Page 205 - Radar Technology Encyclopedia

195 filter, spatial filter, temporal [time]

tial filtering is realized in the spatial section of phased-array A stagger-tuned filter is one in which successive stages in a
radars, forming in the absence of interference a matched spa- receiver are tuned to slightly different frequencies, broaden-
tial filter. The sole difference between spatial and temporary ing the bandwidth and achieving an approximation to the
filtering is presence of an additional operation for integration rectangular filter response.
of spatially filtered signals relative to all coordinates of the
A strip(line) filter is based on sections of strip transmission
antenna aperture, which actually is an inverse Fourier trans-
lines. Stripline filters are used as high-pass and low-pass fil-
form. The term spatial filtering also is used in terrain-map-
ters, providing different frequency responses (e.g., Cheby-
ping airborne radars, understood to mean simultaneous
shev filter, elliptical filter). In terms of type of line sections,
(parallel) processing of separate image elements. IAM
one distinguishes between filters with distributed and quasi-
Ref.: Baklitskiy (1986), p. 11.
lumped parameters. The first use sections of a stripline that,
Spatial-adaptive filtering is spatial filtering with a change depending on the wave resistance, perform capacitive or
over time of spatial filter parameters in accordance with the inductive functions. Filters with quasi-lumped parameters use
variation in radar signal inputs. Spatial-adaptive filtering is line sections in the form of a coil (inductance coil), lines with
used widely in phased-array radars for spatial selection of a opposing-pin structure (capacitor), or inductive-capacitive
useful signal observed against the background of an interfer- loops.
ing signal. Filtering involves optimum evaluation of the The manufacturing technology of striplines limits the
weighted vector of phased-array elements performed by a fil- passband of the filters to 20 to 60% depending on the struc-
ter-compensator in accordance with the algorithm selected. ture. To increase the bandwidth (with the corresponding
(See ALGORITHM, Widrow; CANCELER, How- increase in losses), strip dielectric and ferrite filters are used.
ells-Applebaum, etc.) The weighted vector is controlled IAM
adaptively by feedback from output to input. The latter is Ref.: Veselov (1988), p. 87.
realized during correlation of total output voltage and the
A surface-acoustic-wave (SAW) filter is an analog filter, the
voltages of individual channels. This leads to automatic for-
basic element of which is a surface-acoustic-wave delay line.
mation of notches in the directivity performance toward
Prior to the development of digital technology, it was widely
sources of interference. Spatial-adaptive filtering is realized
used as a bandpass filter for matched filtering of pulse-com-
both in digital and analog forms. IAM
pression waveforms. SAW filters are used in the 10-MHz to
Ref.: Shirman (1981), p. 352; Nitzberg (1992).
2-GHz band with a 100-kHz bandwidth all the way to 50% of
Spatial-extent filtering is a technique in which clutter is the central frequency. Minimum bandwidth is constrained by
rejected by filtering all returns which extend over more than n physical dimensions. The level of sidelobes reaches -60 dB,
range cells, when targets are known to extend over fewer than bandwidth ratio (at the -40 dB level compared with the -3 dB
n cells. level) reaches 1:1, and response realization accuracy in the
Ref.: Currie (1987), p. 283. bandwidth is about 0.7 dB.
Filter stability. A filter is considered stable if under any con- A SAW filter for matched filtering of pulse-compression
ditions of operation an arbitrary limited input signal |x(t)| £ A waveforms comprises a tapped delay line, phase shifters, and
results in a limited output signal |y(t)| £ B. Practically, it an adder. Programmable transversal SAW filters with signal
means that phase and amplitude control at output of each tap have found
wide use. The time-bandwidth product of the signal the filter
P
()
,
,,
Z < 1 k , = 12 ¼ M – 1
k processes usually is limited to a value of 1,000 due to an
increase in insertion losses. The level of the sidelobes may be
P ()
where Z is a filter pole, or
k brought to -45 dB. IAM
¥ Ref.: Matthews (1977); Jordan (1985), p. 28.26.
×
(
£
å hn Dt ) C A synchronously tuned filter is one in which successive
n = 0 stages are tuned to the same frequency, rather than being stag-
ger-tuned. The bandwidth for m synchronously tuned stages,
where h(nDt) is the impulse response of a signal sampled at
each of bandwidth B , is
1
rate Dt, and C is a constant.
These criteria show that a nonrecursive filter is always ln2
B = B --------
stable, and to be always stable a filter must have a finite 1 m
impulse response. The recursive filter can be either stable or
unstable depending on its transfer function. The unstable fil- tapped delay-line filter (see transversal filter).
ter cannot operate when an input signal has infinite duration
Temporal [time] filtering is filtering of signals, the evalu-
but can be used in some cases when the signal has limited
ated parameters of which are functions of time. Temporal fil-
time duration. SAL
tering is the main type of filtering in surveillance radars and is
Ref.: Gol’denberg (1985), pp. 54, 59.
used in all types of non-phased-array, tracking, and scanning
radars. In scanning radars with temporal filtering, the