Page 275 - Radar Technology Encyclopedia

265 measurement, angular METEOROLOGY, radar

Angular measurement is the estimation of target angular which gives a direct measure of time delay. This method is
coordinates based on analysis of the spatial structure of the also used in CW radars.
radar signal. The coordinates of angular position are azimuth The main advantage of the time-related method is the rel-
and elevation (in ground radar), yaw and pitch (in airborne ative simplicity of hardware implementation, and the ability
radar), or direction cosines relative to an array face. (See to measure range on many targets within the same beam. The
COORDINATES, radar.) Angular measurements are based main disadvantage is the inherent inability to measure very
on directional properties of the radar antenna, using either of short ranges (those with time delay less than the transmitted
two signal parameters: amplitude, or phase. In the first, the pulse width), since the receiver cannot operate during the
amplitude of received signals is used to extract information transmission. The main advantage of the frequency-related
about angle of arrival of the wave; while in the second the method is its ability to measure short ranges, and the lower
signal phase is used. (See INTERFEROMETER, radar.) In peak power of the transmitted CW signal, as compared with
early radar the main technique for tracking radar was conical the time-related method. Its main disadvantage is the com-
scanning, but the monopulse technique is now used to provide plexity and size of the hardware (separate transmitting and
more accurate data. (See MONOPULSE.) SAL receiving antennas are necessary for high-power CW radars),
Ref.:Barton (1964), p. 47; Leonov (1988), p. 13; (1986); Dulevich (1978), and the stringent requirements placed on linearity of modula-
pp. 253–281. tion (when multiple targets are measured) and on the noise
sidebands of the transmission. The phase-related method pro-
antenna measurement (see TESTING, antenna).
vides high measurement accuracy but produces many ambi-
Doppler (frequency or velocity) measurement is the esti- guities, since phase can be measured unambiguously only in
mation of target radial velocity by measurement of doppler the range 0 - 2p. Multiple modulation frequencies may be
frequency shift. The most common technique is to use a set of imposed on the carrier to reduce ambiguity, but the practical
doppler filters distributed over an observable frequency inter- implementation becomes even more complicated than with
val (a filter bank) and to determine the radial velocity based frequency ranging. SAL
on the filter in which the strongest signal appears. (See also Ref.: Barton (1964), p. 37; Leonov (1988), p. 13; Dulevich (1978), pp. 215–
DOPPLER EFFECT; ERROR, doppler.) SAL 223.
Ref.: Barton (1969), Ch. 4.
METEOROLOGY, radar. Radar meteorology is the exploi-
measurement error (see ERROR, measurement). tation of meteorology and radio physics employing radar
measurement of precipitation and atmospheric phenomena.
Optimum measurement is the estimation of signal parame-
Typically, radar applied to the observation of meteorological
ters using the optimum receiver structure. (See SIGNAL
phenomena provides two types of information:
parameter estimation.) SAL
(1) The signal arriving from a small region (defined by
RCS measurement (see RADAR CROSS SECTION). the resolution volume) gives information on the properties of
scattering medium (e.g., reflectivity).
Range measurement is the estimation of target slant range
(2) The signals arriving from extended targets make it
by analysis of the temporal structure of the received signal.
possible to define the contour of the targets (e.g., clouds,
Any of three basic methods may be used: (a) time-related, (b)
thunderstorm areas), and the internal structures of different
frequency-related, or (c) phase-related. The first, used in
scale, their evolution and movement.
pulsed radars is based on the time delay t of the received sig-
d
nal relative to that transmitted. The target range R is com- The first type of information typically is referred to as
quantitative, and the second, because of the difficulty of
puted as
incorporating it into quantitative models, is referred to as
ct d qualitative. Meteorological radar can be ground based or
R = -------
2 located on movable platforms: ships, aircraft, or spacecraft.
8
where c = 2.9979 ´ 10 m/s is the velocity of light. The fre- Spaceborne meteorological radars make it possible to gather
quency-related method uses CW signals in which the carrier global meteorological data, but they have limitations because
frequency changes as a function of time in a specific way of the earth’s surface background masking the signals from
(e.g., sawtooth modulation). When the modulation function is the hydrometeors. Frequency agility can be used to decrease
linear, a transmitted frequency f = f + at is mixed in the the effect of the earth background because of the dependence
t 0
receiver with the echo frequency f = f + a(t - t ) to obtain of scattering properties of hydrometeors and surface from
r 0 d
wavelength do not coincide. The main features determined by
Df = f – f = at
t r d meteorological radars are the height of clouds, detection and
The phase method uses the difference f in phase between classification of thunderstorm and hail clouds, intensity of
the transmitted and received waveforms that are modulated at precipitation, and air humidity. IAM
a frequency f : Ref.: Sauvageot (1992); Bogush (1989); Stepanenko (1973), p. 142; Belov
m
(1976), pp. 5, 210; Mel’nik (1980), p. 223.
f = 2pf t
m d