Abstract:
A laser-based tracking system for tracking the position of a targeted moving object. The tracking system includes two lasers: a reference laser and a slave laser. Each laser is a weapon, and when locked on a target, single laser effectiveness may be doubled without a thermal blooming performance loss associated with a single laser operating at twice the power. The slave laser beam is dithered relative to the reference laser beam in a direction along the longitudinal axis of the target. The system includes an optical receiver for repetitively scanning the irradiance profile reflected by the target. Since the slave laser beam is dithered relative to the reference laser beam, both laser beams will jitter and drift together providing a gain factor of two in average irradiance on the moving target.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of copending parent patent application, Ser. No. 08/729,108 filed on Oct. 11, 1996 for a LASER ALONG BODY TRACKER (SABOT I), by Peter M. Livingston now U.S. Pat. No. 5,955,724 and patent application Ser. No. 08/920,538 filed on Aug. 27, 1997 for A Magic Mirror Hot Spot Tracker by Peter M. Livingston now U.S. Pat. No. 5,900,620. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to systems for tracking a moving object and, more particularly, to a conventional imaging laser-based system for tracking a moving object that employs two lasers; a reference and slave laser. Each laser is a weapon, and when locked together on a target, single laser effectiveness may be doubled without a thermal blooming performance loss associated with a single laser operating at twice the power. The slave laser is dithered relative to the reference laser in a direction along a longitudinal axis of the target. The system includes an optical signal processing system which sums the pixel irradiance values from the target to create an error signal to lock the slave laser to the reference laser. 
     2. Description of the Prior Art 
     Various types of systems are known for tracking moving objects, such as rockets and missiles. Such systems can be categorized as either imaging or non-imaging. Imaging types systems normally utilize an imaging device, such as an electronic camera, for detecting and tracking the position of a targeted moving object. While such imaging systems are effective in tracking targeted moving objects, such imaging systems are known to have limitations when used in combination with high power laser beam weaponry. For example, in such systems, the high power laser beam is known to interfere with the imaging system potentially causing loss of the track of the targeted moving object. Although various systems are known compensate for such an interference problem, such systems do not effectively eliminate the interference. 
     As such, non-imaging laser type tracking systems have been developed. An example of such a system is disclosed in copending U.S. patent application Ser. No. 08/631,645 filed on Apr. 2, 1996, now U.S. Pat. No. 5,780,838 entitled LASER CROSS BODY TRACKER (LACROSST), assigned to the same assignee as the assignee in the present invention. The system disclosed in the &#39;645 patent includes a laser generator for generating a single beam of laser energy and a beam steerer for steering the beam of laser energy to track a targeted moving object. The beam steerer steers the beam of laser energy in a oscillatory fashion in two orthogonal directions at a first dither frequency and a second dither frequency, respectively. The system also includes a telescope for receiving reflected laser energy from the targeted object and detecting the amount of reflected energy received. The detected energy is filtered to form first and second dither frequencies for each channel. The filtered signals are synchronously detected by multiplying each channel by a sinusoidal function derived from the laser mirror generator for that channel. A bias signal is generated from the received reflected synchronously detected power proportional to the beam centroid displacement from the target midline which allows the beam steerer to steer the laser beam to center it on the target, thereby tracking the targeted object. 
     Unfortunately, laser based tracking systems are subject to what is known as thermal blooming. Thermal blooming results in a change in the refractive index of the beam path as a result of heating the beam path temperature by the laser. Change of the refractive index creates a lens effect that causes the radiation to spread relative its original direction. As such, thermal blooming increases the diameter of the laser beam as it moves away from the laser source. A detailed explanation of the thermal blooming is disclosed in U.S. Pat. No. 5,198,607, hereby incorporated by reference. 
     The problem of thermal blooming also reduces the effectiveness of high power laser weaponry. In order to overcome the thermal blooming problem for high power laser weaponry, the &#39;607 patent discloses the use of two independent lasers separated by a sufficient distance to prevent interference therebetween, focused onto a single moving object, such as a missile. The &#39;607 patent discloses the use of a known imaging type system for tracking the location of the targeted moving object. 
     SUMMARY 
     It is an object of the present invention to solve various problems of the prior art. 
     It is yet another object of the present invention to provide a tracking system for tracking the position of a targeted moving object. 
