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BACKGROUND OF THE INVENTION
Radar and sonar systems are frequently utilized for measuring the distance
between scatterers or reflectors of radiant energy as well as for
measuring the radial components of the velocities of the scatterers or
reflectors by means of Doppler frequency shifts. These measurements may be
obtained with either electromagnetic radiation or sonic radiation. While
various wavelengths and modulation patterns are selected for particular
situations, in the case of measurements of clear air turbulence,
electromagnetic radiation having a wavelength on the order of ten microns,
such as is provided by a carbon dioxide laser, is preferred. Such lasers
are operated either on a continuous wave (cw) or pulsed basis.
A problem arises when it is desired to obtain good range resolution and
good Doppler resolution at close range. As is well known, a precise
Doppler measurement requires a cw signal, or at least a long pulse signal.
In contrast, good range resolution by a pulsed laser radar requires the
use of relatively short pulses. At moderate ranges, a compromise can
usually be reached wherein the length of the laser pulse signal is
adequately long to achieve good Doppler measurements while still being
sufficiently short to permit good range measurements. However, at very
short ranges, such as one thousand feet, even a reasonably short laser
pulse signal would have a duration longer than the round trip propagation
time from the radar to the reflector and back to the radar. The existence
of the transmitted and received signal pulses simultaneously at the radar
precludes or greatly degrades a range measurement at the short ranges.
This problem is particularly acute for measurements in the vicinity of an
airport runway since such measurements may well fall within the
aforementioned one thousand foot range.
SUMMARY OF THE INVENTION
The aforementioned problem is overcome and other advantages are provided by
a radar system which, in accordance with the invention, combines a laser
with an optical focussing system wherein a variable focus is produced so
that the radiation from the laser can be focussed at varying distances
from the radar, the depth of field of the focus determining the range
resolution element or range cell while the distance of the focus
determines the range of the scattering center or reflector. Thereby, at
close-in ranges, such as the aforementioned one thousand foot region in
front of the radar, the radar is able to operate with a cw signal which
provides excellent Doppler resolution independently of the range
measurements, the range measurements being accomplished by the focussing
of the laser beam. Since the depth of field is a function of the focal
length or range to which the radar is focussed, the range resolution
element or range cell increases in size for increasing values of the focal
range. Accordingly, the invention includes circuitry for providing a
transition from the cw signal to a pulsed signal for use at longer ranges
wherein the range resolution capability of the pulsed signal exceeds that
of the cw signal.
The optical radar system of the invention further comprises circuitry for
the measurement of the radial component of the velocity of the aerosol
scatterers and, hence, the radial component of the velocity of the air
which carries the scatterers. The velocity data is obtained from a
measurement of the Doppler frequency shift induced upon the light
reflected by the moving scatterers. An interferometer combines signals
reflected back to the radar with a reference beam and a detector produces
the difference or beat frequency signal. The beat frequency is equal to
the Doppler shift. A comb filter circuit is disclosed for extracting
numerical values of the Doppler frequency and velocity from the beat
frequency signal. The transmitted light beam may also be scanned by a
mirror to provide directional data of the scatterers.
It is interesting to note distinctions between the instant optical radar
and that of a conventional radar, such as a radar operating at a frequency
of from one to ten gigahertz, with respect to a pulsed Doppler mode of
operation. The instant radar is concerned with range and velocity
measurement of aerosols. The aerosols are objects such as pollen or dust
such as, for example, volcanic dust which remains in the air for many
years because of its small particle size. A ten micron wavelength provides
a capability for observing the foregoing aerosols. The aerosols are
carried along in the air by the wind and have a velocity equal to the
velocity of the wind.
The pulse mode of operation, referred to above for the longer ranges
wherein Doppler data is obtained with pulsed laser signal, should be
distinguished from the conventional pulsed Doppler radar operating in the
frequency range of one to ten gigahertz. The conventional pulsed Doppler
radar utilizes a stable oscillator for detecting a succession of coherent
echo signals. Such coherence depends of the ratio of wavelength to target
size. While the conventional radar utilizes radiation with a wavelength on
the order of one-tenth meter, the radiation of the instant radar is much
smaller, on the order of ten millionths of a meter. As a result, the
conventional pulsed Doppler radar requires pulse to pulse coherence of the
carrier with many pulses being required for a Doppler measurement. In
contrast, the instant radar provides a Doppler measurement with each echo
pulse since there are many Doppler cycles occuring within the duration of
each echo pulse and, accordingly, no pulse to pulse coherence is required.
