Airborne Lidar Simulator for the Lidar Surface Topography (LIST) Mission

Anthony W. Yu 1, Michael A. Krainak2, David J. Harding 3 , James B. Abshire4, Xiaoli Sun 5,

John Cavanaugh 6, Susan Valett 7, and Luis Ramos-Izquierdo 8

1NASA Goddard, Code 554, Greenbelt, MD 20771 USA, anthony.w.yu@nasa.gov2NASA Goddard, Code 554, Greenbelt, MD 20771 USA, michael.a.krainak@nasa.gov

3NASA Goddard, Code 698, Greenbelt, MD 20771 USA, david.j.harding@nasa.gov4NASA Goddard, Code 690, Greenbelt, MD 20771 USA, james.b.abshire@nasa.gov

5NASA Goddard, Code 694, Greenbelt, MD 20771 USA, xiaoli.sun-1@nasa.gov6NASA Goddard, Code 554, Greenbelt, MD 20771 USA, john.f.cavanaugh@nasa.gov

7NASA Goddard, Code 554, Greenbelt, MD 20771 USA, susan.r.valett@nasa.gov8NASA Goddard, Code 551, Greenbelt, MD 20771 USA, luis.a.ramos-izquierdo@nasa.gov

ABSTRACT

In 2007, the National Research Council (NRC)completed its first decadal survey for Earth science atthe request of NASA, NOAA, and USGS.[1] TheLidar Surface Topography (LIST) mission is one offifteen missions recommended by NRC, whose primaryobjectives are to map global topography and vegetationstructure at 5 m spatial resolution, and to acquire globalsurface height mapping within a few years. NASAGoddard conducted an initial mission concept study forthe LIST mission in 2007, and developed the initialmeasurement requirements for the mission.

1. INTRODUCTION

In the Decadal Survey, the NRC recommended a totalof fifteen missions with benefits ranging frominformation for short-term needs, such as weatherforecasts and warnings for protection of life andproperty, to the longer-term scientific understandingnecessary for future applications that will benefitsociety in ways still to be realized. Most futuretopographic mapping missions will likely utilizemultiple laser beams in the cross track orientation tofacilitate surface slope measurements and reduce thetime needed to globally map surface topography. Themost ambitious is a NRC recommended tier-3 missionnamed LIdar Surface Topography (LIST). In thispaper, we will discuss our effort in developingmeasurement techniques and technologies for the LISTmission.

2. LIST MISSION

The key attributes of the LIST mission, as described inthe NRC Earth Science Decadal Survey report, are: (1)a medium cost mission to be launched by NASAbetween 2016-2020; (2) a single-instrument payloadcarrying an imaging lidar at low Earth orbit; (3) one-time global mapping of land, ice sheet and glaciertopography and vegetation structure through theduration of the mission; (4) observe topography andvegetation structure change through time in selected

areas; and (5) achieve 5 m horizontal resolution, 0.1 mvertical precision, and decimeter-level absolute verticalaccuracy for ground surface topography includingwhere covered by vegetation.

The measurement requirements for the mission weredeveloped in an advanced mission concept study forLIST, which was carried out by NASA Goddard inmid-2007. The LIST Science Working Group (SWG)report defined the traceability linking scienceobjectives and measurement requirements for landtopography, vegetation structure, ice sheets andglaciers, and inland water bodies. The results of thestudy highlighted the key challenges for any lidarapproach. The lidar must be capable of: (1) mapping aswath with a width of at least 5 km to acquire globalcoverage in a reasonable amount of time; (2) rangingaccurately to the surface through thin to moderatecloud cover in order to acquire complete coverage inregions that ar e frequently cloudy; (3) operate withsolar background noise to accomplish mapping duringboth day and night conditions (even for a dawn-dusksun-synchronous orbit the solar zenith angle is largeduring parts of the year); (4) large dynamic range toaccommodate highly varying apparent reflectanceconditions due to changes in surface reflectance,atmospheric transmission and canopy cover; (5) highsensitivity in order to detect returns from the groundthrough dense vegetation cover; (6) an effective pulserates of 10 kHz or less to allow atmospheric profilingand unambiguous surface ranging through clouds; andmost importantly (7) highest efficiency in order tominimize required power, mass, size and cost.

