SLWG Feb 2007

Progress on the TWiLITE Direct Detection Doppler Lidar Instrument Incubator Program

B. Gentry1, G. Schwemmer6, M. McGill1, M. Hardesty2, A. Brewer2, T. Wilkerson5, R. Atlas2, M.Sirota3, S. Lindemann4

1NASA GSFC; 2NOAA; 3Sigma Space Corp.; 4Michigan Aerospace Corp.; 5Space Dynamics Lab; 6SESI

Lidar Working Group on Space Based Lidar WindsFebruary 6-9, 2007

Miami, FL

SLWG Feb 2007

• 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 Instrument Incubator Program (IIP)

SLWG Feb 2007

TWiLiTE Measurement Requirements

Parameter WB57

Velocity accuracy (HLOS projected) (m/s) 2.0

Range of regard (km) 0 to18

Vertical resolution (km) 0.25

Horizontal resolution (km) (scan pattern cycle) 25

Groundspeed (m/s) 200

Nadir angle (deg) 45

Scan pattern Up to 16 pt step-stare

Horizontal integration per LOS (seconds)//ground track (km)

10//2

* Assumes scanner average angular velocity of 12 deg/sec

SLWG Feb 2007

Wavelength 354.7 nmTelescope/Scanner Area 0.08 m2

Laser Linewidth (FWHH) 150 MHzLaser Energy/Pulse (6 W @ 355nm) 30 mJ @ 200 ppsEtalon FSR 16.65 GHzInterference filter BW (FWHH) 120 pmPMT Quantum Efficiency 25%

TWiLiTE Instrument Parameters

SLWG Feb 2007

0 5 15 2010Altitude (km)

det

ecte

d p

hot

ons

104

105

106

107

black = overlap corrected, no max. count rateblue = overlap corrected, 50 MHz max. count rate

Photocounts Detected in each Edge Channel10 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.

SLWG Feb 2007

0 5 15 2010Altitude (km)

0.0

0.5

1.0

1.5

2.0

L-O

-S w

ind

err

or (

m/s

)

black = no solar backgroundblue = 50 MHz max. count rate, with solar background

Simulated Shot noise limited L-O-S wind error10 sec (2000 shot) integration; z=250 m; 45 deg nadir

Presence of solar background is only apparent at low altitudes due to the laserenergy and small effective passband of the interference filter and etalon.

SLWG Feb 2007

Specification WB57

Max. Altitude 18 km

Duration 6.5 hours

Cruise Speed 210 m/s @ 18 km

Payload mass 1814 kg (incl. pallets)

Payload Electrical Power

4 X 25 A , 3 phase, 400 Hz

Payload mounting Modular palletNadir view

Window diameter; viewing orientation

45 cm; nadir view

NASA Johnson WB57 Aircraft

3’ palletPallet integration

SLWG Feb 2007

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

SLWG Feb 2007

-0.5

0

0.5

1

1.5

2

2.5

3

-6 -4 -2 0 2 4 6

Lidar Signal vs. Etalon Transmision

Frequency (GHz)

Edge2 Filter

Locking Filter

Edge1 Filter

Aerosol Spectrum

Rayleigh Spectrum

Double Edge Etalon Channels

SLWG Feb 2007

Triple Aperture Step Etalon - Michigan Aerospace Corp

Steps in etalon resonant frequency are created by vapor deposition of fused silica on one plate.

Full Field Fringe Pattern

SLWG Feb 2007

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.

SLWG Feb 2007

TWiLiTE HOE Telescope Design

HOE

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

SLWG Feb 2007

Laser Optics Module

Fibertek Laser Design Full Assembly

• 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

SLWG Feb 2007

TWiLiTE Subsystems

Laser

HOE telescope

Receiver

Electronics

SLWG Feb 2007

TWiLiTE Integrated on WB57 Pallet

SLWG Feb 2007

Project Milestones

START:

AUG 2, 2005

SYSTEM REQ

WORKSHOP

DEC 1, 2005

CONCEPT

DES REVIEW

FEB 16, 2006

2006

2007

2008

CRITICAL

DES REVIEW

MAR, 2007

ASSEMBLY

INTEG & TEST

3Q/2007- 2Q/2008

TEST FLIGHTS

LATE SUMMER

2008

FINISH:

AUG 1, 2008

TELESCOPE

SUBSYS PDR

MAY 22, 2006

PRELIM DES

REVIEW

JUL 20, 2006

ETALON

DELIVERY

APRIL 2006

RECEIVER

DELIVERY

MAR 2006

LASER

DELIVERY

JUN 2007

RECEIVER

SUBSYS PDR

(GSFC IRAD)

MAR 2005

TELESCOPE

DELIVERY

AUG 2007

SLWG Feb 2007

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

SLWG Feb 2007

8 point conical step stare scan pattern

Top view

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)

0 10 20 30 40 50 60-40

-30

-20

-10

0

10

20

30

40

Distance (km)

Veloctiy (m/s)

Aircraft motion

SLWG Feb 2007

• Focal Length 993 2mm• Diffraction Angle 45.26 0.5 deg.• Diffraction Efficiency 64 2%• 160-200µm 1/e2 focal spots

Holographic Optical Element