Method and apparatus for measuring distance to a target

The invention performs coordinate measurement employing multiple-frequency intensity-modulated laser radar. A laser diode source is intensity modulated by variation of its excitation current. Its output beam is directed to a target using scanning mirrors or other opto-mechanical means, and the light returned from the target is detected. The modulation frequency is alternated between two or more values, creating a dataset of several relative phase measurements that uniquely determine the distance to the target without ambiguity. A device for carrying out such a method includes a laser whose output is modulated by a high frequency signal generator, optics for directing the output signal to the target to a detector, a signal generator which generates reference signals offset in frequency from the intensity modulation frequencies by a predetermined amount; mixers for combining the return signals with the reference signals to form a first set of intermediate frequency signals, and for combining the modulation signals with the reference signals to form a second set of intermediate frequency signals, and a computer which calculates phase differences between the output beam and the return signals for each modulation frequency from the intermediate frequencies, and determines the distance to the target from the phase differences.

FIELD OF THE INVENTION 
This invention relates generally to electro-optic range finders for mapping 
physical coordinates in three dimensions, and particularly to laser radar 
techniques for performing this mapping. 
BACKGROUND OF THE INVENTION 
An important part of the manufacturing process is coordinate measurement. 
It is used to map a test piece's shape and to coordinate jig tools. For 
some manufacturing applications, such as automobile and airplane 
manufacturing, coordinate measurement with an absolute RMS accuracy of 
0.001 inch over a range of 0 to 40 feet would be highly desirable. Such 
accuracy has been difficult to obtain with currently available measurement 
techniques. Typically, the measurements are done mechanically, by using 
calipers or other mechanical gauges, or optically, by using geometric 
optics or laser radar. 
Geometric optical techniques involve some form of triangulation, or 
determination of distance by comparing angular measurements from different 
points of view. Triangulation with theodolites may be computer controlled 
for speed and accuracy. At least two theodolite instruments are required, 
and setup and operation may be cumbersome and slow. Photogrammetry is 
another geometric technique involving computer analysis of photographs of 
the test piece taken with a special high resolution camera from three or 
more points of view. Photogrammetry can be much faster to set up and yield 
higher accuracy than triangulation with theodolites, but time-consuming 
development and analysis of the photographs is required. 
Laser radar refers generally to "time-of-flight" sensors that determine 
distance by the propagation time for laser light. They have an advantage 
with respect to geometric techniques of coordinate measurement in that 
each measurement involves only one line of sight and the data acquisition 
does not involve photographic film or other materials that must be 
processed and analyzed, delaying results for long periods of time. The 
following articles discuss various examples of laser radar techniques for 
distance measurement. 
An article entitled "Laser Radar Range Imaging Sensor for Commercial 
Applications" by K. G. Wesolowicz and Robert E. Sampson, Proceedings of 
SPIE, Vol. 783, p. 152 (1987), describes an imaging laser radar system 
employing a single frequency intensity modulation of a GaAlAs laser diode. 
The target range L is obtained from the following equation: 
##EQU1## 
where .phi.=measured phase delay due to time of flight; 
c=speed of light; and 
.upsilon.=modulation frequency. 
Since the phase delay has an implicit 2.pi. ambiguity, the range 
measurement has a corresponding ambiguity interval L.sub.a given by 
##EQU2## 
For example, for a modulation frequency .upsilon.of 0.72 GH.sub.z, the 
interval L.sub.a is 8.2 inches. The 8.2 inch ambiguity interval must be 
resolved by some other means if this device is to be used for large-scale 
coordinate measurement on the order of 0 to 40 feet. 
The article entitled "Laser-diode Distance Meter in a KERN DKM 3A 
Theodolite" by A. Greve and W. Harth, Applied Optics, Vol. 23, No. 17, p. 
2982 (1984), describes an intensity-modulated laser radar that uses a 
phase locking technique to measure the relative phase. By varying the 
modulation frequency, the authors were able at least in principle to 
remove the ambiguity in the range measurement. However, it appears that 
this method results in an inadequate degree of accuracy for some 
applications. 
