Note: Descriptions are shown in the official language in which they were submitted.
1341295
This invention relates to an optical scanning,
tracking, surveillance and laser weapon firing apparatus
and system and subcombinations thereof.
The need for accurate location and surveillance
of a scanned moving object manifests itself in a number of
military and civilian contexts. The system to be described
will be reviewed in a number of specific contexas, but it
will readily be understood that the system and apparatus
according to the invention can be considered for use wherever
there is a problem of precise location and tracking of a
moving object which is sightable by optical (including
ultra-violet and infra-red) means.
For example, improved precision. in the firing
of guns and rockets requires accurate angular information
(both azimuth and elevation) for control of they firing of
such weapons. A number of the previous proposals for
scanning and tracking are limited by reason of inferior
angular resolution, difficulty of adaptation to night opera-
tion or operation in fog, lack of capability for passive
operation only, problems associated with the display of the
received information, problems in coupling to manual or
automatic tracking devices, etc. A drawback with many prior
systems is their inability to function unless t:he equipment
stays in a fixed (usually land borne) position, which prevents
their utility in ships, airborne equipment, etc.
It is an object of the present invent:ion to
provide an optical scanning,tracking, surveillance and
weapons firing system and apparatus which avoids some of
the problems associated with previous equipment. proposals.
It is a specific object to provide such a system and apparatus
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1341295
in which at least limited movement of a vehicle in which
the apparatus is used can be tolerated. It is a further
object to provide alternative active and passive modes of
operation of such system and apparatus.
In its broadest aspect, the present invention
provides optical scanning apparatus comprising an optical
detector sensitive to optical radiation and providing an
electrical analog signal representative of the detected
optical radiation, scanning means for scanning optical
radiation within an adjustable field of view and directing
the scanned radiation to the detector, an anolog-digital
converter responsive to the electrical analog signal pro-
vided by the detector and producing digital information
representative of information content of the detected optical
radiation, display means for visually displaying the digital
information stored in the memory, a memory for receiving the
output of the analog-digital converter and staring the
digital information, processing means for processing the
digital information stored in the memory, and means for
updating the information stored in the memory with digital
information representative of subsequent. scans. of the
field of view.
The present invention is intended for optimum
application in situations in which the field of view is
confined to a relatively small angle (e.g. 2° x 2°). It is
accordingly proposed that the apparatus and system according
to the invention be used in conjunction with other scanning,
tracking, locating and positioning apparatus that can
locate the target object within a viewing angle corresponding
to the field of view of the system according t:o the
invention.
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1341295
It is further proposed that in order to avoid
the disadvantages associated with night operat_Lon, fog,
etc., that the sensing capability of the apparatus extend
into the infra-red wavelength range (e. g. 8-l3~am using
MCT or PbSnTe detector, or 3-Sum using an InsB detector)
and much of the description to follow will presume that
infra-red sensing and detecting apparatus is employed.
However, it is to be understood that in situations in which
night viewing, fog, etc. are not required, the sensing
and detecting equipment could be selected for use in
monitoring radiation in the visible range or even the ultra-
violet range, if so required by any particular application.
In active mode, it is proposed that t=he apparatus
include a directed laser beam whose reflected radiation
from a target is sensed and detected. However" it is some-
times desirable that the monitoring system operate in passive
mode, i.e. sense and detect only that radiation produced by
the target itself. For example, it may be des_Lrable that
ships be able to operate under radio-silent and radar-silent
orders so as to avoid long-range detection.
A preferred embodiment of the tracker according to
the invention is directed to the detection of a target
within a spatial cell of a few degrees in elevation and
azimuth (a typical one being 2° x 2°). A search system
(radar, infra-red or optical) can be used to provide coordinates
to center the tracker along the cell axis and :From then on,
after acauisition, the tracker locks itself on the target.
The optical head scans the full observation ce:l1 continuously
and the signal is fed on a push-through memory of small size.
This memory is read rapidly for presentation on a suitable
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141295
display, e.g. a television-type display on a s~~reen
monitor. From the display, the operator can possibly
obtain target identification and/or make a choice of a
single target in a cluttered field. Target tracking can
be done for example manually with a joy-stick looking at
the target movement on the display screen, or 'through a
penlight designation on the monitor screen, or through the
memory itself.
The optical system can be designed to illuminate
the tracking field with a continuous wave laser and thus
provide an active "signature" from the target casing the
reflected light. The system can be designed to have no
parallax and, using two detectors, both active and passive
signatures could be obtained simultaneously. 'They could
be subtracted or added before loading the memory, or else
loaded alternately in the memory and shown on the screen.
