Page 116 - Radar Technology Encyclopedia

106 DECIBEL DEFRUITER

U . When the gain or loss of a network or device is given, the DECODER. A decoder is a device for decoding a series of
0
reference power P is the input power and P is the output. coded signals and transferring them into the initial form of
0
When P is taken as 1W, P is expressed in dBW (decibels rel- representation. In radar applications decoders are usually
0
ative to 1W) and when P is 1 mW, P is expressed in dBm. used in secondary radars receiving the signals from transpon-
0
In radar applications, decibel notation is used to describe ders and in radars employing phase-coded signals. SAL
not only power but other quantities that are proportional to Ref.: IEEE (1993), p. 317.
2
power (e.g., radar cross section of the target in dBm ). Quan-
DECORRELATION is the reduction of correlation that is
tities in decibel notation are often used to describe signal-to-
usually expressed in a reduction of correlation coefficient
noise ratio, losses, sidelobe levels, and so forth.
during the (de)correlation time.
A decibel conversion chart is used to translate power (P/P )
0
and voltage (U/U ) ratios from decibels to dimensionless DECOY, radar. A radar decoy is a device used to divert or
0
units and vice versa (Table D1). With the widespread use of mislead radars and to prevent them from performing their
pocket calculators with scientific notation, reference to such functions with required performance. It may take the form of
charts is not often required. a radiating (active) or reflecting (passive) decoy and is con-
Example: 3 + 0.7 = 3.7 dB = 1.995262 ´ 1.174898 = sidered a form of electronic countermeasure. SAL
2.344299 as a power ratio, or 1.412538 ´ 1.083927 = An active decoy is a decoy operating at locations separate
1.531088 as a voltage ratio. SAL from real targets, and providing beacon signals to suppress
Ref.: IEEE (1993), p. 315; Leonov (1998), p. 175. the real target echoes. Typically it is the miniaturized repeater
jammer deployed on board or towed by military vehicles (air-
Table D1
borne platforms, naval platforms, etc.). SAL
Decibel to Ratio Conversion
Ref.: Barton (1991), p. 12.7; Neri (1991), p. 407.
dB X = P/P 0 X = U/U 0
a
a
An expendable decoy is a minimissile-shaped object that is
0 1.000000 1.000000 ejected from a military vehicle and whose payload generates
0.1 1.023293 1.011579 deceptive signals. To increase the effectiveness, systems
0.2 1.047129 1.023293 based on forward-fired active decoys have been developed.
0.3 1.071519 1.035142 The main problem in using these decoys is the timing of ejec-
tion. Before launching the decoy, one should be sure that the
0.4 1.096478 1.047129
sensor to be confused, for example a missile seeker, is already
0.5 1.122018 1.059254
approaching and is at the right range. In the absence of this
0.6 1.148154 1.071519 information, expendable decoys should be launched at regular
0.7 1.174898 1.083927 intervals when the threat is detected. That is not always possi-
0.8 1.202264 1.096478 ble because of a limited supply of such decoys. SAL
0.9 1.230269 1.109175 Ref.: Neri (1991), p. 410; Chrzanowski (1990), p. 156.
1.0 1.258925 1.122018 A passive decoy is a nonradiating, reflecting object that is
2.0 1.584893 1.258925 typically used to increase the number of apparent targets seen
by radar. The most common types of passive decoys are radar
3.0 1.995262 1.412538
reflectors (e.g., corner reflectors and Luneburg lens reflec-
4.0 2.511886 1.584893
tors) and special false targets (e.g., reentry vehicles), typically
5.0 3.162278 1.778279
used to mask the warhead and to mislead an ABM radar. SAL
6.0 3.981072 1.995262
Ref.: Barton (1991), p. 12.9; Neri (1991), p. 399.
7.0 5.011872 2.238721
A towed decoy is a decoy that is tied to the military platform
8.0 6.309573 2.511886 (airborne, spaceborne, naval, etc.). Unless it includes an
9.0 7.943282 2.818383 active deception repeater, it produces an echo at the same
10 10 3.162278 doppler as the true target. Used against attacks from the front
2 and rear sectors, it may fail to provide a satisfactory defence.
20 10 10.00000
3 SAL
30 10 31.622780
4 2 Ref.: Neri (1991), p. 408.
40 10 10
5
50 10 316.2278 A DEFRUITER is the equipment that deletes random non-
6 3 synchronous unintentional returns in a beacon system. (See
60 10 10
7 FRUIT.)
70 10 3162.278
Ref.: IEEE (1993), p. 320; Stevens (1988), p. 114.
8 4
80 10 10
9
90 10 31622.78
10 5
100 10 10