Moving target indication is a mode of operation of a radar to discriminate a
target against the clutter. In contrast to another mode, stationary
target indication, it takes advantage of the fact that the target moves with
respect to stationary clutter. The most common approach takes advantage of the
Doppler effect. For a given sequence of radar pulses, the moving target will
change its distance from the radar system. Therefore, the phase of the
radar reflection that returns from the target will be different for successive
pulses. This differs from a stationary target which will cause the reflected
pulses to arrive at the same phase shift.
Radar MTI may be specialized in terms of the type of clutter and environment:
airborne MTI, ground MTI, etc., or may be combined mode: stationary and moving
target indication. Operation
The MTI radar uses Low Pulse repetition frequency to avoid range ambiguities.
Moving target indicator begins with sampling two successive pulses. Sampling
begins immediately after the radar transmit pulse ends. The sampling
continues until the next transmit pulse begins.
Sampling is repeated in the same location for the next transmit pulse,
and the sample taken with the first pulse is rotated 180 degrees and added
to the second sample. This is called destructive interference.
If an object is moving in the location corresponding to both samples, then the
signal reflected from the object will survive this process because of
constructive interference. If all objects are stationary, the two samples
will cancel out and very little signal will remain.
High power microwave devices, like crossed-field amplifier, are not
phase-stable. The phase of each transmit pulse is different from the previous and
future transmit pulses. This phenomenon is called phase jitter.
In order for MTI to work, the initial phase of both transmit pulses must be
sampled and the 180 degree phase rotation must be adjusted to achieve
signal cancellation on stationary objects.
A secondary influence is that phase rotation is induced by Doppler, and that
creates blind velocities. For example, an object moving at 75 m/s will produce
180 degree phase shift each 1 millisecond at L band.
If the pulse repetition interval is 0.002s between transmit pulses, then the
MTI process will produce phase rotation. That is the same as a
stationary object, which renders the system blind to objects traveling at
this radial velocity. MTI requires 3 or 4 pulses to reduce the
effect of blind velocities. Multi-pulse strategies use staggered pulses with
irregular pulse repetition intervals to prevent signal cancellation on moving
objects. The summation process is slightly different so as to accommodate
the additional samples. Phase jitter, Doppler effects, and
environmental influences limit MTI sub-clutter visibility Measure of
Performance to about 25 dB improvement. This allows moving objects about 300
times smaller to be detected in close proximity to larger stationary objects.
Pulse-Doppler signal processing is required to achieve greater sub-clutter
visibility. Characteristics
A target is traveling at velocity at a maximum range with elevation angle and
azimuth in respect to a bistatic MTI radar.
= Probability of Detection = The probability of detecting a given
target at a given range any time the radar beam scans across it, Pd is
determined by factors that include the size of the antenna and the amount of
power it radiates. A large antenna radiating at high power provides the
best performance. For high quality information on moving targets the Pd
must be very high. = Target Location Accuracy =
Location accuracy is a dependent on the certainty of the position of the radar,
the radar-pointing accuracy, azimuth resolution, and range resolution. A long
antenna or very short wavelength can provide fine azimuth resolution. Short
antennas tend to have a larger azimuth error, an error that increases with
range to the target because signal-to-noise ratio varies inversely
with range. Location accuracy is vital to tracking performance because it
prevents track corruption when there are multiple targets and makes it possible
to determine which road a vehicle is on if it is moving in an area with many
roads. The target location accuracy is
proportional to the slant range, frequency and aperture length.
= Target Range Resolution = Target range resolution determines
whether two or more targets moving in close proximity will be detected as
individual targets. With higher performance radars, target range
resolution—known as High Range Resolution—can be so precise that it may
be possible to recognize a specific target and to place it in a specific
class. This would allow more reliable tracking of specific vehicles or groups
of vehicles, even when they are moving in dense traffic or disappear for a
period due to screening. = Minimum Detectable Velocity MDV =
The MDV comes from the frequency spread of the mainlobe clutter. MDV determines
whether traffic will be detected. A GMTI radar must distinguish a moving target
from ground clutter by using the target’s Doppler signature to detect the
radial component of the target’s velocity vector. To capture most of this
traffic, even when it is moving almost tangentially to the radar, a system must
have the ability to detect very slow radial velocities. As the radial
component of a target’s velocity approaches zero, the target will fall
into the clutter or blind zone. This is calculated as:
Any target with a velocity less than this minimum cannot be detected because
there is not sufficient Doppler shift in its echo to separate it from the
mainlobe clutter return. = Area Search Rate =
The area coverage rate is proportional to system power and aperture size. Other
factors which may be relevant include grid spacing, size of the power amp,
module quantization, the number of beams processed and system losses.
= Stand-off Distance = Stand-off distance is the distance
separating a radar system from the area it is covering.
= Coverage Area Size = Coverage area size is the area that the
system can keep under continuous surveillance from a specific orbit. Well
known design principles cause a radar’s maximum detection range to depend on the
size of its antenna, the amount of power radiated from the antenna, and the
effectiveness of its clutter cancellation mechanism. The earth’s
curvature and screening from terrain, foliage, and buildings cause system
altitude to be another key factor determining depth of coverage. The
ability to cover an area the size of an army corps commander’s area of interest
from a safe stand-off distance is the hallmark of an effective, advanced GMTI
system. = Coverage Area Revisit Rate =
This equates to the frequency with which the radar beam passes over a given area.
Frequent revisits are very important to the radar’s ability to achieve track
continuity and contribute to an increased probability of target
detection by lessening the chance of obscuration from screening by trees,
buildings, or other objects. A fast revisit rate becomes critical to
providing an uncorrupted track when a target moves in dense traffic or is
temporarily obscured, if only by trees along a road.
See also Doppler radar
Pulse-Doppler radar References