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Aerospace App ions of Optical SensingS. M. Beck, J. A. Gelbwachs, D. A. Hinkley, D. W. Warren, and J. E. WesselThe Aerospace Corporation

P. 0.Box 92957Los A ngeles, California 90009steven.beck@aero.org(310) 336-1534

Abstract-Remote optical sensing with lasers,known as lidar, has been show n to be a usefultechnique for civilian meteorological andenviron men tal applications. Recently, thepotential of lidar for addressing applications ofinterest to aerospace systems has beenrecognized. Ground- based lidars are ideallysuited for the calibration of instrumentsonboard satellites. Other aerospaceapplications include monitoring the impact oflaunch vehicles upon th e environment. Futuresatellite systems are envisioned carryingonboard lidars for atmosphericcharacterization. In this paper we willdiscuss briefly the lidar technique, outlineimportant aerospace applications for whichlidar is ideally suited, and describe the mobilelidar system under construction at TheAerospace Corporation.

1. INTRODUCTIONLaser remote sensing of the atmosphere,known as lidar, has been shown to be apowerful means to acquire range-resolveddensities of atmospheric species[11. Whencoupled with scannable optics threedimensional profiles can be constructed.Important meteorological parameters such asdensity, temperature, humidity, aerosols, andwind velocity as well as cloud properties have

been acquired by lidar to altitudes of 100 km.Environmental species such as ozone,nitrogen dioxide, carbon mon oxide, and otherurban pollutants, as well ais stack effluentssuch as sulfur dioxide and HC1, have beenmonitored at the sub ppm concentration levelout to several kilometers range. Lidar havebeen developed that m onitor critical indicatorsof climatological change such as stratosphericozone and particulates as well a s m esospherictemperature. In this paper we will discussbriefly the lidar technique, outline importantaerospace applications for which lidar isideally suited, and describe the m ulti-purposemobile lidar system under construction at T heAerospace Corporation.

2. PRINCIPLES OF LIDARA schematic representation of a mob ile lidarunit appears in Fig. 1. A short pulse laserbeam is propagated into the atmospherethrough transmission optics. The faintbackscattered light from molecules andparticles at a distance is collected by atelescope collocated with the transmitter. Thecollected light is spectrally analyzed and thephotons in individual wavelength channels areconverted to electronic impulses byphotodetectors. The electronic signals arethen processed and stored in small com puters

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for subsequent display and analysis. A time-of-flight analysis yields range informationwhile the strength of the optical returns isrelated to the molecular or particle density.The most common optical interactionsexploited for acq uisition of species profiles arefluorescence, absorption, Raman scattering,Mie scattering, and Rayleigh scattering.

3. AEROSPACEPPLICATIONSAtmospheric sensing instruments onboardmeteorological satellites that rely up on passivemicrowave radiometry require ground basedcalibration techniques. Among theseinstruments is the SSWT-2 (Special SensorMicrowave/Temperature-2) microwavehumidity radiometer onboard DMSP (DefenseMeteorological Satellite Program) andinstruments onboard UARS (UpperAtmosph eric Research Satellite). Currentcalibration techniques involve the use ofradiosondes. However, these devices sufferfrom hysteresis effects and degradedperformance at high and very low humidityand at low temperature. Because ground-basedlidar can be made to sample the sameatmospheric region that a passing satellitesenses from above it is considered an idealcalibration technique for this application.Furthermore, as mentioned above, ground-based lidars can monitor importantmeteorological parameters. Recently theyhave been proposed as instruments onboardsatellites. From a space platform it isanticipated that they will provide worldwidecoverage of important weather variables as afunction of altitude down to ground level.

