Communications: directive radio wave systems and devices (e.g. – Plural radar
Reexamination Certificate
2002-02-28
2003-09-02
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Plural radar
C342S02500R, C342S082000, C342S089000, C342S160000, C342S161000, C342S190000, C342S191000
Reexamination Certificate
active
06614386
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to radar systems. Specifically, the present invention relates to bistatic radar systems.
2. Description of the Related Art
In a mono-static radar system, the transmitter and the receiver are co-located. In a bistatic radar architecture, the transmitter and receiver are substantially separated. In addition, both the transmitter and the receiver may be mounted on either fixed or moving platforms. Bistatic radar is therefore distinguished from monostatic radar where the transmitter and receiver are mounted on the same platform and move together.
A characteristic feature of bistatic radar systems is that the transmitter, the receiver and the target, form an iso-range ellipsoid with the transmitter and receiver at the foci of the ellipsoid. In addition, the transmitter, the receiver and the target define the bistatic plane.
Further, the receiver, the target and the receiver's motion relative to the target instantaneously define a plane, known as the “receiver slant plane.” Similarly, the transmitter the target and the transmitter's motion relative to the target also define a plane, the “transmitter slant plane”, which is, in general, different from the receiver slant plane.
Any planar section through the ellipsoid is an ellipse. In particular, a plane tangent to the Earth's surface cuts the ellipsoid in such a way as to produce an ellipse. Similarly, the bistatic plane, defined by the transmitter, the receiver and the target, cuts the ellipsoid in such a way as to create an ellipse.
In conventional airborne, as well as most ground based, bistatic systems the bistatic plane is nearly parallel with, and close to, the tangent plane of the Earth's surface. In this circumstance, the ellipsoid approximately reduces to an ellipse which contains the bistatic plane and the velocity vectors of the transmitter, the receiver and the target. In this simplifying approximation, the receiver slant plane and the transmitter slant plane are practically coincident and both are practically coincident with the bistatic plane. It has therefore become a standard approximation of bistatic radar systems, that the motion of the transmitter and receiver lie within the bistatic plane. This approximate reduction of all system elements to a single plane greatly simplifies the analysis of bistatic radars. Unfortunately, the requirement that velocity vectors lie within the bistatic plane imposes significant constraints on the system and limits the operational flexibility.
A further limitation of conventional radar systems is that, in order to reach long ranges the beam must be narrow (i.e., the antenna gain must be high) so that the intensity of illumination falling on the target is sufficiently large for detection. This has led to a standard design approach whereby the narrowest illuminating and receiving beam is always considered the best.
In addition, when the ground is being illuminated from the air, long range observations result in a very shallow angle of illumination. The footprint of illumination is therefore spread out in a very long and narrow ellipse. For long range observation the parts of the ellipse that are near illuminate territory which is typically not of interest. Thus, much of the energy in the beam is wasted because it does not reach distant targets.
In some cases, such as air search radar, the beam is deliberately broadened so as to detect the presence of targets within a substantially larger volume of space. But the penalty for such broadening is a significant reduction in detection range. Thus, beam broadening is only occasionally pursued—and then only for specialty radars.
One consequence of this conventional design philosophy is that only a very small slice of territory can be examined by the radar at any given time. In order to survey a large range of territory the narrow beam is usually swept through an arc. As a result of this beam sweeping technique, only a small fraction of the accessible territory will be observed at any given time. Events in the un-illuminated territory are unobservable.
Bistatic radar consists of a separate transmitter and receiver. In normal ground observing bistatic radars, both the transmitter's illumination and the receiver's direction of observation are usually at a very shallow angle to the surface of the Earth. The intersection of the two beams is usually a very small patch because the angle of intersection of the two beams is usually substantially large. If the target area of interest is small, this patch can be continuously observed and the signal to noise ratio of the observation can be satisfactory out to a substantial range. However, since in general, both the transmitter and receiver are moving with respect to the target, special coordination between the illumination beam, receiver observation beams and the directions to the target must take place. This introduces a beam coordination problem known as the Scan-On-Scan beam coordination problem.
In the Scan-On-Scan operational mode a conventional bistatic radar illuminates a small region with a very narrow beam. When the transmitter beam moves, the receiver beam must move in a coordinated way to track the transmitter beam and follow a single target or small patch of territory. Alternatively, with Scan-On-Scan operation, the receiver beam can be fixed. In this case only a small area of territory is observed during the transmitter scan. Similarly, when the receiver beam scans, only a small area of territory is observed during each instant of the receiver scan.
FIG. 1A
displays a Scan-on-Scan operation, including a transmitter and receiver.
FIG. 1A
highlights the transmitter scan operation.
FIG. 1B
displays a Scan-on-Scan operation, including the transmitter and receiver shown in FIG.
1
A.
FIG. 1B
highlights the receiver scan operation. Both FIG.
1
A and
FIG. 1B
display a transmitter beam overlapping a receiver beam (e.g., item
120
). In
FIG. 1A
a transmitter
100
generates a narrow transmitter beam in a first position
102
and then scans through an angle depicted by
104
to a second position
106
. A receiver
108
is also shown generating a very narrow beam
118
. The transmitter
100
and the receiver
108
are also shown in FIG.
1
B. In
FIG. 1B
the receiver
108
generates a narrow beam
110
in a first position and scans through an angle depicted by
112
to a second position shown by
114
. During the respective scanning operations, the beam from the transmitter
116
, overlaps with the beam from the receiver
118
, in a very narrow overlapping region
120
as shown in both
FIGS. 1A and 1B
. The very narrow overlapping region
120
, is the observable target region of the system.
As shown in
FIGS. 1A and 1B
, if a bistatic radar is to be used to observe a large territory, the transmitter beam and the receiver beam must be separately scanned across the landscape. During these scans only a small fraction of the illuminating energy will find its way to the receiver at any given time. This means that for broad area observation, bistatic radars tend to be very energy inefficient.
When either the transmitter or the receiver is in motion, a bistatic radar system can create a high resolution two dimensional image of the landscape. (If the target is moving, but the transmitter and receiver remain stationary, a high resolution image of the target can similarly be constructed.) With motion of either the transmitter or the receiver the reflected signal will be Doppler frequency shifted as a function of relative motions and positions of the transmitter and receiver and the position of the target. In effect, the motions and positions of the transmitter and receiver paint the landscape with a spatial Doppler frequency gradient. If the transmitter and receiver are both moving in a similar direction the Doppler gradients add (in a vector sense) thereby creating a stronger Doppler gradient at the target. From the Doppler shift produced by this Doppler gradient it is possible to derive the a
Chen Pileih
Moore Kenneth L.
Richards Chester L.
Alkov Leonard A.
Gregory Bernarr E.
Lenzen, Jr. Glenn H.
Raytheon Company
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