A colored display could also be used to show both images
simultaneously.
One advantage of the system is the large observa-
2G tion field presented to the operator. This should permit
the relaxing of rigid stabilization requirements (of
importance at sea). A differential sensor can be used to
record the very precise movement of the sensor between
frames (or better, between the moments where the target
itself is picked by the sensor) and this information could
be used to put an electronic bias on the display so that
the selected target point will appear at a very precise
and stable position. The information could also be used to
obtain exact target coordinates describing the target
30 position and movement.
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1341~~~
A preferred embodiment can be described generally
as follows:
An optical detection unit, similar t:o a telescope,
is used to focus the radiation on a far infra-red detector.
A mechanical scanner centers the field to be ~~canned within
360° in azimuth and say, some 70° in elevation. Around
this reference line, an optical field of say __+1° x ~1° is
looked at and the temperature variations in this field are
transformed into electrical pulses through a detector/ampli-
fier unit. If passive operation only is desired small mirrors
(or prisms) near the focal point can be used t:o scan the
tracking cell. The detector signal, after amplification,
is sent to an analog-to-digital converter and a buffer, the
sampling action being determined by position reference
pickups on the moving head to ensure that the sampling of
the digitized point is independent of any scanning speed.
The digital signal is accumulated line by lines in a buffer
and dumped in a circulating push-through memory. The memory
unit can be relat.ivel.y small (about 5K bytes for a total
field-of-view of 2° x 2° and an instantaneous field of 0.5
mrad). The rapidly circulating memory is real, by a pick-up,
and the signal after passing through a digital.-to-analog
converter is fed to a screen display, with proper X and Y
positioning, at a rate of 20-30 frames per second. A
flicker-free display is obtained with excellence grey scale
contrast.
After the tracker is positioned around the target,
as soon as the detection is made, the target point appears
on the screen, and can be followed and centered by a selected
means, such as one or more of the following means:
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1341295
(a) One target point could be selected and centered on
the screen through a manual displacement of the central
optical axis using a joy-stick.
(b) A penlite on the screen with automatic feedback could
be used to designate one target, and from the spot
position on the screen a feedback loop could be used
to center the optical axis of the system.
(c) Using the memory itself any target designated by a
penlite could be located exactly in an additional bit
plane and this exact location could be read to provide
very precise coordinates of a many target field without
having to center the optical axis. The memory itself
could be used to center a single point on the optical
axis.
The advantages of this last feature are significant in an
environment where fine stabilization is difficult and
expensive (e. g. at sea). For example, it may be desirable
that ships be able to operate in radar and radio silence to
avoid detection at very large ranges in threat: conditions.
On the other hand, the sensing station may itself
be a target, since it may not be able to avoid producing
infra-red radiation, and may thus be vulnerable to infra-red
seeking missiles, for example. Active countermeasures may
require accurate missile detection.
It is therefore desired to utilize t:he laser beam
for active target detection, and optionally, for target
destruction. One of the most difficult problems is the
pointing of the laser very accurately so that the narrowest
beam and the maximum energy concentration are realized.
The only technique previously known to achieve
an extremely accurate positioning is through a slave system
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1341295
in which one unit (search or track) gives out the target
coordinates very precisely and these coordinates are used
to position the laser beam. Any calibration .error between
the angular position of one of the two .instrwnents means
that the target is missed when the error is larger than
about 0.5 mrad. This is even more difficult .in an environ-
ment such as at sea where a platform stabilization better
than 0.5 to 1 mrad is very difficult and expensive. In
most previously described cases, there :is no coordination
1.0 between the search/track unit and the laser faring so that
damage assessment is difficult, if not amposs:ible.
The principles of this aspect of thc= invention
can be summarized as follows: A small space cell (ex. 2° x
2°) is scanned in elevation and azimuth by a mirror outside
the telescope unit. The signal is fed on a push-through
memory, the content of which is refreshed with each frame
scan. The memory itself is scanned very rapidly and an
image of the track field is displayed e.g. in a flicker-free
way on a screen. Using at least some of the same optics
20 as used for optical tracking, a continuous wave laser can
be used to illuminate the space cell and reflE>_cted signals
from the target can be detected.
