VME 2100 Apr 02 PDF - VITA Technologies

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the signal. Phase jitter in the video A/D sampling clock also causes imperfect clutter cancellation. Sensitivity time control attenuation for range normalization of nearby small targets contributes to amplitude and phase distortion. The phase, amplitude, and timing of the pulse code transmitter driver must also be considered. The MTI improvement factor has a direct effect on the stability of the constant receiver phase shift θ Rx in Equations 6-7. A steadily moving target moving at v d will Doppler shift by ω d the carrier frequency ω c , thereby altering the complex envelope of the radar waveform. This is defined in Equation 8, where ω c is the speed of light. The Doppler frequency is simply the time derivative of the complex envelope’s phase function, thus the emphasis on constant receiver phase offset. This Doppler shift is found by sampling the quadrature phase angle at the pulse repetition frequency. It is this angle α, defined in Equation 9 and illustrated in Figure 2, that is measured using consecutive IQ samples at the pulse repetition interval. This is averaged over many continuous Q (I 0 , Q 0 ) d (I 1 , Q 1 ) Figure 2. Quadrature Doppler phase angle measurement I range bins. The measured α, pulse repetition interval at the time of measurement, and carrier frequency determine a Doppler velocity that, combined with any other pulse repetition interval, determines the weights of the broadband filter when used at that pulse repetition sampling. Log detection in the IF stage with wide instantaneous dynamic range (80 dB) can be used to determine range bin phase samples for appropriate measurement of the Doppler phase angle over many samples using averaging. c v d = 2fc f d (8) 2πfd 4π f α d = fpr = . c . c f v pr d (9) Static Clutter Pulse Cancellers The fundamental MTI filter component is the high-pass FIR feed-forward pulse canceller. These filters are guaranteed to be stable, i.e., all zeroes and no poles in the transfer function. They are easy to implement in direct form using a tapped delay line. The binomial weights, given in Equation 10, provide a symmetric or anti-symmetric unit sample response, depending on the order of the weights being odd or even. This symmetry guarantees a linear phase transformation and constant group delay, which is essential for presenting the filtered pulse compressed signal to the matched correlator. The nth power of two in the denominator of Equation 10 normalizes the maximum filter gain to unity. This is normalized for fixed-point DSP mapping to avoid accumulative overflow, where the inputs are normalized to Reprinted from VMEbus Systems / April 2002 Copyright 2002 / VMEbus Systems
avoid multiplicative overflow. Equation 11 gives the time domain response of the filter, where the order of the filter is q+1. Figure 3 shows the unity gain frequency response of the filter. Given a pulse repetition frequency, only a small range of Doppler frequencies can be resolved at that sampling rate. Aliasing in the MTI filter will periodically stopband at integer multiples of the sample rate any incoming Doppler signal at that frequency. These periodic ambiguous rejection stopbands are known as blind speeds. Pulse-to-pulse staggering eliminates use of feedback to improve the blind speed response, thereby avoiding the stability issues of IIR filters. Staggering can be described as cascaded interpolation and decimation filter stages. Sample rate conversion maintains detection of targets by scaling the frequency domain spectrum envelope into the passband of the MTI filter. Low frequency components are not significantly scaled, so they always fall in the stopband of the MTI filter. Figure 4 illustrates a fourthorder response with varying stagger schedules compared to the uniform sampled filter response. Many stagger ratios have been described by other researchers on this topic, and they are dependent on the order of the filter. Those shown in Figure 4 use 10- 20 percent increases in subsequent sampling intervals. A good stagger ratio can result in only a 10 dB loss of signal in the blind speed zones while maintaining the low-frequency stopband. Dynamic Clutter Pulse Cancellers Clutter-locked MTI uses a measured average Doppler shift of a given volume of clutter that is used to control an offset frequency oscillator that shifts clutter into the rejection notch. The first stage of the process is to take the complex conjugate of the synchronous detected signal to obtain a symmetric weight distribution in the completed derivation. Both h 1 and h 2 in Figure 5 are high-pass FIR transfer functions as defined by Equations 10 and 11 and can have any order as needed. The frequency oscillator performs a down con- dB 0 -10 Order 2 3 4 5 -20 -30 -40 0 0.1 0.2 0.3 0.4 0.5 Figure 3. MTI high-pass filter response vs. normalized frequency f f pr dB 0 Stagger Ratio 1 Stagger Ratio 2 Stagger Ratio 3 Stagger Ratio 4 No Stagger -20 -40 -60 0 0.5 1 1.5 2 2.5 3 Figure 4. MTI fourth-order HPF response with stagger vs. normalized frequency f f pr Reprinted from VMEbus Systems / April 2002 Copyright 2002 / VMEbus Systems
Page 1: Broadband FIR Moving-Target Indicat
Page 5: dB 0 -10 6th order clutter-locked 3

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