Page 215 - Radar Technology Encyclopedia

205 gain, obstacle gate, rectangular

nities for passing clutter or echoes from other targets into the
tracking channel. DKB
Ref.: Blackman (1986), pp. 4–10.
early gate (see split gate).
A guard gate is a range or velocity gate placed adjacent to
the signal gate to detect the approach of an interfering signal.
When the interference enters the guard gate, it initiates a
change in tracking mode (e.g., entering the coast mode)
designed to prevent loss or contamination of the track. This
alternative mode is used for a predetermined time or until the
interference is detected in a guard gate on the other side of the
Figure G1 Inverse-gain jamming waveform (from Neri, 1991,
Fig. 5.33, p. 381). signal gate. Multiple guard gates may be used to develop
information optimizing the alternative tracking and coast
Obstacle gain is the increase in received field over an modes. DKB
obstructed path resulting from knife-edge diffraction. It is
late gate (see split gate).
defined as the ratio of the power density beyond the obstacle
to the density that would occur in the absence of the obstacle. A range gate is (1) a circuit passing signals during a specific
DKB interval of range delay after a transmission, or (2) the signal
channel corresponding to that range delay interval. The latter
Ref,: Meeks (1982), p. 34.
usage is synonymous with the term range cell. In digital pro-
swept gain (see SENSITIVITY time control). cessing systems, where a short strobe samples the signal in an
analog-to-digital converter, the equivalent range gate is the
GARBLE in a secondary radar system is “a term applied to
convolution of the strobe width with the impulse response of
chance overlapping of two replies so that the pulse positions
the prior receiver circuits, which should be approximately
of one reply fall close to the pulse positions of the other reply,
matched to the signal waveform. In FMCW doppler systems
thereby making the decoding of reply data prone to error.”
and pulse-compression systems using the stretch processing
The situation arising and persisting when two aircraft are
technique, the equivalent range gate is a filter channel passing
within the same 3-km slant range interval from an interroga-
echo signals within a specific delay interval.
tor and at similar bearings, causing replies to overlap in the
In search radar, fixed range gates may cover the entire
receiver is called synchronous garble. SAL
pulse repetition interval, each passing signals from a given
Ref.: Stevens (1988), pp. 288, 293.
range delay to an integrator or pulsed doppler filter bank. In a
GATE, GATING. A gate is “(1) an interval of time during tracking radar, a single gate (often divided into a pair of gates
which some portion of circuit or display is allowed to be for split-gate tracking) often suffice, the single or sum gate
operative, or (2) the circuit that provides gating,” such as an passing signals for gain control and angle tracking while the
electronically controlled switch having the capability of pass- split gate provides a range error channel. The range gate is
ing or inhibiting a signal. The gate is normally controlled as a usually matched to the processed pulse width, and when a
function of time (see range gate) to select signals appearing simple pulse (without pulse compression) is used the matched
within a given interval of time delay, but gating is also used to filter may be implemented with a wideband IF amplifier fol-
select signals during a given interval in a scan, or in a given lowed by such a matched gate and a bandpass (or low-pass)
angle or doppler frequency interval. (See angle gate, velocity filter.
gate.) DKB To reduce cost and complexity, range gates wider than
Ref.: IEEE (1993), p. 549. the processed pulse may be used, at the expense of mismatch
or collapsing loss. The matching loss is proportional to the
An angle gate is a circuit capable of selecting and passing
square of gate width when a gate wider than the pulse width is
signals arriving from a given angle sector. In a sector-scan-
used in IF stages preceding a narrowband filter and envelope
ning radar, the selection may be based on the timing of the
detector. When such a gate is used following an IF matched
signal relative to the start of the scan. In a tracking radar, the
filter or at video, a collapsing loss results. DKB
amplitude of the error signal provides the data used in con-
trolling the gate. DKB Ref.: IEEE (1993), p. 549; Barton (1964), p. 9.
Ref.: Barton (1988), p. 391. range gate pull-off (see ECM, range-measurement).
A correlation gate is used in track-while-scan radars to select A rectangular gate is used in range or angle tracking when it
a specific target signal in range and azimuth after initial is unnecessary to provide controlled weighting of signals
detection, providing validation of the target and inputs to the received from different portions of the gating interval. For
tracking filter from successive scans. The size of this gate example, a rectangular range gate following a wideband IF
must be such as to accommodate prediction errors caused by amplifier provides matching to a rectangular pulse of equal
uncertainties in target velocity, while minimizing the opportu-