     It is yet another object of the resent invention to provide a type tracking system which reduces the effects of thermal blooming. 
     Briefly, the present invention relates to laser tracking system for tracking the position of a targeted moving object. The tracking system includes two lasers: a reference laser and a slave laser. Each laser is a weapon, and when locked together on a target, single laser effectiveness may be doubled without a thermal blooming performance loss associated with a single laser operating at twice the power. The slave laser beam is dithered relative to the reference laser beam in a direction along the longitudinal axis of the target. The system includes an optical processing system for summing the pixel irradiance values from the target in order to create an error signal to lock the reference laser to the slave laser. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other objects and advantages of the present invention will become readily apparent upon consideration of the following detailed description and attached drawing, wherein: 
     FIG. 1 is a diagram illustrating the reflected radiation from a uniformly reflecting body illustrating two overlapping laser spots on a portion of a missile target body. 
     FIG. 2A is a graphical representation of the irradiance profile as a function of the distance along the reflected body for two non-overlapping laser spots. 
     FIG. 2B is a graphical illustration of the irradiance profile as a function of the distance along the reflecting body where the two overlapping laser spots are overlapping as illustrated in FIG. 1. 
     FIG. 3 is a diagram of a single laser beam overlapping some highly reflecting feature denoted by a star on a target. 
     FIG. 4 is an overall block diagram of the non-imaging laser tracking system in accordance with the present invention. 
     FIG. 5 is a block diagram of an optical receiver and image derotator in accordance with the present invention. 
     FIG. 6 is a block diagram of a signal processor for use with the tracking system in accordance with the present invention. 
     FIG. 7A is a graphical illustration of the received scan signal for four complete scan cycles of the tracking system in accordance with the present invention. 
     FIG. 7B is a differentiated version of the signal illustrated in FIG. 7A. 
     FIG. 8 is a graphical illustration of the power spectral density of the synchronously detected scan signal at twice the frame rate for the tracking system in accordance with the present invention. 
     FIG. 9 is a graphical illustration of the detected dither modulation signal for the tracking system in accordance with the present invention. 
     FIG. 10 is a graphical illustration of an error characteristic signal following the second detection of the 5 Hz dither modulation signal. 
     FIG. 11 is a block diagram of an imaging tracking system in accordance with the present invention. 
     FIG. 12 is a graphical illustration of the focal plane of a target showing the slave and reference hit spots and the beginning and end of the symmetrical field of regard for the tracking system illustrated in FIG. 11. 
     FIGS. 13a-13b are a pictorial representation of a pixel scan along the reference line when the slave spot leads the reference spot. 
     FIGS. 14a-14c illustrate the results shown in FIG. 12 with a negative value assigned to the scan from the beginning to the middle of the field of regard. 
     FIG. 15 is a block diagram of the processing algorithm in accordance with the present invention. 
     FIG. 16 is a graphical illustration of the summation of the pixel irradiance values and the effect of a low pass filter in accordance with the present invention. 
     FIG. 17 is a block diagram of a signal processor in accordance with the present invention. 
     FIG. 18 is a diagram of the error characteristic of the image tracking system illustrated in FIG. 11. 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to a laser based tracking system for tracking a targeted moving object. In order to minimize the effects of thermal blooming, the tracking system in accordance with the present invention employs two laser beams to track and kill a targeted moving object. One laser beam is configured as a reference beam and is directed toward the targeted moving object. The other laser beam is configured as a slave and is dithered (oscillated with a small amplitude) relative to the reference laser beam on the target surface in a direction generally parallel to a longitudinal axis of the target. Various alternative systems are provided. FIGS. 1-10 relate to a non-imaging type laser tracking system while FIGS. 11-18 relate to an imaging type laser tracking system. 
     Referring to FIG. 1, such a system will result in overlapping laser spots on the moving target. Since the reference and slave laser beams are locked together, both beams will jitter and drift together producing a gain in the average irradiation profile of two as illustrated in FIG. 2B. More particularly the irradiance for separated laser spots on a target is illustrated in FIG. 2A. By causing the laser spots on the target to overlap as illustrated in FIG. 1, the irradiance gain illustrated in FIG. 2B is about twice the normal gain as illustrated in FIG. 2A assuming equidistant laser paths and equal laser powers. 