The article entitled "High-Precision Fiber-Optic Position Sensing Using 
Diode Laser Radar Techniques" by G. Abbas, W. R. Babbitt, M. De la 
Chapelle, M. Fleshner, J. D. McClure, and E. Vertatschitsch, Proceedings 
of SPIE, Vol. 1219, p. 468 (1990), describes a linear position sensor with 
fiber-optic signal distribution. The sensor uses a frequency-chirped, 
intensity-modulated laser diode with an intensity-modulation bandwidth of 
6 GHz. Absolute distance is obtained by determining the beat frequency 
between the laser modulation and the delayed modulation of the return 
signal. The beat frequency is found by high-speed digital Fourier 
transform of the beat signal. This approach has the important advantage 
that several sensor heads may be connected by fiber optics to the same 
source and detection module, provided that the possible variations in 
range to each of the heads do not overlap. However, absolute accuracies of 
0.001 inch over 40 feet would require frequency chirps of very high 
linearity and chirp rates controlled to 2.5 ppm. These specifications may 
not be practical or cost-effective for this system. 
In an article entitled "Utilizing GaAlAs Laser Diodes as a Source for 
Frequency Modulated Continuous Wave (FMCW) Coherent Laser Radars" by A. 
Slotwinski, F. Goodwin and D. Simonson, Proceedings of SPIE, Vol. 1043, p. 
245 (1989), the authors describe an instrument that uses optical 
interferometry to generate beat signals between local and time-delayed 
optical frequencies. The frequency modulation is achieved by thermal 
tuning of a laser diode cavity length. The thermal tuning is easily 
effectuated by precisely controlled variation of the laser excitation 
current and is thus much easier to obtain over large bandwidths than an 
intensity-modulation chirp. However, this system has a maximum operational 
range of about 10 feet, which is inadequate for many applications and, 
like all coherent laser radars, it is sensitive to target surface 
roughness. Also, high accuracy and reliability can only be obtained with 
carefully characterized and monitored single-mode laser diodes. 
The above exemplary measurement systems do not adequately meet the 
simultaneous requirements of very high absolute accuracy and large 
operational range necessary for the coordinate measurement applications 
which the present invention addresses. Further, these radars are not 
incorporated into an optical scanning system specifically designed for 
large-scale coordinate measurement using retro-reflectors or decals on the 
test piece as target points. 
It is thus an object of this invention to meet accuracy and operational 
range requirements of 0.001 inch accuracy over a range of 0 to 40 feet 
using a reliable, cost-effective apparatus, that can be conveniently 
incorporated into a complete coordinate measurement system.

SUMMARY OF THE INVENTION 
The invention provides a method and apparatus for performing coordinate 
measurement employing multiple-frequency intensity-modulated laser radar. 
In accordance with one embodiment of the inventive method, a first step 
intensity modulates a laser diode source by variation of its excitation 
current. A second step directs the beam to a target using scanning mirrors 
or other opto-mechanical means. A further step detects the light returned 
from the target. The final step alternates the modulation frequency 
between two or more values, creating a dataset of several relative phase 
measurements that uniquely determine the distance to the target without 
ambiguity. 
A preferred embodiment of a device for carrying out such a method includes 
a source for an optical output signal which is intensity modulated at a 
plurality of frequencies (for example, a laser whose output is modulated 
by a microwave signal generator), optics for directing the output signal 
to the target and back; a detector for detecting the return signal 
reflected back from the target; a signal generator which generates 
reference signals offset in frequency from the intensity modulation 
frequencies; mixers for combining the return signals with the reference 
signals to form a first set of intermediate frequency signals, and for 
combining the modulation signals with the reference signals to form a 
second set of intermediate frequency signals; and a computer which 
calculates phase differences between the output beam and the return 
signals for each modulation frequency from the first and second set of 
intermediate frequencies, and determines the distance to the target from 
the phase differences. 
The invention provides highly accurate measurements of distance. 
Readily-available crystal-controlled oscillators operating at high 
frequencies can be used to achieve this. Absolute range determination of 
up to 40 feet or more is obtained with three frequencies to resolve the 
ambiguity in the phase measurement. The invention can be made to operate 
with a wide variety of targets. The laser used in the invention may be of 
any type capable of providing spatially coherent light modulated at 
approximately 6 GHz. The constant-frequency system of the invention is 
easier and less expensive to implement than chirped systems and does not 
require expensive signal processing hardware. Signal processing may be 
performed using an ordinary personal computer with suitable input/output 
capabilities. The laser beam may be conveniently oriented in space with 
motorized scanning mirrors and computer control for target acquisition. 
