2.2 Radar Constraints

ELINT signals of interest include radar signals of all types. Sometimes, people concerned about ELINT attribute properties to radar signals that are contrary to the constraints under which radar systems must function. Avoiding this pitfall is an important aspect of ELINT work. Understanding the fundamental limitations faced by radar designers and the associated ELINT implications is important. Consider this statement: Radars of the future could transmit noise waveforms over gigahertz bandwidths and the undetectable by ELINT receivers. Should ELINT equipment be developed to intercept and process this kind of signal? Probably not – because signals like this would not be useful for tracking or search radars in military applications. Assume that the technology for transmitting noise waveforms over wide (> 1 GHz) bands and the digital signal processing (DSP) hardware to store and correlate the transmitted waveform with the echoes received is available. Will such radars be deployed? The answer depends on the job the radar is designed to do and the characteristics of the target and target motion it must detect or track. To be “undetectable” by ELINT means that the power must be low. After all, a high power noise-like waveform would be detectable like the signal from a jammer. Low peak power means the radar must use a long integration time in order to get sufficient energy back from the target. Long integration time may restrict the radar’s ability to keep up with target motion (the target could move out of a range or Doppler cell if the integration time is too long). Transmitting a wide instantaneous bandwidth means that the radar has fine range resolution. If the range cells are smaller than the range extent of the target, the target echo will be spread across many range cells (a bandwidth of 1 GHz provides range cells every 15 cm). This reduces the energy received in a single range cell. For example, if one wanted to recover all the energy from a target 15 m long, it would be necessary to coherently add the energy of 100 range cells. Therefore, the ability to transmit and receive a particular waveform does not by itself make it a good choice for a radar designer. ELINT work must be done with these factors in mind.

对于这一节的题目“Radar Constraints”，如果直接翻译是很难理解的，当仔细读完这一段文字，作者想明确的仍然是一个反复强调的问题：ELINT接收机的设计者应该充分了解雷达的设计，而雷达的设计者也必须充分了解ELINT接收机的设计。作者提出了这样一个问题：是否有必要设计一种用于截获宽带噪声雷达信号的ELINT接收机？作者没有肯定也没有否定，但作者并不认为噪声雷达具有特别的LPI能力。如果这样的雷达信号不能够被ELINT接收机截获，那么它是否能够完成指定的目标探测任务？

总之，任何较量的胜利都只是暂时的，不要期望一劳永逸。