Page 320 - Radar Technology Encyclopedia

POYNTING(’S) VECTOR propagation, ionospheric 310

POYNTING(’S) VECTOR. The Poynting vector of an elec- PROBABILITY (see DETECTION probability, FALSE-
tromagnetic wave is “the vector product of the instantaneous ALARM probability).
electric and magnetic field vectors”:
PROCESS, random (see FUNCTION, RANDOM).
P = E × H
PROPAGATION, wave. Wave propagation is “the travel of
It gives the power density of antenna radiation in the far-field
waves through or along the medium.” In radar applications,
region and is the density of power flow at a specified point in the most important media are the atmosphere and ferrites.
2
W/m . In the single-frequency case, the complex Poynting
(See propagation medium.) Atmospheric propagation is the
vector is used:
primary consideration for radar and is divided into free-space,
1 * tropospheric, and ionospheric propagation, with special con-
P = --- E × H
c 2 sideration to propagation over Earth. In this last case the char-
* acteristics of free-space propagation are modified by
where H is the conjugate to the complex vector H .
proximity to the surface, introducing multipath (reflection)
SAL
and diffraction effects, accounted for, in the range equation,
Ref.: ITT (1978), p. 45; Fink (1982), p. 1.41.
by the pattern-propagation factor. The basic effects associated
PRECISION is the measure of ability of a system to operate with wave propagation are interference, scattering, (atmo-
with a preset level of errors. In radar the term precision pri- spheric) attenuation, refraction, and diffraction, all of which
marily refers to the random spread of measurement errors. are discussed below.
(See ERROR, measurement.) SAL Wave propagation is dependent on frequency. There are
four different types of waves, depending on the propagation
Ref.:Barton (1964), p. 235; Leonov (1988), p. 25.
mode:
PRECLASSIFICATION is the stage of target classification (1) Direct or freely propagating waves, traveling along
in which targets of insufficient interest for further analysis by straight (or almost straight) lines, characteristic of bands from
classification algorithms are excluded from further consider- UHF to millimeter waves.
ation. This process often is based on clustering techniques. In (2) Tropospheric waves that propagate within a
this case the proximity of an incoming signature to all signa- waveguide-like tropospheric structure (ducting), characteris-
tures that are under consideration is measured, and if the tic of VHF and higher frequencies.
incoming signature is sufficiently close to this cluster (in (3) Ground waves that follow the curvature of the Earth
accordance with preset threshold) it passes for the further pro- over some limited range due to the effect of diffraction (VHF
cessing; if not, it is excluded from further processing. An band).
example is deletion of the light decoys accompanying a bal- (4) Ionospheric waves, propagating as a result of single
listic missile warhead at the stage of preclassification, when or multiple reflections from ionospheric layers (HF band).
only heavy decoys and warheads pass for ultimate classifica- The latter two waves constitute the basis for over-the-horizon
tion with identification algorithms. Sometimes this technique radar. SAL
is called alien separation. (See TARGET RECOGNITION Ref.: IEEE (1993), p. 1017; Burroughs (1979); Skolnik (1980), Ch. 12; Blake
AND IDENTIFICATION.) SAL (1980), Ch. 6; Barton (1988), Ch. 6; Rohan (1991).
Ref.: Long (1992), p. 461.
Anomalous [abnormal, nonstandard] propagation occurs
PRINCIPLE when the state of the atmosphere departs from its normal or
standard condition. The main cause of such irregularities is a
The principle of uncertainty in radar is defined by the fact
change in the index of refraction profile, leading to superre-
that the volume covered by ambiguity functions is constant
fraction, ducting [trapping], or subrefraction. (See ATMO-
and does not depend on the shape of the radar waveform. By
SPHERIC ducting.) SAL
varying the type of the waveform, one can change only the
shape of the ambiguity function diagram. For a fixed time- Ref.: Skolnik (1980), p. 450; Blake (1980), p. 224.
bandwidth product, it is possible to increase the resolution Free-space propagation occurs in a vacuum in the absence
and accuracy of the range measurement only at the expense of of proximity to a surface or other medium that can modify
decreasing the resolution and accuracy of the doppler velocity propagation conditions. Departure from free-space conditions
measurement, and vice versa. But by increasing the time- can take the form of (1) refraction, (2) atmospheric attenua-
bandwidth product, it is possible to improve the quality of tion, or (3) scattering of waves from the underlying surface.
measurement both in range (time delay) and velocity (fre- (See pattern-propagation factor, propagation over the
quency). The term has been introduced by analogy with the earth.) SAL
uncertainty principle in quantum mechanics that states that Ref.: Skolnik (1980), p. 441.
the position and the velocity of atomic particles cannot be
Ionospheric propagation is characterized by the strong
simultaneously determined to any degree of accuracy desired.
dependence of propagation effects on radar frequency. Deci-
IAM
meter, centimeter, and millimeter waves are practically not
Ref.: Skolnik (1980), p. 408; Vakman (1965), p. 5.
scattered by the ionosphere, but some attenuation is intro-