3. LIST TECHNOLOGY DEVELOPMENT

In 2009 we started a three-year Instrument IncubatorProgram (IIP) project, funded by NASA’s EarthScience Technology Office (ESTO), for definition andearly technology development for LIST. The purpose isto develop and demonstrate the techniques andtechnologies for a next- generation, efficient, swath-mapping space laser altimeters.

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The instrument requirements associated with the LISTscience objectives far exceed those of existing space-laser-altimeter technologies. A viable LIST instrumentneeds to be able to generate a swath width of 5 km,image this swath onto a detector array and produce animage that describes the topography of the sampledarea, including through foliage if covered byvegetation, and the 3-D structure of the vegetationcover. Our pushbroom photon counting approach hasmuch higher performance and efficiency than recentsingle-beam scanning laser altimetry systems andleverages investments by various technology sectorsinternationally.

Figure 1. Concept drawing of the LIST satellite generatinga 5 km swath containing 1000 beam spots at 5 m per spot.

The instrument concept for LIST is shown in Figure 1.A swath 5 km wide composed of 1000 laser beams in alinear array is oriented in the cross -track direction. Thedivergence of each beam yields 5 -meter diameterfootprint on the ground from a 400 to 425 km orbitaltitude that are contiguous cross-track. Figure 2shows conceptually the echo pulse structure withinformation containing the canopy and foliage structureas well as the ground return. As seen in Figure 1, theground pattern of the illuminated spots is arranged in astaggered fashion to mitigate any crosstalk fromatmospheric scatter from adjacent spots at the detectorarray. In this configuration, each pixel on the detectorarray can have a larger field of view (FOV) than theilluminated spot to aid alignment while eliminatingcrosstalk from adjacent channels. At 10 kHz laserrepetition rate and a nominal spacecraft groundvelocity of 7 km/sec in low Earth orbit, laser footprintsare spaced 0.7 m along track yielding 7 pulses per 5 mpixel. This over -sampling along track enables asufficient density of detected ground returns underadverse observing conditions (low atmospheric

transmission due to thin clouds and/or aerosols andground obscuration by vegetation cover). Ourmeasurement approach differs from the traditionalsingle pulse lidar altimeters in which laser pulses onorder of ten’s m illijoules at relatively low repetitionrate are used. We use a micropulse photon counting,approach with a ~10 kHz pulse rate laser, shorter pulsewidth (as shown conceptually in Figure 2) toaccumulate a few hundred photons from each 5 m pixelfor information processing.

Figure 2. Approaches for measuring the time-of-flightinformation of laser echo pulses into vegetation structureheights and ground return.

4. LIST INSTRUMENT INCUBATORPROGRAM (IIP)

In 2009 we began a three-year program on technologydevelopment for LIST. Our approach will ultimatelyallow for simultaneous measurements of 5-m spatialresolution topography and vegetation vertical structurewith decimeter vertical precision in an elevation-imaging swath several km wide from a 400 km altitudeEarth orbit. Our IIP objective is to demonstrate themeasurement technique and key technologies for ahighly efficient surface lidar to meet the goals of theLIST mission.

During the first two years, we are concentrating ourwork in developing some critical subsystems (laser,detector, optics and receiver processing systems) inpreparation for an airborne demonstration of a multi-beam swath mapping altimeter system in the final yearof the IIP. Our ultimate goal is to develop a >15% wallplug efficient laser system coupled with a highlysensitive multi -element detector for the space mission.

4.1. LASER TRANSMITTER

For LIST a swath of 5 km can be generated using 1000beams each having 5 m footprint. Our goal of this workis to show a viable efficient path for generating the1000 laser and photo-detector channels. According toour analysis, with the current photon counting detectorsensitivity, the energy requirement per channel is ~100µJ at 10 kHz with ~ 1 nsec pulse widths. To meet this,we are pursuing a master oscillator power amplifier(MOPA) laser architecture. Assuming that we need 10MOPA lasers, each laser then subdivide to generate100 beams will produce a 1000 beam swath. Thuseach MOPA laser will need to deliver an energy of>10mJ. At 10 kHz pulse rate, the average optical power is>100 Watt per laser. If the lasers have wall-plugefficiencies of >15%, the prime power for the LISTlasers will be manageable with <7 kW of prime powerfrom the spacecraft. During the first two years of theIIP we are developing a MOPA laser that willdemonstrate the wall plug efficiency and necessaryattributes to meet the measurement requirements. Thefinal product will be able to generate 16 beams eachhaving 100 µJ pulse energy.

