ICESat-2 (Ice, Cloud and land Elevation Satellite-2)
ICESat-2 is a NASA follow-up mission to ICESat with the goal to continue measuring and monitoring the impacts of the changing environment. The ICESat-2 observatory contains a single instrument, an improved laser altimeter called ATLAS (Advanced Topographic Laser Altimeter System). ATLAS is designed to measure ice-sheet topography, sea ice freeboard as well as cloud and atmospheric properties and global vegetation. The requirements call for a 5-year operational mission with a goal of 7 years. 1) 2) 3) 4) 5) 6) 7)
Rational and discussion of mission goals: The mass balance of Earth's great ice sheets and their contributions to sea level are key issues in climate variability and change. The relationships between sea level and climate have been identified as critical subjects of study ib the IPCC (Intergovernmental Panel on Climate Change) assessments, the CCSP (Climate Change Science Program) strategy, and the U.S. IEOS (International Earth Observing System). Because much of the behavior of ice sheets is manifested in their shape, accurate observations of ice elevation changes are essential for understanding ice sheets' current and likely contributions to sea-level rise.
ICESat-2, with high altimetric fidelity, will provide high-quality topographic measurements that allow estimates of ice sheet volume change. High-accuracy altimetry will also prove valuable for making long-sought repeat estimates of sea ice freeboard and hence sea ice thickness change, which is used to estimate the flux of low-salinity ice out of the Arctic basin into the marginal seas. Altimetry is best (and perhaps only) technique for change studies, because sea ice areas and extends have been well observed from space since the 19070s and significant trends have been shown, but there is no such record for sea ice thickness.
As climate change proceeds, continuous measurements of both land-ice and sea-ice volume will be needed to observe trends, update assessments, and test climate models. The altimetric measurement made with the lidar instrument, along with a higher precision gravity measurement (such as GRACE-FO), would optimally characterize changes in ice sheet volume and mass and directly enhance understanding of the ice sheet contribution to sea-level rise. Coupled with the interferometric synthetic aperture radar in the DESDynI mission, the instrumentation would provide a comprehensive data set for predicting changes in Earth's ice sheets and sea ice.
In addition to studies of ice, the proposed instrument could be used to study changes in the large pool of carbon stored in terrestrial biomass. In particular, the proposed lidar could be used to measure canopy depth and thus estimate land carbon storage to aid in understanding the responses of biomass to changing climate and land management. 8) 9)
Figure 1: Schematic view of mission goals (image credit: NASA)
Figure 2: Changes in Greenland ice from 1986 to 2006 (image credit: NASA)
• Quantify the polar ice sheet mass balance to determine contributions to current and recent sea level change and impacts on ocean circulation
• Determine the seasonal cycle of ice sheet changes
• Determine topographic character of ice sheet changes to assess mechanisms driving that change and constrain ice sheet models
• Estimate sea ice thickness to examine ice/ocean/atmosphere exchanges of energy, mass and moisture.
• Measuring vegetation canopy height as a basis for estimating large-scale biomass and biomass change
• Enhancing the utility of other Earth observation systems through supporting measurements.
The instrument will use micro-pulse multi-beam photon-counting approach. Science and ancillary data will be collected, stored on-board and subsequently downlinked to ground stations via an X-band communications link. This link will also include stored housekeeping telemetry. The observatory will also receive and store/execute commands and transmit real-time housekeeping telemetry via an S-band link to the NASA Ground Network.
The ICESat-2 mission is assigned to NASA/GSFC. The spacecraft is being procured under the GSFC RSDO (Rapid Spacecraft Development Office). In August 2011, NASA selected Orbital ATK, former OSC (Orbital Science Corporation of Dullas, VA, to built the ICESat-2 spacecraft. The contractor is responsible for the design and fabrication of the ICESat-2 spacecraft bus, integration of the government-furnished instrument, satellite-level testing, on-orbit satellite check-out, and continuing on-orbit engineering support. The ICESat-2 spacecraft is being designed, assembled, and tested at Orbital's satellite manufacturing and test facility in Gilbert, Arizona.
