A spaceborne lidar for high-resolution topographic mapping of the Earth's surface
In 2007, the National Research Council (NRC) completed its first decadal survey for Earth science at the request of NASA and the US Geological Survey.1 It recommended development of the Lidar Surface Topography (LIST) mission as one of 15 projects to take forward. The mission's primary objectives are to map global topography and vegetation structure at a spatial resolution of 5m and acquire global coverage within a few years. NASA Goddard Space Flight Center conducted an initial mission-concept study and developed the LIST measurement requirements in 2007.
Although most previous spaceborne lidar (light detection and ranging) instruments projected and imaged a single laser spot for altimetry measurements,2–4 the recent Lunar Orbiter Laser Altimeter uses a nonscanned five-beam approach for mapping lunar topography.5 Using multiple laser beams enables surface-slope measurements and greatly reduces the time needed to globally map surface topography. In 2009, we started a three-year project to develop and demonstrate technologies for a next-generation, efficient, swath-mapping spaceborne laser altimeter.
Our objective is to demonstrate key capabilities for a new, highly efficient laser altimeter for the LIST mission. The mission's lidar needs to generate a swath with 5m pixels 5km wide, image this onto a detector array, and produce a range image that includes the topographic height of the sampled area. This involves measuring through foliage if covered by vegetation and measuring the 3D structure of the vegetation cover. Our pushbroom approach (see Figure 1) uses an area 5km wide composed of 1000 laser beams in a linear array, oriented in the cross-track direction. The divergence of each beam yields a 5m-diameter footprint on the ground from a 400–425km orbit altitude. The spots are contiguous cross-track. At 10kHz laser-pulse rate and a nominal spacecraft ground velocity of 7km/s, footprints are spaced 0.7m along-track, yielding seven illuminating laser pulses per pixel. This oversampling allows detecting ground-echo pulses under realistic observing conditions (e.g., attenuation by thin clouds and ground obscuration by vegetation).
Our approach is flexible and scalable in area and pixel width, laser power, and telescope size. We have developed a high-efficiency, high-repetition-rate master-oscillator power-amplifier laser transmitter, using a diffractive optical element (DOE) after the laser to produce multiple beams. Other laser requirements include linearly polarized output, beam divergence better than 1.5 × diffraction limited, and spectral width and wavelength stability <20pm to keep the background photon rate from the sunlit surface to a minimum. We collect the laser backscatter from the surface with a diffraction-limited telescope and image the spots from the swath onto a sensitive detector array. We are currently working with various vendors to develop suitable high-sensitivity, low-noise avalanche-photodiode (APD) detectors that operate in a quasi-analog mode. Desired detector characteristics are nearly single-photon sensitivity, high quantum efficiency, and >1GHz bandwidth. Candidate detectors for our prototype instrument include mercury cadmium telluride, cadmium zinc telluride, and impact-ionization-engineered indium aluminum arsenide APDs, and multi-element anode indium gallium arsenide phosphide intensified photodiode detectors.6,7 The output from each detector element is converted into histograms and analyzed using electronics (such as readout-integrated circuits). Desired data includes the range to the surface and the echo-pulse waveforms that characterize the vertical structure of the surface and any vegetation.
We are also taking advantage of our previous developments, which demonstrated a dual-wavelength micropulse lidar with photon-counting detectors from an aircraft.8 This lidar used a pulse energy of 1μJ per beam, an 8kHz laser-pulse rate, and single-photon-threshold Geiger-mode APD detectors. Our new demonstration lidar for LIST will use 16 laser beams, a pulse energy of 5μJ per beam, a 16-channel single-photon-sensitive detector, and a 16-channel waveform digitizer as timing electronics. We will characterize the measurements radiometrically so that we can scale the airborne performance to space.
Our main development activity will take place in the project's second year. In the third and final year we plan to perform airborne testing of the swath-mapping concept. We will select candidate geographic regions for field tests to demonstrate measurements that satisfy the LIST science objectives for topographic mapping, focusing on the cryosphere, water cycle, and vegetation structure. Table 1 summarizes some of the instrument characteristics and approaches we are using to reduce risks for LIST based on our airborne instrument development and its demonstration measurements.
|Spatial resolution||5m||5m||Use same footprint (not scaled by angular divergence)|
|Swath width||5km (1000 beams)||~80m (16 beams)||Scale: 62.5×|
|Detection scheme||Analog photon counting||Analog photon counting||Waveform analysis|
|Telescope size||2m diffraction-limited telescope||0.13m diffraction-limited telescope||Scaled by altitude: 1/40× with margin|
|Laser energy||100μJ per beam for 1000 beams at 10kHz||100μJ per beam for 16 beams at 10kHz will be demonstrated, 5μJ likely needed for airborne demonstration||Demonstration of full energy per beam for 16 beams meeting LIST requirement, although much lower energy (~5μJ) is needed for the airborne demonstration|
|Spectral linewidth||<20pm||<20pm||Demonstrate technical approach to stabilize laser wavelength and spectral width when used with narrow receiver filter|
|Laser efficiency||>15%||~15%||Demonstrate laser wall-plug efficiency|
|Prime power requirement||6.7kW assuming 1kW optical power||110kW assuming 16W optical power||Demonstrate laser wall-plug efficiency|
|Detector||1000 pixels with >1GHz bandwidth per pixel||16 pixels with >1GHz bandwidth per pixel||Demonstrate the necessary bandwidth in multiple-pixel detector array with photon-counting sensitivity and waveform digitizing|
|Platform speed||7000m/s||200m/s||Scale: 35×|
|Number of samples per footprint||7||250||During the airborne campaign, we will select every 35th sample to simulate space environment|
|Footprint separation||0.7m||0.02m||Airborne will oversample by 35×|
|Beam-dividing optics||In one scenario we have 10 lasers, each with 1×100 beam-divider diffractive optical element (DOE)||Single beam divides into 16 beams using a DOE||Demonstrate efficiency of beam-division technique using DOE|
We gratefully acknowledge support from the NASA Earth Science Technology Office's Instrument Incubator Program and the NASA Goddard Internal Research and Development program.
Anthony Yu received his MS and PhD degrees in physics from the Georgia Institute of Technology. He worked at GTE Labs, Hughes, Raytheon, and Northrop Grumman Electronic Systems. He is currently the laser and electro-optics branch technologist at NASA GSFC, where he has worked on development of several spaceborne laser systems.
Michael Krainak received his BS in electrical engineering from Catholic University, and MS and PhD degrees in electrical engineering from Johns Hopkins University. He worked at AT&T Western Electric and the National Security Agency. He is the laser and electro-optics branch head at NASA GSFC, where he has worked for 20 years.
David Harding received his BS and PhD in geological sciences from Cornell University. He is a member of the Planetary Geodynamics Laboratory at NASA GSFC, where he has worked for 19 years. His expertise is the scientific exploitation of airborne and spaceflight laser-altimetry data.
Jim Abshire is the senior scientist for laser sensing for the Solar System Exploration Division. He has helped lead the development of space lidar at NASA GSFC, and was instrument scientist on the Mars Observer Laser Altimeter and the Geoscience Laser Altimeter System on the ICESat mission. He received his BS in electrical engineering from the University of Tennessee, and his MS and PhD from the University of Maryland.
Xiaoli Sun received his MS and PhD in electrical engineering from the Johns Hopkins University. He is a member of the Laser Remote Sensing Laboratory at NASA GSFC where he has worked on the detectors and receiver modeling for various spaceborne lidars.