Full report

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General Disclaimer One or more of the Following Statements may affect this Document This document has been reproduced from the best copy furnished by the organizational source. It is being released in the interest of making available as much information as possible. This document may contain data, which exceeds the sheet parameters. It was furnished in this condition by the organizational source and is the best copy available. This document may contain tone-on-tone or color graphs, charts and/or pictures, which have been reproduced in black and white. This document is paginated as submitted by the original source. Portions of this document are not fully legible due to the historical nature of some of the material. However, it is the best reproduction available from the original submission. Produced by the NASA Center for Aerospace Information (CASI)

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r Z IACCEiltlnN NUM0Ei2) a k ^a „w IPAOEeI W ^^lL\ ^ ^^2W^ V i r ,^ ^ ^^.^ ^^ 17NRU1 G 3 ^ (CODE) OR TMX OR AD NUM0ER1 ICATti00RY1 I. ELECTRONICS RESEARCH CENTER ^, NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

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a A 0 0 i PM-^6 ^,PPLICATION OF LASER fiECNNOLOGY 1 TO GEODETIC MEASUREb1ENTS AND MAPPING H A Technical Summary Report 1 By Janis Bebris Optical Systems and Co.nponents Branch 1 Optics Laboratory Electronics Research Center 1 0 0 1 April 1968 This publication contains information prepared in the interest of rapid dissemination to a select 1 distribution. The information contained here^^n is preliminary in nature and subject to change. i- 1 0 AERONAUTICS AND SPACE ADMINISTaATION NATIONAL 9

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P^.GL I3L^'^'^1^ .'BUT b'ILb^L P ^Iti^:;^^^3 ^F^ 1.0 INTRODUCTION 2.0 SUMMARY ANd CONCLUSIONS 3.0 RECOMMENDATIO;^JS d.0 DISCUSSION Rangefinder-Altimeter Systems S 4.1 Available Laser and Rub Y Laser S Y stems 5 4.1.1 Korad YAG ^i I CONTENTS _^ 1 2 4 5 4.1.2 Spectra-Physics Geodolite 7 Laser RangefinderAltimeter- Systems . 7 4.2 Developmental 4.2.1 Lunar Mapping Altimeter (MSFC/TRG) 9 4.2.2 Optical Spacecraft Ranging and Automatic System (MSFC/ITT) 11 Docking Tracking System (AFCRL/Hughes) 11 4.2.3 Lunar 4.3 Advanced Techni ques and Conce Pis 13 Fh1-Subcarrier Laser Radar 14 . 4.3.1 4.3.2 Geode*.ic Laser American) 4.3.3 Mode-Locked S.0 REFERENCES SurveJ^ System (North 15 Nd;YAG Laser .System 18 19 •lll-

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i 1.0 INTRODUCTION The objective of this report is *.o summarize the results of a study-review which, was conducted on published tecrnical information on rangefin^Jers. The report includes observations laser alri^aeters and and discussions with some Experts in the field obtained during a few, but selected, visits to industrial organizatio^^s which have been noted contributors to the advancement of laser technology, in particular, to the development of the laser altimeter-rangefinder. This eff,"ort was undertaken to update the available background information on laser altimeter-rangefinder technc^^,^es and new approaches, of the tectnical com,etency in this area and to establish the foundation in the Gptical.Systems and ^^;^iponents Branch, Optics Laboratory of the Electronics Research Center (ERC). efforts and the antici p ated outside ERC- The projected in-house supported research and development work in these last:-advancing laser technology application areas will complement the already established laser research and development work being done in ERC's Optics Laboratory. Section 2.0 of the report presents the conclusic^r;:s which resulted from this study and associated reviews and discussion:,. Section 3.0 presents the writer's short-range recommendations. Section 4.0 describes and discusses the commercially available laser range.f.inder-altimeter systems (A.1), systems presently under development (4.2), and some advanced concepts and techniques (4.3). Section 5.0 presents directly related references.