physical properties. Additionally, it isparticularly useful for locating clou ld bases.Another important area for lidar is monitoringof the environmental impact of rocketlaunches. The current generation of launchvehicles powered by solid rocket motorsinjects large quantities of chlorine and aluminaparticles into the stratosphere. It is estima tedthat chlorine emission into the 15 - 40 kmaltitude region by the Titan IV vehicle and thespace shuttle is 48 tons and 79 tons,respectively, per mission. Active chlorine inthe stratosphere is known to rapidly catalyzeozone destruction. Hence, environmen talconcerns have been raised over the impact ofthese launches upon the natural ozone layer.Com prehensive chemical and transport modelshave been constructed to understand rocketexhaust effects in the stratosphere. Recentmodels predict almost complete ozonedepletion in stratospheric tracks that extendover several kilometers and persist for up to aday. However, the size and persistence of thehole are very sensitive to the rate ofdissipation of the rocket plume, which ispoorly understood at present. Hence, directmeasurements of ozone densities are critical toconfirm the spatial and temporal evolution ofthe hole and for model validation. Thu s far,observations have been elusive because o f thetransient nature of the localized plume c oupledwith its rather inaccessible location. Recentlythe Titan Program Office of the USAF Spaceand Missile Center has sponsored thedevelopment of a mobile lidar unit by thePhillips Lab Geophysics Directorate which isto be taken to Cape Canaveral to monitor forstratospheric ozone depletion in the exhaustplume of Titan IV vehicles. Measurem entsare scheduled for FY 96.

A related application involves clouds.Understanding of cloud properties is importantfor meteorological and surveillanceapplications. Lidar is an excellent techniqueto monitor cloud heights, attenuation, and Another area of concern is the toxic groundcloud associated with the launch of solidrocket motors. These vehicles emit copious

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quantities of HC1 upon launch in the form of aground cloud that poses a health hazard.Lidar is an excellent tool to track the drift ofthe ground cloud and m ap the HC1 densities asa function of time. A related applicationinvolves the validation of wind dispersionmodels that are used to determine when it issafe to launch. In this application a tracerspecies such as sulfur hexafluoride is releasedat the launch site and tracked by the lidar. Thethree dimensional density maps are thencompared with the predictions of the winddispersion models for v alidation purposes.

4. DESCRIPTIONF AEROSPACE MOBILELIDAR

The mobile lidar system is expected toaccommodate a multi-purpose mission. Tothat end, the system has been designed to beversatile, easily converted from one type ofmeasurement to another. A sch ematic of theAerospace Mobile Lidar (AML) is shown inFigure 1. The major compo nents of thesystem are a 30 inch ape rture, full-sky rotatingbeam director and receiver, a 30 inch receivertelescope, detection op tics and electronics, anda variety of laser transmitters. The system ishoused in a mobile facility capable of beingtowed or transported by military aircraft.Below is a more detailed description of themajor compo nents of the AML .ReceiverAn important feature of the system which addsto its versatility is the large aperture receivingsystem. This provides the ability to observesignals from inherently weak scatters, lowconcentrations of scatterers, or large ranges.The receiver consists of a 30 inch aperturetelescope coupled to a two mirror, 30 inchaperture beam director and receiver. Thetelescope is a folded f/7.5 cassegrain. Theprimary mirror, constructed of Astrositall (a

low thermal expansion material developed inthe former Soviet Unio n), is f/2.5. A tertiaryfold mirror is incorporated into a central borein the primary mirror so that the primary focusof the telescope occurs at the side of thetelescope tube. The fold mirror can be rotatedinto four de tent positions, each sep arated fromthe next by 90 degrees. Tlnis allows quickconversion from one detection scheme toanother with virtually no resetting orrealignment of optics required. The plannedlarge-aperture beam-director and receiverscanning optics will con sist of two 45 inch by32 inch elliptical flat mirrors, mounted in atubular housing. The mirrors also will bemade from Astrositall. The azimu th degree offreedom is supplied by a large horizontallymounted crane bearing having an integrallymounted ring gear for driving the mount. Thesame type of bearing is mounted vertically,with its ow n ring gear, and supplies theelevation degree of freedom. The mountswings 360 degrees in azimuth, and 160degrees in elevation, 80 degrees each side ofzenith. Laser transm itter relaiy mirrors will bemounted at the center of each of the largeelliptical beam director mirrors using a spidermechanism. This allows facile changes in thetransmitter mirrors as required by changes intransmitted wavelengths. The surface figuresof the m irrors in the receiving optical path arespecified to permit 100% of the image energyto be contained within a spot 0.1 mrad indiameter (approximately 0.6 mm at thetelescope focal po sition).