DETAILED DESCRIPTION OF THE DRAWINGS 
The invention determines distance by measurement of the phase delay of the 
laser intensity modulation incurred by traveling round trip to a target 
and back. Range ambiguity is resolved by the use of multiple frequency 
synthetic wavelength techniques. Referring to FIG. 1, in a first 
illustrated embodiment, the source of the intensity modulation of the 
laser beam is a microwave signal generator 1 that operates approximately 
in the range of 7 GHz and provides modulation frequencies accurate to 1 
part in 10.sup.6. The exact frequencies are not critical and may vary 
substantially from those used here for purposes of illustration. In this 
embodiment, only one tone is generated at a time. A second signal 
generator 2 which is preferably locked to the same reference oscillator as 
generator 1 for accuracy, provides a frequency that is offset from the 
current frequency provided by first generator 1 by a fixed amount. The 
signal from the second generator 2 is used as the local oscillator signal 
in the phase detection process, as described below. For this particular 
embodiment, an offset frequency of 100 kHz could be chosen, but other 
offset frequencies may be utilized as desired. If, for example, modulation 
frequencies of 7 GHz, 7.010 GHz, and 7.3 GHz were used, there would be 
modulation frequency-offset frequency pairs of 7.0 GHz and 7.0001 GHz; 
7.010 GHz and 7.0101 GHz; and 7.3 GHz and 7.3001 GHz. 
The signal generator 1 is the modulation drive to a laser 4. A 
temperature/bias current controller 3 may be provided to maintain stable 
operation of the laser 4. In this illustrated embodiment, the laser 4 is a 
7 milliwatt, fiber pigtailed, 1.3 micron multi-mode InGaAsP diode laser 
with an integrated optical isolator. The optical isolator ensures that 
light reflected back into the laser does not increase the laser's 
intensity noise or introduce a slight phase shift in the laser's intensity 
modulation. Alternatively, a laser with a separate optical isolator may be 
used. Under some circumstances, the optical isolator may not be required. 
An optical fiber pigtail 6 extending from the laser 4 is connected to an 
output coupler 7 that roughly collimates the beam. The beam then passes 
through beamsplitters 8, 9 and 10 to a mirror 12 mounted on a two axis 
gimbal 13. The gimbal mounted mirror 12 directs the beam to different 
targets 14a, b, c. Although three targets are shown for the sake of 
illustration, any number of targets may be used. In this embodiment, the 
targets 14a, b, c. are preferably open retroreflectors mounted in a 
compact housing, and are interchangeable with the targets currently used 
for photogrammetry and theodolites. Closed retroreflectors can also be 
used, as can other types of reflective targets, provided the signal to 
noise ratio is sufficiently high. The targets are mounted on the test 
piece to be measured by any convenient means. 
From the target 14b (chosen for the sake of illustration only), the light 
beam retraces its path to beamsplitter 8, where the light is directed onto 
a high speed detector 16, which is sensitive to signals in the range of 
those generated by the microwave signal generator 1. In the illustrated 
embodiment, this is in the range of 7 GHz. The detected signal and the 
local oscillator signal from the second signal generator 2 are both input 
into a mixer 17a. This creates an intermediate frequency at the output 18a 
of the mixer 17a. The output signal 18a from mixer 17a is then filtered 
and amplified by processing electronics 19a. The signals from generators 1 
and 2 are used as RF and LO inputs respectively to a second mixer 17b, 
which produces an output signal 18b which is filtered and amplified by 
processing electronics 19b, identical to those which filter and amplify 
the output 18a from the first mixer 17a. The purpose of the signal on 
reference channel 18b is to act as a reference phase, as will be evident 
from the discussion below. 
The outputs from processing electronics 19a and 19b are digitized by high 
speed analog to digital converters (ADC) 21a and 21b. In the illustrated 
embodiment, two ADC's which operate in sync with each other are shown. 
