The Global Ecosystem Dynamics Investigation (GEDI) instrument was designed, built, and tested in-house at NASA’s Goddard Space Flight Center and launched to the International Space Station (ISS) on December 5, 2018. GEDI is a multibeam waveform LiDAR (light detection and ranging) designed to measure the Earth’s global tree height and canopy density using 8 laser beam ground tracks separated by roughly 600 meters. Given the ground coverage required and the 2 year mission duration, a unique optical design solution was developed. GEDI generates 8 ground sampling tracks from 3 transmitter systems viewed by a single receiver telescope, all while maximizing system optical efficiency and transmitter to receiver boresight alignment margin. The GEDI optical design, key optical components, and system level integration and testing are presented here. GEDI began 2 years of science operations in March 2019 and so far, it is meeting all of its key optical performance requirements and is returning outstanding science.
NASA’s Global Ecosystem Dynamics Investigation (GEDI) instrument was launched Dec. 5, 2018, and installed on the International Space Station 419 km from Earth. The GEDI is a Light Detection and Ranging (LIDAR) instrument; measuring the time of flight of transmitted laser beams to the Earth and back to determine altitude for geospatial mapping of forest canopy heights. The need for very dense cross track sampling for slope measurements of canopy height is accomplished by using three individual laser transmitter systems, where each beam is split into two beams by a birefringent crystal. Furthermore, one transmitter is equipped with a diffractive optical element splitting the two beams into four, for a total of 8 beams. The beams are reflected off of the features and imaged by an 800 mm diameter Receiver Telescope Assembly, composed of a Ritchey-Chrétien telescope, a refractive aft optics assembly and focal plane array which collects and focuses the light from the 8 probe beams into the 8 science fibers, each with a field of view on the Earth subtending 300 μrad. The dense cross-track sampling mandated a custom designed dual-fiber interface. The science fibers had to be aligned to the nominal, projected laser spots. The alignment was highly dependent on optimization and co-positioning of the fibers pair-wise due to mechanical constraints. This paper presents the end-to-end alignment and metrology of the full optical system from transmitter elements through receiver telescope, aft-optics, focal plane and receiver fibers performed at NASA Goddard Space Flight Center.
The sole instrument on NASA’s ICESat-2 spacecraft shown in Figure 1 will be the Advanced Topographic Laser Altimeter System (ATLAS)1. The ATLAS is a Light Detection and Ranging (LIDAR) instrument; it measures the time of flight of the six transmitted laser beams to the Earth and back to determine altitude for geospatial mapping of global ice. The ATLAS laser beam is split into 6 main beams by a Diffractive Optical Element (DOE) that are reflected off of the earth and imaged by an 800 mm diameter Receiver Telescope Assembly (RTA). The RTA is composed of a 2-mirror telescope and Aft Optics Assembly (AOA) that collects and focuses the light from the 6 probe beams into 6 science fibers. Each fiber optic has a field of view on the earth that subtends 83 micro Radians. The light collected by each fiber is detected by a photomultiplier and timing related to a master clock to determine time of flight and therefore distance. The collection of the light from the 6 laser spots projected to the ground allows for dense cross track sampling to provide for slope measurements of ice fields. NASA LIDAR instruments typically utilize telescopes that are not diffraction limited since they function as a light collector rather than imaging function. The more challenging requirements of the ATLAS instrument require better performance of the telescope at the ¼ wave level to provide for improved sampling and signal to noise. NASA Goddard Space Flight Center (GSFC) contracted the build of the telescope to General Dynamics (GD). GD fabricated and tested the flight and flight spare telescope and then integrated the government supplied AOA for testing of the RTA before and after vibration qualification. The RTA was then delivered to GSFC for independent verification and testing over expected thermal vacuum conditions. The testing at GSFC included a measurement of the RTA wavefront error and encircled energy in several orientations to determine the expected zero gravity figure, encircled energy, back focal length and plate scale. In addition, the science fibers had to be aligned to within 10 micro Radians of the projected laser spots to provide adequate margin for operations on-orbit. This paper summarizes the independent testing and alignment of the fibers performed at the GSFC.
