass band characteristics. In primary radar, the IF pass band characteristics are not only
matched to the bandwidth of the transmitted pulse, but the phase characteristics are also
matched to achieve the optimum signal to noise ratio. Radar system bandwidth is also
covered in section 3.2.5.13.
Note that because the RF bandwidth is quite large, receiver front-end saturation and/or
desensitisation plays an important part in determining the impact of interference.
Also supported by theoretical and experimental findings is an extreme sensitivity of
detection probability to changes in the radar receiver’s signal to noise ratio (S/N), in this
case caused by adding interference signal power to inherent receiver noise. This is
covered in more detail in the section on band sharing.
3.2.2.7 Signal and Plot Processing
The basic function of a signal processor/plot extractor is to maximise the detection of
wanted targets in the presence of unwanted ground and weather returns. This is achieved
by the analysis of the Doppler content of the received signals. Ground returns have little
or no Doppler content and are rejected in favour of aircraft returns. Similar processes are
applied to weather returns although fast moving weather may cause false alarm problems
in the more basic type of signal processor.
The output of the Doppler filter is normally put through a detection threshold process to
control false alarms. The nature of this thresholding process and the ability of the system
to discriminate against non- synchronous pulse interference is a very important factor
when evaluating the feasibility of band sharing. This is covered in more detail in section
3.2.7.3.
The final stage is to determine target position from the hit pattern (plot extraction). This
process involves correlating the hit patterns in range and azimuth to determine the target
position. The type of Doppler filtering and the threshold process vary widely from one
type of radar to another.
3.2.2.8 Radar Data Processing and Display
The processes outlined so far are generally carried out at the radar sensor. The target
plots (often combined with secondary radar data) are then transmitted to the air traffic
control centre or airport. The plots are then further processed into tracks (=correlated
plots) for display to the operational controllers. Again the approach varies from one
system to another.
3.2.2.9 Technology Summary
It can be seen that the primary radar chain from sensor to the controller’s display is
designed to optimise the performance of a given type of radar. It is the bespoke nature of
primary radar systems that makes generalisations of performance under differing
conditions difficult to predict.
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