The depth of field and range resolution for short ranges are obtained with
the aid of a detector of optical signals such as an amalgam of cadmium and
tellurium, the detector having a diameter of magnitude equal to many
wavelengths. The combination of a reference signal of the laser with a
received optical signal produces a beat frequency signal having a
magnitude dependent on the distance from a scatterer to the focus. A
strong beat frequency signal is obtained from a scatterer at the focus
while a scatterer lying outside of the depth of field produces no more
than a weak signal of the detector. The focussed signal produces a plane
wavefront which combines with the plane wavefront of the reference signal
at the detector to produce the strong signal. The out-of-focus signal has
a curved wavefront which produces constructive and destructive
interference signals at the face of the detector with a resulting
diminished amplitude of signal from the detector. Thereby, the radar is
responsive only to signals near the specific range to which the radar is
focussed. At long range where the depth of field is large, the range
resolution is determined by the width of the transmitted pulse.
The depth of field varies in a quadratic manner with the focal range from
the radar. Thus, by way of example, considering the aforementioned ten
micron wavelength of the radiation, and utilizing a 12 inch f/2 optical
system, at a range of 300 meters the depth of field or range resolution
capability is 52 meters. However, at a range ten times as long, namely,
3,000 meters, the depth of field or range resolution capability has
increased by a factor of 100 to 5,200 meters. With respect to the
foregoing example, a focal range of 2,000 feet (600 meters) would serve as
an upper limit to the focussing region with pulse radar operation being
utilized at greater ranges, the region of the pulse operation being
referred to hereinafter as the pulsing region.
BRIEF DESCRIPTION OF THE DRAWINGS
The aforementioned aspects and other features of the invention are
explained in the following decription taken in connection with the
accompanying drawings wherein:
FIG. 1 is a block diagram, partly schematic, of an optical system for use
in the invention, the figure also showing timing circuitry, ranging
circuitry and Doppler circuitry utilized in the focussing of a lens of the
optical system and for processing received optical echoes for obtaining
Doppler and ranging data;
FIG. 2 shows the relative widths of transmitted pulse signals as compared
to the focal length or focal range at the upper limit of the focussing
region;
FIG. 3 is a block diagram of a range unit of FIG. 1 which provides a
transition between cw and pulsed operation of the system of FIG. 1;
FIG. 4 shows a diagram of the focussing unit of FIG. 1 including lenses of
the optical system utilized in producing the focussed radar beam;
FIG. 5 is a block diagram of a modulator of FIG. 1; and
FIG. 6 is a diagram of an optical scanner for use with the optical system
of FIG. 1 to produce a scanning optical beam.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, there is seen a radar system 20 embodying the
invention, the system 20 including an optics unit 22, a range unit 23 and
a data processor 24. The optics unit comprises a laser 26, a modulator 28,
an amplifier 29, an interferometer 30 comprising three beam splitters
31-33, mirror 34 and a half-wave plate 35, a polarization rotator 36,
mirrors 37 and 38 coupled mechanically and optically via lines 39 and 40
respectively to the rotator 36, a drive unit 41 for positioning the
mirrors 37 and 38 and the rotator 36, a detector 42, a negative or concave
lens 44 mechanically coupled via line 46 to a focussing unit 48 and a
positive or convex lens 50 supported by the focussing unit 48. The lens 50
is shown directing a cone of light 52 toward a focal point positioned on
an aerosol 54 which reflects radiant energy back toward the lens 50, a
second aerosol 56 being shown in an out-of-focus position between the
aerosol 54 and the lens 50. A dotted rectangle 58 represents a cylindrical
region enclosing the depth of field. Wavefronts 60 and 62 of reflected
waves propagate respectively from the aerosols 54 and 56 to the lens 50.