Alternatively, a single ADC multiplexed to sample two channels could be 
used. In FIG. 1, there are two inputs 22a, 22b to the two ADC units 21a 
and 21b, with 400 KHz sampling on each input signal 22a, 22b. The 
intermediate frequency in this particular embodiment is the previously 
mentioned offset of 100 kHz. After being digitized, the signals from ADC 
units 21a, 2lb are digitally IQ detected at 100 kHz in the computer 24. 
The I and Q outputs of each signal are the arguments to an arctangent 
routine which yields the phase of the signal relative to a digitized 100 
kHz cosine wave. The output signal from the reference ADC 21b is used to 
determine the relative phase between the output of generator 1 and the 
digitized 100 kHz wave. By subtracting the phase of the output signal from 
reference ADC 21b from the phase of ADC 21a, the relative phase between 
the signal that traveled round trip to the target 14b and the signal at 
the output of generator 1 is determined. If the phase is measured at a 
zero point range (i.e. when the laser 4 is pointing at a target whose 
range is assigned to be zero), this zero point phase can be subtracted 
from the relative phase obtained above. The resultant corrected phase is 
the relative phase difference between the zero point range and the target 
range. 
In this illustrated embodiment, the zero point range is established by 
steering the gimbal mounted mirror 12 into a retroreflecting position. 
This defines the zero point range to be at the surface of the mirror 12 on 
the gimbal 13. Distance can be determined from the phase measurements. The 
manner of calculating the target distance from the phase information will 
be discussed with reference to the following symbol definitions: 
______________________________________ 
intensity modulation 
.upsilon..sub.i 
frequencies: 
intensity modulation wave- 
.lambda..sub.i = c/.upsilon..sub.i 
lengths: 
synthetic wavelengths: 
.LAMBDA..sub.ij = c/(.upsilon..sub.i - .upsilon..sub.j) 
measured target phase for 
.theta..sub.i .sup.t 
.lambda..sub.i : 
measured reference phase 
.theta..sub.i .sup.r 
for .lambda..sub.i : 
relative phase for .lambda..sub.i : 
.phi..sub.i = .theta..sub.i .sup.r - .theta..sub.i .sup.t 
relative phase for .LAMBDA..sub.ij: 
.PHI..sub.ij = .phi..sub.i - .phi..sub.j 
phase offsets at zero point: 
.PHI..sub.ij (0); .phi..sub.i (0) 
zero-point corrected 
.PHI.'.sub.ij = .PHI..sub.ij - .PHI..sub.ij (0); 
.phi.'.sub.i = .phi..sub.i - .phi..sub.i (0) 
phases: 
modulation "fringe" 
m.sub.i = integer + .phi.'.sub.i /2.pi. 
number: 
synthetic fringe number: 
M.sub.ij = m.sub.i - m.sub.j = integer + .PHI.'.sub.ij 
/2.pi. 
refractive index of air: 
n 
distance to be measured: 
L = m.sub.i .lambda..sub.i /2n 
______________________________________ 
The algorithm used in calculating the distance from the phase measurements 
is as follows for the case of three frequencies: From these measured 
values, calculate the relative phases .phi.'.sub.1, .PHI.'.sub.21, 
.PHI.'.sub.31, as discussed above and record the relative phases as 
numbers between 0 and 2.pi. (note that these values involve fixed 
zero-point phase offsets .phi.'.sub.1 (0), .PHI.'.sub.21 (0), 
.PHI.'.sub.31 (0)). In this embodiment, .LAMBDA..sub.31 &lt;.LAMBDA..sub.21. 
Now calculate 
EQU M.sub.21 =.PHI.'.sub.21 /2.pi., 
##EQU3## 
where I() is a function which returns the integer nearest to its argument. 
The distance is then 
EQU L=m.sub.1 .lambda..sub.1 /2n. 
To make a range measurement, the phase differences are measured at all 
three modulation frequencies. To initialize the measurement process, these 
phase differences have to be measured at a zero-point range. The frequency 
generators 1, 2 are switched to the different frequency pairs and phase 
measurements are made. These measured phases are the zero point phases 
that will be subtracted from relative phase measurements from the targets. 