Once the target is detected and identified, it
is possible to predict the target position in the next frame
scan very exactly, and when passing over that position, it
is possible to fare a pulsed laser of any power level with
a very good probability of success. The image on the
display can be frozen in the memory after firing, if desired,
and an assessment of the damage can be done irnmediately.
30 If the target is moving, it should be possible to follow
the path by processing the memory content of consecutive
1341295
frames and thus track a single target, or one target that
has been designated among many using, for example, a
penlite. With this information, it should be possible to
include in the prediction the target movement, as well as
any movement of the tracker to center the target in the
optical field, and to fire the laser at the exact point
in the optical field where the target will be. A third
correction should be possible to cope with any fluctuation
in the platform. Assuming a stabilization of 3-5 mrad is
1.0 possible, fine sensors on the platform can measure the
continuous movements between frame scans - there would be
no need for long-term accurate reference - and feed that
correction to the prediction unit. The target point could
be taken as a reference to apply the correction so the point
should appear without any blurr on the screen, except for
the changes in the optical axis orientation o:r the target
movement itself.
The present invention offers a uniq,ae means of
positioning a laser beam on a target without centering an
20 optical system, or bore-sighting it to a second search/track
unit. Ranging can be done while scanning, and distance
can be obtained on more than one target in the=_ field-of-view.
Damaging by laser weapon - if feasible - can also be achieved
effectively.
The invention will now be described with
reference to the accompanying drawings in which:
Figure 1 is a schematic view of the optical head
according to one embodiment of the present invention.
Figure 2 is a schematic view of an alternative
~0 optical head for use in an embodiment of the present
invention.
_ g _
141295
Figure 3 is a schematic block diagram showing
the signal processing system for use in association with
the optical head of Figure 2.
In Figure 1, a scanning mirror 10 is shown
pivotally mounted between upright support posits 12 and 14.
The angular position of the mirror 10 (i.e. angle of
elevation of the scanning or scanned beam) is adjusted by
elevation drive motor 28. An elevation angle position sensor
may suitably be incorporated into the same structure generally
indicated by reference numeral 25. The suppor t posts are
mounted on a rotatable turntable 16 having a central
aperture 18 through which light received from the scanning
mirror 10 can pass through a lens 20 to optical detector 22.
The turntable 16 is driven by a servo-drive motor 24 and
the angular position of the turntable may be :>ensed by a
separate sensing element 26 or instead by a sE~nsing element
(not illustrated) associated directly with the drive motor 24.
It is contemplated that the scanning mirror 10
will be oscillated so as to provide a scan of the field of
view within a narrow spatial range, e.g. 2° x 2°. For
this purpose a mechanical oscillator 25 is shown schematically
in Figure 1 for oscillating the mirror rapidly within say
1° of a central reference axis.
The overall mounting for the unit i7_lustrated
schematically in Figure 1 is not shown but it is contem-
plated that the unit would be gyroscopically nnounted if
located on a moving vehicle, for example. Thp~s would tend
to avoid inaccurate azimuth or elevation detei:minations
from the scanning unit.
The optical head of Figure 1 is capable of
detecting passive radiation from a target. However, it may
_ g _
1341 X95
be desired to direct a beam of radiation at the target
and to monitor the reflected radiation. For :such purpose,
the apparatus schematically illustrated in Figure 2 is
preferable to that illustrated in Figure 1. .Cn Figure 2
the scanning mirror :LO is shown located above telescope
mount 30 which should be isolated from the scanner mount 32
so as to avoid transmission of any vibrations or the like
from the scanner mount. On the isolated mount 30 a
telescope generally referred to by reference numeral 34
associated with a prism structure generally referred to
by reference numeral 36, and a laser 38 are mounted. The
optical axis is folded by means of the prism structure 36,
which is thus adapted to pass received radiation from the
mirror 10 to the telescope 34 and adapted to t=ransmit to
the mirror 10 (and thence outwardly to the target) radiation
generated by the laser 38. To this end the prism structure
36 is provided with an outer set o.f prismatic reflectors
40 which pass radiation received from scanning mirror 10
through the telescope 34 to optical detector 22. An
interior space 42 is provided within the prisrn structure 36
through which radiation received from laser 313 may be
directed via prism reflector 44 to the scanning mirror 10
and thence to the target. A masking element 43 is provided
within the telescope 34 to block any stray radiation from
the laser 38. The radiation provided by the .Laser 38 may
simply be scanning radiation intended to be reflected back
from the target received by the scanning mirror 10 and
directed to the optical detector 22. Alternal~ively, if the
laser 38 is a very high powered laser, the beam provided
may be of sufficiently high intensity to damage the target,
in which case the laser 38 in conjunction with the optical
head constitutes a weapon.