The data processor 24 comprises an amplifier 64, a set of band pass filters
65 of which four filters 65 are shown by way of example with individual
ones of the filters 65 being further identified by the suffixes A-D when
it is desired to refer to individual ones of the filters 65, amplitude
detectors 66A-D, analog-to-digital converters 67A-D and a selector switch
68. Each of the filters 65 with its corresponding detector 66 and
converter 67, such as the filter 65A, the detector 66A and the converter
67A, form a signal processing channel for extracting data relative to a
specific Doppler spectral region, the signal processing channels being
selected sequentially by the switch 68. The data processor 24 further
comprises an adder 70, a memory 72, an address generator 73 and a display
74 of Doppler spectral data as a function of the range of scatterers from
the lens 50 and also as a function of the scan angle of an optional
scanner to be disclosed with reference to FIG. 6, the scan angle being
coupled to the display 74 via line 75.
The memory 72 is portrayed as having a set of storage cells arranged in
rows and columns, the rows being identified in the figure by the legends
f1-f4 for the storage of frequency data, there being four rows shown by
way of example with each of the four rows corresponding respectively to
the four Doppler signal processing channels. The columns are identified by
the legends r1-r5 with each column representing a specific range from the
lens 50 for which Doppler data is being obtained. By way of example, five
such columns are shown in the figure, it being understood that, in
practice, many more of such rows and columns would be utilized. Each cell,
therefore, represents a stored value of a spectral region of the Doppler
data which is obtained at a specific value of range.
As shown in FIG. 1, the rectangle 58 representing the extent of the depth
of field is smaller than the focal range or distance of the aerosol to the
lens 50. This is a portrayal of the short range situation wherein the
extent of the depth of field is less than a distance to which the optics
unit 22 is focussed.
In accordance with the invention, the light beam coupled via line 76 from
the interferometer 30 to the lens 44 is pulsed at the longer ranges and
operated in continuous fashion at the shorter ranges. For pulsed
operation, the modulator 28 is operated by a signal at terminal T1 to
modulate the continuous beam of light from the laser 26 resulting in the
formation of pulses of radiant energy which are then amplified by the
amplifier 29 to a suitable power level for illumination of the scatterers
such as the aerosols 54 and 56. In the short range situation wherein cw
operation is utilized, the modulator 28 and the amplifier 29 are bypassed
by an optical path, or bypass, comprising the mirror 37, the line 39, the
rotator 36, the line 40 and the mirror 38. The optical bypass is activated
by inserting the mirrors 37 and 38 in position along the optical path from
the beam splitter 31 via the modulator 28 to the beam splitter 32. The
foregoing insertion of the mirrors 37 and 38 is accomplished by a drive
unit 41 which, in reponse to a signal on line 77, physically moves the
optical assembly comprising the mirrors 37 and 38 and the polarization
rotator 36. Use of the optical bypass prevents overheating of the
modulator 28 which might occur under a condition of continuous
illumination by the laser 26, the overheating resulting in a variation in
the optical properties of the modulator 28. As will be described
subsequently with reference to FIG. 5, the modulator 28 introduces a
rotation of the polarization vector of the optical signal incident
thereupon and, accordingly, the rotator 26 is employed in the optical
bypass to impart the same amount of rotation to the polarization vector of
the signal on line 39 so that the output optical signal of the
interferometer 30 appearing on line 76 has the same polarization whether
the optical signal be pulsed or cw. It is noted that the optical bypass
also bypasses the amplifier 29 with the result that the cw signal is of
lower intensity than that of the pulsed signal. However, a sufficient
amount of total energy for a Doppler measurement is produced during the
time of a measurement interval due to the continuous nature of the optical
signal on line 76 as compared to the relatively short duration, high
intensity signal produced by the pulses of the long range situation.
Considering the case of cw operation, the beam of light from the laser 26
propagates along the upper leg of the interferometer 30 via the beam
splitters 31 and 32, mirrors 37 and 38 to the lens 44 which diverges or
spreads apart the rays of light to illuminate the lens 50. Typically, the
lens 50 has a diameter of 12 inches to provide a directive beam of light
which is depicted in FIG. 1 by the cone of light 52 directed toward the
focal point at the aerosol 54. The beam splitters of the interferometer 30
are constructed of germanium so as to be transparent to the propagation of
the light of the laser 26, the beam splitters being coated so that a
portion of the light reflects from the surfaces of the beam splitters
while the remainder of the light passes directly through the beam
splitters. Thus, while light passes from the laser 26 through the beam
splitter 31 to the rotator 36, a fraction of the laser light is reflected
from the beam splitter 31 and via the mirror 34 through the beam splitter
33 to serve as a reference beam for illuminating the detector 42. The
half-wave plate 35 rotates the polarization vector of the reference beam
to coincide with the polarization of light reflected from the aerosols.