Table 1: Overview of key spacecraft parameters
Figure 3: Artist's view of the ICESat-2 spacecraft (image credit: Orbital)
Project development status:
• August 16, 2017: Lasers that will fly on NASA's ICESat-2, are about to be put to the test at the agency's Goddard Space Flight Center in Greenbelt, Maryland. 15)
- The sole ICESat-2 instrument, ATLAS (Advanced Topographic Laser Altimeter System) will measure the elevation of ice sheets, sea ice and glaciers by sending fast-firing laser pulses to the surface and timing how long it takes individual photons to return. With a scheduled launch date of 2018, the instrument now faces several months of testing at Goddard in which engineers will ensure it is ready to operate in the harsh environment of space. This is an intermediate stage of ICESat-2's testing regimen, and will focus on the flight lasers.
- Starting this fall, ATLAS will go into a test chamber at Goddard where engineers simulate the vacuum of space and can dial temperatures up to 50 C to - 30 C. Engineers will also turn on the two lasers — one primary and one backup — at different power levels to ensure they function correctly, said Anthony Martino, ATLAS instrument scientist at NASA Goddard. One test will include putting the instrument through its paces at different temperatures and taking pictures of the laser pulses to ensure they form a smooth, consistent circle, Martino said, with no rough edges, or dark or light spots.
- "When it's well behaved like that, it's much easier to analyze the results that we'll get," he said. Other tests involve using mirrors to reflect the laser back into the detector portions of the instrument — but only after decreasing the strength of the beam of light by 13 orders of magnitude (about 10 trillion times), to simulate the weakening of the laser beam as it is scattered by the atmosphere, bounces off Earth and returns.
• September 2016: ICESat-2 Technical Status Summary. 16)
Beyond the ATLAS instrument, all other ICESat‐2 systems are nearing completion including spacecraft, launch vehicle, algorithms, operations planning, and ground systems.
- The mission requirements remain intact through the ongoing flight Laser002 repair.
- The ATLAS management and engineering team has crafted and is implementing a conservative plan to address the recent Laser002 optical slab fracture.
Table 2: ATLAS Instrument – Technical Issue with Laser002
Figure 4: The integrated ATLAS instrument (image credit: NASA, Ref. 16)
• Feb. 18, 2016: ICESat-2 passed its Mission CDR (Critical Design Review)! Now, on to building and testing software and hardware for flight. 17)
• Jan. 17, 2016: ICESat-2 passed its Instrument Critical Design Review! The project is now moving full-speed ahead to Mission CDR and instrument I&T start.
• Dec. 10, 2015: NASA engineers tested the ATLAS instrument's pinpoint accuracy. ATLAS (Advanced Topographic Laser Altimeter System) will send laser pulses to the ground about 480 km below and then catch the handful of photons that bounce off the surface and return to its telescope mirror. There's very little margin for error when it comes to individual photons hitting on individual fiber optics - so this November, engineers conducted a series of tests on the ground, to ensure that they could hit that mark when ICESat-2 is in orbit. 18)
- This is the first time Goddard has built an automatically correcting and steering mechanism like this for flight. It was necessary for ATLAS, however, because both the receiver's field-of-view and the laser beam diameter are significantly smaller than on previous instruments, so there is less room for the laser to drift off-target. So the AMCS (Alignment Monitoring and Control System) team spent several weeks in November 2015 testing the steering mechanism and the software that controls it.
• February 2015: A NASA team tested part of the ATLAS instrument in a temperature-controlled vacuum chamber at Goddard, ensuring that its interconnected components worked together and functioned as expected. 19)
• November 3, 2014: Engineers at NASA/GSFC fitted the mirrored telescope of ICESat-2 into its place. In a Goddard cleanroom, teams are working in parallel on two sections of ATLAS: the box structure, which holds electronics that control the instrument, and the optical bench, which supports the instrument's lasers, mirrors, and the 0.8 m, 20.8 kg beryllium telescope that collects light. 20)
- Each ATLAS laser pulse contains more than 200 trillion photons, but only a dozen or so return to the telescope, where they're sent via optical fibers to the instrument's detectors. To catch those few photons, the telescope and its associated equipment, called the RTA (Receiver Telescope Assembly), need to align perfectly to the laser.