- 10 -
1 341 29 5
A principal disadvantage of the arrangement of
Figures 1 and 2 is that the scanning mirror 10 is relatively
large, thereby giving rise to difficulties in generating
a rapid and precise mechanical scanning action. A small
mirror (not shown) positioned for oscillation just before
the focal plane should enable the scanning rate to be
increased. However, oscillation of the larger mirror 10
in the embodiment of Figure 2 does entail these advantages:
(a) The detector is always on (or very close to) the optical
1C1 axis of the telescope; therefore, off-axis aberrations
do not affect the performance of the system. Simple
and low cost telescopes can be used. Chromatic
aberrations are eliminated by the use of :reflective
optics. Since the detector is outside thf>_ scanning
head, its signal is not carried through s:Lip rings,
possibly generating additional noise.
(b) When properly adjusted, the laser beam should coincide
with the optical axis of the system, whersaver it is
directed. There is no parallax problem. This property
20 is the basis for the laser weapon firing to be discussed
below.
(c) The image is always erect. If instead a :Focal scan
were to be used, the image would rotate w_Lth the
azimuth position, and it would be necessary to compen-
sate exactly and continuously in the disp7Lay and the
operator control or programming circuitry for that
image rotation. Such disadvantage might be less
important in the tracking mode, but proper inversion
compensation would be required for proper tracking.
30 (d) The design is very versatile in the choice' of detector
size, operating wavelength and detector mixes. For
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1341295
example, the following modes of operation are
possible:
Single Detector - This option includes a single
detector of a given size within a given wavelength band.
A filter can be used for further selection.
Multi-Spectral - This option permits the choice of
various detectors covering different wavelengths.
This is especially worthy of consideration if one
detector is matched to a laser (CW or pulsed) while
7.0 a second one provides a passive image.
Multi-Spatial - This option compensates for the scan
speed in, fox example, the passive surveillance mode.
A small detector could be surrounded by a much larger
one at the focal plane. A low-resolution moving map
presentation cou:Ld be shown on the display: if any
anomaly or target were detected, a sector scan at high
resolution could provide proper identification.
Figure 3 illustrates schematically an associated
signal processing and control system for use with the optical
20 head of Figure 2 (it being understood that a ;similar system
could be used in conjunction with Figure 1, the major
difference being that the system of Figure 3 :includes means
for controlling the operation of the laser unit 38 whereas
no such unit is illustrated in Figure 1 and thus that part
of the apparatus of Figure 3 would not be required for use
in conjunction with the Figure 1 optical head).
The scanning unit of Figure 2 is indicated
generally by the reference numeral 46 in Figure 3, the
laser unit 38, however, being illustrated separately for
30 purposes of explanation. The received optical signal and
any transmitted optical signal are processed 'through the
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1341295
scanning unit 46, the signal paths being shown in further
detail in broken lines in Figure 2. Three es:aential types
of information are generated from the scanning unit 46,
namely the optical signal received, the angle of azimuth,
and the angle of elevation. For coarse angular information,
separate azimuth sensor 26 and elevation sensor 50 are
illustrated in Figure 3, although a combined :sensing unit
for both azimuth and elevation might conceivably be provided
in an appropriate case. It will be further appreciated that
the transient scanning position of the scanning mirror
both in elevation and azimuth must be determined as well
as the average angular position, the former being governed
by the oscillation of the scanning mirror 10 by the mechanical
oscillator 28. The transient scanning azimuth and elevation
information will generally be detected in association with
the detection of the optical information, and passed to the
signal-processing equipment as raster information along with
a brightness or intensity signal.
An analog-digital converter 58 receives as an
input the sensed optical information and at least the
scanning position information or raster information which
has been received from scanning unit 46 by the>_ optical
detector 22 and which may be amplified by an <~mplifier 60
prior to passage to the analog-digital convert=er 58.
(Because of the difficulty in operating directly upon
the analog signal, the analog-digital converter 58 provides
the essential angular and optical information in digital
format.)
The digital information is fed sequentially to
a push-through memory 62, which may receive the information
at a relatively slow or even irregular input rate. If the
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1341295
image field is progressively changing and is scanned
sequentially, each new line being entered in 'the fully
loaded memory pushes the oldest entry off the memory
(moving map presentation). If, however, the .image field
is fixed (sector scan), whenever the oldest en try line is
scanned again, a new line is fed in, and the oldest drops
out; the replacement proceeds until a fully rc=_freshed
image is available in the memory.
