ISS Utilization: GEDI (Global Ecosystems Dynamics Investigation Lidar)
GEDI is NASA's selection of a laser-based instrument that will provide a unique 3D view of Earth's forests, helping to fill in missing information about their role in the carbon cycle. The system is one of two instrument proposals selected in 2014 for NASA's EVI (Earth Venture Instrument) program and is being led by the University of Maryland, College Park (PI: Ralph Dubayah). The instrument will be built at NASA's Goddard Space Flight Center in Greenbelt, Maryland. 1)
The overall objective of GEDI is to characterize the effects of changing climate and land use on ecosystem structure and dynamics to enable radically improved quantification and understanding of the Earth's carbon cycle and biodiversity. Focused on tropical and temperate forests from its vantage point on the ISS, GEDI uses lidar to provide the first global, high-resolution observations of forest vertical structure. GEDI addresses three, core science questions: 2) 3)
1) What is the above-ground carbon balance of the land surface?
2) What role will the land surface play in mitigating atmospheric CO2 in the coming decades?
3) How does ecosystem structure affect habitat quality and biodiversity?
Answering these questions is critical for understanding the future path of global climate change and the Earth's biodiversity.
GEDI has four science objectives that address its core questions:
• Quantify the distribution of above-ground carbon at fine spatial resolution
• Quantify the changes in carbon resulting from disturbance and subsequent recovery
• Quantify the spatial and temporal distribution of forest structure and its relationship to habitat quality and biodiversity
• Quantify the sequestration potential of forests through time under changing land use and climate.
GEDI strongly supports the Science Focus Areas articulated in the Science Plan for NASA's Science Mission Directorate, particularly for Carbon Cycle and Ecosystems. GEDI science goals respond directly to Decadal Survey observational priorities, which emphasize the need for lidar vertical structure measurements to address key challenges in carbon cycling and biodiversity. GEDI's structure measurements also substantially enhance existing and planned NASA missions. GEDI's high biomass observations may be combined with the NASA-ISRO low biomass measurements and ICESat-2's boreal observations to produce global maps of biomass and provide important constraints and validation for regional scale estimates from OCO-2. GEDI further complements existing and planned pol-InSAR missions, such as DLR's TanDEM-X, JAXA's ALOS-2 and ESA's BIOMASS through its contribution of precisely geolocated canopy profile measurements for fusion with radar observations. Lastly, GEDI revolutionizes the previous forty years of the NASA land remote sensing archive by providing the missing vertical structure information, greatly extending land cover observations from NASA's Landsat and MODIS programs.
The GEDI Lidar employs 3 laser transmitters, beam-splitting optics, and an optical dithering device to produce 14 ground tracks spaced 500 m across-track (total swath width of 6.5 km). The transmitted pulse shape and returned waveform is recorded for each 25 m diameter footprint. The telescope, laser power, and detector sensitivity are designed to provide vegetation canopy penetration in even the densest forests. 4) 5)
In each year of operations, GEDI will fire 16 billion laser pulses at the Earth's surface between ± 51.6 º latitude (the inclination of the ISS orbit) and provide multiple passes through each 500 m cell on the surface. Approximately 50% of those footprints will return sufficient energy to provide high-fidelity return signal waveforms with a high SNR (Signal to Noise Ratio), while the remaining 50% will be degraded by or lost to cloud cover.
All of the required technologies are mature and flight-ready. The laser transmitters were developed in-house at NASA Goddard and have been tested to the equivalent of 6 years of operational lifetime without any damage and only minimal power degradation. GEDI will be mounted on the JEM-EF on ISS and will utilize an active pointing control system to systematically layout its ground tracks to achieve high-resolution coverage of the surface. GPS, star trackers, and an inertial measurement unit will provide precise position and pointing knowledge to achieve precise geolocation of each and every laser footprint on the surface.