Figure 5: Engineers and technicians check the fit of ICESat-2's telescope to its sling, before moving it into place on the instrument's optical bench (image credit: NASA)
• Sept. 1, 2014: Due to cost overruns, the launch of ICESat-2 has slipped to June 2018. ICESat-2's overrun was driven primarily by technical difficulties with the ATLAS (Advanced Topographic Laser Altimeter System) instrument. 21)
• May 2014: The box structure of the ATLAS instrument was delivered to a clean room at NASA/GSFC (Figure 16). A team of 250 engineers, fabricators and scientists has now started the official integration and testing stage of the laser instrument (Ref. 30).
Figure 6: An engineer checks the ATLAS box structure, shortly after its arrival in a NASA clean room in May 2014 (image credit: NASA, Kate Ramsayer)
• February 18, 2014: ICESat-2 passed its Mission Critical Design Review! Now, on to building and testing software and hardware for flight.
• January 17, 2014: ICESat-2 passed its Instrument CDR (Critical Design Review)! The project is now moving full-speed ahead to Mission CDR and instrument I&T start.
• In December 2013, NASA notified Congress of expected budget increases ($200 million overrun) on the ICESat-2 mission. NASA is required by law to inform Congress when a mission appears likely to overrun its approved budget by more than 15%. This may cause possible launch delays. 22)
• Sept. 6, 2013: ICESat-2 passed its Ground Systems CDR (Critical Design Review). An independent review board met Sept. 3-5 , 2013at Goddard Space Flight Center in Greenbelt, MD, to examine details of the entire design of the mission's ground system, including the MOC (Mission Operations Center), the ISF (Instrument Support Facility), and the Science Investigator-led Processing System.
• The ICESat-2 mission was assigned Phase C status on December 17, 2012.
• The ICESat-2 project passed instrument PDR (Preliminary Design Review) on Nov. 18, 2011.
• The ICESat-2 team passed the SRR (System Requirements Review) on May 25, 2011.
• The ICESat-2 team passed the ISRE (Instrument System Requirements Review) on December 1, 2010.
• The ICESat-2 team passed the Key Decision Point A (KDP-A) review at HQ on December 11, 2009. Since then the project started officially in Phase A.
Launch: A launch of ICESat-2 is scheduled for 2018 from VAFB, CA on a Delta-2 7320-10C vehicle. The launch service provider is ULA (United Launch Alliance).
Orbit: Near polar LEO frozen orbit, altitude =496 km, inclination = 92º, repeat cycle of 91 days with subcycles of 29, 29, and 33 days (Figure 7).
Figure 7: Illustration of monthly subcycles (image credit: NASA)
Sensor complement: (ATLAS)
ATLAS (Advanced Topographic Laser Altimeter System)
ICESat-2 will use a new type of laser altimeter instrument, ATLAS, for measuring elevation, and will acquire far more data. To test the instrument concept, and develop accurate software to process the data, NASA has been flying an instrument called MABEL (Multiple Altimeter Beam Experimental Lidar) on high-altitude aircraft (ER-2) to simulate measurements that the ATLAS (Advanced Topographic Laser Altimeter System) — GLAS's successor–will be making from space. 23) 24)
Table 3: ATLAS instrument science measurement requirements (Ref. 28)
MABEL and ATLAS are photon-counting laser altimeters, meaning they measure distance by detecting just a few photons from each laser pulse and timing their round-trip travel from satellite to earth and back extremely accurately. While GLAS used millions of photons to make a single distance measurement, MABEL and ATLAS gather a data set of just a few dozen photons at most, and produce a cloud of points describing the snow or land or vegetation surface structure. Sophisticated software will determine the location of the surface track, the tops of the tree canopy, or the amount of dust or fog in the air.
The original design of the ATLAS instrument for ICESat-2 evolved as a modified version of the ICESat GLAS instrument concept. More specifically, for ICESat-2, the original ATLAS design was a single-beam altimetry system with the laser transmitter operating at a slightly higher repetition rate (50 Hz), lower energy per pulse (50 mJ) and similar 6-7 ns pulse width at the near-infrared (NIR) wavelength of 1064 nm when compared to GLAS. These changes would have provided higher derating on the lasers and potentially longer mission life.
