VME 2100 Apr 02 PDF - VITA Technologies

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