The Advanced Topographic Laser Altimeter System (ATLAS) will be the only instrument on the Ice, Cloud, and Land
Elevation Satellite -2 (ICESat-2). ICESat-2 is the 2nd-generation of the orbiting laser altimeter ICESat, which will
continue polar ice topography measurements with improved precision laser-ranging techniques. In contrast to the
original ICESat design, ICESat-2 will use a micro-pulse, multi-beam approach that provides dense cross-track sampling
to help scientists determine a surface's slope with each pass of the satellite. The ATLAS laser will emit visible, green
laser pulses at a wavelength of 532 nm and a rate of 10 kHz and will be split into 6 beams. A set of six identical,
thermally tuned optical filter assemblies (OFA) will be used to remove background solar radiation from the collected
signal while transmitting the laser light to the detectors. A seventh assembly will be used to monitor the laser center
wavelength during the mission. In this paper, we present the design and optical performance measurements of the
ATLAS OFA in air and in vacuum prior to their integration on the ATLAS instrument.
We have developed and successfully flown a 16-beam, non-scanning laser altimeter instrument with a swath width of 80 m and spatial resolution of 5 m. The Airborne Lidar Surface Topography Simulator (ALISTS) instrument was developed to demonstrate key technologies and a measurement approach achieving the efficiency required for the Lidar Surface Topography (LIST) mission. The approach employs a 10 kHz, near-infrared, microchip laser transmitter, beam splitting optics and waveform capture using a photon-sensitive, linear-mode detector array. In this paper we will present the instrument development effort and access the performance achieved during our two airborne campaigns.
In 2008 we began a three-year NASA Earth Science Technology Office (ESTO) funded Instrument Incubator Program
(IIP) focused on technology development for the Lidar Surface Topography (LIST) mission. The LIST mission is one of
the Earth Science Decadal Survey missions recommended to NASA by the National Research Council (NRC). Our IIP
objective is to demonstrate the measurement approach and key technologies needed for a highly efficient swath mapping
lidar to meet the goals of the LIST mission. To demonstrate the concept we are developing the Airborne LIST Simulator
(A-LISTS) instrument. In this paper we summarize the A-LISTS instrument characteristics and the approaches we are
using to advance lidar capabilities and reduce risks for LIST.
In this paper we will discuss our development effort of an airborne instrument as a pathfinder for the LIdar Surface
Technology (LIST) mission. This paper will discuss the system approach, enabling technologies, instrument concept,
final assembly and the preparation for flight with this new multi-beam non-scanning, swath mapping laser altimeter
We discuss past, present and future spaceborne laser instruments for high-resolution mapping of Earth and planetary
surfaces. Previous spaceborne-laser-altimeters projected and imaged a single laser spot for surface-height
measurements. In contrast, the recent Lunar Orbiter Laser Altimeter (LOLA) instrument on the Lunar Reconnaissance
Orbiter (LRO) uses a non-scanning multi-beam system for surface topography mapping. The multi-beam instrument
facilitates surface slope measurement and reduces the time-to-completion for global high-resolution topographic
mapping. We discuss our first-year progress on a three-year swath-mapping laser-altimetry Instrument Incubator
Program (IIP) funded by the NASA Earth Science Technology Office (ESTO). Our IIP is a technology development
program supporting the LIdar Surface Topography (LIST) space-flight mission that is a third-tier mission as
recommended by the National Research Council (NRC) for NASA's Earth Science programs.
The design and construction of wide FOV imaging polarimeters for use
in atmospheric remote sensing requires significant attention to the
prevention of artificial polarization induced by the optical elements.
Surface, coatings, and angles of incidence throughout the system must
be carefully designed in order to minimize these artifacts because the
remaining instrumental bias polarization is the main factor which
drives the final polarimetric accuracy of the system. In this work, we
present a detailed evaluation and analysis to explore the possibility
of retrieving the initial polarization state of the light traveling
through a generic system that has inherent instrumental polarization.