     FIGS. 1 and 2 illustrate the irradiation for a uniformally reflecting body. FIG. 3 illustrates the use of the invention with a non-uniformally reflection body. In particular, referring to FIG. 3 a single laser beam can be locked on to shiny or dull feature of a target. Even though the total number of photons scattered from the target is constant, the invention will process the information as to lock the single beam onto the distinguishing feature as will be discussed in more detail below. 
     A system for directing a reference laser beam at the target of interest is described in U.S. Pat. No. 5,780,838 assigned to the same assignee as the assignee of the present invention and hereby incorporated by reference. The present invention as described below is related to locking a second laser beam (slave beam) relative to the reference laser beam on the target surface in a direction generally parallel to the longitudinal axis of the target. 
     Referring to FIG. 4, a targeted moving object such a missile or rocket 20 is illustrated defining a longitudinal axis 22. A first laser 24 is used to direct a first laser beam 26 toward the target 20. For purposes of illustration herein, the first laser 24 is designated as the reference laser. In accordance with an important aspect of the invention, a second laser 28 is used to direct to a second laser beam 30 toward the target. The second laser beam 30 is dithered (i.e. oscillated with a relatively small amplitude) in a direction as indicated by the arrows 32 that is generally parallel to the longitudinal axis 22 of the target. 
     The tracking system forms a closed loop that forces the laser spots from the laser beams 26 and 30 to overlap as illustrated in FIG. 1 for all disturbance frequencies falling within the loop bandwidth. The closed loop system in accordance with the present invention is generally identified with the reference numeral 34 and includes an optical receiver 36, a signal processor 38, a dither generator 40 and a dither mirror 42. Unlike the single laser tracking system disclosed in the copending application 08/631,645 for the LASER CROSS-BODY TRACKER (LACROSST), now U.S. Pat. No. 5,780,838 the tracking system 34 does not depend on the time variation of the total number of scattered photons but rather information derived competitively scanning the irradiance profile created the two laser beams 26 and 30 directed toward the target 20. The signal processor 38, as will be discussed below in more detail is used to process the scan data from the optical receiver 36 in order to lock the beam 30 from the slave laser 28 to the beam 26, generated by the reference laser 24. 
     The optical receiver 36 is illustrated in detail in FIG. 5. The optical receiver 36 is used to detect the irradiance profiles for example as illustrated in FIG. 2B, reflected from the target 20 by imaging a target 20 with its laser spots at the laser site using a rocking mirror scanner assembly as discussed below. The optical receiver 36 includes a primary afocal telescope 44 for receiving scattered laser energy from a relatively wide field-of-view, for example, a field of regard of several hundred microradians. The scattered laser energy from the target 20 is directed by the afocal telescope 44 to an image derotator 46. The image derotator 46 is used to cause a rocking mirror scanner assembly 48 to scan the scatter laser energy in a direction generally parallel to the longitudinal axis 22 of the target 20. More particularly, the optical receiver 36 includes a single detector 50 having a field stop slit 52 which defines the system instantaneous field-of-view. The rocking mirror scanner assembly 48 causes the image of the overlapping spots on the target 20 to be swept over the slit 52. The image derotator 46 thus is used to force the dither direction to be generally perpendicular to the field stop slit 32. The field lens 54 is used to direct the image on the single detector 50. In order to improve the signal to noise ratio, a zoom lens assembly 56 may be used in conjunction with a system radar represented by the dashed line 58, to fill the field stop slit 52 to the greatest possible extent. In order to ensure that the spot image on the detector 50 remains centered, bias signals from the signal processing system 38 may be used. The bias signals may be generated by the signal processing system 38 resulting from the action of the tracking system servoloop 34. The action of the servoloop causes the optical signal to be centered on the detector and eliminates signal drift perpendicular to the direction of scan. 
     The optical receiver 36 may be mounted to the coarse gimbals of a laser beam pointer system. The laser beam pointer system is described in detail in U.S. patent application Ser. No. 08/631,645, filed on Apr. 2, 1996, now U.S. Pat. No. 5,780,838 assigned to the same assignee as the present invention and hereby incorporated by reference. 