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r 2.0 SUMMARY AND CONCLUSIONS This section presents the results of a preliminary review of the laser rangefinder and altimeter technology. It is not necessarily allinclusive, but it represents the initial attempt to acquire the basic understanding of the present state and the direction of the laser rangefinder-altimeter technology and its applicability to geodetic measurements and mapping. This type of review will continue and will be updated more or less periodically. The review revealed that several ground-based and airborne r- a ging-altimeter systems are offered commercially by several industrial anizations (Spectra-Physics, Korad, Hughes). A:.1 these systems are intended for rangis up to 50 kilometers at favorable atmospheric conditions with a range accuracy of t10 feet. Several longer range systems are presently under development, but only one (TRG/MSFC) is intended and designed as spaceborne laser a altimeter for cooperative operation with photographic lunar mapping and Earth resources surveying cameras. in the initial pre - prototype phase. The development of this altimeter is It appears that at least 2 more years of intensified development work will be required in order to deliver a qualified flight altimeter system with 80- to 100-km lunar - and 460-km Earth-oriented ranging capability. the invention of the ruby laser by It was concluded that since T. H. Maiman l , 2 in 1960, the widest and the most practical application in ranging and altimetry. Present laser of the new technology had been providing a wide selection of differtechnology is already capable of characteristic solid-state,3,4 gas',5 ent power level and other major and injection lasers 6` for future ranging and altimetry requirements. and development efforts are continuing Basic and applied laser research a high level. It is concluded that no throughout the industry at encouragement are required in this area at the additional support or present time. and the discussions with the workers The review of the literature altimeter field indicated that not enough i .he laser rangefinder and spaceborne system environmental qualifia,,.ention is being directed to If nbt corrected, this situation may cation requirements and problems. bring about the repetition of the bitter qualification experience of systems for the TIROS, RANGER, and the early spaceborne operational effort, and money will be saved later NIMBUS projects. Very much time, if early consultations are held and the past environmental qualification experience is transferred to the laser altimeter design and development the flight prototype designs. A efforts before the finalization of new group of workers has entered laser technology research without any first-hand experience in spaceborne system design and environmental problems. There exists a need for qualification requirements and efforts in the new laser technology immediate support and additional field. -2-

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1 Several ground-based laser tracking and ranging systems exist or are being developed. A few of these systems have been operational for some time. Also some international capability exists in this area. The French, in particular, have accumulated optical satellite ranging and tracking information from two stations near San Michel, France and San Fernando, Spain. It is known that the Russians have done some satellite ranging, but not much more is known about it. Most of the results have been published. Studies and the generation of design specifications are being planned by NASA Headquarters (SAG) for mobile (transportable) laser tracking and ranging stations which could meet the requirements of makng operational geodetic measurements from remote sites. The Optical Systems and Components Branch of the Optics Laboratory, ERC, is interested and ready to participate in this area of tasks. A small service contract ($2.5,K) has been let to ADCOM, a division Mass., under the title: "Design Criteria of Teledyne, Inc., Cambridge, for Radar Tracking.' The emphasis of this effort will be an analytical evaluation of (1) fundamental theories, and (2) design and performance of practical systems. This review will be published as an addendum to this report. Additions to this report will be submitted as new technology and/ or results become available. -3-

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i Y 3.0 RECOMMENDATIONS On the basis of this study and review, and for the purpose of identifying the technical areas which need additional support and attention so that the advancement of the laser rangefinder-altimeter technology would be well coordinated and balanced, recommendations are made: the following 1. Increased support and in-:louse technical'efforts show''ld be directed towards the space environmental qualification of various applicable laser sources. Preferably, this should be accomplished before the final design of a spaceborne altimeter system. 2. Whys and means of transferring without delay the available past experience of spaceborne operational system design criteria and the environa;ntal design requirements to the a should be sought. laser altimeter technology 3. New and more accurate laser ranging and altimeter technique encourrged, particularly in the development should be supported and return signal detection and processing areas. /