Figure 1: Overview of Earth science instruments on the ISS (installed or planned) in the second decade of the 21st century (image credit: NASA) 6)
Launch: The launch of the GEDI instrument is planned for 2018 in a SpaceX Dragon Capsule.
Orbit: Near-circular orbit, altitude of ~ 400 km, inclination = 51.6º, period of ~ 92 minutes.
Figure 2: GEDI will be accommodated at the JEM-EF (Japanese Experiment Module-External Facility), image credit: NASA
GEDI Lidar Instrument:
The scientific goal of the GEDI lidar instrument is to characterize the effects of changing climate and land use on ecosystem structure and dynamics to enable radically improved quantification and understanding of the Earth's carbon cycle and biodiversity. Focused on tropical and temperate forests from its vantage point on the ISS (International Space Station), GEDI uses lidar to provide the first global, high-resolution observations of forest vertical structure. GEDI addresses three, core science questions: (Q1) What is the aboveground carbon balance of the land surface? (Q2) What role will the land surface play in mitigating atmospheric carbon dioxide in the coming decades? (Q3) How does ecosystem structure affect habitat quality and biodiversity? Answering these questions is critical for understanding the future path of global climate change and the Earth's biodiversity.
NASA/GSFC is building the GEDI lidar instrument. GEDI will use 3 NASA-developed laser transmitters to produce 12 parallel tracks of 25 m footprints on the Earth's surface, via an active optical system, necessary for accurate global biomass assessment. The Lasers and Electro-Optics Branch at GSFC (Goddard Space Flight Center) has been tasked with building these units as well as a dedicated unit for life testing, and taking them all through environmental qualification. 7) 8)
The fundamental architecture of the GEDI laser makes use of the HOMER (High Output Maximum Efficiency Resonator) class laser system. The HOMER laser has been developed for over a decade; designed to reduce part count and maximize reliability for space flight applications. The system has evolved from an Armandillo et. al design since its first bread-boarded results in 1997.9) 10)
This laser architecture has largely remained unchanged through many years of efforts to achieve this high level of reliability and long demonstrated life. Mechanically, the laser has evolved and tested continually through several improvements, both at the component and system levels. All the cavity components for the HOMER design have been tested to the TRL (Technology Readiness Level) 6 level, or have flown on previous missions (TRL9). HOMER's success was leveraged for the GEDI program and has since been modified again to achieve the required laser output performance to achieve the science mission goals. 11) 12)
Figure 3: GEDI Laser cross section and interface overview. This shows the orientation for assembly prior to installation aboard GEDI (image credit: NASA)
The GEDI laser overview and interface summary can be seen in Figure 3. The GEDI laser is primarily responsible for providing 10 mJ Q-switched laser pulses at 1064 nm. The oscillator-only laser consists of the optical bench and pump module, discussed later, and is kept in a pressurized laser enclosure. Inside the enclosure will be the high voltage Q-switch electronics, a start pulse optical pick-off, pressure and temperature sensors, and internal harnessing. Electrically, the laser interfaces with the LEU (Laser Electronics Unit), which provides the Q-switch voltage and trigger along with laser diode drive current, and receives the temperature and pressure level telemetry. A liquid-based chiller plate and TEC (Thermal Electric Cooler) will hold the diodes at 35ºC, and mounted to the housing exterior. Optically, there is a monolithic mini-beam expander to provide the output divergence needed for the mission. Finally, the 3 titanium flexures mount to the GEDI optical bench. The JEM module and GEDI instrument can be seen in Figure 4.
Figure 4: The GEDI lasers will be integrated onto the GEDI Instrument, seen on the right. This will be delivered to the JEM on the ISS module, seem on the left (image credit: NASA)
Table 1 summarizes the driving requirements for the GEDI laser design. The GEDI project has been very careful to flow down requirements to the laser team that maximizes science with achievable hardware goals from an engineering perspective. These requirements along with a list of driving test parameters will be trended through the build of the ETU (Engineering Test Unit) and flight units.