     The signal processing system is illustrated in FIG. 6. A scan mirror generator 53 (FIG. 6) as well as the scan mirror drive 55 form part of the signal processor 38 discussed below which drives the rocking mirror scanning assembly 48. The scan mirror generator 53 causes the scan mirror to move an approximately constant angular rate scanning the image back and forth over the fixed slit 52. The detector 50 thus records a voltage proportional to the irradiance filling the slit 52. The slit width and scan extend together with the zoom lens assembly 56 causes the image to be moved completely out of the field of view and returned. The detected voltage from the detector 50 thus represents a running integral of the irradiance distribution. 
     The output from the detector 50 is applied to a detector preamplifier 52. The signal processor 38 is used to develop a scan signal for each complete scan cycle as illustrated in FIG. 7A. The signal processor 38 includes a pair of differential amplifiers 54 and 56 as well as a pair of commutating switches s 1  and s 2 . As mentioned above, the scan mirror moves at a constant angular rate scanning the image back and forth over the fixed slit 52. In a forward scanning direction, the commutating switches s 1  and s 2  cause the signal to be applied to the differential amplifier 54. 
     In particular, when the commutating switch s 1  is closed, a signal from the detector/preamp 52 is applied to a noninverting input of a differential amplifier 54 and compared with the output of the differential amplifier 56 which is zeroed by a capacitor c 2 , connected between the output of the differential amplifier 56 and ground. A discharge resistor R 2  is used to discharge the capacitor such that output of the differential amplifier 56 is zero when the commutating switch s 1  is closed. Thus when the commutating switch s 1  is closed, the signal from the detector/preamplifier 52 will be positive. The commutating switch s 2  is used to connect the output of the differential amplifier 54 to a differentiator 56 in the forward direction of the scan cycle to produce the portion of the signal illustrated in FIG. 7A with a positive slope. 
     In the return direction of the scan cycle, the commutating switch S 1  causes the signal from the detector/preamplifier to be connected to an inverting input of the differential amplifier 56. As mentioned above, the capacitor c 2  in combination with the discharge resistor R 2  causes the output of this amplifier to be zero prior to the rocking mirror scanning assembly 48 moving in a return direction a noninverting input of the differential amplifier 56 is connected to the output of the differential amplifier 54. In a return direction, a capacitor c 1  and parallel connected discharge resistor R 1  force the output of the differential amplifier to be zero in the return direction. Thus, in a return direction, the detector preamp signal 52 is merely inverted as illustrated by the negative slope of the scan signal for the last half cycle as illustrated in FIG. 7. 
     The scan mirror generator 53 is connected between the commutating switches s 1  and s 2  to control their operation. More particularly, the scan mirror generator 53 is coupled to a scan mirror drive which, as discussed above causes the rocking mirror scanner assembly 48 to scan the image back and forth over the fixed slit 52. The scanning mirror generator 53 also controls the operation of the commutating switches s 1  and s 2  as discussed above. The waveform illustrated in FIG. 7A, shown for four complete scan cycles is thus produced at the output of the commutating switch s 2 . 
     A scan signal, as illustrated in FIG. 7A, is applied to the differentiator 56. The differentiator 56 differentiates the scan signal to produce a differentiated scan signal as illustrated in FIG. 7B. Except for the line reversal of the odd half cycles, the differentiated scan signal illustrated in FIG. 7B is similar to the irradiance profile for partially overlapping laser spots on a target illustrated in FIG. 2. Since the differentiation eliminates the DC component, operation of the system does not depend on accurate DC restoration. 
     After the scan signal is differentiated, the dither modulation portion of the signal, which shows up an FM component, is recovered. In particular the differentiated signal illustrated in FIG. 7B is synchronously detected by multiplying it with a cosine signal from the scan mirror generator 53. More particularly, referring to FIG. 6, the output of the scan of mirror generator 53 is applied to a frequency doubler 58. The output of the frequency doubler 58 is essentially a sine wave at twice the sweep frequency of the scan mirror generator 53. The output of the frequency doubler 58 is applied to a 90° phase shifting device 60 which, generates a cosine signal at twice the sweep rate. The output of the phase shifter device is applied to a multiplier 62. 
     The power spectral density of the synchronously detected scan signal at twice the frame rate is illustrated in FIG. 8 at a 0.6 beam width separation. As shown in FIG. 8, the frame rate is 40 frames or 80 scans per second. Thus, for a scan rate of 80 Hz synchronous detection only detects the sidebands 40 Hz above and below the 80 Hz above and below the 80 Hz signal within the synchronously detected differentiated scan signal at the dither frequency of 5 Hz. 