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i ., , ..w"i.r.*^ 4.0 DISCUSSION One of the first, and still the major, application areas of the laser is that of rangefinding and altimetry. To date, a number of laser rangefinders have been designed and fabricated by both military and industrial organizations for application to ground and airborne surveillance, target acquisition, and fire control systems. Most of these rangefinders have utilized a Q-switched ruby laser as a transmitter. liowever, some of the most recent systems are using neodymium as the laser element, and it is anticipated that this trend will continue in the future. A preliminary review of industrial and Government organizations which are active in the laser de g-elopment and application work indicate that a significant effort is being devoted to laser altimeters and rangefinders. Some 17 organizations were identified as having performed design studies, built; experimental and operational systems, and performed measu , ements on ground, sea, and from air. Table I lists these organizations. In addition to organizations listed in Table I, a few Government installations and Government-supported organizations have established laser satellite and/or lunar tracking and ranging capabilities. Except for the Frankford Arsenal, which develops and builds its own laser sources, almost all other Government laboratories and installations have had their laser systems developed and installed by experienced industrial organizations. The most completely equipped and instrumented locations are: AFCRL, Bedford, Mass., NASA/GSFC, Greenbelt, Md., JPL, Pasadena, California, NBS, Boulder, Colo., and the Smithsonian Institution Astrophysical Observatory, Cambridge, Mass. The latter operates three laser tracking stations on Mt. Hopkins, Arizona, Mt. Haleakala, Maui, Hawaii, and in Dionysos, outside Athens', Greece. 4.1 AVAILABLE LASER RANGEFINDER-ALTIMETER SYSTEMS Because of the increasing scope of laser applications for distance terrain-profile recording, industrial organizations are measurement and commercial laser ranging systems. So far, Spectramaking available Geodelite CW laser ranging system ($79,000.00), Physics has announced its ruby laser systems, which will be described and Korad, two YAG and one in more detail in the following subsections: 4.1.1 Kor a d YAG and Ruby Laser Systems Korad's Model KLRF-23 YAG laser system is a compact, high-power, repetition-rate system designed for airborne deployment or applications where minimum size is required. The system consists o'f two units: the laser head assembly and power supply control unit. Its performance specifications are listed in Table II.

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TABLE: I 6 ORGANIZATIONS INVOLVED IN LASER DEVELOPMENT AND APPLICATIONS Industrial No. Organization Study 1. Avco X 2. Boe ing X 3. Douglas X 4. EG$C X S. General Electric X 6. Hughes X 7. I.B.M. X b. Korad X 9. Lockheed X 10. Lear Siegler X 11. North American X 12. Perkin-Elmer X 13. Raytheon X 14. R.C.A. X 15. Spectra-Physics X 16. TRG X 17. Sylvania X Area of Participation Hardware Devel. Experimental X X X X X X X X X X X X X X X X X X X X X X X X X -6-

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TABLE I1 PERFORMANCE SPECIFICATIONSICATIONS KORAD MODEL KLRF -23 YAG LASER SYSTEM Laser Material Wavelength Range Ringo Resolution Ream Divergence Output Power Pulse Width P.R.F. Pulse Energy Q-Switch Input Power Neodymium-!loped YAG 1.06v >20,000 m t10 feet 4 mead (uncorr.) 8-10 MW 8-10 nsec 10 pps NO MJ Pockel Cell 28 V-dc, 20 A Weight: Laser Head 10 lb Supply 12 lb Power Ground-operated laser rangefinder performance specifications are listed in Table III. 4.1.2 Spectra-Physics Geodolite The Spectra-Physics Geodolite system is the result of 4 years of development work on a modulated CW laser distance-measuring instrument. The Geodelite 3G system has been proven by field measurements. High resolution profiles of the surface of the Earth and the ocean waves have been recorded from an altitude of 15,000 feet. The published Model 3G performance specifications are shown in Table IV. 4.2 DEVELOPMENTAL LASER RANGEFIIDER-ALTIMETER SYSTEMS The General Electric Company has developed a small and relatively inexpensive ($95.00) pulsed gallium arsenide injection laser rangefinder system for use-as an automobile safety device. It can warn the driver of a car when the distance between his car and the car immediately ahead of him becomes less than a predetermined value. This laser safety system could also operate*the brakes of the car automatically. The General Electric Company has demonstrated this system to the Chrysler Corporation: and the General Motors Corporation. It is known that Ford is developing a similar laser automobile safety device. RCA has developed a gallium arsenide early warning system for application on high-speed trains. This system has been experimentally tested on an electronics testing ground near Denver, Colors.do. When fully developed, this ranging system will be capable of detecting a 1 cubic inch obstacle on the railroad tracks from 5 miles sway while traveling at 200 mi/hr. D -7-

