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Progress on the TWiLITE Direct Detection Doppler Lidar Instrument Incubator ProgramB. Gentry1, G. Schwemmer6,M. McGill1, M. Hardesty2, A. Brewer2, T. Wilkerson5, R. Atlas2, M.Sirota3, S. Lindemann41NASA GSFC; 2NOAA; 3Sigma Space Corp.; 4Michigan Aerospace Corp.; 5Space Dynamics Lab; 6SESI Lidar Working Group on Space Based Lidar Winds February 6-9, 2007 Miami, FL
TWiLiTE Instrument Incubator Program (IIP) • High altitude airborne molecular direct detection scanning Doppler lidar • Serves as a system level demonstration of key technologies and subsystems • Leverages significant technology investment from multiple sources • On development path for future space based direct detection and ‘hybrid’ Doppler lidar implementations
TWiLiTE Measurement Requirements * Assumes scanner average angular velocity of 12 deg/sec
TWiLiTE Instrument Parameters Wavelength 354.7 nm Telescope/Scanner Area 0.08 m2 Laser Linewidth (FWHH) 150 MHz Laser Energy/Pulse (6 W @ 355nm) 30 mJ @ 200 pps Etalon FSR 16.65 GHz Interference filter BW (FWHH) 120 pm PMT Quantum Efficiency 25%
107 black = overlap corrected, no max. count rate blue = overlap corrected, 50 MHz max. count rate 106 detected photons 105 104 0 5 10 15 20 Altitude (km) Photocounts Detected in each Edge Channel 10 sec (2000 shot) integration; z=250 m; 45 deg nadir Includes effects of lidar overlap function and the use of 3 PMTs sharing the incoming signal in the ratio 90:9:1 to increase linear counting dynamic range.
2.0 1.5 L-O-S wind error (m/s) 1.0 0.5 black = no solar background blue = 50 MHz max. count rate, with solar background 0.0 0 5 10 15 20 Altitude (km) Simulated Shot noise limited L-O-S wind error 10 sec (2000 shot) integration; z=250 m; 45 deg nadir Presence of solar background is only apparent at low altitudes due to the laser energy and small effective passband of the interference filter and etalon.
3’ pallet Pallet integration NASA Johnson WB57 Aircraft
TWiLiTE Direct Detection Wind Lidar Key Technologies • High spectral resolution all solid state laser transmitter • High spectral resolution optical filters • Efficient 355 nm photon counting molecular Doppler receiver technologies • Novel UV Holographic Optical Element telescopes and scanning optics
Steps in etalon resonant frequency are created by vapor deposition of fused silica on one plate. Triple Aperture Step Etalon - Michigan Aerospace Corp Full Field Fringe Pattern
Receiver Mechanical Assembly Overall Dimensions (LxWxH) : 0.468m x 0.446m x 0.30m • Fiber coupled input from HOE telescope • High QE (>25%) low noise PMT’s used in photon counting mode. • Multiple PMT’s per channel to increase dynamic range. • Environmentally controlled (T, P vib) for high altitude aircraft operation.
HOE TWiLiTE HOE Telescope Design • Designed and built by Utah State Univ./SDL • 355 nm HOE fabricated by Wasatch Photonics • Mass 43 kg • Power 35 W • Volume 60 cm diameter x 60 cm long • ~Isothermal environment, active control
Fibertek Laser Design Full Assembly Laser Optics Module • Dual compartment design with oscillator and amplifier on opposite sides • Hermetic sealing for low pressure operation • An ~ 31 cm x 25 cm x 14 cm canister will accommodate the required modules • Design is derived from one being developed for a NASA Langley High Spectral Resolution Lidar system
TWiLiTE Subsystems Receiver Electronics HOE telescope Laser
Project Milestones LASER DELIVERY JUN 2007 TELESCOPE DELIVERY AUG 2007 TELESCOPE SUBSYS PDR MAY 22, 2006 RECEIVER DELIVERY MAR 2006 CRITICAL DES REVIEW MAR, 2007 TEST FLIGHTS LATE SUMMER 2008 CONCEPT DES REVIEW FEB 16, 2006 START: AUG 2, 2005 SYSTEM REQ WORKSHOP DEC 1, 2005 PRELIM DES REVIEW JUL 20, 2006 ASSEMBLY INTEG & TEST 3Q/2007- 2Q/2008 FINISH: AUG 1, 2008 RECEIVER SUBSYS PDR (GSFC IRAD) MAR 2005 ETALON DELIVERY APRIL 2006 2006 2007 2008
TWiLiTE Summary • TWiLiTE is a three year R&D project to design and build an airborne scanning direct detection Doppler lidar • The TWiLiTE Doppler lidar will be serve as a testbed to validate critical technologies in a fully autonomous, integrated Doppler lidar as a stepping stone to space. • The instrument is designed to measure full profiles of winds from a high altitude aircraft and many of the design elements may be transitioned to UAV or other suborbital platforms for mesoscale and hurricane research. • Acknowledgements: NASA ESTO IIP Program; Goddard Space Flight Center IRAD program
8 point conical step stare scan pattern Top view Aircraft motion • Scanning parameters: • Constant dwell of 10s/LOS • Fixed azimuth increments of 45 deg steps Radial HLOS wind speed measured in a single range bin for 3 cycles of the 8 point step stare scan pattern. Assumes constant velocity (maximum = 40 m/s)
Holographic Optical Element • Focal Length 993 2mm • Diffraction Angle 45.26 0.5 deg. • Diffraction Efficiency 64 2% • 160-200µm 1/e2 focal spots