     The synchronously detected differentiated scan signal is applied to a low pass filter 64 as illustrated in FIG. 6. The output of the low pass filter 64 is applied to another multiplier 66 used for synchronous detection at the dither frequency to recover the signed envelope of the dither signal. A signal from a dither mirror scan generator 68 is applied to the multiplier 66. The dither mirror scan generator 68 is used to drive a dither mirror drive 70, which, in turn, drives the dither mirror and, in turn, the slave beam 30 at the dither frequency. 
     FIG. 9 illustrates five synchronously detected 5 modulation signals shown for 7 values of spot separation 0.0 through 0.6 at 0.1 increments. As shown, the dither modulation envelopes illustrated in FIG. 9 are signed. Thus, referring to FIG. 1, if the dither spot is to the left of the reference spot and moves toward as the high power beam fast steering mirror 42 FIG. 4, moves from left to right, the envelope sign is positive. However, if the dither spot is to the right of the reference spot, the same motion of the high power fast steering mirror 42 causes the sign to be negative. 
     The sign of the dither modulation signal then may be used as an error signal to lock the reference beam 30 from the slave laser 28 relative to the reference beam 26 from the reference laser 24. As shown in FIG. 10, the error signal changes magnitude with the mean spot displacement measured in beam radii. FIG. 10 is an example of the detected 5 Hz dither modulation signal recovered for seven values of mean spot displacement. As shown in FIG. 10, the system will cause the beam 30 to walk onto the reference beam 26 for least plus or minus 0.6 beam-radius separations. As shown in FIG. 6, this detected signal is applied to an integrator 70 which forms a closed loop with the dither mirror scan generator 68 drive the dither mirror with a signal proportional to the integral of the detected envelope. 
     ALTERNATE EMBODIMENT 
     An imaging type tracking system in illustrated in FIGS. 11-18 and generally identified with the reference numeral 200. A block diagram of the imaging type tracking system 200 in accordance with the present invention is illustrated in FIG. 11. As shown, the imaging type tracking system 200 includes an image tracker 202, a signal processor 204 and a fast steering mirror 206 forming a closed loop. 
     The block diagram for the imaging type tracking system 200 illustrated in FIG. 11 is similar to the block diagram for the non-imaging tracking system illustrated in FIG. 4. However, the imaging type tracking system 200 in accordance with the present invention eliminates the need for the one dimensional scanning receiver system 36 illustrated in FIG. 5. Rather the imaging type tracking system 200 develops an error signal in order to lock a slave beam from a slave laser 210 to a reference beam from a reference laser 208 at a point 212 on a moving target 214 such as a missile. As will be discussed in more detail below, an error signal is created by imaging the moving target 214 and the reflected radiation from the laser hit spots and summing the pixel irradiance value reflected from the target 214. The error signal, in turn, is used to lock the slave beam from the slave laser 210 onto the reference beam of the reference laser 208 by closed loop control. 
     The image tracker 202 may be an imaging type tracker which provides a focal plane image of the target 214 as well as the slave and reference laser hit spots on the target 214 as illustrated in FIG. 12. A suitable image tracker 202 is disclosed in copending application Ser. No. 08/920,538, filed on Aug. 27, 1997 now U.S. Pat. No. 5,900.620 for A Magic Mirror Hot Spot Tracker, by Peter M. Livingston, assigned to the same assignee as the assignee of the present invention and hereby incorporated by reference. As disclosed therein, the image tracker 202 includes a focal plane detector array which converts the images to corresponding electrical signals. As shown in FIG. 12, the target 214 is imaged on a focal plane array (not shown) which forms a portion of a image tracker 202. The image illustrated in FIG. 12 of the missile 24 also illustrates 2 laser hit spots; a reference spot 216 at coordinates (m 2 , n 2 ) and a slave spot 218 at coordinates m 1 , n 1 . As will be discussed in more detail below, the signal processor 204 scans the pixel irradiance values along an axis 222 of the target 214 to develop an error signal. 