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TABLE III GROUND-OPERATED LASER RANGEFINDER PERFORMANCE SPECIFICATIONS Model KLRF-20 Model K LRF-21 Material Ruby iavelength 6943 Range 200- 20,000 m Range Accuracy 12.S m Beam Divergence 1.0 mrad utput Power S.0 MW Pulse Width 20 nsec P.R.F. 4.0 ppm Neodynium-Doped YAG 1.06N 45-5500 m t8.S m 1.0 mrad 1.0 MW 20 nsec Single shot and/or continuously variable from 1 to 10 pps Self-contained re- 28 V-dc; 350 W Input Power chargable battery for 200 shotps Range Readout Digital 1100F 0 to -50oC Temp. Operating Range -10 0 F to Digital and Analog (including 18,5 lb Weight 40 lb batteries) shots with >1000 hr Operating Life >3S0,000 scheduled maintenance Physical Configuration 1 ,Unit 2 Units Mist other present laser rangefinder-altimeter development programs are intended for military applications on the ground. Few systems are being developed for airborne applications, and still fewer for use on the sea. The major contributors to the present state of this technology have been Hughes Aircraft, Korad, TRG, Lear Siegler, and F_'ankford Arsenal. The requirements for the range capability of the military systems have been limited from 2 to 15 miles. The majority of them have employed a pulsed ruby laser (6943 42 8-

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6 TABLE IV MODEL 3G SYSTEM PERFORMANCE SPECIFICATIONS Laser Wavelength Power-Output Range (in clear air) Oscillator Stability Mounting Dimensions (overall) and Weight- Ambient Temperature Input Power He-No 6328 100 mw SO miles at night 20 miles in full sun 1 in 10 6 per day; 6 1 in 10 year Alti-azimuth mount with horizontal and vertical tangent screws; graduated vertical and horizontal circles with verniers reading to 1 minute. Telescope Assembly: 34x2Ox:6 in., 100 lb Control Unit: 17xl6x5-1/4 in., 40 lb Digital Readout: 17x2lxS-1/4 in., 33 lb Operating: -20,Q F to 120OF 115 t10 V, 50 to 400 Hz, 400 VA Presently, three known unclassified developmental programs are in progress. Two are intended for space application, and the third for ranging to the moon from Earth. These programs are discussed in more detail below. 4.2.1 Lunar Mapping Altimeter (MSFC/TRG) Preceding the present hardware development program at TRG, NASA/ MSFC sponsored a study and design specification program on spaceborne laser altimeter systems with the Raytheon Company, Space and Instrumentation Systems Division, Sudbury, Mass. The work was performed -9-

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from September 1966 to January 1967 under contract NAS 8-21013. a The results of this study showed that a laser altimeter could be developed to meet the space mission requirements for photogrammetry and geodesy. Based upon the analysis conducted and a decision to utilize presently available laser transmitters, a ruby laser system operating at 6943 X was recommended to perform the Earth-orbiting missicn over land and sea. For lunar operation a neodymium-doped YAG laser system operating at 10,600 R was recommended. This decision whs predicated upon the lower power, lower weight, and smaller volume requirements of the YAG-type system as compared to the ruby. The subsequent hardware development contract was awarded to TRG. The goal of the present program is to develop a flight pre-prototype laser altimeter system which will be tested in an aircraft. A design review was conducted on March 11, 1968, at TRG. The Optics Laboratory participated with two observers. The major subject was the discussion and approval of the redesigned optics. An on-axis optical system was proposed in order to meet the size and performance specifications. . The TRG/MSFC laser altimeter specifications are outlined in Table V. TABLE V TRG/MSFC LASER ALTIMETER SPECIFICATIONS Wavelength Energy Beam Angle Pulse Repetition Rate Pulse Width Range Accuracy Orbit Altitude Illuminated Area on- Ground Mounting Spacecraft Stability -10- 6943 0.140 J 40 urad 6 (per min) 20 nsec t2.5 m 460 km 18 ft dia. Fixed--boresighted with metric camera ±1/2 deg