Wavefronts 78 of the reference beam are shown as solid lines in the figure
while the wavefronts 60 are shown by a dot-dash representation and the
wavefronts 62 are represented by dotted lines. Focussing of the cone of
light 52 is accomplished by physically moving the lens 44 relative to a
fixed position of the lens 50, the mechanical movement being accomplished
by the focussing unit 48 as will be described with reference to FIG. 4.
Normals to the wavefront 60 coincide with the rays of the cone of light 52
and are directed via the lenses 50 and 44 to the beam splitter 32, which
directs the wavefront 60 via the beam splitter 33 to the detector 42. Both
the wavefronts 60 and 78 appear as planar wavefronts at the face of the
detector 42. Similarly, the wavefront 62 is also directed by the lens and
beam splitters to the detector 42, but with a significant distinction.
Since the aerosol 56 is not located at the focal point as is the aerosol
54, normals to the wavefront 62 do not coincide with the rays of the cone
of light 52 with the result that the wavefront 62 appearing in front of
the detector 42 lies on a curved surface rather than a planar surface.
The planar wavefronts 60 and 78 can interact constructively over the entire
face of the detector 42 when the wavefronts are in phase, or destructively
when the wavefronts 60 and 78 are out-of-phase. Assuming some motion of
the air carrying the aerosol 54, the waves reflected therefrom experience
a Doppler shift relative to the waves directed thereto by the lens 50. As
a result, the waves represented by the wavefronts 60 and 78 in front of
the detector 42 have different frequencies so that the wavefronts 60 and
78 periodically interact constructively and periodically interact
destructively to produce a beat frequency signal on line 80 from the
detector 42. In the preferred embodiment of the invention, a
cadmium-telluride amalgam is utilized for the material of the detector 42,
the amalgam producing a voltage in accordance with the intensity of the
sum of the wavefronts 60 and 78.
With respect to the wavefront 62 which has a curved surface in front of the
detector 42, and wherein the dimensions of the detector 42 are many
wavelengths in length, the interaction of the curved wavefront 62 with the
planar wavefront 78 results in alternate regions of constructive and
destructive interference across the face of the detector 42, this being
analogous to the phenomenon of Newton rings observed in the optical
studies of thin films. As a result, relatively little signal voltage is
produced on line 80 in response to the wavefront 62 while a strong signal
is produced in response to the wavefront 60. Thereby, the optics unit 22
is able to distinguish between aerosols lying within the depth of field
represented by rectangle 58 and aerosols lying outside the depth of field.
Thus, the optics unit 22 produces range discrimination wherein the range
cells or range resolution element is equal to the length of the depth of
field. The particular value of range to which the optics unit 22 is
responsive is set by the focussing unit 48 which positions the lens 44 to
produce the desired focal length for the combination of the lens 44 and
50. The beat frequency of the signal on line 80 is equal to the Doppler
frequency shift induced on the wave reflected from the aerosol 54 by the
wind which blows the aerosol 54, the beat frequency on line 80 therefore
being a measure of the wind speed at the focal range of the optics unit
22.
With respect to the foregoing continuous operation, as distinguished from
pulsed operation of the optics unit 22, it is noted that any leakage of
the transmitted light from the beam splitter 32 to the beam splitter 33 is
combined with the light in the bottom leg of the interferometer 30 at the
beam splitter 33 to contribute in the formation of the aforementioned
reference beam. The Doppler signal on line 80 appears continuously and is
coupled via the amplifier 64 to the filters 65. The amplifier 64 includes
a band pass filter for extracting and amplifying the beat frequency
component of the signal on line 80 for coupling the beat frequency
component, this being the Doppler frequency shift, to the filters 65.
The signal on line 80 is a cw or pulsed signal depending on the use of
either the rotator 36 or the modulator 28 in producing the signal
transmitted by the optics unit 22. In either case, it is noted that the
movement of the aerosols in the air result in a continuous Doppler
frequency spectrum representing the nominal wind speed. Complete Doppler
data, therefore, is obtained by examining not only the mean Doppler
frequency representing the nominal wind speed, but, is obtained by
examining various portions of the aforementioned continuous spectrum.
