Radar surveillance can be improved through the use of narrow beams, but this may lead to there being more beam positions to be searched than is possible in the time available. Multibeam systems can help with this problem, but they put more pressure on the data processing activities, which are often already stretched, even with today's technology. The RCS of real targets fluctuates and its statistical nature must be taken into consideration if the radar detection performance is not to be overestimated. The problem of clutter and the need for sub-clutter visibility is often severe and leads to a need for doppler processing. Careful design of the transmitted waveform is needed to avoid range and/or doppler ambiguities.

Target detection is a probabilistic idea; noise and clutter prevent us from being certain to find the targets we are looking for, and will normally present us with plenty of 'targets' we are not looking for. We can only define the probabilities of detection and of false alarm that we are prepared to live with. These determine the signal-to-noise ratio that is required for detection. Optimal detection performance is achieved by maximizing the SNR at the output of the receiver. The receiver that does this uses the correlation or matched filter principle. As long as a matched filter is being used, the form of the transmitted pulse is irrelevant for detection purposes. All that matters is the ratio of the signal energy to the noise power per unit bandwidth on input to the receiver. By developing expressions for the PDFs of signal plus noise and noise only, relatively straightforward calculations of detection and false-alarm probabilities can be carried out for single pulses. These become more complicated when fluctuating targets and multiple pulses need to be considered, so that graphical or numerical techniques are needed.

Detection performance is optimized and range and doppler both improved by maximizing the SNR. Since this needs the pulse to have high energy, long pulses are required unless a very high power transmitter is available. Long pulses also permit good doppler accuracy, but, for simple pulses, give poor range accuracy. The system designer can avoid this apparent dilemma by using phase-modulated pulses. These can be constructed to give the large bandwidth that, after pulse compression, leads to good range accuracy. Whatever transmitted waveform is chosen, the resolution constraints imposed by the ambiguity function cannot be escaped. All that the designer can do is to attempt to move the significant areas of ambiguity into regions of the range-doppler plane where targets are unlikely to be present.

At low elevation angles, the propagation of radio waves is degraded by both atmospheric effects and scattering by the terrain. For many general-purpose surveillance requirements, the four-thirds earth approximation is a sufficiently good correction for atmospheric refraction (the strongest effect) and the other problems are ignored. When precision target tracking is required, all atmospheric effects have to be considered, and appropriate corrections made, based on local climatic information. The electron content of the ionosphere also causes attenuation and refraction effects, and a rotation of the plane of polarization. Radio waves passing through the ionosphere may suffer a form of scintillation that can affect space-borne earth-imaging radars.

There are two forms of 0TH radar, surface-wave and skywave. Surface-wave systems are relatively inexpensive and have found applications in sea sensing (see next chapter), for the defence of localized areas against low-flying missiles and to some extent for monitoring ship traffic. Skywave radars are used to monitor very large areas of land and sea to search for air targets, ballistic missiles during launch phase and some types of surface target. They also have remote sensing capabilities, especially storm tracking and ocean wave monitoring. These radars are large, powerful, expensive and require sophisticated frequency management systems in order to operate via the ever-changing ionosphere.

Ground-probing radar, also known as subsurface radar, is becoming an important subject. The number of applications for ground-probing radar is growing and the technology is beginning to be employed in other radar fields; for example, impulsive or carrier free radar, which is in widespread use for ground probing, is now being developed by the military for its anti-Stealth capabilities. Ground-probing radar is one of the few ways of inspecting geological features and locating hidden objects and structural flaws. The technique is not new, but it is only since the advent of fast digital processing and microprocessors that sufficient low-cost signal and data processing has been available to solve many of the problems raised by this type of radar.

The very success of radar at detecting and tracking military targets means that it has become the target of electronic attack itself. As soon as a military radar system is turned on, its EM emissions will be detected unless it uses spread spectrum LPI techniques to evade detection. Any radar signals detected will be analysed by ESM receivers and an ECM strategy formulated; this may include the use of Stealth to reduce the RCS of the target. While electronic countermeasures are effective, and expensive to defend against using ECCM techniques, the balance at present probably resides on the side of radar, provided the waveforms are controlled intelligently.

The future of radar does not lie in larger and more powerful systems, but rather in slightly smaller systems that are more agile, intelligent and difficult to detect because of the larger bandwidths that will be used. The resolution of radars, and the number of targets that can be tracked, can be expected to increase as large amounts of low-cost computer power become available. The factor limiting radar performance is likely to remain technology (rather than any law of physics) for some time to come. Until recently, it was the ability to process the large amounts of data that created an upper limit to performance, but with the advent of parallel processing the weakest link in the chain is almost certain to become the A/D converter because of the problem of increasing the sampling rate while maintaining a high dynamic range. The recent trend of moving the point at which the A/D conversion takes place further and further up the receiving chain towards the antenna only exacerbates this problem. Radar seems certain to provide challenging engineering, mathematical and computational problems to be solved for years to come. We hope that this book has conveyed the main ideas and helped you to understand the underlying principles of radar. We hope also that you have gained an appreciation of the importance of radar in many diverse areas, and sensed some of the excitement of working in this field.

Appendix 2 provides an index of acronyms and abbreviations used within the text.

Decibels can be one of those little things that you get a mental block about. It is not that there is anything difficult about the maths, but rather the concept of handling voltage and power ratios in this way. However, it is worth learning to use decibels, because they make life so much easier for the radar engineer.