Page 180 - Radar Technology Encyclopedia

error, platform-dependent error, radar-dependent 170

The factors and errors of nonradar character can be
described through the introduction of different reference
frames corresponding to the appropriate distortion factor and
evaluation of target coordinate measurement errors as the
function of ambiguity in evaluation of origin and orientation
of corresponding frames (Table E6) The errors of radar-per-
formance character can be evaluated applying the general
approaches of the radar and antenna theory. The resultant
errors of the target coordinates measurement for the radars
located on the movable platforms (shipborne, airborne, and
spaceborne radars) are the statistical sum of radar-dependent,
target-dependent, platform-dependent, and propagation
errors. Platform-dependent errors can be prevalent for preci-
sion movable radars operating in the lower portion of wave-
lengths (e.g., millimeter wave airborne and spaceborne
radars). SAL
Ref.: Leonov (1988), p. 29; Leonov (1990), pp. 174–203
polarization error (see cross-polarization error).

Probable error is the value exceeded 50% of the time. For an Figure E16 Elevation bias error vs. range for exponential refer-
error with normal distribution and standard deviation s , the ence atmosphere, N = 313 (from Barton, 1988, p. 306).
s
x
probable error is x = 0.6745s . For a two-dimensional error
50
x
with normal distribution and standard deviation s in each
x
coordinate, the circular probable error is r = 1.177s . DKB
x
50
Ref.: Barton (1964), pp. 330–333.
Propagation error results from atmospheric refraction along
the radar-target path. The error is evident primarily in eleva-
tion and range coordinates. The error is proportional to the
refractivity N , measured at the radar site. The refractive bias
s
errors in elevation and range, for a radar at sea level operating
through the exponential reference atmosphere, are shown in
Figures E16 and E17.
There are also small fluctuating errors in all three radar
coordinates, resulting from variations in refractive index in
the atmosphere. The tropospheric components of these errors
are usually a few hundredths of a milliradian.
The ionospheric errors are strongly dependent on fre-
quency. Bias errors in elevation and range are shown in Fig-
ures E18 and E19, for daytime conditions at zero elevation
angle. DKB
Ref.: Barton (1969), pp. 366–393.
Figure E17 Range bias error vs. range for exponential refer-
Quantization error results from the granularity of digital ence atmosphere, N = 313 (from Barton, 1988, p. 307).
s
processes as the result of data quantization, typically in steer-
ing the antenna beam (see beam-steering error), reading the
angles of a mechanically steered antenna, or converting signal A
voltages to digital form in an analog-to-digital converter. s = -----------------
a
m
2 12
When a shaft angle encoder having m bits and a minimum
m
/2 is
angle quantum D = 360° used, the peak-to-peak error is Note that m bits plus a sign bit are needed to obtain this noise
D and the rms error is. on a sine wave of amplitude A. DKB
Ref.: Barton (1969), pp. 187–193.
D 180°
s = ---------- = -------------- Radar-dependent error is the portion of error dependent pri-
12 2 m 3
marily on design of the radar, independent of target-induced
A similar error appears when voltage is quantized with m bits
errors such as glint and dynamic lag, and of propagation
representing a maximum amplitude A, in which case there
errors (see angular error, doppler error, range error). DKB
will be an rms noise introduced:
random error (see error model).