A range measurement to an arbitrary target is made by moving the 
gimbal-mounted mirror 12 to point the beam to the target, and making phase 
measurements at all modulation frequencies. By subtracting off the zero 
point phases from the relative phases for a target, the corrected phases 
for that target are obtained. From these corrected phases, the unambiguous 
range can be determined by the above calculation for ranges that span less 
than half the maximum synthetic wavelength .LAMBDA..sub.21. 
Determining the absolute range with optimal accuracy requires a precise 
knowledge of the refractive index of air. This is obtained by monitoring 
the temperature and pressure of the ambient air with monitoring unit 26. 
Such units are commercially available and can supply data which can be 
used to calculate the refractive index to better than 1 part per million. 
For higher accuracy, the humidity can also be measured by commonly 
available means. 
The illustrated embodiment of FIG. 1 includes the provision of a mechanism 
for accurately aiming the optical beam at the targets 14a, b, c. The 
direction of the outgoing optical beam 27 is determined by the position of 
the mirror 12 mounted on two axis gimbals 13. An angular accuracy on the 
order of 3.5 microradians can be achieved by well-known, commercially 
available gimbal systems. Provided that beam 27 can be pointed accurately 
at the apex of a target, the transverse position accuracy that can be 
achieved is the product of the range times the angular accuracy. 
Accuracies of 0.003 inches can be achieved for ranges out to 70 feet, 
assuming perfect pointing accuracy. The tracking algorithm used in the 
preferred embodiment to maintain accurate pointing of the beam at the 
target's apex is described below. 
Before making a measurement of a target, the alignment of the optics should 
be checked and noted. Referring to FIG. 1, this can be done automatically 
by the computer 24 controlling the gimbals 13 and data acquisition, thus 
eliminating the chance for human error. The computer 24 is interfaced to 
the gimbals 13 via a control box 28. The gimbals 13 are first put in a 
retroreflecting position such that the light from laser 4 reflects off the 
mirror 12 and back into the fiber 6. A coupler 29 between the fiber 6 and 
the laser 4 picks off a small percentage of the light and transmits it 
through a fiber 31 to a power meter detector 32 whose output is fed to the 
computer 24. The computer 24 instructs the control box 28 to adjust the 
gimbals 13 until the power meter 32 shows that the power being returned is 
maximized. 
Correct alignment will also yield a maximum return on detector 16 at this 
point. This could optionally be used to check alignment, if desired. 
The elevation and azimuth axis position is then noted. With the gimbal in 
the retroreflecting position, the coordinates of the centroid of the 
returned laser spot on the quadrant detector 33, which receives light 
reflected by beamsplitter 9, is noted in the computer 24. These will be 
referred to as the null positions. 
The illustrated embodiment also includes a provision to aid the user in 
pointing the mirror 12 at a new target. A visible light point source 34 is 
directed in a diverging beam by beamsplitters 36 and 10 out toward the 
desired target, say 14c for purposes of illustration. The visible light 
from point source 34 is retroreflected by the target 14c, retraces its 
path, and is reflected by dichroic beamsplitter 10 onto a visible camera 
37. A commercial video camera with autofocus works well in this 
application. Camera 37 will show the image of the target and its 
surroundings as illuminated by ambient light on a video monitor 35 (shown 
in FIG. 4) as well as the light from point source 34 that is 
retroreflected, which causes a bright spot on the camera's image where a 
target is located. In the illustrated embodiment, the video image is input 
to the computer 24. The computer 24 then commands the gimbal 13 to move 
until the bright spot of the target is centered in the image from the 
visible camera 37 (alternatively, this could be done manually, checking 
the centering of the bright spot by visual inspection). This assumes that 
camera 37 is aligned such that when the light beam from laser 4 is 
pointing directly at the apex of a target, the target is centered in the 
image on the visible camera 37. This step roughly aligns the gimbal 13 
such that the laser beam is at least overlapping enough of the target 14c 
to create a return beam. The return beam is reflected by beamsplitter 9 
onto the IR quadrant detector 33. 
