FINAL REPORT - Stakeholders - Ofcom

signals which populate the range cell samples. A simple example of this type of detector
is shown in Figure 3-4.
Input
Figure 3-4: Constant False Alarm Rate Detector
The detection threshold is designed to maintain a constant false alarm rate with respect to
the back ground noise level, which has essentially homogeneous Rayleigh statistics. If
the interfering signal does not have these characteristics, then the false alarm rate is
likely to rise. Furthermore if the interfering signal fills the range cell samples then the
threshold will rise, degrading the probability of detection.
3.2.7.4 Interference to Noise Ratios
In the specialist community, there are differences of opinion regarding the impact of band
sharing proposals and the protection levels for primary radar are still under discussion.
Primary radar system performance is very susceptible to changes in signal to noise ratio
as is illustrated in Figure 3-5.
Probability
of
detection
.95
.8
.5
.2
M taps in each
delay line
Taped Tapped delay delay line line
Sum
Test cell cel
Sum
Non fluctuating
Taped Tapped delay line line
10-4 10-8 10-12 10-4 10-8 10-12 10-4 10-4 Sum
10-8 10-8 Video (range under
test)
10-12 10-12 Comparator
x K
Probability of false alarm
10 12 14 16 18 20 22
Signal to noise ratio
Output
Detection
Threshold
Swerling
1
Page 53