Page 291 - Radar Technology Encyclopedia

281 MTI, clutter referenced MTI, limitations to performance

cell is used to control the center velocity of the rejection double-, and triple-delay cancelers as a function of the nor-
notch. DKB malized clutter velocity spread, z = 2ps/v , are
v b
Ref.: IEEE (1990), p. 8. 1 2
I m1 = -------------------------------------- » ----
2
Coherent MTI is “a form of MTI in which moving a target is 1 – exp – ( z ¤ ) z 2
2
detected as a result of pulse-to-pulse change in echo phase
relative to the phase of a coherent reference oscillator.” Typi-
1 2
cally it is a system in which a coherent oscillator (COHO) at I m2 = ---------------------------------------------------------------------------------------------------- » ---- 4
2
2
2 +
intermediate frequency provides the reference for phase 1 – ( 4 3 ¤ ) exp – ( z ¤ ) 1 3 ¤( ) exp – ( 2z ) z
detection of the signals passed to the canceler or filter. The
COHO may be continuously running, providing the offset of 4
I m3 » --------
the transmitter frequency from the receiving local oscillator, 3z 6
or in coherent-on-receive MTI it may be locked in phase to a
where s is the standard deviation of the clutter spectrum, v b
v
downconverted sample of each transmitter pulse. Coherent
is the blind speed of the waveform, and the canceler notch is
MTI is distinguished from area MTI and from noncoherent
assumed centered on the clutter spectrum.
MTI, in which clutter at or near the target provides a phase
There is a dependence between improvement factor and
reference. DKB
other basic parameters describing MTI performance:
Ref.: IEEE (1993), p. 206; Skolnik (1990), p. 101.
I = G mti CA
m
Digital MTI uses an A/D converter to process phase-detected
signals for coherent MTI (or envelope detected, for noncoher- I = D SCV
m
xc
ent MTI) before their passage to a digitally implemented filter - 1 - 1 - 1
I m actual = I m ideal CR
circuit (usually a delay-line canceler or filter bank). DKB
Ref.: Skolnik (1980), p. 119. where G mti is the MTI gain, CA is the clutter attenuation,
SCV is the subclutter visibility, D is the clutter detectability
xc
externally coherent MTI (see noncoherent MTI).
factor, and CR is the cancellation ratio determined by system
MTI gain is the average response of the MTI to targets, instabilities. The ideal improvement factor is computed on the
defined as the ratio of signal power at the MTI output S to assumption that instabilities have no effect on the clutter
o
that at the input S averaged over all target radial velocities of returns: CR ® ¥ .
i
interest: The ratio of MTI improvement factor at a specific dop-
pler frequency to that averaged over all frequencies is often
S o ö
G mti = æ ----- termed the MTI velocity response. DKB, SAL
è
S ø
i Ref.: IEEE (1993), p. 825; Schleher (1991), p. 108; Skolnik (1980), p. 129.
Because the MTI improvement factor is defined as
Limitations to MTI performance are defined in terms of the
( S ¤ C ) maximum attainable improvement factor, I in the presence
o
o
I = -------------------- m
m
¤
( S C ) of a specific source of limitation. There are two basic types of
i i
limitation:
it can also be written as (1) Those determined by the characteristics of input
( S ¤ ) clutter.
S
o
i
I m = -------------------- = G mti CA (2) Those determined by the parameters of the radar
¤
( C C )
i
o
channel itself.
where CA is the clutter attenuation. The limit set by the first type is expressed in terms of the
The effective MTI gain for targets distributed uniformly spectral spread of the clutter, s = 2s /l, where s is the clut-
fc
v
v
over the response is the same as the gain for white noise, and ter velocity spread defined by the clutter model. (See CLUT-
hence there is no gain (or loss) in signal-to-noise ratio in pas- TER spectrum.) The limitations due to radar parameters can
sage through an MTI canceler. There may be, however, a loss be divided further into (1) antenna scanning modulation (or
in detection capability caused by introduction of correlation scanning fluctuation, the causing a spectrum of nonzero width
between successive noise samples, loss of quadrature compo- at the radar input because of the finite time on target as the
nents of both signal and noise, and nonuniformity in target antenna beam scans over it); (2) use of PRF stagger; and (3)
response. (See LOSS, MTI processing.) SAL radar equipment instabilities.
Ref.: Blake (1980), p. 63. The effect of antenna scanning modulation for an m-
delay canceler is to limit the improvement factor to
The MTI improvement factor I is a measure of MTI per-
m
formance, defined as “the signal-to-clutter ratio of the output 2 m f r 2m
æ
I = ------ ------------ ö (1)
of the clutter filter divided by the signal-to-clutter ratio at the ma m! 2ps ø
è
f
input of the clutter filter, averaged uniformly over all target
2
radial velocities of interest.” Equations giving I for single-, where f is the PRF, s = s + s 2 fa , and
m
r
f
fc