Page 159 - Radar Technology Encyclopedia

DIVERSITY DOPPLER EFFECT 149

DIVERSITY is the mode of radar operation when the basic Time diversity is the mode of radar operation in which inde-
radar parameters are changed in time, space, frequency, or pendent samples are received at intervals equal to the correla-
polarization to improve radar performance. SAL tion time of the target. It requires that the integration time t o
exceed the correlation time t of the target. The number of
c
Combined diversity is the mode in which both time and fre-
samples available is
quency diversity are used to increase the number of samples
t
to reduce the fluctuation loss in the detection of fluctuating n = 1 + ---- 0
e
targets. The total number of samples can be expressed as t c
t 0 öæ Df ö For rigid targets, the correlation time may be estimated as
n = æ 1 + ---- 1 + -----
è
t øè
f ø
e
c c l
where t is the integration time, t is the correlation time, Df is t = ----------------
c
o c 2w L
a x
the frequency bandwidth, and f is the correlation frequency
c
of the target. DKB, SAL where l is radar wavelength, w is the rate of rotation of the
a
Ref.: Barton (1989), p. 86, (1991), p. 4.21. target about the line of sight, and L is the target dimension
x
normal to the line of sight and the axis of rotation. DKB, SAL
Frequency diversity is the mode of radar operation in which
Ref.: Barton (1989), p. 86.
parallel channels or burst-to-burst frequency changes are used
to obtain independent samples to reduce the fluctuation loss DIVIDER: A power divider is “a device for producing a
in detection of fluctuating targets. It offers a solution to the desired distribution of power at a branch point.” Typically, it
problem of large fluctuation loss, without compromising MTI is an assembly of transmission lines for the distribution of the
or doppler performance. A number of channels (often two) power fed to the input between several output channels. As a
may be operated in parallel, spaced by at least the correlation rule, power dividers are reciprocal units (i.e., they are capable
frequency f within the radar band, or the frequencies may be of dividing and summing power). Depending on the their
c
transmitted by changing from one burst to the next. Pulses are function, dividers perform even and uneven division into two
received in each channel to increase the integration gain and or a greater number of channels.
to decrease the fluctuation loss. DKB, SAL The quality of operation of a power divider in a fre-
Ref.: Barton (1989), p. 8, (1991), p. 4.21. quency band is evaluated by the voltage standing-wave ratio
of each input and the transmission coefficient. Sequential and
Diversity gain is “the reduction in predetection signal-to-
parallel (in the form of a star) power dividers are used. A
interference ratio required to achieve a given level of perfor-
power divider with two outputs is a T-junction with absorbing
mance, relative to that of nondiversity radar, resulting from
elements for the matching and isolation of the outputs.
the use of polarization, space, time, frequency, or other char-
Bridges and directional couplers are widely used in divider
acteristics.” The usual notation is G (n ), where n is the
d e e
number of independent samples provided by diversity. When networks. IAM
the task is to estimate the reduction in fluctuation loss for Ref.: IEEE (1993), p. 986; Veselov (1988), p. 68; Sazonov (1988), p. 113.
radar detecting a fluctuating target due to taking samples over A voltage divider is “a network consisting of impedance ele-
intervals in time and frequency, the diversity gain can be ments connected in series, to which voltage is applied, and
defined as from which one or more voltages can be obtained across any
L 1 () 1 – 1 n e portion of the network.” Typically, the output voltage is
¤
f
G n ( ) --------------- = L 1 ()
º
d e L n ( ) f reduced in relation to the input voltage by a specific factor.
f e
The most simple and widely used voltage divider is con-
where L (1) and L (n ) are the target fluctuation losses for one structed on resistors. For the division of alternating voltage,
e
f
f
and n number of samples correspondingly. In dB notation, dividers with reactive resistors are often used; for example,
e
the expression for the diversity gain can be written as
capacitance dividers, consisting of two or several capacitors
1 ö
G n ( ) = æ 1 – ----- L 1 () connected in series. IAM
d e dB è n ø f dB
e
Ref.: IEEE (1993), p. 1,467; Popov (1980), p. 104.
DKB, SAL
Ref.: IEEE (1993), p. 379; Barton (1988), p. 85. Dolph-Chebyshev weighting (see WEIGHTING).
Polarization diversity is the mode of radar operation in DOPPLER EFFECT. The doppler effect is “the effective
which multiple polarizations are used to avoid interference, to change of frequency of a received signal due to the relative
increase signal detectability, or to identify targets. SAL velocity of transmitter with respect to receiver.” In radar it
Ref.: Brookner (1977), p. 55. primarily manifests itself in the effect that the carrier fre-
quency f of received signal differs from the carrier frequency
r
Space diversity is the mode of radar operation in which the
f of the transmitted signal when reflected (or retransmitted)
0
transmitting and/or receiving antennas are displaced in space.
by the moving target.
(See RADAR, multistatic.) SAL
Ref.: Barton (1964), p. 501.