Although the memory loading can proceed at a
random rate, the reading of the memory can be made at a
very fast rate, even one compatible with a television
raster. One thus obtains on such display a flicker-free
image of very high quality as well as a unique tool for
advanced signal processing.
Since the digital memory 62 can accommodate a
slow or irregular rate of input, while the oui=put can be as
fast as desired. the possibility arises of using the azimuth
and elevation axis information to command the sampling and
synchronization. In this way, assuming no vibration, any
given point can be referred to an absolute sp<~ce position.
Two encoders 52, 54 are used with fine angular resolution,
and the encoder marks are simply counted relative to an
arbitrary reference point. Thus, any identif_Led point in
space can be relocated precisely once the insi=rument is
operating. This pro<:ess is independent of scanning speed,
overshoot and speed variations. The image on the display
(to be described below) is always linear in a:,~imuth and
elevation. This feature is especially important for co-
ordinate transfer in a fire control role for :Laser 38, for
example.
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~~4~zs5
To this end, the markers from the encoders 52,
54 are added in a programmer 72 and this information is
used for the azimuth and elevation drive control, provided
by azimuth and elevation drive control units 24, 25
respectively. The elevation encoder also provides sampling
command and synchronization references in unit 56 for the
required analog-to-digital sampling. This information is
passed by unit 56 to analog-digital converter 58. The
programmer 72 itself is controlled by either a manual input
via operator control unit 68, or by an automatic servo
loop including signal-processing and computational unit 72.
Horizontal displacement may be controlled through
the programmer 72 by the vertical position of the mirror
10 and the angular width of the vertical scan line. Lt
is thus possible to have an optimum line-to-line match
strictly tied to the vertical scanning position independent
of the vertical scanning speed and its fluctuations. This
is quite useful whenever two detectors of different
horizontal width are used in the instrument (zoom effect);
the horizontal speed may thus be automatically controlled
by the selection of any one detector.
The display unit is illustrated generally by
reference numeral 64 and may include an appropriate digital
analog converter for the continuous presentation of the
received and stored optical information. The display
unit 64 may take its input directly from the ;push-through
memory 62 or from a stable storage medium such as tape or
disc storage 66 whose recorded information has been obtained
from the push-through memory 62. The storage facility
may be useful for later interpretation and a valuation
although of course if the system of Figure 3 is being used
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1 341 29 5
actively for tracking a target and possibly controlling
weapons brought to bear on such target. the tape or disc
storage 66 would not be used and the display 64 would be
fed directly through the push-through memory 62.
Since the memory 62 when loaded, can be read at
any speed, an obvious choice for display is a format
compatible to commercial television, so that commercial
equipment could be used for recording or displaying the
memory content. The digital data in the memory is thus
:LO preferably read in a television scanning format, converted
to analog and displayed directly. In advanced signal
processing, false or pseudo colors may be used to improve
detectability of targets: the processed image can be fed
to a regular color television unit.
The presentation on the display may be varied
depending upon the mode of operation. For example, the
following display presentations may be provided:
(a) A Moving Map type presentation showing the horizon
line around, say, a ship, moving very slowly on the
20 display.
(b) A Sector Scan type presentation, where a horizon sector
of, say, 10°, centered at any azimuth point, is displayed
continuously while being scanned. The information
regarding this sector could be refreshed rapidly, say
every few seconds.
(c) A Tracking Mode where any space sector (e.g. 2° x 2°)
around, say, a ship (using for example -10° to +20°
elevation; 360° azimuth) can be imaged on the display
and refreshed every few seconds. This approach should
30 permit a good quality flicker-free presentation of the
full tracking field at every instant, and permits the
following means of operation:
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1341295
MANUAL TRACK: This could be achieved using a simple
joy-stick to center the target as seen on the display.
Selection of one of many targets should be feasible.
DISPLAY TRACK: Using the display itself, one target
could be designated in the field by a pen7.ite sensor
or a movable crosshair indicator, and the system optical
axis could be moved automatically to center that
target on the display and in the field of view.
MEMORY TRACK: Using the digital memory and its
inherent ability for signal processing and data location,
it is possible to conceive a simple hands-off operation,
wherein a well-identified target point in the memory
is displaced towards a central position through
appropriate signal processing and automatic servo-
displacement.
