Spaceborne 3D Imaging Lidar

John J. Degnan

Geoscience Technology Office, Code 920.3

Code 900 Instrument and Mission Initiative Review

March 13, 2002

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)

Heritage: IIP Airborne Microlaser AltimeterRaw Sample Data From 1st Engineering Flight, Jan 4, 2001IIP Airborne Multi-kHz Microlaser Altimeter

Sample Profiling Data From 1st Engineering Flight, Jan 4, 2001• Engineering Flight Parameters

– NASA P-3 Aircraft, Wallops Flight Center

– Locale: Chincoteague, VA & Chesapeake Bay

– Flight Altitudes: 3.5 to 6.7 km (11,000 to 22,000 ft)

– Early afternoon (maximum solar background)

– Laser Energy: < 2 J @ 532 nm

– Laser Repetition Rate: 3.8 kHz

– Laser Power: ~7 mW

– Effective Telescope Diameter: 14 cm

– Mean Signal Strength per Laser Fire: ~ 0.88 pe

Shallow Water BathymetryBuildings and Trees

Tree Canopy Heights

Bay Surface

Bottom

Tool ShedTwo StoryDwelling

One StoryStructure

Ground

Ground

Building

Instrument Heritage and Technology PathwayIIP 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, SFunding

PIDDP,Discovery?

Funding Sources

ESTO

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 Pixel Probability of Detection

Probability of Detection

pd

Mean Photoelectrons per Pixel (Gaussian )

np

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 90o 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.

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 (0.47o), the starting phase between counter-rotating plates (90o), 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 1.23o.

• 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

400 275 150 25 100 225 350 475 600400

300

200

100

0

100

200

300

400Ground Pattern (0.1 sec)

Along Track (m)

Cro

ss T

rack

(m)

0Dg

0 Dg

Dg 40m

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.

Lunar Mission ParametersParameter ValueOperating Wavelength, 532 nm (green)Surface Reflectivity, 0.12 (Soil at 532 nm)One-way Atmospheric Transmission, T0 1.0Detector Quantum Efficiency, q 0.4 (GaAsP)Receiver Optical Throughput, r 0.4Photon Energy, h 3.74 x 10-19 JRange to Surface, R 25 kmTransmitted Energy, ET 1.2 mJ (at 532 nm)Laser Fire Rate 872 HzLaser Output Power ~ 1 W (@532 nm)Receiver Area, Ar 0.008 m2 (10 cm primary diameter)Detector Pixels/Timing Channels 64 (8x8 Segmented Anode Photomultiplier)Mean Signal Count per Laser Fire, ns 4 pe per pixelMean Ranges per Laser Fire ~58Exoatmospheric Spectral Irradiance, N0 0.2 W/m2-Ao (532 nm)Receiver FWHM Spectral Bandwidth, 2.5 Ao (GLAS filter)Ground Horizontal Resolution, 5 mVertical Resolution, ~10 cm