In 2009, the ATLAS instrument on ICESat-2 underwent a complete redesign during the pre-Phase A activities to accommodate more science objectives and incorporate recommendations from the ICESat-2 science workshop (June 2007). For ice sheets, improved pointing will reduce the uncertainty in the ice sheet elevations introduced by the cross-track surface slope. In addition, for land topography and vegetation, improved pointing will provide observations along exact repeat ground tracks, and sampling along uniformly spaced ground tracks will provide well-sampled grids of topography and biomass. Based on this and other recommendations, a new instrument concept was proposed and accepted by the ICESat-2 program. 25) 26) 27) 28) 29) 30) 31)
The new baselined instrument is a high repetition rate (10 kHz), micropulse laser altimeter system. GSFC has begun an in-house program to investigate various potential laser technologies to meet the laser requirements for the ATLAS instrument.
A single laser transmitter having sufficient laser energy will be split into multiple beams using a DOE (Diffractive Optical Element) similar to the one used on LOLA. The current instrument architecture consists of a 9-beam system arranged in a 3 x 3 configuration.
Figure 8: Measurement concept of the ATLAS instrument (image credit: NASA)
In contrast to the first ICESat mission, ICESat-2 will use micro-pulse multi-beam photon counting approach to provide:
- Dense cross-track sampling to resolve surface slopes on an orbit basis
- High repetition rate (10 kHz) generates dense along-track sampling (~70 cm)
- Different beam energies to provide necessary dynamic range (bright / dark surfaces)
The advantages are:
- Improved elevation estimates over high slope areas and very rough (e.g. crevassed) areas
- Improved lead detection for sea ice freeboard.
The ATLAS instrument is a multi-beam micropulse laser altimeter with the following features:
• Single laser beam split into 9 beams
• 10 m ground footprints
• 10 kHz repetition rate laser (~1 mJ)
• Multiple detector pixels per spot
• On-board boresight alignment system
• LRS (Laser Reference System) gives absolute laser pointing knowledge.
Figure 9: Schematic of the ATLAS instrument (image credit: NASA)
ATLAS will employ a micropulse laser transmitter frequency doubled to 532 nm (visible green) with a 1 ns FWHM pulse width and operating at a 10 kHz repetition rate (0.7 m along-profile footprint sampling). A narrow 20 µrad beam divergence from a 500 km orbit altitude will yield 10 m diameter footprints. To improve spatial sampling ATLAS will employ a DOE (Diffractive Optic Element) that will split the single transmit beam into 6 beams, creating a pattern consisting of 3 sets of 2 closely spaced (< 100 m) beams. The closely-spaced beam pairs will resolve local slope, enabling determination of real elevation change from a single repeat of a reference track (Ref. 4).
With ICESat-2 operating in a 91 day repeat orbit and ATLAS operating continuously, seasonal observations of inter-annual ice sheet elevation change will be possible. The beam pairs will be separated cross-track by 3 km, providing improved spatial coverage as compared to that of ICESat. Over land, rather than repeating reference tracks, spacecraft pointing will be used to systematically displace the profiles cross-track through time in order to build up dense global sampling of topography and vegetation over the course of the mission.
In the traditional analog Si:APD detection approach used by GLAS of ICESat-1, thousands of photons reflected from the Earth's surface were acquired per laser fire (for clear atmospheric conditions) in order to obtain waveforms with sufficient SNR to achieve the 3 cm ranging precision. In the ATLAS micropulse approach the transmit pulse energy will be significantly lower such that only a few to ~10 photons will be detected per footprint per laser fire using a 0.8 m diameter telescope and photon-sensitive Photomultiplier Tube (PMT) detector arrays. Laser fire times and the arrival time of each photon, those reflected from the surface as well as from solar background noise, will be time tagged with 0.15 ns precision yielding < 20 cm single-photon range precision. Post-processing on the ground will yield "point clouds" of geolocated single photon surface returns. An advantage of this approach is that the combination of small, oversampled footprints, narrow pulse width and high-precision timing can yield elevation data of higher spatial and vertical resolution. In addition the geospatial information content of the point cloud is amenable to a greater diversity of analysis approaches than afforded by analog waveforms, opening up possibilities for new ways to characterize the vertical structure of the Earth's surface (Ref. 4).
Legend to Figure 10: Single laser pulse, split into 6 beams. Redundant lasers, redundant detectors.