POSITION AND RANGE INDICATION: Although n.ot an imagery
presentation, it would be relatively easy to use the
memory for obtaining the exact position of a target
anywhere in the tracking field. The target could be
designated on the scope and precisely positioned in
the memory, or else the memory could do an automatic
target selection. If the instrument. orientation is well
established, very accurate digital values for azimuth
and elevation should be available for fire control and
displayed on an indicator. Values for range could also
be displayed if a laser range finder is used. If many
targets appear in the tracking field, the coordinates
of each could be displayed in sequence or on a series
of digital indicators. Angular rates and range rates
could also be obtained.
As suggested above, the interaction of the operator
with the display information is variable depending upon the
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1 341 29 5
kind of display presented and the kind of control that is
desired over other operations. Accordingly, a wide
variability of system and circuit design can be provided
to govern these interrelationships. Genera115r, the
operator control unit 68 permits the operator to receive
information from the display 64 and optionally to control
the display in some manner as by stopping the displayed
optical picture at a specific point in time (for example,
for the purpose of evaluating damage done to a target).
But the information provided by the push-through memory 62
may instead be automatically processed by the signal-
processing and computational unit 70, or this unit 70 may
be wholly or partially under the control of the operator via
control unit 68.
The computational unit 70 may include circuitry
for the provision of target enhancement and false target
elimination. Means for improved target visibility for an
operator subject to fatigue can also be provided by the
use of false or pseudo color, or by moving target designa-
tion.
An exemplary system may include the following
system parameters:
MOVING HEAD CHARACTERISTICS
Clear Aperture through Mount 8 in Dia.
Elevation Limits -10° t:o +20°
Azimuth Limits none
Maximum Elevation Tracking Rate 3 rad/s
Maximum Elevation Acceleration 560 rad/s2
Maximum Azimuth Tracking Rate .15 rad/s
Maximum Azimuth Accelleration 85 ra~i/s2
Maximum Azimuth Slew Rate 300°/:>, 5 rad/s
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1341295
Elevation Incremental Sensor 100 lines/°
Azimuth Incremental Sensor 100 lines/°
Elevation Position Accuracy ~.O1°,/0.17 mrad
Azimuth Position Accuracy ~.Ol°,/0.17 mrad
OPTICS
Cassegrain Reflecting System
Primary Minor Diameter 8 in
Focal Length 12 in
Effective Focal Ratio f/1.9
Focus infinity
Telescope Limiting Resolution 0.1 mead
DETECTOR
Atmospheric Window 8-13 p
Cooling Temperature 77°K
Photovoltaic Type PbSnTe
Element Size 0.15 i_ .02 mm2
Cold Shield 32° ~ 2°
Cooled Optical Filter (a at 50~) 8-12.4 a
D* (10.6 u, 20 KHz, 1 Hz, 32°) 2 x 1010 or better
Optimum Sensitivity Range
Wavelength (T 850) 8.4 to 12.0 a
Preamplifier Gain 200
Noise Figure 2.0 dE9 or better
Bandwidth 5 Hz t:o 150 KHz
DIGITAL PULSE MEMORY
Elevation Sampling Resolution 50 elements/°
Azimuth Sampling Resolution 50 lines/°
Elevation Memory Format 128 el.ements/line
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1341295
Azimuth Memory Format 485 lines
Detector Level Format 6 BITf.
Input Line Frequency ~'60 li.nes/s
Input Frame Time for 9.6° Field of
View in Azimuth 8 s
TV DISPLAYS
Full Scan Picture Format (Full Memory) 2.56° x 9.6°
Horizontal Line Rate 15750/s
Vertical Frame Rate 30/s
Line Interlace 2:1 positive
Geometric Picture Distortion less than 1.5~
IMAGING PERFORMANCE
Resolution at the display 1.2 c/mrad
Sampling Density 2,86/mrad
Optic System IFOV .5 mrad
Such system would be capable of the following
performance:
(a) Station-Keeping/Collision Avoidance
360° azimuth coverage / 2.56° elevation
Moving map display / 9.7° x 2.56° sector scan
Time to complete one rotation in azimuth: 5 min
(b) Tracking
2.56° x 2.56° space cell coverage
Centered from -10° to +20° elevation / 360°azimuth
Time to update the space cell display: 2.2 s
Circuit and mechanical details for the blocks of
Figure 3 will vary widely from case to case and will therefore
not be further described. Their design will be achievable,
for any particular application, by a person skilled in the
art.
