Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to optical radar of the type
which illuminates targets by means of a laser beam and derives
target information from the reflected laser beam. Such radars
usually include a scanning and tracking capability. The
scanning system moves the transmitted laser beam over the field of
view, usually in some systematic manner, for example with a
sawtooth scan system of the type used in television or with spiral
type scanning. If such a radar is provided in addition with a
tracking capability for moving targets, the scan format must be
randomly programmable so that random target movements can be
followed.
Scanner/trackers for laser beams may include a coarse
scanner, for example a wide angle, low speed, low resolution
scanner; with a narrow field, high resolution, high speed dither
scanner in series with the coarse scanner. With such a dual mode
scanner/tracker system, the coarse scanner may for example scan in
a sawtooth fashion with gaps between the scanning lines, with the
high speed dither scanner filling in the gaps. Thus the two
scanners complement each other. In the tracking mode both of
these scanner/trackers move in a programmed coordinated manner to
achieve target tracking.
Scanners of this type usually achieve laser beam
movement by means of moving optics such as rotating prisms or
wedges through which the beam passes or electrically driven moving
mirrors from which the beam is reflected. High speed tracking,
such as is required for the aforementioned dither scanner/tracker
requires extremely high power if the moving optics are located at
a point where the laser beam has its largest diameter. Laser
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radars normally include a means to expand the beam diameter before
transmission to improve angular resolution or provide greater
range.
According to one aspect of the present invention, there
is provided a dual mode scanner/tracker system for an optical
radar, said radar comprising a laser beam having magnified and
demagnified portions, a first scanner/tracker located in said
demagnified portion of said laser beam, said first scanner/tracker
being high speed, high resolution and having a narrow field; a
Ill second scanner/tracker located in the said magnified portion of
said laser beam, said second scanner/tracker being lower speed,
lower resolution and having a wider scan angle, relative to the
same characteristics of said first scanner/tracker.
According to another aspect of the invention, there is
provided the beam steering telescope of Claim wherein said first
scanner/tracker comprises a pair of orthogonally mounted
electrically driven reciprocating mirrors and said second
scanner/tracker comprises a pair of transparent rotating wedges
through which the said magnified laser beam passes. The beam
steering telescope is thus capable of scanning its field of view
360D in azimuth while the dual mode scanner/trackers are
operating.
In the accompanying drawings, which illustrate an
exemplary embodiment of the present invention:
Figure 1 is a block diagram illustrating the concept of
the novel dual mode scanner/tracker;
Figure 2 shows one way in which the novel concept of
Figure 1 can be implemented;
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Figure 3 shows additional details of the apparatus of
Figure 2; and
Figure 4 is a pictorial view of a beam steering
telescope in which the novel dual mode scanner/tracker is
integrated with the beam expanding telescope of the optical
radar.
The diagram of Figure 1 shows a portion of an optical
radar including narrow field scanner/tracker 5, a beam expanding
telescope 7 which receives the output of scanner/tracker 5 and
applies the expanded laser beam 13 to wide field scanner/
tracker 9, which radiates the laser beam into space, and receives
laser target echoes. The narrow field scanner/tracker receives
the narrow transmitted laser beam from the optical radar
transceiver circuitry to the left thereof, now shown, and also
applies the target echo signals passing there through to said radar
circuitry.
A known method of achieving efficient scanning/tracking
of optical radar beams is to provide two scanner/trackers in
series or cascade along the beam with one scanner/tracker having a
wide field of view, for example 60, low resolution and low
scanning speed. Such a scanner/tracker must necessarily be
located in the wide or expanded beam region of the laser beam.
The low resolution and low scanning speeds result in moderate
power requirements for moving the necessarily large optics over
such a large field of view. The low speed and low resolution of
such a scanner/tracker can be enhanced by a narrow field, high
resolution, high speed scanner/tracker in series therewith, with
the beam movements of the two scanner/trackers designed to
complement each other. For example, the narrow field
scanner/tracker may have a 1 yield of view, referenced to the
optical system output, which means that this scanner/tracker is
capable of high speed, high resolution movement of the beam over
this angle anywhere within the large field of view of the wide
field scanner/tracker. In Figure 1, the angle 15 at the radar
system output indicates the overall wide field of view of the
radar with the smaller angle 17 representing the field of view due
to the action of the narrow f old scanner/tracker. For example
the angle 15 may be 60 and the angle 17, 1. The narrow field
scanner/tracker could be located in the wide or expanded portion
of the laser beam to the right of the beam expanding telescope,
however the size of the moving optics required for such a location
would have to be at least equal to the beam size Since moments of
inertia of reciprocating or rotating mirrors or prisms go up with
the square of the diameters thereof, the power requirements for
achieving high speed, high resolution performance even over a
small angle can be prohibitive. Significant power and consequent
weight saving can be realized by locating the narrow field
scanner/tracker in the narrow or demagnified portion of the laser
beam, to the left of the beam expanding telescope as shown in
Figure 1. At this location, the scanner/tracker optics can be
scaled down to the approximate diameter of the narrow or
demagnified laser beam, however the scanned field of view must be
increased by the magnification of the beam expanding telescope.
