Page 182 - Radar Technology Encyclopedia

error, scintillation EXCITER 172

1
S
ö
æ -------- n = ----------------------- Tuning error can refer to the error in frequency setting of the
è I ø e 2b Wf () radar receiver circuits, relative to the received echo signal, or
D n s
to errors in angle or range that result from the receiver tuning.
The latter definition includes the effects of phase and time
where b is the tracking bandwidth and W(f ) is the power
s
n
spectral density of the target fluctuation at the scan frequency. delay change in the receivers, resulting from a signal not cen-
tered in the bandpass. DKB
The corresponding scintillation error is
q 3 Weighting error refers to the failure of a receiving array net-
s = ----- b Wf ()
s k n s work to apply accurately to the array elements the weighting
s
function desired for low-sidelobe performance. The rms
For example, if the fluctuation power density is W(f ) = 0.01 errors can be expressed as an amplitude component s (nor-
s
a
per hertz at the scan frequency, with k = 1.4 and b = 1 Hz, malized to the intended element amplitude at the center of the
n
s
the rms error is s = 0.07q . array), and a phase component s , in radians. For random
s
3
A similar error occurs in sector-scanning radar, including f
errors, the resulting mean sidelobe power level, relative to
search radars rotating through 360° For the usual Rayleigh
.
isotropic gain, is
fluctuating target (Swerling case 1 or 2) with correlation time
2 2
(
t , the error depends on the number of independent ampli- G = G s + s )
e
f
s
c
a
tude-difference samples received during the observation time,
where G » p is the array element gain. DKB
or time-on-target t , according to Fig. E20. This is sometimes e
o
called fluctuation error. DKB Ref.: Nitzberg (1992), p. 38; Mailloux (1994), pp. 395–399.
Ref.: Barton (1969), pp. 171–182. Wind-torque error is the result of wind forces on a mechan-
ically pointed antenna. The wind may cause both mechanical
deflection of the antenna surface and servo tracking error, in
which the target error signal must generate sufficient torque at
the servo output to balance the wind torque. DKB
Ref.: Barton (1964), pp. 331–335.
velocity (measurement) error (see doppler error).
ESTIMATOR (see TRACKER).
EXCITER. A radar exciter is a unit that generates wave-
forms used in the transmitted signal. It consists of two parts: a
waveform generator that creates and shapes the waveform
Figure E20 Scintillation error in sector-scanning radar (from modulation, and an upconverter that translates this waveform
Barton, 1969, p. 180). to the transmitted carrier frequency. An important require-
ment on the exciter, especially in a coherent radar, is its fre-
Survey error is the error resulting from failure to locate
quency stability. High stability is usually achieved using
accurately the position of the radar antenna with respect to the
crystal oscillators driving frequency multipliers in stable local
geodetic coordinate system to which the data are referred. In
oscillator chains. Exceptionally high stability for certain
modern systems, accurate location using such techniques as
classes of pulsed doppler and continuous-wave radars is
GPS reduces this error to a negligible level, even for trans-
achieved using cavity-stabilized microwave oscillators (usu-
portable and shipborne radars. DKB
ally klystrons) at or near the carrier frequency. The crystal
oscillator provides better long-term stability, at the expense of
Systematic error (see bias error).
greater phase instability, because multiplication increases the
Target-dependent error refers to error components that
phase instability of the basic crystal oscillator by the square
depend on target parameters and trajectory. See angular
of the multiplication ratio (phase noise sidebands increase in
error, doppler error, range error. DKB
power by 20log(frequency ratio)). A typical crystal oscillator
Tracking error is the component of error resulting from operates at 90 MHz, and is multiplied by a factor of 32 to
inability of the radar tracker loops to follow accurately the obtain an S-band carrier output. Tunability of the crystal
centroid of the target echo in angle, range, and doppler. See oscillator is very limited (e.g., 0.25%), and frequency change
angular error, doppler error, range error. DKB is usually obtained by changing crystals. The configuration of
a typical exciter for coherent radar is shown in Fig. E21. The
Translation error is the component of error resulting from
exciter can be included in the transmitter or the receiver sub-
inability of the radar data system to report accurately the indi-
system of the radar. SAL
cations of the tracking loops in angle and range. See angular
Ref.: Fink (1982), p. 25.70
error, doppler error, range error. DKB