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The delivery of the pre-prototype lanar altimeter system is an icipate d in October, 1968. t 4.2.2 Optical Spacecraft Ranging and Automatic Docking System ( MSFC/ITT)B A guidance system for rendezvous and docking, using galliumarsenide injection lasers and a gallium - arsenide spontaneous source, has been developed for NASA /Mc,1C Laboratory, San Fernando, ;;y ITT Calif. It is intended for use in either manned or unmanned spacecraft to provide the necessary intelligence to the guidance computer to effect a complete rendezvous and docking operation automatically. This system is the culmination of 4 years of study, experimentation, and hardware development. The system uses uncooled gallium-arsenide laser arrays operating in a pulsed mode for initial acquisition and, subsequent measurement of X and Y angles, angle rates, range, and range rates. When the distance between the two spacecraft has been reduced to less than 3 km, greatly increased range and range rate accuracy are obtained by the use of an incoherent diode source continuously modulated at a gallium-arsenide high frequency, and using phase-locked detection techniques in the receiver. Control of the spacecraft from this sensor continues until the docking operation is completed. In its present configuration, the laser guidance system utilizes equipment on both spacecraft, which are designated the chaser vehicle and the targ et vehicle. Normally, the target vehicle will remain in a fixed orbit, while the chaser vehicle will use attitude control as necessary to maintain the proper orientation for the chaser vehicle approach. The sensing element in the receiver is an image-dissecting multiplier phototube with an S-1 photo-emissive surface, which has at approximately 9000 R A narrow band 'its peak spectral sensitivity interference filter in the optical path is used to minimize interference from the sunlit background. specifications are summarized in Table VI. This The system spaceborne laser guidance system has been successfully tested and in a space at the Martin Company, Denver, Colorado, operated simulator facility. 4.2.3 Lunar Trackingg S y stem ( AFCRL/ Hu ghes )9 A feasibility study of laser-ranging systems working from an Earth-based astronomical observatory against an optical retroreflector placed on the moon by a spacecraft or an astronaut was performed by Hughes Aircraft Company, Culver City, California, from Aprii ; 1965, to April, 1966. It was concluded that, from consideration of the restraints imposed by present and the optics and kinematics of the technology 10-nsec, Q-switched ruby laser with Earth-moon system, a 10-joule, operating through a single, 60--inch photomultiplier receiver, both telescope, might be selected for the system design.

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TABLE V1 46 01 SYSTEM SPECIFICATIONS FOR GaAs LASER Wavelength Power: Peak Pulsed 9000 R 1000 W Average (m Watts) 200 mW Pulse Width Pulse Repetition Rate, Range--Pulsed Range--CW 100 nsec 2000 (per sec) 120 km 3 km Range Accuracy 120 to 3 km 1/2 percent Range Accuracy 3 to 0 km 10 cm Angle Accuracy Size Weight i Power--Input 10 arc sec 1 ft3 35 lb 15 W The analytical evaluation of such a system showed that it would be capable of measuring range with a precision of a few meters at all hour angles and at almost all phases of the moon. Information obtained by laser ranging between an Earth station and points on the moon will be valuable to several scientific disciplines, including geodesy, and celestial mechanics. important results systematic program of lun g laser ,which are likely to come from a ranging include improved knowledge of the size of the moon and its orbit, determination of the gross figure of the moon with more assurance than it is now known, greatly improved libration data, and accurate Earth station coordinates that are independent of geodetic surveys. While the above study was still in progress, AFCRL contracted for the construction of a telescope with a 60-inch diameter aperture, to be installed in the Catalina mountains of Arizona at a site maintained by the Lunar and Planetary Laboratory of the University of Arizona. The laser system has been designed around the above telescope, and for a specific experi:aent. Far this reason, commercially available e quipment has been used whenever possible in preference to special signs which might be slightly more effective but more expensive and less flexible. Table VII summarizes the parameters of the systems for rangefinding and for photographing the retroreflectors with the separate 60-inch photographic telescope. At the time of this report, the 60-inch telescope has been delivered'to the Arizona site and the mount installed. The ruby laser system is being built by the Hughes Aircraft Company and will be completed by the end of this year (1968). The installation and debugging -12-

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1 SUMMARY OF DESIGN PARAMETERS 1 1 Ruby Laser 0 Ruby Diameter and Length Laser Output Energy Pulse Duration 1 Laser Output . Power Laser Beam Divergence Repetition Period (minimum) Average Laser Output Power Telescope ' Diameter Ranging Photography Q-switched Ordinary- 15 mm x 15 cm 15 mm x 15 cm 10 J 40 J 10-8 sec 4 x 10 -4 sec. 109 W 105 W 0.3 - 1.0 mrad 0.3 - 1.0 mrad 5 sec 5 sec 2 W 8 W 1.5 m 1.5 m Transmit 0.75 0.75 Optical Efficiency on 1 Receiver Bandwidth 10 10-100 Receive 0.4 0.4 Optical Efficiency on 1 Detector Quantum Efficiency, Travel Time Counter Rate Pump Energy 1 Head 2 kW 2 kW Average Power into Laser Operating Power 1 Weight of Telescope--Mounted 0.1 - 100 me - 104 J 104 J I5.16 kW 5.6 kW Unit 100 kg 100 kg of the lunar ranging system is scheduled for another year. The experiments are planned to start in 1970. 1 4.3 ADVANCED TECHNIQUES AND CONCEPTS t As mentioned earlier in this report, the newly emergin? laser technology found its first major application in optical ranging systems. In the meantime, the research and advanced development efforts have 1 been devoted towards the increased range and range accuracy capabilities. The ultimate range accuracy of the measurement is determined by the accuracy of the measurement of light speed. All practical and reliable 0 laser distance measuring systems are still far from this final limitation. The atmospheric variations, the laser output variations, the detector sensitivity, and response time limitations and the received 9 signal processing capabilities are still the major limiting factors in the range accuracy measurements. 9 -13-