Once the mirror 12 is pointing to a target such that there is a return beam 
on the retroreflectors, the gimbal 13 is scanned across the target while 
the power of the return beam is monitored by either the high speed 
detector 16 or the power meter 32. From the transverse coordinate 
information and the power of the return beam at each location, the angular 
position of the centroid of the target is calculated. The gimbal 13 is 
then steered to that angular position. A range measurement is then taken 
and recorded in the computer 24 for use in deriving the transformation 
matrix. The azimuth and elevation angles are then individually stepped a 
small precise amount and the centroid of the return beam is noted. From 
the centroid positions in the two transverse steps and the range, a 
transformation matrix that scales offsets on the quadrant detector to 
deviations in pointing can be computed. Once computed for one target, the 
transformation matrix can be used for all targets on a test piece provided 
the alignment is sufficiently correct. The matrix must be scaled by the 
range and be rotated as the gimbal rotates. Alternatively, the 
transformation matrix can be computed for each target, or the matrix can 
be permanently stored, provided the instrument's alignment is unchanged. 
For subsequent targets, the mirror 12 is roughly centered on a target (by 
scanning for maximum power return) and the range is measured. The 
transformation matrix can then be either rederived by stepping the gimbal 
13 or transformed and scaled from the matrix derived from the initial 
target and the measured range. 
Tracking of the target is performed by measuring the centroid position on 
the quadrant detector 33 (or other suitable detector) and comparing it to 
the null positions measured during initial alignment. The error is 
transformed into an angular correction by the transformation matrix and 
this correction is added to the current gimbal position. The gimbals are 
then steered to the corrected positions and the range is then measured. 
The angular and range information may be displayed on a monitor 40 (shown 
in FIG. 4) as spherical coordinates or transformed to Cartesian 
coordinates. The tracking can then be repeated or another target can be 
selected. To reduce the noise in the centroid measurements of the quadrant 
detector, the measurements can be averaged or a gain can be multiplied by 
a correction factor that is less than unity. 
The components of the illustrated embodiment can be packaged for ease of 
use on a factory floor. As shown in FIG. 1, the components can be 
separated into signal components 38 and processing components 39. With 
reference to FIG. 4, the processing components 39 are preferably mounted 
on a rolling operator's cart 50, which also caries the video monitors 35 
and 40. The signal components 38 are preferably incorporated into a 
compact tracking head 55. The cables 41a-i which form connections between 
components in the operator's cart 50 and tracking head 55 can be bundled 
in a single umbilical cord 57. This allows the compact tracking head 55 to 
be easily mobile relative to the cart 50. The configuration illustrated in 
FIG. 1 minimizes the effects of phase measurement errors due to path 
length differences in the cables 41a-i between the signal components 38 
and processing components 39. It should be noted that, for clarity of 
illustration, only signal and control lines between the signal components 
38 and processing components 39 are drawn. Additional power lines could be 
included in the umbilical cord 57 as needed. 
Several alternative embodiments are possible, replacing one or more parts 
of the FIG. 1 embodiment with alternatives described below as illustrated 
in FIGS. 2 and 3. In these figures, like elements have been given like 
reference numbers. 
The quadrant detector 33 of the FIG. 1 embodiment could be replaced by a 
camera 42, shown in FIG. 2, that is interfaced with a frame grabber in the 
computer 24. Centroid estimates are then done digitally. In the FIG. 2 
embodiment, the first signal generator 44 could simultaneously output 
three signals, each at a different modulation frequency. The three signals 
are combined by power combiner 46 before modulating the laser beam from 
the laser 47 through an external amplitude modulator 43. Thus, the output 
laser beam is modulated by three tones simultaneously. The signal detected 
by detector 48 includes all three tones and is split by power splitter 49. 
The individual signals could be phase detected by mixing each of them with 
one of three local oscillator frequencies generated by a second signal 
generator 51. The local oscillator frequencies are each offset at a fixed 
frequency from the three modulation frequencies, as was described above in 
connection with the FIG. 1 embodiment. Each of three mixers 52a, 52b, 52c 
has a different local oscillator frequency. The output of the mixers 52a, 
52b, 52c are signals at the offset frequency. The signals from the mixers 
52a, 52b, 52c are bandpass filtered and amplified by processing units 53a, 
53b, 53c before being recorded by analog to digital converters 54a, 54b, 
54c, respectively. Simultaneously, the analog to digital converters 54a, 
54b, 54c are recording signals generated by mixing the outputs of the 
first signal generator 44 with each of the three local oscillator signals 
generated by the second generator 51 in mixers 56a, 56b, 56c. The signals 
from the first signal generator 44 are matched with the respective local 
oscillator signals from the second generator 51. The six recorded signals 
are then used in the algorithm described in connection with the FIG. 1 
embodiment to obtain the absolute range of the targets. 
