Page 175 - Radar Technology Encyclopedia

165 error, beem-steering error, geodetic

where q is the half-power beamwidth, G (q) is the normal-
d
3
ized pattern derivative, or monopulse pattern for the scan q 3 ö c ö s c
D
æ
angle, and G is the gain for uniform illumination of the s = æ ------ ----- ------
è
q
k è
ø S ø
0
2s
antenna aperture. m
where q is the half-power beamwidth, k is the monopulse
The average angular error for phase-shift errors s f 3 m
radian, which are independent over T elements in an array, is difference slope, S is the on-axis sum pattern voltage
response for the intended polarization, s is the target RCS in
q s
3 f
s = ------------------ the intended polarization, and s that in the orthogonal polar-
c
q
1.61 T ization.
DKB While the ratio s /s is usually < 1 for reflective targets,
c
Ref.: Barton (1969), pp. 196–197; Barton (1988), p. 171. the presence of a jammer may greatly increase this ratio,
causing large errors or breaking the track. DKB
Bias error is the systematic component of error, which is
constant in time or which varies slowly, preventing its reduc- Ref.: Barton (1988), pp. 415–416.
tion by averaging of sequential readings (smoothing) of the Diffraction error is a component of multipath error that is
radar output. DKB the result of signal components arriving at the radar antenna
via diffraction from an obstacle underlying the target-radar
Boresight error is the displacement of the antenna electrical
path. DKB
axis, or monopulse null axis, from the mechanical axis to
which the output data are referred. DKB Doppler (measurement) error is the error in radar measure-
ment of target doppler (frequency) shift. The many sources of
Clutter error refers to the radar tracking error caused by
doppler error may be divided into tracking errors, which
presence of uncanceled clutter at the output of the receiver
cause the radar tracking filter (velocity gate) to depart from
and signal processor. When random clutter sources extend
the centroid of the target echo spectrum, translation errors,
across the radar beam, producing uncanceled clutter of power
which cause the frequency of the filter or tracking oscillator
C in a monopulse difference channel, and C in the sum chan-
D
nel, the resulting angular clutter error component is to be reported incorrectly, and propagation errors. The several
components of each type of error may further be divided into
q 3 æ C D ö æ C ö radar-dependent and target-dependent classes, and into bias
s = ------ ------------ 1 + ----
q k è 2Sn ø è S ø and noise components, as shown for a tracking radar in
m c
Table E5. DKB
where q is the half-power beamwidth, k is the monopulse Table E5
3
m
difference slope, S is the signal power in the sum channel, and Sources of Doppler Error
n is the number of independent samples of S/C averaged
D
c
during the tracking time constant. If clutter is concentrated at Error Class Bias Components Noise Components
a particular region in the difference pattern of the antenna, Radar- Discriminator zero Thermal noise; multi-
producing correlation between a component C in the sum dependent setting and drift; path; clutter; jamming;
c
channel and C in the difference channel, an additional bias tracking gradient of receiver variation in receiver
Dc
error may appear: errors delay delay
Radar- Transmitting oscilla- Range-doppler cou-
q 3 C × C
Dc
s = ------ -------------------- c dependent tor (velocity of light) pling; VCO frequency
q2 k 2
m 2S translation measurement; radar fre-
errors quency stability
This error is not reducing by averaging over successive
Target- Dynamic lag Glint (target rotation);
pulses. DKB
dependent target modulation
Ref.: Barton (1988), pp. 531–533.
tracking
Collimation error refers to the boresight error component errors
resulting from failure to perform initial alignment of the elec- Propagation Gradient of refraction Irregularities in refrac-
trical and mechanical axes of the antenna. DKB errors of troposphere and tion of troposphere and
ionosphere ionosphere
Cross-polarization error is the boresight error component
resulting from reception of a signal component whose polar- Ref.: Barton (1988), pp. 551–552.
ization is orthogonal to that of the intended antenna pattern.
dynamic error (see lag error).
The antenna is collimated on a signal having the intended
polarization, with response D in the difference channel going fluctuation error (see scintillation error).
to zero at the axis. A component received with the orthogonal
Geodetic error refers to the error in target position resulting
polarization is passed to the receiver as a result of undesired
from discrepancies between the model of the geoid (the aver-
antenna response D in the antenna difference pattern. The age earth’s surface) and the actual average surface, or
c
resulting cross-polarization error is
between models used at the radar and in systems using radar