Page 212 - Radar Technology Encyclopedia

frequency band, X-band frequency stability 202

tracking, shipboard navigation, weather avoidance by aircraft,
Table F8
and other miscellaneous short-range roles, such as vehicle
Use of Radar Frequencies by Application
speed measurement. The combination of wide available band-
width and narrow beamwidths in small dimension antennas Band Application
yield high resolution capabilities in range, doppler, and angle,
HF Over-the-horizon radar, combining very long
with manageable hardware configurations. Atmospheric
range with low resolution and accuracy
attenuation at X-band can be overcome, if necessary, by a
high (but expensive) power-aperture product, but the rain VHF and Long-range, line-of-sight surveillance with low
attenuation and backscatter effects are severe, even at moder- UHF to medium resolution and accuracy, free of
weather effects
ate rain rates. For this reason, X-band is generally used only
over short-to-medium ranges, typically 150 km or less. PCH L-band Long-range surveillance with medium resolu-
tion, slight weather effects
K u , K, and K a Bands. The so-called K-band, defined by a
24-GHz radar developed during World War II, was subdi- S-band Short-range surveillance, long-range tracking
vided into two bands on either side of this frequency after it with medium accuracy, subject to moderate
weather effects
was recognized that the original frequency lay near a water
vapor absorption band. The lower region, K -band, is cen- C-band Short-range surveillance, long-range tracking
u
tered approximately at 17 GHz, and the upper region, K - with high accuracy, subject to increasing weather
a
band, lies nominally at 35 GHz. As one would expect, radars effects in light to medium rain
in these frequencies, especially those at K -band, are suitable X-band Short-range surveillance in clear weather or light
a
only for short range radar, and where high angle resolution is rain; long-range tracking with high accuracy in
required. To date, it has been difficult to generate high power clear weather, reduced to short range in rain
at K -band and applications at this frequency are limited, in K - and K - Short-range tracking, used especially when
a
u
a
general, to radars requiring a few hundred watts of average band antenna size is very limited and when all-weather
power in a small physical package (e.g., active radar missile operation is not required; wider airborne use at
seekers and other airborne applications). The use of K -band altitudes above most weather
u
has been limited to special-purpose radars, such as slaved
mm-wave Limited to short ranges in atmosphere, very short
range-only radars and has seen service, in a similar role, in a bands ranges in rain; used generally for tracking with
few airborne intercept (AI) and ground-based antiaircraft fire very small antennas; possible airborne and space
control radars (AAA). PCH use.
The millimeter band, roughly from 40 GHz to 300 GHz, The frequency shift between direct and multipath compo-
encompasses the V-, W-, and mm-band designations shown nents of a target signal is the result of different target velocity
in Table F7. Because of the variability of atmospheric absorp- components projected on the lines of sight to the radar and to
tion characteristics in this region, only a relatively few fre- the surface reflection region. Referring to the geometry of
quencies are suitable as serious candidates for radar Fig. F39, the frequency shift for specular reflection is
application. One of these lies at 94 GHz, although the attenu-
v v 2 2 v h h
t
t t r
ation in the vicinity of this frequency is greater than the Df = ---- cos q – cos q ) ------ q – q ) --------------
t
(
»
=
(
r
t
r
t
water-absorption line at the original K-band (24 GHz), which l 2l lR 2
precluded further consideration of the latter’s use. Some For a typical subsonic, low-altitude aircraft (v = 250 m/s, h =
t
t
development has been accomplished in the design of an 100 m) observed by a surface-based, X-band radar (l =
active radar seeker at this frequency, but there is no trend in 0.03m, h = 10 m) at R = 10 km, the resulting frequency shift
r
this direction. PCH is very small: Df = 0.083 Hz. DKB
Ref.: Button (1981); Currie (1987).
Summary of frequency by application. Table F8 summa-
q t
rizes the use of the radar frequency spectrum by application.
q t
v t
Note that although this chart is, in general, representative of R
actual radar developments and deployments, notable excep-
h t
tions do exist. PCH h q r
r
Ref.: Skolnik (1980), p. 7.12; Skolnik (1990), pp.1.13–1.18; Barton (1991),
App. K.
Rated frequency is the frequency of oscillator under the nor-
Figure F39 Low-altitude target geometry.
mal conditions of its operation in the absence of frequency
drift. AIL Ref.: Barton (1988), p. 522.
Ref.: Popov (1980), p. 251.
Frequency stability refers to the stability of reference fre-
quency sources within a radar system. For example, in mod-