圖2.14描述了頻率捷變導(dǎo)致的RCS變化問題。

Figure 2.14illustrates the ability of frequency agility to force RCS variations.

Figure 2.14. 從固定不變的視角觀察，頻率捷變帶來的RCS變化Variation in RCS due to frequencyagility for a constant viewing angle. See text for details.

對于具有20個(gè)5m x 10m隨機(jī)散射體的目標(biāo)，從固定的大約54°視線角進(jìn)行觀察，則有效深度大約為10sin(54°) = 8.1 m。

A 20 scatterer, 5 m by 10 m random target was observed from a fixed aspectangle of about 54°, making its effective depth approximately 10sin(54°) = 8.1 m.

如果每個(gè)脈沖的工作頻率都相同，那么目標(biāo)RCS和接收到的回波功率應(yīng)該完全相同。

If the same RFfrequency was used for each pulse, the RCS and thus received power would beexactly the same on each pulse.

然而，在上圖中兩個(gè)相鄰脈沖的RF頻率是按照18.5MHz步進(jìn)的（根據(jù)式（2.63）計(jì)算），其起始頻率為10.0GHz。

However, in this casethe RF frequency was increased by 18.5 MHz [calculated from Eq. (2.63)] fromone pulse to the next, starting at 10.0 GHz.

不同頻率點(diǎn)之間的相對RCS波動(dòng)約為38dB，線性值約為6300。

The resultingrelative RCS measurements vary by 38 dB, a factor of about 6300.

在第六章中可以看到，當(dāng)連續(xù)的目標(biāo)測量之間互不相關(guān)時(shí)，某些情況下的檢測性能將會(huì)得到改善。

It will be seen inChap. 6 that in certain cases detection performance is improved when successivetarget measurements are uncorrelated.

因此，一些雷達(dá)采用頻率捷變技術(shù)來實(shí)現(xiàn)連續(xù)測量數(shù)據(jù)之間的解相關(guān)(Ray, 1966)。

For this reason, someradars use a technique called frequency agility to force decorrelation ofsuccessive measurements (Ray, 1966).

在這一過程中，相鄰脈沖之間的雷達(dá)頻率以ΔF Hz或更高的頻率遞增，其中ΔF根據(jù)式（2.63）計(jì)算，從而確保相鄰脈沖的目標(biāo)回波之間的測量解相關(guān)。

In this process, theradar frequency is increased by ΔF Hz or more between successive pulses, whereΔF is given by Eq. (2.63), ensuring that the target echo decorrelates from onepulse to the next.

一旦獲得期望的不相關(guān)測量次數(shù)，則下一組測量將循環(huán)使用設(shè)定好的頻率增量周期。

Once the desirednumber of uncorrelated measurements is obtained, the cycle of increasingfrequencies is repeated for the next set of measurements.

式（2.63）是基于高度簡化的目標(biāo)模型以及相關(guān)間隔存在的假設(shè)前提。

Equation (2.63) isbased on a highly simplified target model and an assumption about whatconstitutes the correlation interval.

不同的相關(guān)間隔定義，例如通過相關(guān)函數(shù)首次下降到其峰值的1/2或1/e的點(diǎn)來定義，將導(dǎo)致對消除目標(biāo)相關(guān)所需角度或頻率變化的較小估計(jì)值。

A differentdefinition, for example defining the interval by the point at which thecorrelation function first drops to 1/2 or 1/e of its peak, would result in asmaller estimate of the required change in angle or frequency to decorrelatethe target.

此外，許多雷達(dá)是對回波幅度的平方進(jìn)行處理的，而不是以上推導(dǎo)過程中假定的幅度值。

Also, many radars operateon the magnitude-squared of the echo amplitude, rather than the magnitude ashas been assumed in this derivation.

平方律檢測器將產(chǎn)生與式（2.60）的平方成比例的相關(guān)函數(shù)（Birkmeier和Wallace，1963）。

A square law detectorproduces a correlation function proportional to the square of Eq.(2.60) (Birkmeier and Wallace, 1963).

因此第一零點(diǎn)的位置仍然是在相同的ΔKθ值處，上述結(jié)論仍然是適用的。

The first zerotherefore occurs at the same value of ΔKθ, and the previousconclusions still apply.

但是，如果使用了相關(guān)間隔的不同定義（例如以50%作為去相關(guān)點(diǎn)），則平方律檢測所需的ΔKθ變化將小于線性檢測器所需的變化。

However, if adifferent definition of the correlation interval is used (such as the 50percent decorrelation point), the required change in ΔKθ is less forthe square law than for the linear detector.

——本文譯自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》

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