Principles of Modern Radar - Volume 2 1891121537

68 CHAPTER 2 Advanced Pulse Compression Waveform Modulationsaddress straddle loss, additional range gates may be inserted within an unambiguousregion, but the result is a higher sampling rate and increased processing.2.5.5.5 Waveform Parameter Trade SummaryAn SF waveform is defined by four principal parameters, whose impact on performanceis interrelated: pulse repetition interval, number of pulses, frequency step size, and pulsewidth. The previous sections addressed some of these relationships. The principal tradesare summarized here for convenience:1. Pulse repetition interval, T :a. The minimum PRI is selected to support the maximum unambiguous range.b. The product of the PRI and the number of pulses, NT, defines the dwell timeassociated with the waveform. The dwell time should be sized to prevent a target frommoving through more than 1/2 a range resolution cell during a coherent processinginterval.2. Product of the number of pulses and the frequency step size, Nf :a. The product defines the waveform’s composite bandwidth.b. The radar system’s range resolution, c/2Nf , dictates the value of Nf .3. Frequency step size, f :a. Defines the size of the unambiguous region, c/2f .b. A small value of f increases the number of pulses required to achieve the desiredrange resolution and correspondingly the waveform’s dwell time.c. When imaging a target of length L, c/2f > L is often enforced to resolve ambiguousreturns.d. In many cases, c/2f defines the range gate spacing.4. Pulse width, τ (CW pulse only):a. With τ = 2L/c, the maximum straddle loss is 6 dB across a target of length L.b. With τ = 1/2f , returns are localized to a region defined by c/2f ; however,straddle loss tends to increase.2.5.6 Impacts of DopplerThe SF waveform has been examined assuming a stationary target and radar. This is avalid assumption in some instances, for example, an instrumentation radar used to measurethe HRR profile of a stationary object. In other instances (e.g., a tactical application), theenvironment is not as constrained, and the effects of relative motion must be considered.To evaluate the impact of Doppler shift on an SF waveform, first consider the samplescollected from a stationary target as defined in equation (2.112). With relative motion, themeasured range to the target changes pulse to pulse. The range varies asR (n) = R 0 − nvT n = 0 ...N − 1 (2.129)where v is the radial component of velocity.