For example, if the telescope 7 of Figure 1 has an focal
magrlification of 20, the beam 13 in the output thereof will have a
diameter 20 times the diameter of the beam 11 applied thereto from
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scanner/tracker 5, but the scan yield ox the expanded beam 13 will
be reduced from the angle 12 at the telescope input, also by a
factor equal to the telescope magnification. Thus if the scan
field or angle 17 at the radar's output is to be lo in this
example, the scanner/tracker 5 would be required to scan the
beam 11 over the angle 12 equal to 20. Even with this larger
scan field, the reduced size of the optics for the narrow field
scanner/tracker results in power and weight savings.
The diagram of Figure 2 is one example of how the
concept of Figure 1 can be implemented. In Figure I a portion of
the optical radar circuitry is shown, including an optical
duplexes 21 which directs the target echoes 37 to a receiver, not
shown, and passes the narrow transmitted laser beam 19 to the
narrow field scanner/tracker 25 via quarter wave plate 23. The
high speed, narrow field, high resolution scanner/tracker 25
utilizes a pair of electrically driven reciprocating mirrors 27
and 31 which rotate around orthogonal axes 29 and 33 respectively
to produce scanning or tracking with a bandwidth from do to over
l.kHz. The mirrors are electrically driven as indicated by the
arrows 35 labeled "Programmable Drives" and readouts 39 are
provided for indicating instantaneous mirror positions. Such a
scanner/tracker may have a 1 scan field, referred to the radar
system output, with 3 x 103 elements per field (circular field
with a diameter of 64 elements) and frame time of 1/30 second.
The effective aperture may be 0.5 cam with 0.02 random access
resolution and 0.26 milliseconds random access time.
The output beam 40 of scanner/tracker 25 is applied Jo
the input of the beam expanding telescope 43 via relay optics 41.
The relay optics may be required to keep the wide scan angle
output of the scanner/tracker 25 within the small entrance pupil
of telescope I The details of the relay optics and telescope
are illustrated in more detail in Figure 3. The output beam 45 of
telescope 43 will be a wide beam, for example 10 cm. in diameter
if the narrow beam 40 is 0.5 cm. in diameter and telescope 43 has
a rnagnifiacation of 20. Also the relatively wide scan field angle
of the beam 40 will be reduced by this factor of 20, for example
from 20 to 1 in the telescope output.
The wide field scanner/tracker 47 may comprise a pair of
in-line rotating wedges or prisms 49 and 51 with apertures or
diameters sufficient to accommodate the magnified scanned laser
beam applied thereto from telescope 43. Such a rotating wedge
scanner can have a total field of view of 60 with .6 random
access resolution and a 20 milliseconds response time. The wedges
49 and 51 are separately driven as indicated by the arrows 53
labeled "Programmable Drives" and each has separate readouts 55
for indicating the position thereof. These versatile programmable
scanner/trackers may be provided with a 16 bit optical shaft
encoder as part of the readout system thereof for accurately
monitoring the instantaneous scanner line of sight to within the
diffraction limited resolution of the radar which is
approximately 250 micro radians. Further details of such rotating
wedge programmable scanner/trackers will be found in a co-pending
Canadian patent application No. 426,681, entitled PROGRAMMABLE
SCANNER/TRACKER, Filed on 25 April 1983.
Dual in-line optical wedges may also be used for the
narrow field high speed scanner 25, because of their superior
performance in high vibration environments, for example such as
would occur in an optical radar installed in a helicopter. Small
aperture wedges for such an application would be competitive in
frequency response to the reciprocating mirrors shown, but would
require more signal conditioning to realize a tracking capability
because of their non-linear transfer function.
Also, rather than using a pair of reciprocating mirrors
for the scanner/tracker 25, a single mirror could be used, mounted
on dual gimbals which are separately driven by the x and y
LUG scanning signals. This arrangement may obviate the necessity for
the relay optics 41.
The details of the relay optics 41 and the telescope 43
are shown in Figure 3. As can be seen the beam expanding
telescope may comprise merely a pair of lenses 63 and 65 arranged
along the optical axis 0-0. A beam directed into ocular or
entrance pupil 63 will emerge from the objective lens I expanded
in diameter and with a reduced scan field as explained above.
The relay optics 41 may comprise, for example, merely a single
positive lens 61 positioned so that the narrow laser beam 40 from
scanner/tracker 25 is concentrated at the entrance pupil 63 of the
telescope 43, indicated by the converging rays 42.
Figure 4 shows a beam steering telescope which embodies
the dual scanner/tracker of the present invention mounted in a
rotatable turret 70 which is mounted on the underside of an
aircraft 71. In this embodiment the narrow field scanner/tracker
is integrated with the beam expanding telescope to reduce the
number of optical components. The narrow laser beam 79 is applied
to device 31 which includes both the narrow field scanner/tracker
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as well as the relay optics, if necessary, and the ocular lens of
the beam expanding telescope, such as lens 63 of Figure 3. The
beam 83 emerging from device 81 is reflected from fixed 45~ mirror
85 which is mounted along the axis of rotation 75 of turret 70.
The beam 87 then passes through the telescope objective lens 89
and is turned by another 90 by means of a second 45 mirror 91.
The beam 88 then passes through the wide angle scanner/tracker
WhiCtl may comprise the two rotating wedges 95 and 97 plus
ancillary apparatus, not shown, and emerges into space as the
scanning beam 77. The arrow 73 represents the rotation of the
turret around the axis 75.
While the invention has been described in connection
with illustrative embodiments, obvious variations therein will
occur to those skilled in this art, accordingly the invention
should be limited only by the scope of the appended claims.
