Accurate measurements of above ground biomass are important to evaluate its contribution as a CO2 absorption source. If global canopy height model can be obtained, we can make an appropriate evaluation. This is MOLI’s mission. Moreover, MOLI can improve the accuracy of digital terrain model. In this paper, we will report on the development status of MOLI for LIDAR observation from space by mounting on the Japanese Experiment Module with International Space Station.
Accurate measurements of forest biomass are important to evaluate its contribution as a source of CO2 absorption. Forest biomass correlates with forest canopy height, and thus global measurements of canopy heights lead to a better understanding of the global carbon cycle. Space-borne lidar has the unique capability of measuring forest canopy height. A vegetation lidar named MOLI (Multi-footprint Observation Lidar and Imager) has been designed to observe canopy heights more accurately, and MOLI is currently being studied in the Japan Aerospace Exploration Agency (JAXA). This paper introduces an overview of MOLI and its current status.
Observation of the Earth’s environment from space is important for resolving issues resulting from global climate change. Increasing anthropogenic carbon dioxide (CO2) and methane are species to climate change. To estimate accurately sinks and sources of CO2 in the biosphere, measurement uncertainties of the column averaged dry air mole fraction of CO2 (XCO2) are expected to be 1-3 ppm (0.3%-1%)[1]. The Greenhouse Gases Observing Satellite (GOSAT) and the Orbiting Carbon Observatory (OCO-2) have proceeded to reveal the global carbon exchange [2, 3]. The GOSAT sensor observes trace gases using a passive remote sensing technique. However, passive techniques using solar light limits observation because 1) the total column CO2 can only be evaluated only during the daytime, 2) solar seasonal dependence reduces global coverage, such as the northern hemisphere in winter, 3) unknowns and variations in broken clouds and aerosol contamination also cause bias errors. To resolve these issues, active remote sensors, such as a differential absorption lidar (DIAL) or a laser absorption spectrometer (LAS), are valuable tools for future trace gas sensing from space as they involve no seasonal dependence, can mitigate the impact of broken clouds and aerosol, and can evaluate XCO2 all day. According to earlier studies [4-8], a precision of up to 0.7 percent has already been achieved. This study aims at demonstrating a sensitivity analysis for a space-borne system and the results of the airborne test to evaluate column-averaged CO2.
Accurate measurements of forest biomass are important to evaluate its contribution to the global carbon cycle. Forest biomass correlates with forest canopy height; therefore, global measurements of canopy height enable a more precise understanding of the global carbon cycle. A vegetation lidar named “MOLI” which is designed to measure accurate canopy height has been studied by the Japan Aerospace Exploration Agency (JAXA) in cooperation with some researchers. MOLI stands for Multi-footprint Observation Lidar and Imager.
The feature of MOLI is to set multi-footprints for improving the precision of canopy height, and we can find out whether ground surface is flat or slope because an angle of inclination affects the estimation of canopy height.
MOLI is going to be mounted on the Exposed Facility (EF) of the Japanese Experiment Module (JEM, also known as “Kibo”) on the International Space Station (ISS). Now, we are carrying out a feasibility study and some experiments. We introduce an overview and a status of MOLI.
JAXA developed the ground test model of DIAL, Differential absorption Lidar, to measure the quantities of the carbon dioxide for the calibration and the validation of the data acquired by the one instrument, TANSO-FTS, aboard on the GOSAT, Greenhouse gases observing satellite. FTS is the Fourier Transform Spectrometer. In addition to using for the calibration and the validation, this DIAL system has the purpose to take the data for the study of the space-borne DIAL. Our CO2 DIAL system adopted the 1.6 micron CW laser, incoherent detection and all fiber optical circuit. The transmitted on-line and off-line signals are coaxial and have the same field of view and the same time oscillation. And the transmitted laser is modulated doubly, intensity modulation by micro wave and phase modulation. This double modulation is adopted to detect the distance between the DIAL system and the target. JAXA is now performing the test of this DIAL to confirm the accuracy of the measurement of the carbon dioxide. This ground test model can be aboard on an airplane, therefore JAXA is planning the test using an airborne as a part of the test of the ground test model. In addition the comparison with the other CO2 DIAL systems is under consideration. Now JAXA does not have the plan to develop the space-borne LIDAR, however the space-borne LIDAR system has been under study recently, therefore JAXA intends to take the data which will be reflected in the design of the space-borne CO2 DIAL system through this test of the ground test model of DIAL.
