MABEL (Multiple Altimeter Beam Experimental Lidar)
MABEL is an airborne instrument of NASA designed for the high-altitude ER-2 aircraft to provide optimal satellite validation and simulation. The instrument was developed as a demonstrator and validation tool for the ICESat-2 photon-counting altimetry concept. 1)
The ICESat-2 measurement concept of photon-counting detection is a major shift in measurement approach from previous NASA altimetry missions. The laser technology required to support the photon-counting approach is considerably different from that required for analog altimetry. The optical co-alignment and pointing requirements are also more stringent than in previous NASA applications. The success of the technical approach to the ICESat-2 mission will enable a new era in precision laser altimetry measurements for science applications.
The ICESat-2 (Ice, Cloud, and land Elevation Satellite-2) mission is currently (2012/13) under development by NASA (scheduled for launch in 2016). The primary mission of ICESat-2 will be to measure elevation changes of the Greenland and Antarctic ice sheets, document changes in sea ice thickness distribution, and derive important information about the current state of the global ice coverage. To make this important measurement, NASA is implementing a new type of satellite-based surface altimetry based on sensing of laser pulses transmitted to, and reflected from, the surface. Because the ICESat-2 measurement approach is different from that used for previous altimeter missions, a high-fidelity aircraft instrument, MABEL (Multiple Altimeter Beam Experimental Lidar), was developed to demonstrate the measurement concept and provide verification of the ICESat-2 methodology. The MABEL instrument will serve as a prototype for the ICESat-2 mission and also provides a science tool for studies of land surface topography. 2) 3) 4)
MABEL instrument design:
The MABEL instrument uses a high repetition-rate pulsed laser fabricated by Fibertek, Inc. The laser repetition rate is variable from 5 kHz to 25 kHz, although typical operations will use 10 kHz to mimic the on-orbit behavior of ICESat-2. The laser pulse length is 2 ns. If the laser is operated in 10 kHz mode, then a pulse is emitted every 2 cm along track given nominal 200 m/s speed of the ER-2.
The laser generates both 1064 and 532 nm output. However, because only one laser is used, a means to divide the output energy into multiple transmit paths is required. To accomplish this feat, the transmitter fiber splitter box was developed. Figure 1 provides an optical layout of the MABEL instrument. Key to the instrument design is a pair of matched telescopes, one to transmit and one to receive. Selecting which fibers in the transmitter fiber select box are active, and matching to the identical fibers in the receiver fiber select box, determines the instrument viewing geometry.
Essentially a series of cascading beam splitters, the transmitter fiber splitter box separates the laser output into the two wavelengths then divides the output pulse energy down to the 5-7 nJ level. The output of the transmitter fiber splitter box is a series of eight 1064 nm output beams and sixteen 532 nm output beams. Each output is focused into a 50 µm diameter fiber. In addition, just prior to coupling into the fibers, each beam path can accommodate a small neutral density filter to be used to vary the energy level for each footprint.
Figure 1: MABEL instrument optical layout (image credit: NASA)
The real heart of the MABEL design, and the aspect that permits great flexibility in selecting and changing the instrument viewing geometry, is the transmitter fiber select box and the corresponding transmitter fiber array. Basically, an array of 215 fibers (107 fibers for each wavelength, plus one center fiber) defines the instrument transmit/receive geometry. The fiber arrays were custom fabricated by Fiberguide, Inc. The instrument geometry as defined by the fiber arrays is illustrated in Figures 2 and 3. At any given time, sixteen of the 107 532 nm fibers can be coupled to the 532 nm transmitter fibers and eight of the 107 1064 nm fibers can be coupled to the 1064 nm transmitter fibers. Selection of a set of transmitter fibers thus defines the instrument viewing geometry. Because each wavelength has a separate set of transmitter fibers, it is possible to have both wavelengths interrogate the same surface area (e.g., the footprints for each wavelength can overlap). While not changeable during flight, the geometry can be changed between flights to permit evaluation of instrument geometry or to optimize validation capability.
MABEL transceiver concept features:
• Use of two identical focal length telescopes with identically spaced fiber arrays in the focal plane of each telescope.
• Use of smaller core fiber in the transmitter telescope and larger core fiber in the receiver telescope.
• The difference in the IFOV (Instantaneous Field of View) 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.