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i The workers in this field are searching for and developing new and improved optical ranging and altimetry techniques. Some of them will be discussed in the following subsections. Most of these techniques are intended for CW operation, and, therefore, are applicable only to systems for shorter range determination. The output power capability of CW lasers is rapidly increasing, and these advanced techniques will become important alternatives. However, wide application of high power CW lasers for ranging and altimetry will not be practical, because of the inefficient use of the radiated power. 4.3.1 FM-Subcarrier Laser Radar 10 Theoretically, a CW optical ranging system employing coherent optical transmission and heterodyne reception, provides exceptional range and velocity,resolution. However, because of the very small wavelength of the optical radiation employed, numerous practical dif- `iculties arise when heterodyne detection is attempted. First, accept- .ble angular misalignment of the optical system is very small; second, atmospheric turbulence causes phase distortion of the optical wavefronts; third, the Doppler shift of a high velocity target is many thousands of Hertz (Hz) and necessitates the use of a broad-band insensitive receiver. These degrading effects can be overcome by utilizing CW laser radiation as a carrier of FM-modulated VHF, UHF, or microwave subcarriers. Frequency modulation of the Subcarrier provides the means for coherent demodulation and information processing. Since the Subcarrier modulation is in the radio frequency region rather than the optical region of the spectrum, the Doppler shift will be a few Hz per knot, rather than several MHz per knot. An experimental FM subcarrier laser radar system has been constructed and tested by Douglas Aircraft Co., Inc., Missile and Space Systems Division, Santa Monica, California. Operation of the system is as follows: (1) A He-Ne laser beam is intensity-modulated by a KDP crystal driven by a frequency swept generator; (2) a portion of the diffused reflection from the target surface is collected, filtered, and then detected by a photomultiplier tube; and (3) the output signal is mixed with the frequency swept source signal to produce sum and difference frequencies. The difference frequency, which is proportional to target range, selected by a low pass filter, and is amplified and displayed on a rrequency counter. Several experiments have been performed to assess the system performance. Range counts were made over one full cycle of the modulating waveform and were found to be invariant as long as the peak-to-peak driving voltage remained constant. This result is significant in that the frequency sweep linearity requirements for a modulator driver are substantially relaxed in this mode of operation. The method of CW optical ranging described above has'several practical advantages. It is free of the alignment difficulties and atmospheric distortion which is present in optical heterodyne detection. -14-

