Multiple Altimeter Beam Experimental Lidar (MABEL)

1. Multiple Altimeter Beam Experimental Lidar (MABEL) An Overview

2. Why do we need MABEL? ICESat-2 is utilizing a relatively new approach for surface altimetry, with multiple transmit/receive beams to permit measurement of cross-track slope. This new approach relies on using high rep-rate, low pulse energy laser(s) and photon-counting detection. It would be good if we had high-altitude airborne measurements to conclusively demonstrate the measurement concept. Normally we have a demonstrator instrument and measurements before mission approval. Not so in the case of ICESat-2. At this point, data from MABEL is critical for model validation and algorithm development. Comfort level for science, engineering, and management alike will be improved with demonstration measurements from MABEL.

3. Measurement objectives Goal: Address measurements that can only be addressed by field measurements – issues related to ice, snow, water, and clouds. Data will support both instrument and science development. • Land Ice: • reflectance of ice surface vs. wavelength • assess importance of surface roughness and slope • effects of blowing snow on surface detection • effects of ranging angle on reflectance, • impulse response • Sea Ice: • lead detection as a function of wavelength • effects of clouds or ground fog • reflectance of water surface vs. surface • conditions • Atmosphere / Clouds: • Transmittance through clouds of varying • optical depth • Ability to discern surface return at high PRF • Vegetation: • Ability of detect surface return through • canopy • Ability to return canopy structure • Solar Background: • Assess background count rate vs. • wavelength, sun elevation, etc..

4. Flexibilities required

5. Our ride: the ER-2

6. Instrument parameters comparison For signal (532 nm): For solar background (532 nm):

7. MABEL transceiver concept 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 • Use two identical focal length telescopes with identically spaced fiber • arrays in the focal plane of each telescope. • Use smaller core fiber in the transmitter telescope and larger core • fiber in the receiver telescope. • The difference in the instantaneous field of view (IFOV) for each • telescope will be a ratio of the fiber sizes used. • Include more than the required 16 fibers in the fiber array to allow • flexibility in choosing the ground track patterns. Transmitter telescope Receiver telescope Conceptual drawing Not to scale 20 km aircraft altitude Ground tracks 1 2 3 4 5 6 7 8

8. Footprint geometry comparison For spaceborne For MABEL ATLAS ground track spacing 6 beams track 532 track 1064 1.05 km 1.05 km ~3 km ~3 km select up to 16 for 532 nm select up to 8 for 1064 nm Specific yaw angle determines beam spacing: Specific fibers selected determines beam spacing: (see also following slide)

9. Footprint spacing & selectability 15 channels at 2m spacing, 90 at 20m spacing yields 3 detailed areas, and 1.05 km width. . . . . . . . . . . . . . . . . . . . …. . …. . …. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . …. . …. . …. . . . . . . . . . . . . . . . . . . . 10m 10m 10m 20m 20m 20m 20m 20m 20m 40m 40m sample for sub-footprint information and to compare reflectivity… 105 fibers in each row - select up to 16 active channels for 532 nm select up to 8 active channels for 1064 nm or use every other channel to further increase spatial extent… or increase spatial extent for cal/val with ICESat-2, or…

10. Footprint geometry for initial flights maximum off-nadir angle is +/- 1.05 km . . . . . . . . . . . . . . . . . . . …. . …. . …. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . …. . …. . …. . . . . . . . . . . . . . . . . . . . for December flights, was set to +/- 100 m angle, in milliradians -2.0 -0.1 +0.1 +2.0 -4.0 +4.0 +5.0 -3.0 -1.9 -1.0 -0.2 0 +0.2 +1.0 +1.9 +3.0

11. MABEL mechanical layout (isometric) Data System Transmit and Receive Fiber Boxes Laser Laser Splitter Box IMU Detector/Filter Assembly Co-Aligned Telescopes MABEL is 52” x 26” x 30”

12. MABEL, completed MABEL airborne demonstrator, ready for shipment.

13. Dec 8: first look at MABEL data 20 MABEL 532 nm data, for one channel only. Image is approximately 9 minutes of data. This is the raw data, noise and all. 15 Cirrus Clouds 10 Altitude (km) 5 The Ground 0

14. Dec 8: first look at MABEL data 20 Spectacular example of cirrus clouds CPL quick look image. Area shown in MABEL image is circled. 15 Altitude (km) 10 Pollution over Central Valley 5 The Ground 0

15. Another example: Dec 9 CPL MABEL Note: images not corrected for aircraft roll

16. Dec 10 MABEL at 10 kHz. CPL shows good alignment with cloud and land features. Note small encircled area where dense cloud overlays ground – provides a good test for automated algorithms to discern “true” ground.

17. Dec 9 10 m Data from clear sky over open desert. The CPL image spans 2 ½ minutes (the ground appears to come-and-go because of aliasing in the image). The MABEL image spans ½ second…and this is only for one channel.

18. Dec 10: MABEL Talks to the Trees

19. Dec 10: MABEL Talks to the Trees

20. Dec 10: MABEL Talks to the Trees A global vegetation grid, interpolated to this area, indicates mean tree height is 32 +/- 5 m

21. MABEL data cross-track view First attempt at displaying data in cross-track visualization format. cloud

22. Summary MABEL was completed in ~12 months (concept to flight). Initial flights with MABEL were successful. Data has to have calibrations applied, and rewritten into “L1” format. - release raw data to Project, processed data to SDT. IMU data has to be post-processed to generate exact geolocations. Retrofit etalon filter to receiver to permit full daytime measurements. Next flights will be March 21-April 22 from Dryden, to get daytime measurements and other ground targets Planning spring 2012 deployment to fly over Greenland for ice, snow, sea ice demonstrations.

23. Cast of Characters From GSFC: Stan Scott / 694 Luis Ramos / 551 Shane Wake / 551 From Sigma Space: Ryan Cargo Eugenia DeMarco Dan Reed Roman Machan Ed Leventhal Rodney Faulkner Pete Dagoda Tony Melak Diane Schuster Marcos Sirota From SSAI: William Hart Andrew Kupchock opto-mechanical optics opto-mechanical mechanical* mechanical* software* electrical/data system electrical/data system mechanical optical alignment thermal thermal blanketing project management data analysis* support* We must also acknowledge the superior support from the ER-2 pilots and support crew – they went out of their way to make this successful. There are also a host of support folks who always do their best to make my efforts succeed, including Ray DiSilvestre and Ken Corry in our staff shop, Rick Eichen who gets our instruments shipped safely and securely, and many others.

24. Acknowledgements MABEL was made possible by contributions from the following companies: Sigma Space Corp. data system, electrical, and systems engineering Fibertek, Inc. laser transmitter Fiberguide, Inc. custom fiber optic assemblies Special Optics custom telescope assemblies