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13~+~ 295
The operation of the system of Figure 3 will
be further elaborated with reference to some of the
operational modes referred to above:
1. Station-Keeping and Passive Surveillance
Passive surveillance at sea covers t:he search and
track of any object, flying or floating, while the radars
are kept silent. Station-keeping is a more restricted
activity the aim of which is to operate many ships effec-
tively while maintaining proper position; collision avoid-
ance is of extreme importance whenever ships have to operate
close together or are in a closing course. Floating objects
on the sea surface are generally moving at a relatively
slow pace, so functions like station-keeping or collision
avoidance can be performed effectively using a scanning
instrument operating at low speed.
In the above-mentioned exemplary in~~trument, the
operation of scanning mirror 10 for an 8-inch telescope
would be limited to an oscillation rate of abc>ut 30 Hz (i.e.
60 lines per second). With 50 lines per angular degree,
5 minutes would be required to cover the full horizon around
the ship; the horizon line being displaced at a rate of
1.2° per second. Assuming a display coverage of 485 lines
with 128 elements per line, a sector of the horizon of
9.7° x 2.56° would be visible shifting around at a rate of
1.2°/s in a moving map presentation with a sampling frequency
of about 2.86 cycles/mrad. Such a system would give 4.2 ft
resolution at 1 mi, 16.8 ft at 4 mi and 42 ft at 10 mi.
With an adequate signal-to-noise ratio, a patrol boat
should be identifiable at more than 1.5 mi and a destroyer
at more than 5 mi.
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1341295
The limit imposed on the scanning speed of the
horizon line by the heavy mirror displacement could be
eased, if two detectors were used - a larger detector
element for coarse detection and a finer one used for
identification in a sector scan mode. For example, if the
horizontal resolution is decreased by a factor of 20, the
entire horizon will be scanned in about 18 s, giving good
detection range (the larger cell noise being possibly
compensated by the narrower bandwidth), and good identifica-
tion should be possible of close-by vessels a:~ well as
excellent protection against collision. Whenever detection
occurs, it would be possible to switch to a high resolution
sector scan (zoom effect). The best operating conditions
in such a compromise would depend upon t:he operational and
tactical requirements.
The sector scan could be used to cover one segment
of the horizon of 485 lines. At a rate of 60js, this
segment could be refreshed once every 8.08 s by using a
horizontal scan of a triangular or sinusoidal form. This
should be more than adequate for following a manoeuvering
ship nearby. High repeatability could be insured through
the stability of the position encoders. If the surrounding
ships are well identified, it is possible that: some range
information (perhaps imprecise) could be obtained from the
size of the ship as shown on the display.
2. Tracker Mode (Passive)
It is preferable to use the information of a
search radar, or that of an infra-red search unit, to
position the instrument of the above-described. exemplary
system within a couple of degrees of the target, and,
whenever the target appears on the display, to perform the
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1341295
fine tracking task using the present invention. The ability
of the system to present a very steady image of the full
tracking field is an obvious advantage in the case of
multiple targets or where it is necessary to precisely
follow the effect of some counter-attack on tlhe target.
The tracker in the above-described exemplary unit
will cover a field of 2.56° x 2.56° centered on any point
from -10° to +20° elevation in a 360° azimuth. The
tracking
cell image will be updated every 2.2 s, and an angular
1.0 displacement of the target of, say, less than 0.1°/s,
would ensure adequate tracking capability. Tlzis limit may
be acceptable for an attacking missile manoeuwering slowly
or for a helicopter being brought in for docking. In those
cases where the azimuthal rate is slow enough, the dynamic
firing of laser can be used in connection with the tracking
mode (see below).
By the addition of a two-dimensional scanning
mirror near the focal point, it would be poss_Lble to
accelerate considerably the scanning rate up i.o say, 100
20 frames per second. It would be possible in this case to
follow a target at the speed limit of the scanner drives.
The display could be direct on a screen without digital
memory. Some means to correct for the c~radual_ rotation of
the imagery as a function of the azimuthal position would
be required in the servo command, if not on the display, to
ensure proper tracking capability.
3. Laser (Active Mode)
The optical head design offers a unique
capability for pointing a laser along the optical axis of
30 the system at any instant during the scanning process. This
means that the full laser energy can be concentrated on the
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1341295
target without any energy spreading to cope with out-of-
alignment difficulties. Similarly the detector and telescope
can match closely the beam spreading of the laser, thus
reducing the overall noise.