ATLAS carries two lasers on the optical bench – one primary and one backup. The laser light is at 532 nm, a bright green on the visible spectrum. It is fast-firing, sending 10,000 laser light pulses per second. 33)
The first step on the laser's path to the ground is just a few inches past the laser, where a fold mirror directs the light 90º around a corner, where it hits the first key component, the PBC (Polarizing Beam Combiner). The PBC has two functions: The first is to make sure that the primary and backup lasers head down the same path. Although the two lasers won't fire at the same time, the laser beams begin at different positions and need to end up at the same place. The second function is to use a periscope to pick off a fraction of the laser light and direct it to the LSA (Laser Sampling Assembly).
At the LSA, one of these fibers starts the 'stopwatch' for that photon pulse. This timing component has to be incredibly precise to get the measurements that scientists need – when a photon returns, its travel time is recorded to the billionth of a second. The LSA also uses the small fraction of the laser to measure the laser's wavelength, ensuring it remains precisely at 532.272 nm. This specific shade of bright green is what the filters on the receiving telescope let pass through to stop the stopwatch, once the laser pulse completes its journey. Any other wavelength gets filtered out as background noise. As the LSA starts the timer, the rest of the laser pulse continues to the BE (Beam Expander).
Shaping and steering the beam: The BE consists of two mirrors, facing each other but slightly angled so that the laser hits one, bounces across to the second one, and then continues on in the same direction. These mirrors are curved to make the laser beam more than four times wider once it bounces off them. Making the beam wider actually makes the photons diverge less as they travel to Earth, tightening the laser footprint on the ground and allowing for a more precise map of surface heights.
"The spot diameter on Earth's surface would have been 66 m, now it's 15 m," according to Ramos-Izquierdo. "By making the laser beam bigger in diameter before exiting the instrument, we actually decrease how much it spreads as it propagates downward through the atmosphere."
The wider laser beam now goes through the BSM (Beam Steering Mechanism), which directs the laser at the ground below, but also has fine control over where the laser is pointing. This mechanism is connected with components on the instrument's telescope receiver, which collects any photons that return. The goal is to automatically point the lasers at the exact spot on the ground, where the telescope is observing. If the telescope and laser are pointed at different spots, the BSM will make slight adjustments to correct the alignment. The BSM is key because as the satellite goes in and out of the sun, changes in temperature could slightly warp the optical bench.
Split in six: The last hurdle for the laser beam is the DOE (Diffractive Optic Element), on the far side of the beam steering mechanism. This optical component is etched with a microscopic pattern of crisscrossed lines, which splits the single laser beam into six. The beams are set at slightly different angles, so they will cover the ground in a specific formation of three pairs of beams.
Once through the Diffractive Optic Element, the photons – lined up perfectly in six beams – are off on their journey to Earth.
Figure 11: ICESat-1 observation spacing at Jakobshavn Isbræ (left) and planned ICESat-2 spacing overlay (right), (image credit: NASA, Ref. 6)
Figure 12: Schematic of analog versus photon counting (image credit: NASA)
Legend to Figure 12: It is important to note that the integrated photon-counting sample ("histogram") looks like the analog wave - but it is not - the information content is different, and the method of analyzing the data is different.
Figure 13: Overview of the ATLAS instrument (image credit: NASA)
Table 4: Current laser transmitter performance requirements for the ICESat-2 micropulse laser altimeter system
Figure 14: Block diagram of the ATLAS instrument (image credit: NASA)
Figure 15: ATLAS functional block diagram (image credit: NASA)
The ATLAS instrument has a mass of 298 kg and a power consumption of 300 W.
Just as with the original ICESat data, ICESat-2 measurements are expected to provide added value to science and applications beyond their primary purpose. NASA's Applied Sciences Program actively seeks to connect NASA's Earth-observing satellite data to societal applications and encourages each mission to come up with a plan to connect its science to user needs. To that end, the ICESat-2 mission established an ICESat-2 Applications Team to organize and develop a mission applications program that will help establish these vital links between ICESat-2 science and society.
