Page 161 - Radar Technology Encyclopedia

doppler navigator duplexer, balanced 151

A doppler navigator is “a self-contained dead reckoning An evaporation duct is one that lies above the surface of the
navigation aid transmitting two or more beams of electromag- sea and results from the water vapor evaporated from the sea.
netic or acoustic energy outward and downward from the This duct exists practically all the time over the ocean. The
vehicle and using the doppler effect of the reflected energy, a height of the duct typically lies between 6 and 30m, and it
reference direction, and the relationship of the beams to the varies with the geographic location, season, time of day, and
vehicle to determine speed and direction of motion over the wind speed. The water surface temperature, air temperature,
reflecting surface.” relative humidity, and wind speed are sufficient measurement
Ref.: IEEE (1993), p. 382; Skolnik (1962), p. 103. parameters to describe ducting conditions. SAL
doppler spread (see CLUTTER spectrum). Ref.: Skolnik (1980), p. 453.
A ground-based [surface] duct is one that lies close to the
Doppler velocity is the radial velocity of target derived from
surface of the ground, typically at a height of 10 or 20m
the measurement of doppler frequency, f . For active radar:
d
(never more than 150 to 200m). Ground-based ducts are more
lf d usual than elevated ducts. Sometimes this duct is called a sur-
v » ------- face duct. SAL
2
2
and for passive radar: Ref.: Skolnik (1980), p. 451.
v = l f d Ionospheric ducting is the long-distance propagation of
1
SAL electromagnetic waves by ducting along field-aligned ioniza-
Ref.: Skolnik (1980), p. 69. tion in the ionosphere. SAL
Ref.: Fink (1975), p. 18.116; Kolosov (1987), p. 31.
DRIVE, antenna. An antenna drive is a device serving for
rotation of an antenna in one or two planes for the spatial dis- DUPLEXER. A duplexer is “a device that utilizes the finite
placement of the beam. An antenna drive comprises one or delay between the transmission of a pulse and the echo
several electrical motors, control equipment, and a system of thereof so as to permit the connection of the transmitter and
mechanical or electrical transmission of motion (rotation). A receiver to a common antenna.” On transmission it protects
power servo often is used in radars. AIL the receiver from the damage by the high power of the trans-
Ref.: James (1947); Popov (1980), p. 39. mitter, and on reception it channels the echo signal to
receiver. Typically, duplexers are divided into devices using
DUCT, DUCTING. Ducting is “confinement of electromag-
gas-discharge tubes (gas-tube or TR-tube duplexers), solid-
netic wave propagation to a restricted atmospheric layer by
state devices (diode duplexers), and ferrite circulators (ferrite
steep gradients in the index of refraction with altitude.” Duct-
circulator duplexers). The most common configurations are
ing occurs when the gradient of the index of refraction dn/dh balanced duplexer and branch-type duplexer. In a typical
- 1
- 7
is less than -1.57´10 m and results in anomalous propa-
duplexer application, the transmitter peak power might be a
gation that can extend radar range considerably. Ducting is
megawatt or more. Ensuring a safe power level for the
usually a fine-weather phenomenon (with the exception of
receiver, typically less than a watt, more than 60 dB of isola-
thunderstorms), occurring when the upper air is exceptionally
tion may be required. Usually a receiver protector may be
warm and dry in comparison with that at the surface.
inserted between the duplexer and the receiver for added pro-
Although radar range increases within the duct, the coverage
tection. SAL
in other directions can be reduced and so-called radar holes
Ref.: IEEE (1993), p. 393; Fink (1975), p. 25.70; Skolnik (1980), p. 4.4;
can appear. A duct (or atmospheric duct) is a natural
Skolnik (1990), pp. 359–368.
“waveguide” formed in atmosphere inside which the ducted
A balanced duplexer is based on the short-slot hybrid junc-
propagation takes place. Elevated ducts, evaporation ducts,
tion that consists of two sections of waveguides joined along
and ground-based (or surface) ducts are distinguished. SAL
one of their narrow walls (Fig. D57). Such a hybrid may be
Ref.: IEEE (1993), p. 392; Barton (1991), pp. 5.16, 8.17; Morchin (1993),
p. 312. considered a broadband directional coupler (coupling ratio is
3 dB). The slot is cut in the common narrow wall of the junc-
The ducting effect is the effect of forming of an atmospheric
tion to provide the coupling. In the transmit condition the TR
duct in particular atmospheric conditions. SAL
tubes fire and reflect the incident power out the antenna arm,
Ref.: Neri (1991), p. 87.
while on reception the TR tubes are unfired and the echo sig-
An elevated duct is one that lies above the ground surface. nals pass through the duplexer into the receiver. The hybrid
An example of propagation in elevated ducts is found in the junction provides an additional 20 to 30 dB of isolation in
“tradewind region” between the midocean, high-pressure addition to attenuation provided by the TR tubes.
cells and the equatorial doldrums. The best known tradewind There is also another configuration of balanced duplexer,
areas lie between Brazil and the Ascension Islands, and called ATR type, that can be employed. This form uses two
between Southern California and Hawaii. The propagation in hybrid junctions and four ATR tubes (Fig. D58). Dashed lines
the elevated ducts is seasonal. SAL show the flow of power. This type of balanced duplexer has
Ref.: Skolnik (1980), p. 453.