Specifically a CW laser could be installed,
adjusted parallel to the optical axis and operated to
illuminate the target. thus generating an active target
signature. (Backscattering might limit the usefulness of
this approach.) A CW or quasi-CW laser could also be used
in a target marker or designation role {see be=low).
The most immediate use of the laser in this mode
is to obtain the range either for station-keeping or while
tracking. The coincidence of the laser beam with the system
optical axis and the prediction of the firing instant permits
a great economy of laser power as well as proi~ection against
the laser beam being intercepted through multiple pulses
and large beam spreading. Means can also be provided to
indicate on the display exactly where (or when) the laser
pulse was released. Ranging on ships during sector scan
sweeping is quite easy and would require a single pulse of
limited energy and, if in the nanosecond length range, the
pulse would hardly be detectable.
A second characteristic of the system is the
possibility of predicting within the memory the exact
location of the target. It is thus feasible t:o control the
firing instant of the laser through the memory while scanning:
this process is called dynamic firing. After proper location
of the target in the digital memory, either through designa-
tion on the display or automatic designation i.n the memory
itself, a pulsed laser could be fired during t:he following
scan at the exact moment when the optical axi~~ crosses the
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134195
target. The full energy of the laser could thus be
delivered directly on target without any appreciable
spoiling and without a difficult boresighting of two
independent pieces of equipment.
A second point to be mentioned is that proper
signal processing in the digital memory not on:Ly permits
exact firing at the proper point, but should also permit
fine correction for the following:
(a) Optical Axis Disp:Lacement while continuously tracking.
(b) Target Movement, since after a few scans and adequate
memory processing it would be possible to predict the
target position for the next few scans.
(c) Platform Movement, since sensitive analog position sensors
could be attached to the instrument suppori:ing platform
to provide fine position corrections relatp.ve to the
exact instant when the target was scanned. This correction
could be applied firstly to the display presentation
(x and y stabilization) so that a selected target would
appear very steady in the field being disp7_ayed, and
secondly, to the memory processor for accurate target
location prediction even if the platform is slightly
unstable. This platform movement correction on the
display arid in the memory is very important. since an
optical sea-borne system might otherwise require very
accurate platform stabilization at an exce;>sive cost.
If the stabilization requirement can be relaxed by a
factor of 5 or 10, the cost saving will be very large.
4. Dynamic Firing as a Weapon
The invention raises as a corollary t:he question of
the feasibility of using a very powerful laser beam in a
defensive role for destroying an incoming attacking vessel,
- 25 -
1341 ~9~
for blinding the pilot or for any other strong interaction
with the attacker. It would be necessary in any case, to
position very accurately the laser beam on the target and
avoid beam spoiling as much as possible. .The potential
of the dynamic firing of the laser while tracking is unique
because it offers the possibility of combining in a single
instrument observation of the tracking field as well as
control of the laser beam position, Damage can also be
assessed immediately in the next scan.
A possible approach would be to use a laser which
is transparent to its own radiation when not excited (e. g.,
the TEA C02 laser). A powerful system including an oscillator,
a preamplifier and a power amplifier could operate in a
ranging mode using only the output of the preamplifier
through the idling power amplifier. Whenever .damaging is
intended, the overall system would be fired in a dynamic
mode, and the full laser beam could be directed without
spoiling and with great precision, on the target.
5. Target Marker
A major threat against naval ships i;s the low
flying missile, or the sea skimmer. The best defence and
active countermeasure appears to be a direct attack against
the missile itself after detection. The possibility of
"marking" the missile with a laser beam (CW or quasi-CW)
and using an anti-missile-missile guided by the=_ reflected
light is a potential approach.
The system described above offers a unique possibility
of accomplishing the foregoing. A general characteristic
of an incoming missile is its very slow angular rate relative
3a to the target. Using focal scan, the missile is kept centered
in the display and a :Laser beam is directed through the
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1341295
optical axis directly onto the missile itself. The imagery
on the display will show immediately if the beam is really
on target by the strong reflection. It should thus be
possible to maintain a laser beam on an incoming missile
with minimum spoiling and, constantly through the display,
be sure of the effectiveness of the laser marking as well
as that of the anti-missile-missile.
Using a dichroic mirror to separate the passive
imagery from the laser return, a quadrant detector could be
used for controlling the tracking head, when the laser
return is properly detected. The passive imagcary would
provide an overriding means of recuperating the' target, if
it escapes from the quadrant detector field of view.
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