Page 174 - Radar Technology Encyclopedia

EQUIVALENCE PRINCIPLE error, beam-steering 164

given by Schelkunoff. It is widely used in antenna theory to Angular (measurement) error is the error in measurement
determine radiation from different types of aperture antennae: of radar angular coordinates (e.g., azimuth and elevation in a
horn antennas, reflector antennas, and so forth. SAL land-based radar, yaw and pitch in an airborne radar or mis-
Ref.: Johnson (1984), p. 2-8. sile seeker, or other angular coordinates depending on the
nature of the radar platform and application). The many
ERROR, measurement. An error is a discrepancy between
sources of angular error may be divided into tracking errors,
the true value of a target parameter and its measured value
which cause the radar antenna axis to depart from the target
(the estimate at the output of a radar estimator). The main
angles; translation errors, which cause the axis angles to be
measured parameters in radar applications are signal time
reported incorrectly; and propagation errors. The several
delay (target range), signal angle of arrival (target angular
components of each type of error may further be divided into
coordinates), signal doppler frequency (target radial velocity),
radar-dependent, target-dependent, and platform-dependent
and signal amplitude (target RCS). The fundamental source
classes, and into bias and noise error components, as shown
of measurement error is the presence of random noise in the
for a tracking radar of the monopulse type, in Table E4. DKB
receiver that introduces stochastic uncertainty in the process
Ref.: Barton (1988), pp. 533–548.
of radar measurement. In the general case the measurement
error can be represented as a random function of time (see Table E4
error model) and of target coordinates, the latter representa- Angular Error Components
tion being much more complicated and applying to measure-
ment in phased-array radars. Based on the representation of Error class Bias components Noise components
the target-radar measurement channel, the errors may be clas- Radar- Boresight axis set- Thermal noise; multi-
sified as target-dependent, propagation, radar-dependent, and, dependent ting and drift; path; clutter; jamming;
if the radar is located on a moving platform (ship, aircraft, tracking torque caused by torque caused by wind
spacecraft, etc.), platform-dependent errors. From the view- errors wind and gravity; gusts; servo noise;
point of the measured parameter, angle, doppler, range, and servo unbalance deflection of antenna
RCS errors are distinguished. From the viewpoint of time and drift caused by acceleration
variation (correlation function), they are classified as system- Radar- Pedestal leveling; Bearing wobble; data
atic (bias) and varying (slow or rapid) errors. The rapidly dependent azimuth align- gear nonlinearity and
varying errors caused by receiver noise are called noise translation ment; orthogonal- backlash; data takeoff
errors, and as they are uncorrelated from pulse to pulse they errors ity of axes; nonlinearity and granu-
pedestal flexure larity; pedestal deflec-
can be reduced by data smoothing. As a result, the final accu-
caused by gravity tion caused by
racy of the data is usually dominated by bias and slowly vary-
and solar heating acceleration; phase
ing errors. Bias error can be reduced by proper radar pointing
shifter error
or calibration. SAL
Target- Dynamic lag Glint; dynamic lag varia-
Ref.: Barton (1964), (1969), (1988); Leonov (1990).
dependent tion; scintillation or bea-
Acceleration error refers to the dynamic lag error in a track- tracking con modulation
ing radar or track-while-scan loop, caused by target accelera- errors
tion. This component of error can be found as the ratio of Propagation Average refraction Irregularities in refraction
target acceleration a to the acceleration error constant K of errors of troposphere and of troposphere and iono-
a
t
the loop: ionosphere sphere
a t
e = ------ An anomalous error is an error in evaluation of parameter
a K
a that exceeds some specified value. It can arise because the
When the acceleration is expressed in a linear coordinate, as conditions under which measurements are made deviate sig-
2
m/s , the error will be in meters in that coordinate, while if nificantly from the standard case (e.g., because of sig-
2
angular acceleration is used, r/s , the error will be in radians nal-to-noise ratio decreases caused by jamming, clutter, or
in that coordinate. other adverse factors). AIL
The acceleration error constant can be expressed in terms
Ref.: Kulikov (1978), p. 29.
of the closed-loop (noise) bandwidth, b , in hertz:
n
Beam-steering error is the error between the commanded
2
K = 2.5b (and indicated) position of a phased-array beam and the
a n
actual position. For a linear array or rectangular array with
For a track-while-scan loop of the a-b filter type, K may be common row or column phase-shift settings, the error due to
a
found as (Blackman, 1986)
granularity of an m-bit phase shifter is
b
K = ----------------------- q
a 2 3 G q()
d
( 1 – a ) T e = ------ ---------------
q m
2 G 0
where T is the data sample interval. DKB
Ref.: Barton (1988), pp. 463–466; Blackman (1986), p. 46.