4.2. RECEIVER

Another critical technology for LIST is highÐsensitivity low-noise detectors that provide single-photon sensitivity. The backscatter laser signals bysurface and biomass (e.g. grass, trees, etc.) at thesatellite altitude are very weak. Detectors with highquantum efficiency (QE) and internal gain are neededto overcome detector amplifier noise and achieve therequired signal-to-noise ratios. We are exploring anear single-photon sensitive detector array operating inanalog mode, with >1GHz bandwidth. We arecurrently working with several vendors to develop highbandwidth multi-element photon- sensitive detectorsduring the first two-year of the program. Candidatedetectors for our prototype lidar include HgCdTe onCdZnTe APDs, impact-ionization-engineered InAlAsAPDs [2] and multi-element anode InGaAsPintensified photodiode detectors (IPDs) [3].

4.3. OPTICAL SYSTEM

The airborne lidar will demonstrate a 16-beam versionof the LIST space lidar. The sixteen beams orient in a4x4 grid pattern with uniform spacing between spots.The overall dimension of the grid is 75 m x 75 m (7.5mrad x 7.5 mrad) with 20 m (2mrad) between spots.The grid will have a 14.5±1 clocking with respect tothe aircraft velocity vector to yield an effective 5m spotcross-track spacing as shown in Figure 3. Adiffractive optical element (DOE) will be used todivide a single beam into 16 beams. A similar DOE isbeing used presently on the Lunar Orbiter Laser

Altimeter (LOLA) instrument on the LunarReconnaissance Orbiter (LRO). [4]

Figure 3. This figure illustrates the baseline footprintconfiguration of the IIP airborne lidar from 10 km altitude.The solid circles are the 5 m laser footprints with the opencircles showing the detector field of view of ~7 m.

4.4. AIRBORNE DEMONSTRATION

In the 3rd year of work, we plan airbornedemonstrations of the swath-mapping concept. Wewill leverage our recent experience on a micropulselidar airplane demonstration. [5] Previously wedemonstrated a lidar with a 1 µJ per beam, a 10 KHzlaser, and a single-photon-threshold detector (Geiger-mode APD) based receiver. Our new lidar usingmicropulse photon-counting approach will demonstratea laser with 100 µJ per beam, a 10 kHz pulse rate, anda receiver using single-photon-sensitive analog-modedetector and waveform-digitizer.

Our plan is to operate the instrument at a 10 kmaltitude. We intend to scale the receive telescope onthis airborne experiment to the LIST mission concept.The LIST concept presently uses a 2-m receivertelescope at an orbit height of 400 km. From a 10 kmairplane at altitude, a receive telescope diameter of 5cm will provide the same detected signal. Table 1shows a comparison between the airborne instrumentbeing developed under this IIP and the LISTspaceborne instrument requirements. A number offlight tests over different regions are being considered.Candidate regions for the tests will be selected todemonstrate measurement concept that satisfy the LISTscience objectives of mapping in cryosphere, watercycle, vegetation structure and solid Earth applicationareas.

Table 1. Comparison of Space- and Air-borne instruments for LIST.