Accordingly, the filters 65, which may comprise inductor-capacitor tank
circuits, have slightly overlapping pass bands with each of the respective
pass bands covering respective portions of the Doppler spectrum. In
response to the beat frequency signal coupled thereto by the amplifier 64,
each of the filters 65 produces an analog signal having an amplitude which
is detected by the respective one of the detectors 66. The signals of the
detectors 66 are sampled by the converters 67 and, in response to a strobe
signal on line 82, are converted to digital signals which are applied to
the switch 68. The control signals for operating the system 20 to produce
ranging data are provided by the range unit 23, these signals including
the strobe signal on line 82, the signal at terminal T1 and the focus
control on line 83. Thus, in the short range cw situation, the range of an
aerosol such as the aerosol 54 is designated by the focus control signal
on line 83, the focussing unit 48 adjusting the position of the lens 44 to
focus the cone of light 52 at the range of the aerosol 54. In the long
range, pulsed operation of the system 20, the range unit 23 provides the
signal at terminal T1 to initiate a pulse of radiant energy, and then at
specific instance thereafter initiates each of a sequence of the sampling
signals on line 82 for strobing the converters 67 to sample the Doppler
data, the times of occurrence of the sampling signal on line 82
determining the ranges of the observed echo signal, this corresponding to
the ranging procedure in a conventional pulsed radar.
The times of occurrence of the successive strobing signals on line 82 are
set in accordance with the response time of the filters 65. Since the
response time of a filter 65 is proportional to the reciprocal of the
bandwidth of the filter 65, the repetition frequency of the strobe signals
on line 82 is relatively low when narrow band filters 65 are utilized and
relatively high when wide band filters 65 are utilized. This is in
accordance with the well known theory of Doppler measurement which states
that a long measuring interval produces greater resolution of the Doppler
spectral lines than does a short measurement interval. At the shorter
ranges of the focussing region wherein cw operation is utilized, the
sampling rate may be selected independently of the range resolution
element since the range resolution is dependent on the depth of field of
the optics unit 22. At the longer ranges of the pulsing region wherein the
range of each scatterer is dependent of the propagation time of the
radiant energy to that scatterer, better range resolution is obtained at
the higher sampling rates using wide bandwidth filters 65 with reduced
range resolution being obtained at the lower sample rates using narrow
bandwidth filters 65.
By way of example, the Doppler frequency shift for the system 20 is
approximately 60 kHz (kilohertz) per foot per second, and in response to
aerosol movement, produces values of Doppler frequency shift typically in
the range of a few hundred kilohertz to 10 MHz (megahertz). Thus, for a
pulse width of 10 microseconds, as produced by the modulator 28, there are
at least several cycles of the beat frequency signal on line 80 from the
detector 42. Assuming the bandwidth of a filter 65 to be 100 kHz, then a
sufficient number of cycles of the Doppler frequency shift in the beat
frequency signal on line 80 occur within the response time of the filter
65 such that the filter 65 produces a signal for the detector 66 which is
readily detectable. Assuming the aforementioned 100 kHz bandwidth of the
filter 65 and a corresponding response time of approximately 10
microseconds, the range resolution element has a value of 1500 meters. The
sample strobe signal on line 82 may then occur at a rate of 100 kHz to
produce Doppler data of contiguous range resolution elements or, at a
lower sampling rate if Doppler data is required only of representative
range resolution elements which are spaced apart from each other within a
general region of interest.
The range unit 23 produces a sequence of digital numbers on line 84
representing successive ones of the frequency bands of the respective
signal channels incident upon the selector switch 68. Line 84 is coupled
to the switch 68 for successively selecting individual ones of the signals
of the respective converters 67A-D for storage within the memory 72, the
line 84 also being coupled to the memory 72 for addressing the
corresponding row of the memory 72 for the storing of the frequency data.
On line 85, the range unit 23 provides digital signals representing the
ranges of the signals sampled by the converters 67, the line 85 being seen
to fan into the line 84 at the memory 72 to provide the complete address
for the individual cells of the memory 72, the range signal on line 85
identifying the specific column of the memory 72 in which the frequency
data is to be stored.
In order to provide increased accuracy to the Doppler spectral
measurements, a succession of range sweeps is initiated by the system 20.