With guidance from the Applications Team, ICESat-2 has developed and implemented a diverse range of mission-specific prelaunch applications activities and strategies for engaging end users. These activities are modeled after the highly successful application strategies implemented for NASA's SMAP (Soil Moisture Active/Passive) mission and are intended to provide a fundamental understanding of how ICESat-2's data products can be best integrated into operational procedures to improve decision-making efforts across multiple disciplines. 34)
Figure 17: Overview of ICESat-2 elements in the ground segment (image credit: NASA)
Table 5: ICESat-2 science data products. The rows are shaded light gray to dark gray to represent Level 0 (light gray), Level 1, Level 2, and Level 3 (dark gray) data products. The ICEsat-2 mission will not have a Level 4 (value-added-model) product.
MABEL (Multiple Altimeter Beam Experimental Lidar)
MABEL is a high-altitude airborne laser altimeter designed as a simulator for ICESat-2. The MABEL design uses multiple beams at fixed angles and allows for local slope determination. The MABEL instrument was developed to: 35) 36)
1) enable the development of ICESat-2 geophysical algorithms prior to launch
2) provide detailed error analysis of the ATLAS measurement strategy
3) provide ATLAS model validation.
MABEL is a photon-counting multibeam lidar sampling at both 532 nm and 1064 nm wavelengths using short (~1.5 ns) laser pulses. MABEL beams are arranged in a linear array, perpendicular to the direction of flight. The system allows for beam-geometry changes between flights with a maximum view angle of ±1 km from a 20 km nominal altitude achieved during the 2010–2012 deployments using a NASA ER-2 aircraft (Figure 18).
Figure 18: Schematic ICESat-2 and MABEL beam geometry (dashed lines) and reference ground tracks (grey lines along icesheet surface). ICESat-2 beam pairs (separated by ~90 m) do not have the same energy in order to keep the required laser energy low; therefore, each beam pair consists of a strong and a weak beam (as indicated by the dash difference). MABEL allows for beamgeometry changes with a maximum ground spacing of ~2 km at 20 km. However, for the 2014 AK deployment, the maximum ground spacing was 0.2 km (Ref. 36), image credit: MABEL Team
The repetition rate of MABEL is variable (between 5 and 25 kHz); most flights during the 2010–2012 deployments used 5 kHz. At this nominal altitude, repetition rate, and an aircraft speed of ~200 m/s, MABEL samples a ~2 m footprint every ~4 cm along track. During these initial MABEL deployments, beam geometry (specifically the spacing between the individual beams) was configured to mimic ICESat-2.
Following engineering test flights in December 2010 and March 2011, MABEL was deployed to Greenland in April 2012 to collect data over polar targets (Figure 19).
Operation IceBridge is a NASA airborne campaign intended to bridge the data gap between ICESat and ICESat-2. Operation IceBridge hosts a suite of instruments, including the ATM (Airborne Topographic Mapper). ATM is a lidar that conically scans at a rate of 20 Hz, with an off-nadir scanning angle of ~15º. Like GLAS, ATM digitizes returned energy as a waveform with derived surface elevations based on 532 nm wavelength pulses and a 5 kHz PRF (Pulse Repetition Frequency). The ATM flights were conducted using the NASA P-3B at an aircraft speed of ~100 m/s, with a nominal elevation of 500 m above ground level. At this air speed, elevation, and repetition frequency, ATM generates a 1 m footprint and a scanning swath width of ~250 m.
Logistics and cloud-free weather allowed for coordinated surveys between ATM and MABEL over the Greenland Ice Sheet (Figure 19). Here, the MABEL multibeam determination of the ice-sheet surface is presented and compared with that determined by ATM, including local slope assessments. These comparisons are made with consideration for the ICESat-2 planned beam geometry and relative signal strength.
Both MABEL and ATM simultaneously surveyed a 150 km "Southern Traverse" of the Greenland Ice Sheet on April 20, 2012 (Figure 19). Additionally, MABEL made three passes over a 50 km stretch of ICESat track 0412 in the vicinity of Summit Station, Greenland, on April 8, 2012. ATM made a pass of the same ground segment on April 11, 2012. This ground segment has been used as a calibration site for ICESat-2.
Figure 19: Location map of the 50 km along-track Summit Area site and the 150 km along-track Southern Traverse site on the MODIS Mosaic of Greenland. (Inset) Operation IceBridge P-3B captured in the NASA ER-2 Cirrus Digital Camera System at the black.