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6 It is adaptable for use with powerful CO2 lasers, and then it is capable of measuring the ranges of hundreds of miles and range resolution of, about a foot. 4.3.2 Geodetic Laser Survey System (North American)11 The existing laser geodetic survey systems are restricted to a precision of several parts per million of the distance measured. The major limiting factor in all electromagnetic distance measuring techniques is the uncertainty introduced by atmospheric fluctuations !hich affect the value of the refractive index. The resultant variations in the velocity of light causes corresponding distortions in distance measurements. The Geodetic Laser Survey System described herein and developed by North American Aviation, Inc., subsequent to an extensive study program of Laser Space Communication Systems (LACE) under contract NAS w -9°77, overcomes these constraints and provides an order of magnitude improvement in accuracy. This dual-beam laser system has direct application to geodesy, earthquake prediction research, and other fields requiring precise distance measurements. The Geodetic Laser Survey System utilizes two lasers operating at obtain simultaneous, independent range measure - different wavelengths to mcnts at two points in the optical spectrum. The light beam from each laser is modulated at a radio frequency, and the range measurement is made using this modulation frequency. By simultaneously measuring the difference in optical range or atmospheric dispersion between two can be determined which provides, to a optical frequencies, a factor first-order approximation, compensation for the temporal and spatial variation in the atmospheric index of refraction. This approach not only eliminates the necessity for measuring discrete points along the optical path, but atmospheric parameters at true average for these atmospheric parameters, allows the derivation of a This produces an order of magnitude improvement in the precision with which long ranges can be measured. ' 1 experimental system is shown in a schematic con- The original figuration in Figure 1 and the modified'second-generation system, presently undergoing design changes and -reassembly into the mobile unit, is shown in Figure 2. Three major improvements have been made in the wavelength, the detection and phase measuremodified system: the laser ment techniques, and the modulating frequency. The use of the new low-power laser at the shorter (visible) wavelength overcame the objectional below-quantum efficiency of availably infrared photodetectors. In the original experimental configuration, considerable phase jitter in the 100-MHz signal was encountered during passage through the photomultipliers. The modified syster. uses a nominal S-kHz frequency obtained by heterodyning at the first dynode of the photomultiplier with a signal that is phase synchronous with, source as the modulating frequency. The an-d derived from, the same effects of transit-time jitter is thus eliminated. -15-

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4 Additional modulating frequencies have been added to permit unambiguous ranging over the total distance, and the original 1004111z modulation frequency has been modified to reflect the constant 5-kNz intermediate frequency in the detection system. The North American group at Downey, Calif., hopes to complete these modifications in the near future, and to be back performing field measurements along the San Andreas fault system by this spring. 4.3.3 Mode-Locked Nd;YAG Laser System The Optical Eystems and Components Branch of the Optics Laboratory at NASA/ERC is procuring a short-pulse (10- i0 sec), mode-locked Nd;YAG laser system for experiments and the development of high-accuracy, distance-measuring techniques. Three potential sources are considered: Raytheon Company, Sylvania-Western Division, and United Aircraft Corp. is estimated that this system will cost approximately $24 to $29,000. .11 major associated instrumentation alr>o has been ordered or selected. It is expected that the in-house experiments and evaluation of new techniques will begin within the next 6 months. Parallel efforts have been initiated by the same Branch at ERC on space qualification of Nd;YAG laser sources. This work is supported under the OTS E programs. Related research and development work is carried on in the Optics Laboratory at ERC in optical physics, optical materials, and generation and detection of optical radiation and modulation. Therefore the laser ranging and altimetry, optical tracking, and high data rate communication system research and development work will be con;plementar r and beneficial to the other work performed by this laboratory. .18.

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5.0 REFERENCES 1. T. H. Maiman, Optical. Maser Action in Ruby, Brit. Commun. and Electr. Vol. 7, pp. 674-675, 1960. 2. T. H. Maiman, Stimulated Optical Radiation in Ruby, Nature, Vol. 187, pp. 493-494 0 1960. 3. Z. J. Kiss and R. J. Pressley, Crystalline Solid Lasers, Appl. Optics, Vol. 5, No. 10, pp. 1474-1486, October 1966. 4. Lasers, Appl. Optics, Vol. S. No. 10, pp..1487- E. Snitzer, Glass 1499, October 1966. Appl. Optics, Vol. 5, No. 10, pp. 1500- 5. A. L. Bloom, Gas Lasers, 1514, October 1966. 6. Semiconductor Lasers, Appl. Optics, Vol. S, No. 10, M. I. Nathan, pp. 1514-1528, October 1966. 7. Technical Study and Design of a Spaceborne Raytheon Company, Laser Altimeter System, Final Report, Volume I, Contract NAS8-21013, February 28, 1967. 8. T. P. Dixon, C. L. Wyman and H. D. Coombes, A Laser Guidance System Docking, J. Inst. Navigation, Vol. 13, No. 3, for Rendezvous and Autumn 1966. 9. R. S. Julian, Feasibility Study of Laser Ranging Systems, Final Report, Contract No. AFl.9(628)-5172, Hughes Aircraft Co., June 1966. 10. R. B. Hankin and E. J. Pisa, Some Aspects of Optical Ranging, Douglas Aircraft Co., Inc., Report DAC-59098, November 1966. 11. R. A. Fowler, Earthquake Prediction from Laser Surveying, Report, Contract NASW-1545 (NAA# SD67-772), 21 August 1967. -19-