Table 1: GEDI laser driving requirements
Figure 5: Schematic view of the GEDI instrument and its elements (image credit: NASA)
The HOMER class design used for the GEDI laser is a flight quality Nd:YAG, diode-pumped solid state oscillator-only cavity producing beam qualities and pulse energies typically associated with MOPA (Master Oscillator Power Amplifier) systems. The HOMER lasers achieve this with high efficiency, low part count, and a demonstrated long life, running continuously for over 2 years (>16 billion pulses). This laser employs a side-pumped zigzag slab in a stable cavity which can make it difficult to achieve a TEM00 mode without the use of passive beam shaping. The use of porro-prisms and/or intra-cavity apertures can be used to produce quality single mode beams. However, these items have proven to generate intense optical diffraction effects and reduce overall optical damage resistance. These induced optical perturbations reduce the effective, average optical damage threshold of all the cavity's components by introducing optical spiking, temporal and spatial, and constructive interference effects within the optical materials and coated surfaces. This hinders the total reliability, introduces intensity jitter, and ultimately degrades the performance of the final laser system. If one is successful with the implementation of these methods in producing damage free operation TEM00 in such a laser, the optical efficiency is always reduced and the many of the zigzag slab advantages are lost. Alternatively, with a Graded Reflectivity Mirror GRM-based cavity design, in concert with an optimally pumped zigzag slab, these issues are greatly reduced and the finer details of safe cavity operation can be studied. 13)
Figure 6: The GEDI Laser optical cavity layout featuring a 22 bounce slab and a GRM can be seen at the top. This is implemented in the laser optical bench (in blue) and the pump module seen below the layout (image credit: NASA)
Using a GRM (Graded Reflectivity Mirror) as the output coupler, TEM00 pulse energies are readily obtainable from a >10 mJ oscillator-only design and are suddenly highly scalable because a relatively large intracavity low-fluence beam is created as the system's beam quality foundation. The final optical design makes use of a 40 cm cavity length. It employs a positive (concave) radius of curvature (ROC), 99.9% HR mirror along with the negative (convex) 237 cm ROC GRM using an effective average reflectivity and peak reflectivity of approximately 32% and 65%, respectively. To produce a gain-switched laser pulse of 1064 nm, an active Q-switch is employed, comprised of an electro-optic (EO) pockel's cell, and a pair of passive polarizing optics; a quarter wave plate and TFP (Thin Film Polarizer). As part of the GEDI program, all delivered optics that have coatings will have witness samples go through LIDT (Laser Induced Damaged Testing). Using the LIDT values along with models and verified experimental results over the past decade allow the laser team to know what energy level is safe for the laser to run at for a specific configuration. The optical layout is seen in Figure 6.
Laser gain pumping scheme:
HOMER's gain module, or laser head, employs 7 stacks of 4-bar 809 nm LDAs which pump a 22-bounce zigzag slab of 1.1% doped Nd:YAG. These highly astigmatic beams from these 28 bars are concentrated with the use of a long, planoconvex cylindrical lens made of undoped:YAG. This produces a highly concentrated gain region of similar dimension across the Nd:YAG's cross section, normal to the cavity optic axis, from which its stored energy is efficiently extracted by the resonant cavity mode. In fact because the system is so efficient the diode pump is significantly derated giving the laser system plenty of margin to recover from any unexpected laser loss. The diodes being employed can run at least as high as 100 A and 200 µs. Comparatively, when the laser is configured to run with an output energy of 10 mJ the diodes are pumped at 50 A and as little as 40 µs. When assembling the diode pump module, the distance between the pump lens to the diode faces is closely monitored, as placing them in regions where small absorption peaks along the pump surface has been proven to produce and trigger optical damage from microlensing effects (Figure 7).