The received backscattered radiation passesthrough a field stop placed at the primaryfocus. The telescope was designed to operatewith a 0.3 mrad field-of-view. This limits thecollection of background radiation to just thatin the solid ang le of the projiected laser beam.After the field stop, the received radiation is

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Aerospace Mobile LIDAR HousingUnittlaser transmitte

Computer stationor system control,data acquisition,an d display

collecting telescopePhotodetector andspectral filter package

Figure 1. Schematic representation of the multi-purpose transportable lidar under dev elopmen tat the Aerospace Corporation.

Figure 2. Photograph of the mobile lidar transportable housing.

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collimated with lenses, separated bywavelength using dichroic beam splitters, anddetected by photomultiplier tubes havingnarrow bandpass filters in front of theirapertures.TransmitterThe transmitter section simply consists of asupport structure for the lasers, beamexpansion telescopes, and steering mirrors.The current laser, used for Raman scatteringmeasurements of water profiles, is a largepulsed Nd:YAG laser with wavelength-doubling and tripling capability. Other lasersand multiple lasers can be accommodated inthe mobile housing. The laser beams areprojected coaxially with the beam director andreceiver assembly using the small lasermirrors mounted at the center of the largeelliptical mirrors.Mobile Housing Unit (MHU)A photograph of the unit that houses themobile lidar system is shown in Fig. 2. Thisunit was originally used as a military, field-deployed radar system. It was acquired byAerospace and the interior was completelyrefitted with a ne w electrical system , lights, airconditioning, and carpeting. The wheels areattached as a dolly system to the container boxwhich can be lowered to the groundhydraulically. This allows the system to berigidly secured to the ground during operationand provides a very stable work platform.The dolly system also provides air suspensionto the box during road transportation. The boxstructure was quite strong and rigid, however,in order to support the heavy rotating beamdirector assembly, internal bracing was added.The telescope and o ther optics are mounted toan aluminum honeycomb structure which isbolted directly to the floor of the MHU. Animportant requirement for the mobile lidar isthat it be transportable by military aircraft

such as C-130s or C-141s. This places a strictheight limit on the system, requiring that thebeam director and receiver assembly bedemountable from the roof duringtransportation. Therefore, a system wasdesigned whereby the rotating beam directo r iscarried on the front of the MHU in a loweredposition, such that the top of the beam directorassembly is at roof level with the MHU. Inorder to deploy the system, a platform on acarriage raises the bottom of the scanningoptics to roof level. The rotating beamdirector is then pushed into position over theazimuthal bearing along a seit of rails mountedto the MHU roof. Once there, it is loweredonto the bearing and secured.

5. SUMMARYRemote optical sensing with lasers hasbecome a valuable tool for acquisition ofimportant atmospheric data :for many civilianapplica tions. Aerospace aipplications havebeen identified that could blenefit from lidar.We have begun construction of a uniquemulti-purpose transportable lidar system toaddress these app lications.

AcknowledgmentsThis work was jointly supported by theAerospace Sponsored Research program andthe DM SP Program Office. The authors wishto thank Dwight Abbott of the SpaceTechnology Applications csEce and DonBoucher of the DMSP Program Office fortheir support and encouragem ent.

