Anthony W. Yu, David J. Harding, John F. Cavanaugh, Susan R. Valett, Xiaoli Sun,
Michael A. Krainak and James B. Abshire,
NASA Goddard Space Flight Center, Greenbelt, Maryland
In 2007, the National Research Council (NRC) completed its first decadal survey for Earth science at the request of NASA, the National Oceanic and Atmospheric Administration, and the U.S. Geological Survey . The Lidar Surface Topography (LIST) mission is one of 15 missions recommended by NRC, whose primary objectives are to map global topography and vegetation structure at 5-meter spatial resolution, and to acquire global coverage within a few years. NASA Goddard conducted an initial mission concept study for the LIST mission, and developed recommended science and measurement requirements for the mission. LIST will serve a diverse array of science and applications objectives, providing foundation data on the Earth’s topography, the three-dimensional structure of vegetation cover, and the height of water, snow, ice sheets and glaciers. This foundational data is fundamental to understanding, modeling and predicting interactions among the solid Earth, hydrosphere, biosphere, cryosphere and atmosphere. LIST enables improved quantitative understanding of volcanic and landslide hazards, terrestrial carbon sequestration, habitat quality and its influence on biodiversity, coastal protection from hurricane and tsunami inundation, ice sheet mass balance and its contribution to sea level rise, changing sea ice contributions to ocean-atmosphere energy exchange, and the storage and transfer of in-land water resources in lakes, wetlands and snow.
Most previous spaceborne lidar projected and imaged a single laser spot along a profile for altimetry measurements [2-6]. This single beam approach is insufficient to fully map the Earth’s surface in a few years. The Lunar Orbiter Laser Altimeter is the first multi-beam spaceflight altimeter using a non-scanned, 5-beam approach for sampling lunar topography . Using multiple laser beams enables surface slope measurements and increases the sampling density of the topography profiles. The ICESat-2 mission scheduled for launch into Earth orbit in 2016 will use six beams to achieve the same benefits . For complete mapping of the Earth’s land, sea ice and ice sheets, the number of beams must be increased by several orders of magnitude.
In 2009, we started a three-year Instrument Incubator Program (IIP) project, funded by NASA’s Earth Science Technology Office, for early technology development for LIST. The objective of our work was to demonstrate the key capabilities for a new, highly efficient laser altimeter for the LIST mission. In order to map the Earth in a few years, the LIST lidar needs to be able to generate a 5-kilometer swath composed of 5-meter pixels. It must image this swath to a detector array and produce a range image with the topographic height of the sampled area. This includes measuring through foliage and measuring the 3-D structure of vegetation cover. An instrument concept for the LIST mission is depicted in Figure 1. A swath 5 kilometers wide is composed of 1,000 laser beams in a linear array oriented in the across-track direction. The divergence of each beam yields a 5-meter diameter footprint on the ground from a 400-425 kilometer orbit altitude. Forty sets of 25 beams, with each set arranged in a five-by-five grid pattern and rotated with respect to the flight direction to produce parallel profiles that are contiguous cross-track. This grid pattern mitigates crosstalk from adjacent spots at the detector array. In this configuration, each pixel on the detector array can have a field of view (FOV) three times larger than the spot size to facilitate boresight alignment.
Figure 2 shows the laser ranging measurement approach obtaining information on canopy vertical structure and height and the topography of the ground beneath. In traditional altimeters, a relative broad laser pulse is used, thereby defining the vertical resolution of the system. The voltage output of a silion (Si) based avalanche photodetector (APD) is digitized to record a waveform of the height distribution of returned energy. The high detector dark-count rate requires that a large number of signal photons are received. A single pulse is used to obtain the measurement, so lasers with high pulse energy and low pulse rate are used. In our approach, we employ a micropulse laser with a pulse 10 times narrower, improving vertical resolution. A linear-mode, photon counting detector array is used that has a low dark count rate. Digitizing its output yields a waveform identifying 1 to n simultaneously arriving signal photons. At a 10 kHz laser repetition rate and a ground velocity of 7 kilometers per /second, laser footprints are spaced 0.7 meters along track, yielding 7 pulses per 5-meter pixel. This over-sampling along track enables signal accumulation to achieve a sufficient density of detected ground returns under adverse observing conditions (such as low atmospheric transmission due to thin clouds or aerosols and ground obscuration by vegetation cover) .
In order to demonstrate this measurement approach from an aircraft, we developed the Airborne LIST Simulator (ALISTS) instrument . Figure 3 summarizes the two ALISTS airborne campaigns conducted in summer 2011 and 2012. The plots below the aircraft pictures in Figure 3 depict along-track waveforms, color coded by amplitude, showing strong ground returns where vegetation is absent and weak ground returns in the presence of vegetation. The measurement objective of the September 2011 ALISTS engineering test flights was to acquire micropulse waveform data over a variety of land cover and terrain types from a flight altitude of 10 kilometers in areas of northern Ohio accessible from Glenn Research Center where the Lear 25 aircraft is based. These data were used to demonstrate that the measurement approach successfully detected the ground surface beneath vegetation cover and characterized the vertical structure of that vegetation, including its height and crown depth. Examples of the acquired waveforms are shown in Figure 4. The data were also used to validate our model prediction of signal strength. The primary objective of the August 2012 science demonstration flights on the P-3 aircraft was to document ALISTS capabilities in comparison to independent lidar data sources available for terrestrial ecology research sites in the mid-Atlantic and Northeast United States. Instrumentation problems limited data collection to targets in the vicinity of Wallops Flight Facility flown at lower-than-expected altitudes. Nonetheless, in comparison to high resolution, micropulse, photon-counting data acquired by the airborne Slope Imaging Multi-polarization Photon-counting Lidar (SIMPL), we demonstrated ALISTS achieved key measurement requirements for the LIST mission: 10-centimeter range precision needed for accurate topographic mapping, quantification of surface relief based on pulse broadening, and measurement of vegetation structure with 1-metervertical accuracy [10-12].
Through link analysis modeling, development of a highly efficient laser transmitter with a power-scalable design , evaluation of photon-sensitive, linear-mode detector arrays [14-16], and demonstration of a multi-beam measurement approach in the ALISTS instrument, we met our IIP objectives to mitigate major risks and develop a highly efficient measurement technique for the LIST mission. The ALISTS 16-beam, push-broom configuration achieves an 80-meter wide swath composed of contiguous 5-meter beams with separation of the receiver FOVs in order to prevent optical cross-talk. Along-track aggregation of micropulse waveforms yields measurements that characterize the vertical structure of the land surface, including vegetation height, crown depth, canopy closure and the relief of the ground. Engineering test flights in 2011 confirmed our prediction of expected photon detection rates. Comparison of ALISTS data collected during flights in 2012 to single-photon ranging data acquired by the SIMPL instrument showed comparable measurements of surface properties. Future work will include algorithm development for the LIST mission using these and additional data collections to establish efficient methods for on-orbit signal processing and data acquisition.
We gratefully acknowledge support from the NASA ESTO IIP program and the NASA Goddard IRAD program.
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