During the short range portion of each sweep, the range unit 23 provides
the aforementioned focus signal on line 83 to the focus unit 48 for
continuously varying the range of the focus from the shortest range until
the transition range wherein a transition is made from cw operation to
pulsed operation. Thereafter, the focus range is held constant, the
optical bypass comprising the mirrors 37 and 38 is retracted from the
modulator 28, and the range unit 23 provides the signals at terminal T1
for pulsing the modulator 28 to transmit pulses of radiant energy to sweep
the range at the longer distances.
Upon the occurrence of the first range sweep, the Doppler data is stored in
the memory 72. Upon the occurrence of a subsequent range sweep, the data
stored in each memory cell is added via the adder 70 to the next sample of
Doppler data designated for that memory cell, the summation of the data
samples of the first and second sweeps being entered into that memory
cell. In the same manner, for each of the sweeps in a sequence of range
sweeps, the data previously stored in a memory cell is added to the
corresponding data of the next range sweep so that, after a sequence of,
for example, 20 sweeps, the digital numbers stored in the cells of the
memory 72 represent an average value of Doppler data as obtained over the
set of 20 sweeps. The data of the memory 72 is then coupled to the display
74 which, in the foregoing sample of the 20 sweeps, includes a divider
circuit (not shown) for dividing the sums in each of the memory cells by a
factor of 20 for displaying the average value of the spectral data as a
function of frequency and range. The address generator 73, in response to
clock pulses C4 of the range unit 23, provides address signals to the
memory 72 and to the display 74 for coupling the data of specific ones of
these cells of the memory 72 to the display 74. It is also noted, by way
of alternative embodiments, that the set of Doppler spectral processing
channels coupled to the switch 68 may be replaced by a fast Fourier
transformer (not shown) in which case the signal from the amplifier 64
would be sampled at a rate at least twice the frequency of the Doppler
shift (the Nyquist criterion) and applied to the fast Fourier transformer.
The output lines from the fast Fourier transformer would then have a set
of digital signals similar to those provided by the converters 67 for the
spectral data.
Referring also to FIG. 2, the relationship between the pulsing operation of
the system 20 at the long range situation is compared to the continuous
operation of the system 20 for the short range situation. Two graphs 88
and 89 are provided in registration with each other, the horizontal axis
in each graph representing range, or equivalently, the time required for a
pulse of radiant energy to propagate through the cone of light 52 to the
corresponding range. FIG. 2 shows the optics unit 22 and the data
processor 24 of FIG. 1. For simplicity, the cone of light 52 is shown as
simply a line in FIG. 2, the graph 88 representing the duration of a
transmission of light in the cone of the light 51. For the short range
situation, the right hand terminus of a transmitted pulse 90 of the graph
88 is deleted since the light is continuously transmitted. In order to
provide as much signal energy as possible for a measurement of the Doppler
frequency, the duration of the pulse 90 is made slightly less than the
time required to traverse a distance equal to twice the focal range of the
short range situation. In the preferred embodiment of the invention, the
transition point between the focussing region of the short range situation
and the pulsing region of the long range situation occurs at a range equal
to approximately 90% of the focal range wherein the depth of field is
equal to the focal range. The focal length or focal range of the optics
unit 22 at the transition point is indicated by line 91 in FIG. 2. The
aerosols 54 and 56 are shown in the graph 89, and are seen to lie at a
distance less than the maximum useful focal range. By way of example, an
aerosol 92 is shown at a distance substantially greater than the maximum
useful focal range. Ranges less than that represented by the line 112 are
in the focussing region of the system 20 of FIG. 1 while ranges greater
than that represented by the line 112 are in the pulsing region of the
system 20.
Referring now to FIG. 3, the range unit 23 comprises a clock 95, switches
96 and 97, counters 98 and 99, memories 100 and 101, a monostable
flip-flop 102, a comparator 103, encoders 104 and 105, an inverter 106, a
timer 107, a pulse generator 108 and a gate 109. The memory 100 is
responsive to an aperture signal on line 97 from the focussing unit 48 of
FIG. 1 and to a digital signal of the encoder 104 representing the pulse
width for providing a range at which the transition takes place between
the focussing region and the pulsing region. Also seen in FIG. 3 are the
terminals C4 and T1 as well as the lines 85, 83, 77, 82 and 84 coupling
the signals referred to previously with reference to FIG. 1.