References1. R. M. Measures, Laser Remoire Sensing, Malabar,Florida: Krieger, 1992.Steven M. Beck wa s born in Oaklimd, CA in 1954. Hereceived the B. S . in chemistry fiom the University ofCalifornia Santa Barbara (1976) and the Ph.D. inChemical Physics from Rice University, Houston TX

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(1980). His graduate research focused on thespectroscopy and dynamics of aromatic molecules insupersonic molecular beams. Following graduation, hewas a postdoctoral fellow at the Bell TelephoneLaboratory, Murray Hill, NJ. (1980-1982) where hestudied molecular processes in liquids using time-resolved Raman Spectroscopy. After leaving Bell, Dr.Beck went to O ccidental Research Corpo ration in 1982and then joined the Aerospace Corporation, ElSegundo, CA in 1983. From 83-94 he was a memberof the technical staff where his research interestsinvolved spectroscopy, dynamics, and chemistry ofsmall metal, semiconductor, and organic molecularclusters in supersonic beams. He also carried outpicosecond time-resolved experiments onsemiconductor surfaces and trace detection of toxiccompo unds using photoacoustic techniques. Dr. Beckbecame manag er of the Lidar, O ptical Propagation, andSpectroscopy Section in 1994 and has recently beeninvolved in development of a mobile lidar system forapplication to Air Force and Aerospace needs. He hasco-authored over 40 publications and a book chapter.Dr. Beck is a life member of the American PhysicalSociety.Jerry A. Gelbwachs received a Bachelors Degree(magna cum laude) from The City College of NewYork. He was awarded a National Science FoundationFellowship to pursue graduate studies at StanfordUniversity where he received a Ph.D. degree in 1970.While at Stanford he participated in research in theareas of lasers and non-linear optics. Dr. Gelbwachsjoined The Aerospace Corporation in 1970 and has heldvarious technical and managerial positions. He iscurrently Senior Scientist in the Lasers and OpticalPhysics Department. His research activities during thistime period have encompassed ultra-sensitive tracedetection, photoacoustic spectroscopy, atmosphericlidar, and atomic resonance filters. In 1 981, Dr.Gelbwachs received the Aerospace Corporation'sPresident's Award for Science. Dr. Gelbwachs has co-authored over fifty publications and one book chapter.He has been granted five U.S. patents. In 1981, heorganized the SPIE symposium entitled "LaserSpectroscopy for Sensitive Detection," and edited theproceedings (SPIE 286). In 1982, Dr. Gelbwachstaught an upper level laser physics course at CalifomiaState University at Long Beach. He has served on theprogram committees for various national laserconferences (CLEO and LEOS) and is currently CLEO'95 subcommittee chairman for "Atmospheric, Space,and Ocean Optics." Dr. Gelbwachs is a Fellowof the Optical Society of America, a Senior Member of

the Institute of Electrical and Electronic Eng ineers, anda member of the American Physical Society.John Wessel received the B. S. degree in chemistryfrom the University of Califomia, Los Angeles (1965)and the Ph.D. degree in chemical physics from theUniversity of Chicago (1970). From 1970 to 1974 hewas a Postdoctoral Fellow and Instructor in Chemistryat the University of Pennsylvania, where he conductedresearch on picosecond and two-photon molecularspectroscopy. He joined the Aerospace Corporation, ElSegundo, CA, in 1974 and while there, has conductedresearch in molecular, atomic, semiconductor, andsurface spectroscopies. He is currently involved inmeteorological remote sensing involving lidar andmicrowav e techniques. Dr. Wessel is a memb er of theAmerican Physical Society, the American ChemicalSociety, the AAAS, and Sigma Xi.David W. Warren received the MS degree in opticalengineering from the University of Rochester in 1977.He subseq uently o ined the technical staff of theAerospace Corporation, where he has specialized in thedesign of instrumen ts for space, astronomical, and earthresources applications.David A. Hinkley was born in Concord, MA in 1967.He received the B.S. degree in m echanical engineering in1989 from the University of Califomia at San Diego andthe M.S. in manufacturing engineering in 1991 from theUniversity of Califomia at Los Angeles. From 1987 to1993 he was an associate to the technical staff at TheAerospace Corporation, El Segundo, CA. During 1992to 1993 he consulted for Lyntone Engineering workingon R&D projects for their main customer, the Rain BirdSprinkler Company. Presently he is an associate memberof the technical staff at The Aerospace Corporationwhere he works on m echanical vibration control in lasercavities, optical component alignment structures,computer aided desigdmanufacturing and computercontrolled projects.

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