Up-link Laser Differential Absorption Sensing: ULDAS, shown in Fig.1, is a new method to measure green house gas concentration with earth observation satellites. Although the measurement area is restricted in only small visible area of an optical ground station, ULDAS has outstanding features as followed:
- Faster: Easy to development, small size and small resource requirements to satellite system
- Better: High accuracy (CO2 observation error of weighted column is <0.3% which corresponds to 1ppm error of atmospheric concentration)
- Cheaper: Simple system, small number of parts and no special parts
The flight segment of the ULDAS is able to be loaded on a marginal resource of green house effect observation satellites, such as Japanese GOSAT-series. In this paper, the feasibility study of the mission concept and field experiments are reported.
Multi-footprint Observation LIDAR and Imager (MOLI) is a candidate mission for International Space Station – Japanese Experiment Module. The mission objective MOLI is to manage forest and to be a good calibrator for evaluation of forest biomass using satellite instrument such as L-band SAR. SAR is the powerful tool to evaluate biomass globally. However it has some signal saturation over 100 t/ha biomass measurement, whereas Vegetation LIDAR is expected to measure higher mass precisely. MOLI is designed to evaluate forest biomass with high accuracy. An imager, that is equipped together in good registration with LIDAR, will help to understand the situation of target forest. Also two simultaneous Laser beams from MOLI will calibrate the relief effect, which affects the precision of canopy height extremely. Using together with L-band SAR observation data or multispectral image, it is expected to have a good “wall to wall” biomass map with its phonological information. Such MOLI observation capability is so important, because both quantity and quality evaluation of biomass are essential for carbon circulation system understandings. Currently, as a key technical development, LASER Transmitters for MOLI is under test in vacuum condition. Its power is 40mJ and PRF is 150Hz. Pressure vessel design for LIDAR transmitter is supressing Laser induced contamination effect. MOLI is now under study towards around 2020 operation.
Accurate measurements of forest biomass are important to evaluate its contribution to the global carbon cycle. Forest
biomass correlates with forest canopy height; therefore, global measurements of canopy height enable a more precise
understanding of the global carbon cycle. Space-borne lidar has the unique capability of measuring forest canopy height.
A vegetation lidar named Multi-footprint Observation Lidar and Imager (MOLI) has been designed to make accurate
measurements of canopy height and is currently being studied in the Japan Aerospace Exploration Agency. This papers
introduces an overview of MOLI and its current status.
IPCC Fifth Assessment Report says that there are still large uncertainties of carbon flux estimations in the interaction between ground and atmosphere. That is because of the uncertainties of “change of land use”, in other words, “change of biomass” such as deforestation. Biomass estimation needs not only area of the forest but also its height information with topological features. In that sense, active sensors are highly expected for precise height measurement. Laser Altimeter or simply LIDAR is able to measure the height of dense forest, where SAR has salutation. ICESat / GLAS is firstly used to measure biomass as satellite LIDAR. However it was reported that there is uncertainty where terrain relief exists. To calibrate terrain relief using multi footprints, a Vegetation LIDAR named MOLI (Multi Observation LIDAR and Imager) was studied by JAXA. The unique points of MOLI are the dual beams with enough small and close footprints to determine terrain relief. Full wave analysis technique is also under development to distinguish canopy heights, crown depth and other forest features. Co-aligned imager will be used for determination of positions where LIDAR measured and observation of phonology. MOLI system design is about to finalize. Regarding Laser Transmitter, Bread Board Model with pressure vessel is being tested under vacuum condition. Target launch year of MOLI is around 2019.