In operation, the counter 98 serves as a range counter by counting clock
pulses, either pulses C1 or C2, coupled via the switch 96 to the counter
98. The comparator 103 compares the digital signal representing the
transition range of the memory 100 with the digital signal on line 85
representing the count of the counter 98, the comparator 103 providing a
signal on line 113 having a logic state of 1 when the range count on line
85 from the counter 98 is equal to or exceeds the transition range from
the memory 100. The signal on line 113 therefore indicates the long range
situation in which the system 20 of FIG. 1 operates in the pulse mode, the
absence of the signal or logic 0 on line 113 indicating the short range
situation wherein the system 20 operates in the cw mode. The signal on
line 113 operates the switches 96 and 97 and is also coupled via line 113
from the comparator 103 to the flip-flop 102, the gate 109, the timer 107
and the inverter 106.
The clock 95 resets the range counter 98 at the beginning of each range
sweep. Thereupon, the counter 98 counts the C1 clock pulses coupled via
the switch 96. The C1 clock pulses occur at a relatively low rate as
compared to the C2 clock pulses, the C1 clock pulses being commensurate
with the rate at which the lens 44 of FIG. 1 is moved for varying the
focal length of the optics unit 22. During the long range situation
wherein the focus of the optics unit 22 remains constant, the long range
signal on line 113 activates the switch 96 to couple the C2 pulses to the
counter 98. The C2 pulses occur at a rate commensurate with the speed of
propagation of the radiant energy from the optics unit 22 to the aerosols.
In the short range situation, the terminals of the switch 97 are coupled as
shown in the figure so that the output signal of the memory 101 is coupled
to line 83. The memory 101 utilizes the numerical value of the range on
line 85 as its input address and, in response to that address, provides a
digital signal for line 83 designating the spacing between the lens 44 and
50 to produce a focal length equal to the range on line 85. An exemplary
graph showing the relationship of lens position versus range is shown in
the memory 101. By way of example, the memory 101 may be a
read-only-memory in which a specific spacing of the lens is associated
with each value of range on line 85. Thereby, during the short range
situation, the focal length of the optics unit 22 follows the range
ordered by the counter 98.
The clock 95 provides C3 clock pulse signals for the timer 107, the timer
107 producing a succession of the sample strobe signals on line 82 in
response to the C3 clock pulse signals. The rate of sample strobe signals
may be altered by a knob 114 affixed to the timer 107. The timer 107 also
provides clock pulses to the counter 99 which, in accordance with the
aforementioned example of the four Doppler spectral data channels coupled
to the switch 68 of FIG. 1, counts modulo four. If, for example, eight
spectral data channels were utilized, then the counter 99 would count
modulo eight. With respect to the foregoing example, of the four spectral
data channels, the timer 107 applies a sequence of four clock pulses to
the counter 99 which, in turn, counts these pulses to produce a sequence
of four digital signals on line 84 which identify sequentially the
individual ones of the spectral data channels. The digital signals on line
84 operate the switch 68 of FIG. 1, as was described previously, to select
the specific channel, and also serve as a portion of the address of the
memory 72 for the identification of the corresponding rows of the memory
cells. The clock 95 also provides the C4 clock pulses for the
aforementioned operation of the address generator 73 of FIG. 1.
At the conclusion of the portion of the range sweep in the focussing
region, activation of the switch 97 by the signal on line 113 couples a
preset value of lens position from the encoder 105 to the line 83 in lieu
of the position previously presented by the memory 101, the preset
position directing the lens 44 to a position which remains fixed during
the pulsing region. The preset position provides for a bracketing of the
pulsing region by the rectangle 58 of FIG. 1. During the time required for
the lens 44 to move to the preset position the flip-flop 102, which is
triggered by the signal on line 113, produces a pulse of fixed duration
which stops the clock 95 during the duration of the pulse. The duration of
the pulse of the flip-flop 102 is sufficient to permit the lens 44 to move
to the preset position. The stoppage of the clock 95 stops all of the
clock pulses C1-C5 so that there is no counting of range by the counter
98, there are no strobing signals on line 82 for sampling data, and the
count on line 84 also remains constant during the stoppage of the clock
95. For operation in the focussing region, the logic state of 0 on line
113 is converted by the converter 106, the logic state of 1 on line 77 for
activating the drive unit 41 of FIG. 1 to position the bypass mirrors 37
and 38 for guiding the laser light along the bypass path around the
modulator 28 and the amplifier 29. For operation in the pulsing region,
the logic state of 1 on line 113 is converted by the inverter 106 | | |