Figure 7: The diodes are mounted to the aluminum pedestal, which thermally mounts to the enclosure floor. The laser slab is mounted to the W:Cu heat sink and to the aluminum bridge. The pump lens lies between the slab and diodes with high precision placement. They are all combined to make the full assembly, seen in the upper left (image credit: NASA)
The GEDI laser mechanical design allows for subcomponent modularity and is divided into 3 major sections: the enclosure, the pump module, and the optical bench. All three sections are primarily made of aluminum with the exceptions of some of the optical mounts and the titanium flexures interfacing with the instrument optical bench. Each are have design requirements principally driven by maintaining GEDI bore sight over the environmental conditions through the life time of the mission. Finite element modeling has been performed on the entire design and was found to meet all the bore sight jitter and alignment requirements. Further the laser enclosure is required to keep the laser pressurized at least 1 atm over the life time of the mission. An identically made enclosure will be made to qualify the pressure seal design. The mechanical design requirements will be confirmed with the ETU during environmental qualification testing as well.
This modular hardware design, levied by the laser team, allows for frequent optical inspections throughout the assembly and alignment process. Any issues that may occur over the laser builds can be quickly resolved with minimum invasive techniques, and without the need for cavity realignment. Furthermore, the optical bench, shown in Figure 6, can be removed from the pump module, Figure 7, fully aligned and operated without the enclosure if necessary. Therefore, both can be manufactured, tested, and characterized independently and in parallel. This nonlinear manufacture capability helps to reduce schedule and cost. The full aluminum assembly, sans the Titanium optical mounts, will be a little more than 9 kg in mass, shown in Figure 5.
Figure 8: The full GEDI laser assembly is seen above. Note; the laser will be built in this orientation, sealed, pressurized, and then inverted 1800 when it is mounted on the GEDI optical bench. This allows for easy access to the laser head thermal interface with which to mount the JEM thermal plates and TECs (image credit: NASA)
The GEDI laser design requires active conductive cooling in order to maintain the desired Nd:YAG absorption pump wavelength from the LDAs (Laser Diode Arrays). Additionally, stable operating temperature must be maintained in order to guarantee the required turn-on time while in orbit. The JEM-provided liquid cooling loop requires additional active control since the fluid's temperature arriving at each laser experiences a 6ºC temperature swing, depending on the other heat loads in its path. Therefore, a pair of TECs (ThermoElectric Coolers) will reside between the laser base and the JEM liquid thermal plate. This will keep the pedestal and LDAs within the 350 (±2)ºC when the laser if off or in standby mode. Otherwise, the optical bench and enclosure are designed such that internal thermal stresses will not affect laser performance. The laser system will be tested over a full survival range of 50ºC to -10ºC during thermal vacuum testing.
Laser performance summary:
Each GEDI laser fires at 242 Hz and produces 11-16 ns Q-switched pulses at 10-17 mJ, with a TEM00 beam quality at 1064 nm. This mission needs a reliable laser that will last a minimum of 3.2 billion shots (33% duty cycle on average per orbit) over the lifetime of the mission. We recently published a complete lifetime dataset where we ran our HOMER-2 laser for over 15 x 109 shots. This was the equivalent of 2 years of continuous operation with a measured decay rate of only ~ 0.1 mJ/Billion. Therefore, this laser design provides plenty of life-time margin for the GEDI mission.
This life test demonstrated the advantages of an oscillator-only design, with a specific set of requirements, over an equivalent MOPA system when producing TEM00 laser pulses under 20 mJ. Even with the extensive development history, qualification, and data sets of the HOMER laser design, the GEDI mission laser performance requirements required that design be modified slightly from the HOMER-2 TRL6 system. These modifications have since been implemented in the HOMER-1 engineering unit which readily achieved 3.2 billion shots (the mission requirement) at the necessary energy output levels in a recent long term run. Pre-, and post-run inspections show damage-free operation at these parameters, further demonstrating the validity of the HOMER oscillator concept. Any remaining laser performance requirements were tested on the GEDI laser breadboard system. It is to be noted that as part of this testing, known damage triggers for the laser system, including longitudinal model beating and small scale self focusing, are monitored continuously through out the testing process. Finally, the highest risk optics, determined from laser induced damage testing and high precision mathematical models, (the GRM and laser slab), are inspected for optical damage after each major experiment or any step change in operation. A summary of the laser performance can be seen in Figure 9.