,LIST' Spacehornell,nstrumerrC +Airbornell,nstrLlmerrC lComments,(Obj'ectiwes

,Spatial' 0 ethe'same,foot'nnt;irather;thanlscaled;byiangular'

ResoI,Aon' i5 nneter' i5'meter' ,diiiergencel

;Altitude] f400 . km' 1^0.km' Scale:[40x,'

i

SwatYi.4Vitltli' i[Q^OOO,h^eams}' (,i6'beams}'. ,Scale:^62?5x,'

Detection' ,Bacl:upOption'—;Geiger,F;to del P,notonCountrng;on'

,S`cneniel nalogP, oton;Counting', nalogP,hoton;Counting', „;irbornelnst'runientl

Re d'irr,TelescopelSiiel ;2meter;DiffractionlL"imitedtelescop Oi

el 127'. meter Diffradion'L' Scaled -tiy;Altitude'-1]40x;witli;margih'';te; l̂escopel

100 uJ per;.beamfor1000beann @ 10kHi 100 uJ per.beamfor16 ' beann Q 101kHi ;Demons$ation of,full'energyper,[ieamlmeeting',

,Laser,EhergY' 1', h'ttoptical;p owe r,Gr617 FA : prime'power' 16r^^Gptical,power,or',110'tprimel ernoisspaceborne'instFumeetreai]rnieti^assunling -,15,, efficient' ;power'

1000;pi'selswith 1iGHidandWidtn'on' 16pikelswith' 1kGHi,bandWidth'on' ,Demonstratethenecessary;oandwidtn'inmultiplel

each;pi el'

,Detector' each;pixel' pixeldetector;arraywithi phi ton counti^ngIsensitivity'a n d,evavef o mild i g itiii n g',

P.Iaffornn Speed' ,7000:misecl 200:misecl ,Scale:35x

Nmpl,es per' T ;2501r' Duringthe'Airborne`campaign';we`cansample`every',

';f o otp ri ntl l35?'^o n eltol s i m u l ate s p a ce `e nvi ro n rrfe ntl

F,o otp ri'4017, m ete r' 101 02 meter' Airb o rn e wi I I'ove rs a m p I'ej byr35XSe parati on'

lane scenaridis,tolhave1011asersjeach'

Beam diVi'ding', ,,with 1x100oeam diVi ecdiffradive optical' ,SingIe`bearri diwdes'iritol61beamsi ,Demonstrate efficienc,,oeann diVisiontechniguel

;network(element(DpE)'. lush abCiE'!, ,mingDOE!

Spectral' ,Dernonstratetne,technical'approachto stabiI'zelaser'

4neewidthl `2'0;pm' s201pnil waveIengthand'spectral,witltfiwhenuse;wdnnarrow;receiver;filter'

5. CONCLUSIONS

In this work we are developing the measurementapproach and lidar technologies for the LIST lidarmission requirements. The objectives are to mitigatethe major risks and developing measurementtechniques for the LIST mission. Our plans are toincorporate the work on the measurement approachand lidar technologies developed during the first twoyears into an airborne lidar simulator, and todemonstrate measurements in 2011.

6. ACKNOWLEDGMENT

We gratefully acknowledge the suppo rt fromNASA’s Earth Science Technology Office (ESTO)and the GSFC IRAD program. We also acknowledgethe technical support and component developmentfrom Raytheon Company, Intevac, Inc. andSpectrolab, Inc.

7. REFERENCES

[1] Earth Science and Applications from Space:National Imperatives for the Next Decade andBeyond, National Research Council of the Nationalacademies, The National Academies Pres,Washington D.C., (2007).

[2] Wang, S., et. al., “Low-Noise Impact-Ionization-Engineered Avalanche PhotodiodesGrown on InP Substrates,” IEEE Photonics TechLetts, Vol. 14, pp. 1722-1724, (2002).

[3] Sun, X., et. al., “Single photon counting at 950 to1300 nm using InGaAsP photocathode – GaAsavalanche photodiode hybrid photomultiplier tubes,”Journal of Modern Optics, Vol. 56, pp. 284-295,(2009).

[4] Smith, J.G., et. al., “Diffractive Optics for MoonTopography Mapping,” Proc. SPIE, Vol. 6223, 2006.

[5] Harding, D.J. et al., “The Swath Imaging Multi-polarization Photon-counting Lidar (SIMPL): ASpaceflight Prototype,” Proceedings of the 2008IEEE International Geoscience & Remote SensingSymposium, 06-11 March, Boston, MA, (2008).

[6] Mallet, C. and F. Bretar, “Full-waveformtopographic lidar: State-of-the-art,” ISPRS Journal ofPhotogrammetry and Remote Sensing, 64, 1, (2009)