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operational requirement. However, the narrower the beam the larger the sidelobe levels
become. Sidelobes constitute unnecessary illumination and need to be minimised for
radar performance reasons. Strong targets (=large aircraft) will be seen by the main beam
as well as the sidelobes resulting in multiple detections of the same aircraft at different
azimuths (if we are considering the azimuth sidelobes). Therefore, in terms of antenna
design, the radar performance objectives coincide with the requirements of optimum
spectrum utilisation. Modern primary radar antenna designs utilising either conventional
antenna designs or phased arrays represent the state of the art in terms of two
dimensional radar systems.
Three dimensional radar systems using phased arrays, which have electronic beam
steering under computer control in three dimensions, offer the possibility of confining the
illumination to the specific areas occupied by the target (noting that a sweep of the full
coverage volume is required to initially acquire targets). Such systems are technically but
not operationally proven and would be very costly to implement across the radar
infrastructure. In view of the benefits to radar performance and the spatial aspects of
spectrum efficiency, the application of 3D (phased array) systems to ATC surveillance
radar should be considered as a research topic.
3.2.5.13 Filters and Bandwidth Requirements
The subject of bandwidth requirements and filters is complex. The bandwidth of every
radar is different depending on pulse width, pulse rise-time, pulse compression frequency
deviation, type of output device, frequency diversity, multi-pulse working etc. Therefore,
the bandwidth of every type of radar must be calculated individually for each of the 3dB
bandwidth, the necessary bandwidth (-20dB) and the out of band limits (-40dB).
In general the introduction of filters at any point in the transmit/receive chain with band
pass characteristics less than the spectrum of the transmitted waveform introduces
distortion to the pulses. This distortion can take the form of time sidelobes, which are
additional pulses either side of the desired pulse. These time sidelobes can cause false
alarms because they have similar characteristics to the basic pulse.
Time sidelobes can be introduced by the filter required by pulse compression (see Figure
3-2) or by filters which are designed to minimise the transmitted spectrum.
Filters which minimise the transmitted spectrum are designed to reduce out of band and
spurious transmissions. Such filters can be useful in reducing the OoB and spurious
emissions of magnetron systems. Typically, a low pass filter could be used at the 40 dB
bandwidth to provide compliance with the 20dB/decade roll off characteristic defined in
Figure 3-3. Application of filters around the necessary bandwidth (20dB) is likely to result
in the introduction of time sidelobes. In general, a band pass filter can be applied based
on the 40dB bandwidth of the pulse without serious pulse distortion, time sidelobes or
impact on radar range performance to meet the unwanted emission mask.
3.2.5.14 Statistical Analysis
The use of statistical techniques in relation to the evaluation of band sharing opportunities
has been considered by ITU Radiocommunication Study Group document 8B/28-E –
“Use of Statistical and Operational Aspects in the Radar’s Protection Criteria”
This document outlines the use of a particular statistical approach to determine what
constitutes an unacceptable degradation of the operational performance of a radar
system in the presence of short term interference. The paper first makes some illustrative
assumptions about the processing techniques included in the whole radar chain including
the controller’s radar processing and display system. It then considers the effect of
permanent interference on the probability of detection. This model is then extended to
consider the effect of sporadic interference. The conclusion is that under the assumed
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