Our case is a wide FOV lens and a splitter device. In particular, we
chose as splitter device a Philips-type prism, because it is able to
divide the signal in 3 independent channels that could be
simultaneously analyze to retrieve the three first elements of the
Stoke vector (in atmospheric applications the elliptical polarization
can be neglected ). The Philips-type configuration is a versatile,
compact and robust prism device that is typically used in three color
camera systems. It has been used in some commercial polarimetric
cameras which do not claim high accuracy polarization measurements
. With this work, we address the accuracy of our polarization
inversion and measurements made with the Philips-type beam divider.
NASA Goddard Space Flight Center (GSFC) has been engaging in Earth and planetary science instruments development
for many years. With stunning topographic details of the Mars surface to Earth's surface maps and ice sheets dynamics
of recent years, NASA GSFC has provided vast amount of scientific data products that gave detailed insights into
Earth's and planetary sciences. In this paper we will review the past and present of space-qualified laser programs at
GSFC and offer insights into future laser based science instrumentations.
We present the final configuration of the space flight laser transmitter as delivered to the LOLA instrument. The laser
consists of two oscillators with co-aligned outputs on a single bench, each capable of providing one billion plus shots.
The Lunar Orbiter Laser Altimeter (LOLA) instrument on NASA's Lunar Reconnaissance Orbiter (LRO) mission,
scheduled to launch in October 2008, will provide a precise global lunar topographic map using laser altimetry. LOLA
uses short pulses from a single laser through a Diffractive Optical Element (DOE) to produce a five-beam pattern that
illuminates the lunar surface. For each beam, LOLA measures the time of flight (range), pulse spreading (surface
roughness), and transmit/return energy (surface reflectance). LOLA will produce a high-resolution global topographic
model and global geodetic framework that enables precise targeting, safe landing, and surface mobility to carry out
exploratory activities. In addition, it will characterize the polar illumination environment, and image permanently
shadowed polar regions of the lunar surface to identify possible locations of surface ice crystals in shadowed polar
This paper presents the design, analysis, and testing of a diffractive optical element (DOE) to be part of the Lunar Orbiter Laser Altimeter (LOLA) instrument scheduled to launch in 2008. LOLA will be one of six instruments to orbit the Moon for a year or more as part of the Lunar Reconnaissance Orbiter (LRO). The various scientific instruments aboard the LRO will map the lunar environment in greater detail than ever before. LOLA will produce a topographic map of the Moon from a nominal 50km orbit during the one-year mission. LOLA works by bouncing laser pulses off the lunar surface as it orbits the Moon. By measuring the time it takes for light to travel to the surface and back, LOLA can calculate the roundtrip distance. Each pulse consists of five laser spots in a cross-like pattern spanning about 50 meters of the lunar surface. The spots are generated by a DOE from the single, collimated LOLA laser input beam. It is projected that LOLA will gather more than a billion measurements of the Moon's surface elevation creating a high resolution three-dimensional map of the surface.
For space-based lidar applications, conductively cooled lasers have been identified as a critical technology for high energy, 2-micron laser transmitter. Effective thermal management is a challenge for high-energy, 2-micon lasers. In this paper, the design of a totally conductively cooled, diode pumped, 2-micron laser amplifier is presented. Based on the successful testing of a conductively cooled oscillator, concepts for a laser amplifier were developed. The newly designed amplifier consists of a 40 mm long Ho:Tm: LuLF rod being pumped by 4 banks of 5-radially arranged diode lasers totaling 80W pump power. Optical and thermal studies for the amplifier head are presented and discussed. Currently, the design of the amplifier head is being integrated into a complete amplifier subsystem for a conductive cooled Master Oscillator Power Amplifier (MOPA) laser.
A lidar system is described that measures laser pulse time-offlight and the distortion of the pulse waveform for reflection from Earth surface terrain features. This instrument system is mounted on a highaltitude aircraft platform and operated in a repetitively pulsed mode for measurements of surface elevation profiles. The laser transmitter makes
use of recently developed short-pulse diode-pumped solid-state laser technology. Aircraft position in three dimensions is measured to submeter accuracy by use of differential Global Positioning System receivers. Instrument construction and performance are detailed.