Page 21 - Radar Technology Encyclopedia

altimeter, CW ambiguity, doppler 11

An FMCW altimeter is one using frequency-modulated con- the radar altimeters used on the Skylab and GEOS-C earth-
tinuous-wave waveforms. The most common modulation orbiting spacecraft.
waveforms are triangular and sawtooth linear frequency mod- The major types of pulse compression altimeters are lin-
ulation. A basic mechanization of an FMCW altimeter is ear frequency modulated (or chirp) altimeters and phase-shift
shown in Fig. A23. The frequency modulator generates a keyed (PSK) altimeters. A variant of the chirp pulse-compres-
modulated waveform that is applied to the voltage-controlled sion system is the stretch technique, which uses a chirped
oscillator (VCO). The VCO and a power amplifier, if need be, local oscillator in conjunction with series of bandpass filters
constitute the transmitter, from which energy is directed to process the received signal and has the potential for resolu-
toward the surface by the transmitting antenna. The linearizer tion of a few centimeters. The PSK altimeter has some advan-
adjusts the scale factor by setting the slope of the frequency tages relative to short-pulse and FMCW altimeters, as the
modulation. waveform usually is digitally generated and processed so that
Backscattered energy arriving through the receiving both modulation and processing errors are minimized.
antenna is applied to the receiver. A sample of transmitted The short-pulse altimeter is one using a pulsed waveform
signal is mixed with the received signal, and the difference with short pulsewidth without intrapulse modulation. The
frequency, containing information about the measured alti- pulsewidth may vary from tens of nanoseconds at low alti-
tude, is amplified and applied to the signal processor. With tudes to several microseconds at high altitudes. The principle
respect to antenna beamwidth, broad- and narrow-beamwidth of operation is common to pulsed radar without pulse com-
FMCW altimeters are distinguished. The first is one using a pression. Any of several implementations of range trackers,
beamwidth of typically tens of degrees. This type of altimeter differing basically in the range-gating arrangement, can be
is used when the altitude has to be measured to the closest used. SAL
point below the vehicle, or when measurements are needed Ref.: Cantafio (1989), pp. 230, 237–242.
while the vehicle is subject to large pitch and roll. The second
AMBIGUITY, in radar measurement, is the effect of erro-
uses a beamwidth of a few degrees. This type of altimeter is
neous measurement of target location when more than one
used when it is necessary to measure range to the surface
value of a target coordinate corresponds to the single value of
along a given axis. An example is the radar altimeter for the
radar return parameters. Usually angle ambiguity, doppler
Surveyor spacecraft, with beamwidth equal to 4°. SAL
ambiguity, range ambiguity, and range-doppler ambiguity are
Ref.: Skolnik (1980), pp. 84–86; Hovanessian (1984), pp. 329–331; Cantafio
(1989), pp. 245–247. distinguished. SAL
Ref.: Barton (1969), p. 12; Hovanessian (1984), p. 332; Cantafio (1989),
Linearizer pp. 237–257.
Angle ambiguity leads to an erroneous angle measurement
due to the periodic character of an antenna pattern resulting
Frequency
Transmitter
modulator
from its multilobe structure. The problem of angle ambiguity
is especially common in measurement by phased arrays,
interferometers, and antenna measurement with multipath
Leading-edge
Low-noise Range-freq spectrum Data Altimeter propagation effects. The elimination of angle ambiguity is
amplifier analyzer interface
tracker(s) data achieved by using special design solutions for the specific
antenna system and conditions of its application. SAL
Figure A23 Block diagram of basic FMCW altimeter (from
Ref.: Barton (1964), pp. 54–56.
Cantafio, 1989, Fig. 7.11, p. 247).
A distal ambiguity is a system response far from the desired
A laser altimeter is one operating in the optical band. In
target location, that is undesirable if additional targets or clut-
practice it is a pulsed laser rangefinder providing accurate
ter may appear at such a location.
ranging information to a few meters maximum error. SAL
Ref.: Nathanson (1990), p. 285.
Ref.: Brookner (1977), p. 354.
Doppler ambiguity expresses the possibility of assigning
A pulse altimeter is one using pulsed waveforms. In princi-
different values of radial velocity v to a given doppler fre-
r
ple it is a pulsed radar making range measurement in the
quency f . It is the result of the periodic character of the dop-
d
direction toward the earth. Usually it has low-altitude and
pler spectrum in pulsed-doppler radars operating with
high-altitude modes in which sensitivity time control (STC)
discrete numbers of pulses (pulse trains). The maximum
voltage varies to control receiver gain. The major types of
unambiguous doppler frequency is
pulse altimeters are the short-pulse altimeter and the pulse-
compression altimeter. A pulse-compression altimeter is one f du = ± f ¤ 2
r
using complex waveforms with intrapulse modulation. It is a where f is the pulse repetition frequency. Correspondingly,
r
useful system for high-resolution radar altimetry; for exam- the maximum unambiguous radial velocity is
ple, for spacecraft vehicles to ensure reasonable values of
v = ± lf ¤ 4
peak transmitted power producing measurement from orbital ru r
altitudes. A pulse-compression technique was employed on where l is wavelength.