2.2.6 Frequency Agility 频率捷变

From one coherent processing interval to the next, the radar can change its carrier frequency without changing its range resolution properties. This type of wideband signal is called frequency agility. The agility band is limited by the designer’s ability to obtain sufficient transmit power and to maintain the antenna beam width and pointing angle over that band. Typical designs have an agility band of about 10% of the center frequency. (For example, a 1 GHz agility band centered at 10 GHz.) What this means for ELINT is that narrowband receivers have a low probability of intercepting the complete radar transmission. If it is sufficient to intercept only portions of the radar transmission, narrowband receivers can be slowly tuned across the radar band and the entire agility band can still be determined if the signals is present for enough time. If not, then a wideband ELINT receiver is needed. These may be of the channelized type with sufficient channels to cover 500 to 1000 MHz at once. Frequency agility (FA) is used primarily to make the radar signal more difficult to recognize and to jam, but it also provides a way to reduce fluctuations of target echoes by, in effect, averaging the observed radar target cross section over several different frequencies. Depending on ELINT system architecture, it may be useful to have sufficient instantaneous bandwidth in the ELINT receiver to cover the entire frequency agility band at one setting of the receiver. The FA bandwidth does not affect the radar’s range resolution. It does provide enhanced target detection because the target’s cross section is examined at several frequencies. Note also that at present the center frequency is not changed during the coherent processing interval or over the time that Doppler processing is performed. That is, if more than one pulse is used for Doppler processing, which is normally the case, the center frequency or RF carrier frequency cannot be changed for the length of the processing interval. And the processing interval determines the Doppler resolution. When FA is used with Doppler processing, the frequency is changed on a pulse-burst to pulse-burst basis, not a pulse-to-pulse basis.

本节内容主要涉及以下几点：

1.       目前典型捷变雷达的频率范围为中心频率的10%。

2.       对于频率捷变雷达，如果我们有足够多的时间，窄带接收机可以将不同频段的信号特征都记录下来，从而还原出整个信号；否则，只能考虑采用足够带宽的宽带接收机。

3.  在一次相干处理过程中雷达中心频率并不发生改变。

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