1

Multiple Altimeter Beam Experimental Lidar (MABEL)

An Overview

2Why 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.

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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..

Measurement objectives

4Flexibilities required

We need… We need it why? How we can do it…

Ability to vary laser repetition rate.

Examine effect of multiple-pulse ambiguity.

Laser rep rate adjustable in discrete steps from 5 to 25 kHz; fly CPL for independent verification of atmosphere.

Ability to time-tag every photon event.

Simulate intended spaceborne operation; use to prototype algorithms.

Leverage Sigma Space electronics development.

Ability to oversample expected spaceborne resolutions.

We always want aircraft measurements oversampled relative to satellite.

Inherent difference between satellite and aircraft speed satisfies this objective.

Ability to vary the viewing geometry/footprint pattern.

Determine optimal geometry; quantify cross-talk; permit validation with differing spatial resolutions.

Use the MABEL switchboard panels to select specific geometries.

Ability to operate at both 532 and 1064 nm.

Examine system performance at each ; examine differences in reflectivity, water penetration.

Operate simultaneously at 532 and 1064 nm (i.e., MABEL is a two-wavelength instrument).

Ability to vary energy level for each footprint.

Explore dynamic range trade space; inform satellite design.

Neutral density filters in transmit/receive path emulate the satellite.

5Our ride: the ER-2

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parameter spacebased airborne

Operational altitude 600 km 20 km

Wavelength 532 or 1064 nm 532 and 1064 nm

Telescope diameter 1 m 6 inches

Laser PRF 10 kHz variable 5 – 25 kHz

Laser pulse energy 25 J per beam 3-5 J per beam

Laser footprint diameter (1/e2)

17 rad (10 m) 100 rad (2 m)

Telescope field of view 66 rad (40 m) 210 rad (4.2 m)

Filter width 532: 30 pm1064: 60 pm

532: ~150 pm1064: ~400 pm

Detector efficiency 532: 1064: TBD

532: 10-15% 1064: 1-2%

Swath width +/- 3 km +/- 1.05 km

Instrument parameters comparison

MABEL

ICESat2

5J25 J

7.6 cm 2

50 cm 2 600 km 2

20 km 2 4For signal (532 nm):

For solar background (532 nm):

MABEL

ICESat2

150 pm

30 pm

7.6 cm 2

50 cm 2 210 rad 2

66 rad 2 1.2

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• 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.

Conceptual drawing

Not to scale

20 km aircraft altitud

e

Ground tracks

Receivertelescope

Transmittertelescope

1 2 3 4 5 6 7 8

8 7 6 5 4 3 2 1

8 7 6 5 4 3 2 1

MABEL transceiver concept

8Footprint geometry comparison

For spaceborne For MABEL

select up to 16 for 532 nm

select up to 8 for 1064 nm

Specific fibers selected determines beam spacing:

track

532

1064

1.05 km1.05 km

(see also following slide)

Specific yaw angle determines beam spacing:

~3 km~3 km

ATLAS ground track spacing6 beams

track

9Footprint spacing & selectability

sample for sub-footprint information and to compare reflectivity…

or use every other channel to further increase spatial extent…

or increase spatial extent for cal/val with ICESat-2, or…

10m

20m

40m 40m

10m

10m

20m20m 20m20m20m

105 fibers in each row - select up to 16 active channels for 532 nm

select up to 8 active channels for 1064 nm

. . . . . . . . . . . . . . . . . . . …. . …. . …. . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . …. . …. . …. . . . . . . . . . . . . . . . . . . .

15 channels at 2m spacing, 90 at 20m spacingyields 3 detailed areas, and 1.05 km width.

10Footprint geometry for initial flights

maximum off-nadirangle is +/- 1.05 km

for December flights,was set to +/- 100 m

. . . . . . . . . . . . . . . . . . . …. . …. . …. . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . …. . …. . …. . . . . . . . . . . . . . . . . . . .

0

+0.1

+0.2 +1.0 +3.0+1.9

+2.0-0.1

-0.2-1.0-1.9

-2.0

-3.0-4.0 +4.0 +5.0

angle, in milliradians

11MABEL mechanical layout (isometric)

Data SystemTransmit and Receive Fiber

Boxes

LaserLaser Splitter Box

Co-Aligned Telescopes

Detector/Filter Assembly

IMU

MABEL is 52” x 26” x 30”

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MABEL airborne demonstrator, ready for shipment.

MABEL, completed

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0

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Alt

itu

de (

km)

The Ground

Cirrus Clouds

MABEL 532 nm data, for one channel only. Image is approximately 9 minutes of data. This is the raw data,

noise and all.

Dec 8: first look at MABEL data

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CPL quick look image. Area shown in MABEL image is circled.

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15

5

Alt

itude (

km

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Spectacular example of cirrus clouds

Pollution over Central Valley

The Ground

Dec 8: first look at MABEL data

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MABEL

CPL

Another example: Dec 9

Note: images not corrected for aircraft roll

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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.

Dec 10

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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.

10 m

Dec 9

18Dec 10: MABEL Talks to the Trees

19Dec 10: MABEL Talks to the Trees

20Dec 10: MABEL Talks to the Trees

A global vegetation grid, interpolated to this area, indicates mean tree height is 32 +/- 5 m

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First attempt at displaying data in cross-track visualization format.

cloud

MABEL data cross-track view

22Summary

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.

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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 FaulknerPete Dagoda Tony Melak

Diane Schuster Marcos Sirota

From SSAI: William Hart

Andrew Kupchock

opto-mechanicalopticsopto-mechanical

mechanical*mechanical*software*electrical/data systemelectrical/data systemmechanicaloptical alignmentthermalthermal blanketingproject 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.

Cast of Characters

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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

Acknowledgements