     The signal processor 204 (FIG. 15) is illustrated in FIG. 17 and may be implemented as a CPU as with an on board analog to digital converter (ADC) for converting the analog detector signals to digital values as discussed above. The signal processor 204 also includes a random access memory (RAM), an electrically erasable programmable read only memory (EEPROM) device or flash ROM device, and executes the algorithm illustrated in FIG. 15 in order to develop an error signal which is supplied to the high power fast steering mirror 206 in order to lock the laser 210 to the laser 208 at a point 212 on a moving target 214 under closed loop control. 
     Electrical signals from the focal plane detector array (part of the image tracker 202) are applied to the signal processor 204 at an input 201. For a condition when the slave spot 218 leads the reference spot 216 as shown in FIG. 12, the relative pixel position as a function of amplitude along a line 222 (FIG. 12) discussed below, is as shown in FIG. 13b. The peaks 217 and 219 correspond to the slave spot 218 and the reference spot 216, respectively. These electrical signals may be amplified by an on board preamplifier 203 (FIG. 17) and converted to digital values by the ADC and multiplied by a squarewave 224 (FIG. 14b) by a square generator 213 (FIG. 17) by way of a multiplier 205 to form a product signal. Multiplying the detector signals by square wave allows for synchronous detection of the signals by dividing the detector signals at the input 201 into two halves 224 and 225, as generally shown in FIG. 14a. The product signal is then integrated by an integrator 207 (FIG. 17), whose output may be filtered by a suitable filter 209. As shown in FIG. 14c, the area under the lower curve is negative and corresponds to an instantaneous error signal, for example, as shown in FIG. 18. The instantaneous error signal, in turn, is applied to the high power beam fast steering mirror 206 (FIG. 11) by way of the mirror driver 211 (FIG. 17) forming a closed loop. Nulling of the error signal provides closed loop control of the scanning mirror 206 to enable slave beam 30 to be locked onto the reference beam 26. 
     Referring to FIG. 15, initially in step 220, a straight line 222 is constructed between the two laser hit spots 216 and 218. In particular, assuming the pixel field has its origin in the upper left hand corner of the grid illustrated in FIG. 12, the coordinates of the line connecting the reference and slave spots 216 and 218 is given by ##EQU1## The line 222 may or may not be parallel to the missile roll axis as shown. 
     After the line 222 is constructed, the field of regard is defined in step 222. The field of regard is defined by measuring equal distances from the reference spot at coordinate m n along the line 222. As shown in FIG. 12, a field of regard of a length equal to 2m 2  is shown. Thus, the field of regard begins at coordinates m 2  -M, n 2  -M and ends at coordinates m+M, n+M along the line 222. Once the field of regard in defined in step 222, the pixel irradiance values from the focal point array are converted to digital values by an analog to digital converter (not shown) and mapped to a memory device (not shown). The digitized irradiance values for each pixel location corresponding to the field of regard is then summed from the beginning of the field of regard (i.e. m 2  -M, n 2  -M) to the midpoint of the reference hit spot (m 2 , n 2 ) in step 224 to define a first partial sum in step 224. The pixel values from the focal point array represent intensity levels which correspond to the reflected radiation from the target 214. The first partial sum is multiplied by a signal, such as the signal 224, for example a -1 in step 226 to define a first product signal and stored. Next, in step 228, the pixel irradiance values from the reference spot 216 (n, m) are digitized and summed as discussed above to the end of the field of regard (m 2  +M, n 2  +M) in step 228 to define a second partial sum. The second partial sum is multiplied by a signal 225, for example a +1 to define a second product signal. The two product signals are summed in step 230 to define an instantaneous error signal at a time t=o. The entire process, (i.e. steps 224-230) is repeated for each imager frame creating an error signal time series. More particularly, assuming that the sums to the left and right of the reference spot 216 are symbolically denoted by L and R, then after product detection with the square wave, the sum of the partial sums is: ##EQU2## 
     The error signal time series is then used to pass through a low pass digital filter with a cutoff at a maximum of about 1/3 the imager frame rate to form an error signal that is applied to the high power fast steering mirror 206 (FIG. 20) of the slave laser 210 to force the slave laser beam 11 to lock onto the reference laser beam under closed loop control. The digital filter is described in &#34;Filtering In the Time and Frequency Domains&#34;, supra, hereby incorporated by reference. The effect of the filter is to smooth out random noise contributions accompanying the sampled data. FIG. 16, illustrates the raw data as a function of frame number as well as the low pass filtered data. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.