The advantage of the simultaneous operation of the FIG. 2 embodiment is a 
significant reduction in latency. This enables the tracker to follow the 
range of targets at significantly higher radial velocities since no 
extrapolation of the phases is required. There are, however, a greater 
number of components compared to the FIG. 1 embodiment and possibly an 
increase in error due to the loss in signal strength that comes from 
splitting the return signal and reducing the amplitude of the input powers 
so that the modulation of the laser beam is not saturated. 
FIG. 3 shows another alternative embodiment in which a visible laser 61 is 
used to produce a tracer beam that is copropagating with the IR beam from 
the laser 62. The visible beam is reflected from beamsplitter 68 and 
dichroic beamsplitter 64 before being directed at one of the targets 14a, 
14b, 14c. The return beam is reflected by the dichroic beamsplitter 64 
into a camera 65. The output of camera 65 is used to estimate the centroid 
of the return HeNe laser beam and the signal sent to the gimbals 13 to 
keep them pointing at the apex of the target. Rather than use the camera 
65, the beam could be reflected by beamsplitter 68 into a quadrant 
detector 69. In either case, the initial alignment and measurement of the 
null position is performed in a manner similar to that described above. 
Adding a visible beam for tracking results in a factor of two reduction in 
the angular beam divergence due to diffraction. It also provides a tracer 
beam that can be seen with the unaided eye when manually steering the 
gimbals, and allows for the use of a visible light camera or quadrant 
detector for tracking rather than more expensive infrared ones. 
Referring again to FIG. 3, the RF reference signal of the FIG. 1 embodiment 
could be replaced by an optical reference signal. This is done by sampling 
the laser beam reflected by beamsplitter 71 and 72 onto detector 73. The 
signal is then mixed in a mixer 76a with the current local oscillator 
frequency. The resultant signal is filtered and amplified by processing 
electronics 77a and recorded by analog to digital recorder 78a. The 
reference phase obtained from this signal is functionally the same as that 
obtained solely from RF signals, as described above. The advantage is that 
if dynamic phase shifts occur in the modulation of the laser or if the 
RF/optical path distance from the signal generator 74 to the beamsplitter 
71 varies in time due to environmental variations, the optical reference 
will compensate for these variations. 
Rather than obtaining the index of air by monitoring the temperature, 
pressure and humidity of the ambient air, a calibration arm could be used 
to measure the refractive index of air. This is done by, in addition to 
the optical reference just mentioned, a second detector 81 which samples 
the beam reflected by beamsplitter 71. The distance from beamsplitter 72 
to detector 81 and the distance from beamsplitter 72 to detector 73 must 
be known and maintained to high precision. This can be done by mounting 
these elements on a thermally stable material such as Super Invar or 
Zerodur glass. The signal from the second detector 81 is mixed with the 
local oscillator frequencies in mixer 76c, filtered and amplified by 
processing electronics 77c, and recorded by analog to digital converter 
78c. The phase information from the analog to digital converters 78a and 
78c are used to obtain the calculated distance in vacuum represented by 
the path difference between the path to detector 73 and to detector 81. 
The known path difference is then divided by the calculated vacuum path 
difference to obtain the current refractive index of air. The calibration 
arm can also be used to compensate for uncertainties in the modulation 
frequencies. 
The return signal is reflected from target 14b to detector 63. It is mixed 
with signals from the local oscillator in mixer 76b. The output of mixer 
76b is filtered and amplified by processing electronics 77b and recorded 
by analog to digital recorder 78b. 
The preferred embodiment includes a phase detection method based on mixing 
of the target signal with a local oscillator signal whose frequency is 
offset from the first. The signal is sampled and digital IQ detection is 
performed. There are several alternative phase detection schemes known to 
those skilled in the art that could be used. 
While preferred embodiments have been described, those skilled in the art 
will recognize modifications or variations which might be made without 
departing from the inventive concept. Therefore, the description and 
claims should be interpreted liberally with only such limitation as is 
necessary in view of the pertinent prior art.