Table 1: Primary instrument parameters of MABEL
Figure 3: MABEL viewing geometry as defined by fiber arrays (image credit: NASA)
Legend to Figure 3: If the outermost fibers were illuminated the swath width would be 1.05 km. For the December 2010 flights, only the tightly arranged inner fibers were used, resulting in a ground swath of ±100 m.
The transmitter fiber array is positioned at the focus of a 12.5 cm diameter telescope. The telescope is f/4.17, resulting in a 100 µrad field of view for the transmitter. From the nominal 20 km operating altitude, this corresponds to a laser footprint on the ground of 2 m diameter. Two matched telescopes, both custom fabricated by Special Optics, Inc., are used in the system. The maximum view angle for the system is ±3º, or about ±1.05 km in cross-track.
The receiver is similar, except the fibers are 105 µm diameter resulting in a 210 µrad receiver field of view. The receiver fiber array, positioned at the focus of the matched receiver telescope, is precisely aligned to the transmitter array. The same fiber positions illuminated on the transmitter fiber select box are selected on the receiver fiber select box. The sixteen 532 nm receiver fibers are routed to a 16-channel Hamamatsu model H7260 PMT (Photomultiplier Tube) detector. The eight 1064 nm receiver fibers are routed to eight individual Excelitas SPCMs (Single Photon Counting Modules). The detector dead time on the SPCMs is not ideal for precision ranging, but choices of photon-counting detectors able to operate in the near-IR region are limited.
The signals collected by the detectors are time-tagged by custom electronics developed by Sigma Space Corporation. As prototype for the ICESat-2 mission, demonstrating the electronics was an important step. The time-tagging electronics have a measured resolution of 83 ps (about 12 mm), which is smaller than the ICESat-2 requirement of 150 ps. The data system used to operate the instrument and collect data is based largely on heritage from the CPL (Cloud Physics Lidar) instrument. Operating on the ER-2 platform requires fully autonomous capability and rugged design. Data is stored to a solid-state hard drive that is removed after flight for data retrieval. A Novatel model HG1700 IMU (Inertial Measurement Unit) is mounted directly to the telescope assembly to permit accurate determination of instrument pointing.
The overall schematic of the MABEL instrument is shown in Figure 4. The instrument dimensions are: 138 cm x 66 cm x 76 cm. The instrument mass is 250 kg. MABEL is accommodated in the nose of the ER-2 aircraft.
Figure 4: Illustration of the MABEL instrument configuration (image credit: NASA)
Figure 5: MABEL instrument, in flight configuration (image credit: NASA)
The airborne MABEL instrument was developed in a mere 12 months and was first deployed for engineering test flights in December 2010. A subsequent series of flights were conduced in March 2011. Initial results from MABEL conclusively demonstrate that the concept of photon-counting detection for surface ranging is a viable approach for the ICESat-2 mission.
MABEL flight campaigns:
• MABEL flies her maiden voyage: On Dec. 7, 2010, the ER-2 lifted off with MABEL from NASA's Dryden Flight Research Center in Palmdale, CA, headed for a forested region in the Sierras. MABEL made its first five flights in December 2010. 5) 6)
• Validation flights in Greenland: NASA's high-flying ER-2 Airborne Science aircraft has concluded its four-week deployment to validate data acquired by the MABEL laser altimeter over the Greenland ice cap and surrounding sea ice fields. In April 2012, the ER-2 aircraft flew more than 100 hours on 16 flights in the MABEL validation campaign, including 14 data collection flights over Greenland and surrounding sea ice areas and two transit flights between Keflavik (Iceland) and its home base in Palmdale. Several of the flights were conducted concurrently and on the same flight tracks as flights of other NASA environmental science aircraft involved in the Arctic Operation IceBridge campaign in order to compare data being recorded by the MABEL with instruments on the other aircraft. 7)
