Page 77 - Radar Technology Encyclopedia

bridge, waveguide calibration, mixed 67

loads connected to branches 3 and 4 are mismatched. The Absolute calibration is the technique in calibration of radar
maximum bandwidth is 10 to 15% for a standing-wave ratio reflectivity measurements in which the target under test is
of 1.2. replaced with a known calibration target at the same location,
Other configurations include a waveguide formed into a and direct comparison of the target returns is performed. Typ-
circle (circular bridge) and the slotted-waveguide bridge. ically a sphere is used as the reference target. SAL
Waveguide bridges are used to decouple generators that Ref.: Currie (1987), p. 74.
operate at different frequencies through a common load, as
Amplitude calibration is the technique in calibration of
sum-and-difference devices in monopulse radars, to measure
radar reflectivity measurements of comparing target RCS
mismatch, and as elements of antenna commutators and bal-
with a calibration standard (direct calibration) or using the
anced mixers. IAM
radar equation with careful measurements of radar and propa-
Ref.: Popov (1980), p. 67; Druzhinin (1967), p. 141; Gardiol (1984), p. 282; gation path parameters to predict the RCS of a target from its
Sazonov (1988), p. 95.
received power (indirect calibration). Since the errors associ-
BURNTHROUGH is the ECCM tactic of increasing the ated with the two methods of amplitude calibration are for the
radar energy in the direction of the jammer in hopes of most part independent, the calibration errors can be estimated
increasing the radar power echo above the jamming noise. by performing both techniques and comparing the results
Typically it may be accomplished with adding reserve trans- (this is called closure). SAL
mitter power or by dwelling longer in the direction of the Ref.: Currie (1989), p. 65.
jammer. The longer dwell reduces the date rate, so it has to be
Calibration closure is the criterion of correspondence of
taken into account when such a tactic is supposed to be used.
reflectivity measurement results obtained using different cali-
SAL
bration techniques. Typically 3 dB difference is considered to
Ref.: Skolnik (1980), p. 549.
be acceptable and 1 dB is excellent. SAL
burnthrough range (see JAMMING, barrage) Ref.: Currie and Brown (1987), p. 763.
BUOY, radar. A radar buoy is a type of anchored marine Direct calibration is the technique in radar reflectivity mea-
buoy on which are mounted special radar reflectors. Because surement when a calibration standard is used for comparing
of large reflectivity, it can be observed at long distances by its return with the signal from an unknown target. Typically a
shipborne radars. The main application is for ship navigation, standard is used whose RCS can be accurately inferred from
marking the places dangerous for ship sailing and indication its physical dimensions (i.e., a sphere or a corner reflector).
of ship paths in seas, rivers, lakes, and channels. SAL The RCS of the unknown target s can be determined from
u
4 4
Ref.: Popov (1980), p. 345. æ R ö F ö
æ
r
u
s = P ------ ÷ ------ ÷ s
ç
u r ç u
è
BURST (see PULSE). è R ø F ø
u
r
where P = ratio of power received from the unknown target
r
to that from the reference target, R and R are ranges to
r
u
C unknown and reference targets, respectively, F and F are
r
u
propagation factors for unknown and reference targets,
CALIBRATION is “the process of adjustment to match the respectively, and s is the RCS of the reference target. The
r
designed operating characteristics with the subsequent mark- accuracy of this calibration method depends of the precision
ing of the positions of the adjusting means.” In radar applica- of s and on the propagation factor being well known. SAL
r
tions, calibrations of different devices in radar subsystems are Ref.: Currie (1989), p. 65.
made at the stages of hardware tuning, testing and service. Indirect calibration is the technique in radar reflectivity
The calibration of radar reflectivity measurement is very measurement involving measurement of radar characteristics
important phase to ensure accurate acquisition of noncoordi- (such as transmitted power, antenna gain, etc.), measurement
nate data in the process of radar operation. of received power from an unknown target, and calculation of
Calibration of radar reflectivity measurement is the pro- the unknown RCS by using the radar range equation. SAL
cess of relating signals that can be extracted from radar mea- Ref.: Currie (1989), p. 66.
surement to the physical characteristics of a target such as its
Mixed calibration is a combination of the absolute and rela-
radar cross section or its polarization properties. Accurate cal-
tive calibration techniques, which uses the absolute method to
ibration is one of the most important phases of radar reflectiv-
establish the value of one point of the relative receiver trans-
ity measurement and it employs several techniques. Generally
fer curve. The RCS of an unknown target at any range R can
un
they are amplitude, phase and polarization calibrations. Also
be determined from the equation
absolute, direct, indirect, mixed and relative calibrations are
distinguished. SAL æ R cal ö
s un = 20log v ( un ) 20log– v ( cal ) s cal – 40log ---------
+
Ref.: IEEE (1993), p. 149; Currie (1987), pp. 760-778; Currie (1990), pp. 64- è R un ø
75.