Figure 9: A summary of the GEDI laser performance while configured to run with 10 mJ output energy. Note; these values are without use of the mini-beam expander (image credit: NASA)
Laser build and test plan overview:
The laser team has been tasked with delivering an ETU (Engineering Test Unit) laser, three flight units, and a flight spare. The GEDI ETU laser hardware has been fabricated and has started assembly. The ETU and flight lasers will go through the same assembly and test processes with the general exception that "lessons learned" during the ETU build will be carefully documented, evaluated and implemented on the flight builds. Both ETU and flight units will use custom designed electrical and optical GSE (Ground Support Equipment). The electrical GSE will have the role of simulating the LEU along with taking all of the required data needed to record laser performance. The comprehensive performance test will be run in this configuration before and after each phase of environmental testing. The optical GSE seen in Figure 7.
The GEDI Laser units will run vibration testing as spelled out in the GEDI Structural/Mechanical Loads Document. The lasers will be tested unpowered in the X, Y and, Z axis. The laser will be performance tested in between vibration runs primarily looking for any change in laser beam quality and any movement on the alignment bore sight.
The ETU and Delivered flight units will carry out TVAC (Thermal Vacuum ) tests as spelled out in the GEDI Thermal Interface Control Document. At this time each flight laser will go through 8 survival cycles. Survival temperatures range from 50 to -10 ºC. During each cycle the laser will pause at the operating temperature ranges to check powered on laser performance. The TVAC test will also serve as another confirmation of the pressurized cavity design.
Once the environmental testing is completed the ETU will start life testing well before the delivery of the flight units. During the testing monitoring laser requirements will be monitored continuously and it will engineering unit interface testing with other hardware such as the BDU (Beam Dithering Unit). The flight units will be delivered straight for integration with the GEDI instrument and then will support instrument level performance and environmental as required.
Figure 10: The optical GSE will be used for the BDU and Laser ETU life testing, along with the ETU laser CPT testing. A similary configuration will be used to test the flight lasers (image credit: NASA)
In summary, the GEDI Instrument installed on the ISS JEM platform will produce a valuable 3D, multi-seasonal, global biomass data set. The GEDI mission is employing a robust HOMER class laser, which has been thoroughly tested over several different models, and meeting key performance parameters. The GEDI laser has large overhead in LDA derating, well known and measureable damage triggers, and quantified margins in thermal and optical space. Sound and proven development processes, qualification plans, and infrastructure hardware is in place or under assembly at this time as the project readies for the completion of the ETU unit, and prepares for the arrival of flight hardware soon after. Finally, based on HOMER's extensive data set, and the GEDI laser's solid foundation and relative performance adjustments, the riskspace for each laser is approximately equal, and thus the project expects a similar life time capability should the GEDI mission be extended.
Figure 11: GEDI laser track coverage (image credit: NASA)
GEDI: Using Lidar Waveforms to Measure Canopy Structure 14)
GEDI will address its mission science questions by making lidar waveform (i.e., vertical profile) observations between 51.6° N and S latitudes. Each GEDI laser shot will result in a waveform that contains information about the vegetation canopy and the topography underneath. Scientists will use this information to quantify canopy vertical structure, canopy height, and ground elevation (Table 2).
The GEDI instrument includes three identical laser transmitters. One will be used at full power, while the output from the other two will be split into two beams each, creating a total of five beams. Each of the beams will be optically dithered across-track to create a total of ten ground tracks, with between-track spacing of ~600 m, and a total across-track width of 5.4 km. The average along-track distance between the footprints will be 60 m). The across-track dithering is fully programmable on orbit so that any pattern of along-track contiguous footprint patterns are possible. By the time the beam reaches the surface it has an approximate diameter (footprint) of 22 m —as illustrated below.