The NASA IceBridge ATM Level-2 Icessn Elevation, Slope, and Roughness (ILATM2) for April 11 and 20, 2012, were obtained from the National Snow and Ice Data Center (NSIDC). This is a resampled and smoothed elevation data set that provides four across-track elevations per timestamp every ~35 m along-track, which allowed for the trivial calculation of across-track slope. The total across-track span for this data set, for the flights used in this analysis, was approximately 150 m.
MABEL data (release 8) for April 8 and 20, 2012, were obtained from the NASA ICESat-2 website. Each data file contains 1 minute of data for every available beam. The data files contain photon arrival times resulting from reflected laser light (i.e., signal photons) and background photons due to sunlight (i.e., noise photons).
In order to discriminate coarse signal photons from noise photons and derive ice-sheet surface elevation, the team developed an algorithm based on histograms of the photon data. Evolving from techniques applied to other photon-counting lidars, such as the SIMPL (Slope Imaging Multi-polarization Photon-counting Lidar), a 0.125 s (~25 m; 625 shots) along-track segments of data was generated and the photon data at 10 m vertical resolution was histogrammed. Signal photons in four sequential steps were identified.
For direct comparison of the surveys, ATM tracks and MABEL beams were chosen to most closely mimic the 90 m spacing of the ICESat-2 beam geometry. ATM tracks 2 and 3 were used for this analysis because they have ground separation of ~85 m. Elevations from tracks 2 and 3 from the same along-track time were then used to calculate the ATM across-track slope. MABEL beam 6 (center of the array) and beam 5 (~85 m ground spacing from the center of the array) were chosen for analysis as they have an across-track ground separation similar to the ATM tracks used. To determine the MABEL across-track slope, signal photons from beams 5 and 6 were interpolated along track to a common time so that, similar to ATM, an across-track slope could be then calculated for each increment of along-track time. The across-track slopes for both ATM and MABEL were then compared.
MABEL beams have variable signal strengths; however, beams 5 and 6 are the most similar to the expected radiometry of the strong beams of ATLAS. The along-track data density differed within and between flights based on variables that affect reflectivity, including weather conditions, time of day, and sun-incidence angle. For the data used in this analysis, the full-rate along-track data density average for both beams was always greater than 4 signal photons/m. For the Southern Traverse flight, the along-track data densities were 3.4 and 3.9 signal photons per 70 cm for beams 5 and 6, respectively. For the Summit Area flight, data densities were 3.1 and 3.4 signal photons per 70 cm for beams 5 and 6, respectively.
A strong-beam/weak-beam pair will be used for ICESat-2 slope determination; the energy associated with the weak beam will be reduced by a factor of 4. Therefore, the expected number of signal photons per laser shot (every 70 cm along track) between the strong beam and the weak beam will also differ by a factor of 4. The current best estimates of expected signal photons per laser shot vary with season and surface type. Based on ICESat-2 engineering models, under similar conditions as the 2012 MABEL survey, the team expects ICESat-2 to record 8.5 and 2.1 signal photons every shot (or 70 cm along track) for the strong and weak beams, respectively. Thus, the MABEL full-rate data used in this analysis suggest data densities of 46% of the expected ICESat-2 data densities. MABEL engineers are currently working to increase signal strength to achieve the expected ICESat-2 data densities, which will facilitate more direct MABEL to ATLAS comparisons.
To further assess accurate ground characterization given the ICESat-2 planned configuration, all photons associated with one of the MABEL beams (beam 5) were subsampled by a factor of 4 and then reprocessed through the ground-finding algorithm, to simulate the expected radiometric relationship between the ATLAS strong and weak beams. After subsampling, the data densities were 0.9 and 0.8 signal photons per 70 cm for the Southern Traverse and Summit Area, respectively. To determine the MABEL across-track slope, the ground-signal photons from beam 6 and the subsampled ground-signal photons from beam 5 were again interpolated to a common time so that an across-track slope could be calculated as described above. Therefore, the beam with the fewest along-track samples (the weak beam, 5) limited the total number of samples that was used in the slope determination.
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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 (firstname.lastname@example.org).