Page 122 - Radar Technology Encyclopedia

112 DEPOLARIZATION, DEPOLARIZER detectability factor, coherent

The fact that bodies of different shape introduce differing DETECTABILITY FACTOR is the ratio of input signal
degrees of depolarization can be exploited to reduce energy to noise spectral density required to achieve a given
unwanted reflections relative to desired targets. The primary detection performance. For pulsed radar it is defined as “the
example of this is the use of circular polarization to discrimi- ratio of single-pulse energy to noise power per unit bandwidth
nate against rain clutter. Because raindrops are nearly spheri- that provides stated probabilities of detection and false alarm,
cal, they reflect circularly polarized waves with a sense of measured in the IF amplifier.” It is used in the radar range
rotation opposite to that incident on them. The spherical sym- equation as the minimum, or required, signal-to-noise ratio
metry of rain is not perfect, and the antenna does not radiate for calculation of maximum range for the given detection per-
and receive perfectly circular polarizations, but the rain return formance. The conventional symbol is D. Depending on
is reduced by the integrated cancellation ratio (over the two- whether beamshape and signal-processing losses are included
way pattern of the antenna in angle space). A complex target in D or as separate terms, different detailed definitions apply,
such as an aircraft introduces significant depolarization, giv- as given below (see also DETECTION curves). DKB
ing nearly equal power in the two senses of rotation. The loss Ref.: IEEE (1993), p. 336.
in target power relative to use of a linearly polarized signal is
The basic detectability factor is the signal-to-noise power
approximately 3 dB. SAL, IAM
ratio given by detection theory for:
Ref.: IEEE (1993), p. 328; Skolnik (1980), p. 504; Vasin (1977), p. 93.
(1) A steady target signal, of which n samples (pulses)
DEPTH OF FOCUS is “the range distance over which the are available for integration, D (n).
0
cross-range dimensions of the impulse response at a fixed (2) Swerling Case 1, 2, 3, or 4 target signals, where
focal range is essentially constant. Applies principally to syn- D (n), ... , D (n) represent the required average
4
1
thetic aperture radar.” SNR.
(3) A generalized fluctuating target signal of which n
Ref.: IEEE (1990), p. 10. e
independent amplitudes are observed within the n
DESIGNATION is “a selection of a particular target and samples integrated, D (n,n ).
e
e
transmission of its approximate coordinates from some exter- This basic detectability factor is the value required at the
nal source to a radar, usually to initiate tracking.” input to the envelope detector, and it represents the input
Ref.: IEEE (1990), p. 10. energy ratio for samples received with full antenna gains in a
receiver matched to the single sample (or pulse), assuming
Jammer strobe designation is the designation in angle of the
optimum video integration with no other signal-processing
jamming vehicle as a target. Then a collocated tracker can
losses. It can be expressed, in the general case, in terms of the
scan at the angle (or fix its beam up the strobe, for a designa-
basic steady-target detectability factor for a single sample,
tion by a 3D search radar). Jammer designation from a net-
D (1) as
work of search radars, using triangulation, provides range and 0
azimuth data to the tracker, permitting decisions on the D 1 () L n () L n ( e )
0
i
f
D nn ,( ) --------------------------------------------
=
degree of threat and whether it is within engagement range. e e n
DKB, SAL where L (n) is the video integration loss and L (n ) is the tar-
i f e
Ref.: Barton (1991), p. 9.18. get fluctuation loss when n independent target samples are
e
Optical designation refers to the designation of targets by a integrated. DKB
collocated optical instrument, eliminating the need for acqui- Ref.: Barton (1993), p. 15.
sition scan by antenna. The acquisition range of the radar is The clutter detectability factor D is the counterpart of the
xc
then found from the basic radar equation, setting the observa- detectability factor D when the interfering background is
x
tion time equal to the time in which a given probability of dominated by clutter. If the clutter probability density func-
acquisition is to be obtained. DKB, SAL tion has a Rayleigh distribution, and the correlation time is
Ref.: Barton (1991), p. 9.18. less than the pulse repetition frequency, t <t , then D = D .
c r xc x
Search radar designation is the designation by a collocated Clutter which is correlated between pulses, t > t , leads to
c
r
or remotely located search radar. If this is a 2D radar, then it D xc >D , because the integration gain will be reduced. Non-
x
can provide range and azimuth designation data to the tracker, Rayleigh clutter requires a higher threshold to control false
but target elevation (and usually radial velocity) will be alarms, resulting in D xc > D (see LOSS, clutter distribu-
x
unknown. The tracker acquisition scan will cover the eleva- tion). The loss may be from a few decibels to as much as 15
tion sector on which targets might be located, and some azi- or 20 dB in the case of land clutter observed at low grazing
muth scanning may also be required to cover the 3s error of angles. DKB
the search radar data. For 3D radar, the designation includes Ref.: Barton (1988), pp. 61–85.
target elevation or height. This reduces the sector that the The coherent detectability factor is the input signal-to-noise
tracker acquisition scan must cover. DKB, SAL energy ratio required on a steady target when ideal coherent
Ref.: Barton (1991), p. 9.18. detection is used: a synchronous detector with its reference
oscillator input matched to the known phase of the signal,