Note: Dithering of the beam creates two parallel ground tracks for each beam instead of one, which increases across-track coverage.
Figure 12: Shown here is a sample GEDI lidar waveform [left]. The light brown area under the curve represents return energy from the canopy, while the dark brown area signifies the return from the underlying topography. The black line is the cumulative return energy, starting from the bottom of the ground return (normalized to 0) to the top of the canopy (normalized to 1). Blue horizontal lines are the RH (Relative Height) metrics, which give the height at which a certain quantile returned energy is reached. The schematic [right] shows incident near-infrared laser beam from GEDI interacting with a canopy (image credit: Ralph Dubayah, University of Maryland, College Park)
Table 2: List of planned GEDI data products
Legend to Table 2: * Several Level-4 "demonstration" products are planned to be produced later in the mission; they will have various gridded sizes that will be specified at a later date.
GEDI data enable important opportunities for vegetation studies other than biomass estimation, by producing example products for specified regions globally.
Figure 13: This diagram represents the relationship between GEDI data products and the GEDI science questions (image credit: Ralph Dubayah, University of Maryland)
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4) J. Bryan Blair, Ralph Dubayah, "Global Ecosystem Dynamics Investigation (GEDI) Lidar for mapping the vertical structure of vegetation from the International Space Station," Proceedings of the IGARSS (International Geoscience and Remote Sensing Symposium) 2015, Milan, Italy, July 26-31, 2015
5) Ralph Dubayah, "Remote Sensing of Ecosystem Structure and Dynamics - GEDI Lidar," URL: http://kiss.caltech.edu/study/ecosystem/Presentation
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8) D. Barry Coyle, Paul R. Stysley, Demetrios Poulios, Greg B. Clarke, Richard B. Kay, "Laser transmitter development for NASA's Global Ecosystem Dynamics Investigation (GEDI) lidar ," Proceedings of SPIE, Vol. 9612, 'Lidar Remote Sensing for Environmental Monitoring XV,' 961208 (September 1, 2015), doi:10.1117/12.2191569
9) Donald B. Coyle, R. B. Kay, S. J. Lindauer, "Design and performance of the vegetation canopy Lidar (VCL) laser transmitter," IEEE Aerospace Conference Proceedings 3,1457-1464 (2002)
10) Errico Armandillo, Callum Norrie, Alberto Cosentino, Paolo Laporta, Paul Wazen, Patrick Maine, "Diode-pumped high-efficiency high-brightness Q-switched ND:YAG slab laser," Optics,Letters, Vol. 22, Issue 15,pp: 1168-1170, 1997, doi: 10.1364/OL.22.001168
11) Donald B. Coyle, Richard B. Kay, Paul R. Stysley, Demetrios Poulios, "Efficient, reliable, long-lifetime, diode-pumped Nd:YAG laser for space-based vegetation topographical altimetry," Applied Optics, Vol. 43, Issue 27, pp: 5236-5242, 2004
12) Paul R. Stysley, D. Barry Coyle, Richard B. Kay, Robert Frederickson, Demetrios Poulios, Ken Cory, Greg Clarke, "Long term performance of the High Output Maximum Efficiency Resonator (HOMER) laser for NASA's Global Ecosystem Dynamics Investigation (GEDI) lidar," Optics & Laser Technology, Vol. 68, pp: 67-72, 2015
13) M. Morin, "Graded Reflectivity Mirror Unstable Laser Resonators," Optical and Quantum Electronics,August 1997, Volume 29, Issue 8, pp: 819-866
14) Suzanne Marselis, John Armston, Ralph Dubayah, "Summary of the Second GEDI Science Team Meeting," The Earth Observer,Volume 28, Issue 6, pp: 31-36, November-December 2016, URL: https://eospso.nasa.gov/sites/default/f
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 (email@example.com).