There is also international cooperation. Data from ESA's CryoSat-2 mission allowed scientists to create detailed maps of sea-ice thickness and ocean circulation in the Arctic. This campaign, known as CryoVEx (CRYOsat Validation EXperiment), involves flying instrument laden planes along CryoSat orbit tracks over the Arctic Ocean and then comparing the data gathered by low flying aircraft and the satellite 700 km overhead. This collaboration is a fine example of what can be accomplished when many nations and organizations team up instead of competing with each other. 8)
Figure 6: The convergence of two glaciers near Thule, Greenland can be seen in this photo from the cockpit of NASA's ER-2 Earth Resources aircraft during a MABEL laser altimeter validation flight (image credit: NASA)
Note: NASA's Operation IceBridge 2012 took place from mid-March to mid-May 2012. A modified P-3 aircraft from NASA's Wallops Flight Facility in Wallops Island, VA, conducted daily missions out of Thule and Kangerlussuaq, Greenland -with one flight to Fairbanks, Alaska and back-to measure sea and land ice. 9)
Figure 7: Artist's view of the 3 missions ICESat, Operation IceBridge and ICESat-2 observing ice elevations in the Arctic (image credit, NASA, Ref. 3)
The IceBridge P-3 has carried out many survey flights over CryoSat tracks this year, but the most noteworthy flights happened on March 29, 2012, the 100th anniversary of Scott's death, and April 2. On these flights, the P-3 joined forces with a Twin Otter from the Technical University of Denmark and a Basler BT-67 from the Alfred Wegener Institute in Germany. The Twin Otter carried a laser scanner and ASIRAS (Airborne SAR/Interferometric Radar System), an airborne version of the radar altimeter aboard CryoSat-2, and the BT-67 was equipped with a laser scanner, cameras and a towed electromagnetic sensor called an EM bird (Ref. 8).
• In September 2012, the ER-2 aircraft arrived at NASA's Wallops Flight Facility in Wallops Island, VA. The objective was to fly two laser instruments, MABEL (Multiple Altimeter Beam Experimental Lidar) and CATS (Cloud-Aerosol Transport System) to measure vegetation along the U.S. East Coast, to provide data useful for developing methods for determining the amount and thickness of vegetation coverage. This involved measuring both the tops of tree canopies and ground level at the same time. - In addition to CATS and MABEL, the ER-2 carried the CPL (Cloud Physics Lidar) instrument, used to detect clouds and aerosols that could hinder MABEL's performance. 10)
1) Matthew McGill, Thorsten Markus, V. Stanley Scott, Thomas Neumann, “The Multiple Altimeter Beam Experimental Lidar (MABEL), an airborne simulator for the ICESat-2 mission,” submitted to the Journal of Atmospheric and Oceanic Technology, 2012, URL: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120016023_2012016303.pdf
3) Kelly M. Brunt, Sinéad L. Farrell, Vanessa M. Escobar, “ICESat-2: A next generation laser altimeter for space-borne determination of surface elevation,” 93rd American Meteorological Society Annual Meeting, Austin, TX, USA, Jan. 6-10, 2013, URL: https://ams.confex.com/ams/93Annual/webprogram/Handout/Paper224015/BruntAMS2013.pdf
4) Multiple Altimeter Beam Experimental Lidar (MABEL) - An Overview,” URL: http://icesat.gsfc.nasa.gov/icesat2/data/mabel/docs/mabel_overview.ppt
5) Kathryn Hansen, “MABEL Flies Her Maiden Voyage,” Jan. 04, 2011, URL: http://www.nasa.gov/topics/earth/features/mabel-maiden.html
6) Matthew McGill, “Multiple Altimeter Beam Experimental Lidar (MABEL) First Flights and Initial Results,” April 2011, URL: http://atmospheres.gsfc.nasa.gov/uploads/science/ppt/2011_04_highlights.ppt
7) Alan Brown, Beth Hagenauer, “NASA's ER-2 Completes MABEL Validation Deployment,” NASA, May 1, 2012, URL: http://www.nasa.gov/topics/earth/features/ER-2_completes_MABEL_deployment.html
8) “NASA's IceBridge Seeking New View of Changing Sea Ice,” August 27, 2012, URL: http://www.nasa.gov/mission_pages/icebridge/news/spr12/index.html
9) “NASA's IceBridge 2012 Arctic Campaign Takes to the Skies,” NASA, March 15, 2012, URL: http://www.nasa.gov/mission_pages/icebridge/news/spr12/arctic_2012campaign.html
10) George Hale, “High-Flying NASA Aircraft Helps Develop New Science Instruments,” NASA, Sept. 17, 2012, URL: http://www.nasa.gov/topics/earth/features/er2-develop.html
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.