1. Spaceborne 3D Imaging Lidar John J. Degnan Geoscience Technology Office, Code 920.3 Code 900 Instrument and Mission Initiative Review March 13, 2002

2. Spaceborne 3D Imaging Lidar Science Objectives: –Globally contiguous high resolution laser mapping of lunar and planetary surfaces Few meter horizontal resolution Decimeter vertical (range) resolution –Range-resolved atmospheric scattering Aerosols, dust Clouds –Other Earth Science Applications Tree canopy heights/biomass estimation Ocean/Ice studies Flood hazard estimation Technical Approach: –Low energy, kHz rate lasers –Photon-counting array detectors –Multichannel timing receivers –Internal dual wedge optical scanner –Multiple spatially-resolved range measurements per laser fire New Technology Disclosure –Filed 01/15/02 –NASA Case No. (GSC-14616-1)

3. Heritage: IIP Airborne Microlaser Altimeter Raw Sample Data From 1st Engineering Flight, Jan 4, 2001

4. Instrument Heritage and Technology Pathway IIP Airborne Microaltimeter (Degnan et al, 2001) Shuttle Laser Altimeter 3 (Harding et al, 2004?) NASA ER-2 Demo (2003) Lunar Explorer (Discovery, 2008?) GLAS-X? MOLA-X? Joint Y, S Funding PIDDP, Discovery? Funding Sources ESTO

5. Maximizing the Range Measurements per Laser Fire The number of range measurements per laser fire is maximized by choosing a laser beam radius which is circumscribed by the projected 8x8 array image on the ground. The mean signal strength in the figure is 4 pe per pixel but is highest at the center of the gaussian laser profile. The probability of detection per pixel has a much flatter distribution and is close to 100% for most pixels. Mean Signal Photoelectrons per PixelProbability of Detection

6. Dual Wedge Optical Scanner Two counter-rotating optical wedges impart no net angular momentum to the spacecraft. Swath width and scan pattern are controlled by magnitude of wedge and relative starting phase. Starting phase difference of 90 o generates near optimum scan pattern on the ground. Wedges are driven by a common motor to maintain starting phase indefinitely. Central transmit wedge is offset in phase relative to outer receive wedge to correct for transmitter point- ahead caused by long pulse roundtrip transit time and fast scan speeds. Can be placed internal to instrument resulting in small device.

7. Optical Scan Pattern on Lunar Surface Contiguous mapping of a planetary surface with few meter horizontal resolution is possible. Swath width chosen to cover mean spacing between ground tracks at the lunar equator (575 m for 2 yr mission) Swath width and scan pattern are controlled by the magnitude of the wedge cone angle (0.47 o ), the starting phase between counter-rotating plates (90 o ), mechanical scan rate (20 Hz), and the laser fire rate (872 Hz). Co-centering of laser circles and detector array squares in the figure are the result of transmitter point-ahead correction using a central wedge phase advance of 1.23 o. Each projection of the square detector array image onto the lunar surface is 40 m on a side. The image is further subdivided into 64 (8x8) elements (5 m x 5 m) representing individual pixels of the photon-counting detector array

8. Lunar Science Explorer Possible submission to Discovery Program in FY02 –Preliminary instrument weight: 18 kg (no redundancy) –Preliminary instrument power: ~100 W –Working with IMDC to define mission and costs Science Goals: –Globally contiguous map of the Moon in two years 5 m horizontal resolution 10 cm vertical resolution –Improved lunar gravity field Why the Moon? –Good science/Good technology demo for future planetary missions –Close enough to Earth for high data rate transmission Instrument generates range data at ~ 1 Mbit/sec continuously (>55,000 range measurements/sec) One meter antenna can transmit at ~4 Mbit/sec with 6 hours daily tracking by DSN. – Low orbital altitude (~30 km) lends itself to a compact instrument –Moon requires ~ 2 trillion range measurements to cover surface with 5 m resolution; planets require an order of magnitude higher data volume.

9. Lunar Mission Parameters