Vegetation LIDAR, which measures an accurate canopy height, has been studied by JAXA. Canopy height is a very important parameter to estimate forest biomass, and global measurement of accurate canopy height leads to better understanding of the global carbon cycle. The vegetation LiDAR is designed based on the assumption that it is to be mounted on the Exposed Facility (EF) of the Japanese Experiment Module (JEM, also known as “Kibo”) on the International Space Station (ISS). The vegetation LIDAR uses an array detector (2x2) for dividing the ground footprint, making it possible to detect the slope of the ground for improving the accuracy of canopy height measurement. However, dividing the footprint may cause a reduction in reflected lights and signal-to-noise ratio (SNR); hence, the vegetation LiDAR system needs high sensitivity and low-noise array detector module. We made a prototype of the array detector module and it satisfied the tentative target SNR which we set. This presentation will introduce the mission objectives, the LiDAR system including experimental prototypes of array detector module, and some results of the study.
Laser remote sensing technologies are valuable for a variety of scientific requirements. These measurement techniques
are involved in several earth science areas, including atmospheric chemistry, aerosols and clouds, wind speed and
directions, prediction of pollution, oceanic mixed layer depth, vegetation canopy height (biomass), ice sheet, surface
topography, and others. Much of these measurements have been performed from the ground to aircraft over the past
decades. To improve knowledge of these science areas with transport models (e.g. AGCM), further advances of vertical
profile are required.
JAXA collaborated with NICT and RIKEN started a new cross-sectional 3-year program to improve a technology
readiness of the critical 1-micron wavelengths from 2011. The efficient frequency conversions such as second and third
harmonic generation and optical parametric oscillation/generation are applied. A variety of elements are common issues
to lidar instruments, which includes heat rejection using high thermal conductivity materials, laser diode life time and
reliability, wavelength control, and suppression of contamination control. And the program has invested in several
critical areas including advanced laser transmitter technologies to enable science measurements and improvement of
knowledge for space-based laser diode arrays, Pockels cells, advanced nonlinear wavelength conversion technology for
space-based LIDIRs. Final goal is aim to realize 15 watt class Q-switched pulse laser over 3-year lifetime.
Continuous validation of data observed by satellites, such as Greenhouse gases Observing SATellite (GOSAT), is
important to qualify the long-term trends of greenhouse gases. High-precision data over the restricted region measured
by ground-based high-resolution Fourier transform spectrometers (g-b FTS), airborne in-situ instruments, and flask
sampling devices have been used for the validation of satellite data. As part of CAL/VAL (Calibration/Validation)
activities of the GOSAT, airborne flight campaigns were performed over Tsukuba and Moshiri using the ground-based
FTS, airborne in-situ and flask devices, and 1.57-μm Laser Absorption Sensor (LAS). Airborne flask sampling and insitu
carbon dioxide (CO2) sensors were carried out to obtain vertical profiles of the CO2 mixing ratio while ground-based
FTS and LAS measured solar direct spectra and weighted column-averaged CO2, respectively. Those results were used
to decide a calibration factor of the ground-based FTS and compared with GOSAT products over Tsukuba. We will
report the comparison results of the aircraft campaign measurements and the retrieval value from the FTS.
A 1.57-μm laser remote sensor using differential absorption spectrometory is being developed as a candidate for the next
space-based mission to observe atmospheric CO2 and/or other trace gases. In a previous study, the performance of a
proto-type system with sinusoidal modulation was evaluated based on ground and airborne measurements. The airborne
measurements showed that the LAS with sinusoidal modulation could detect strong CO2 plume, and suppress the impact
of an aerosol layer over high surface reflectivity. Based on those results, an outline of LAS system on the space platform
such as the International Space Station Japan Experimental Module (ISS-JEM) was desined. However, an elevated layer
in the observation path is still remain, which leads to reduce effective observed data as long as current sinusoidal
modulation is employed. In order to prevent the impact of elevated layer, different modulation schemes such as random
or frequency modulation are capable. We are currently improving the LAS system with a chirp modulation scheme for
the purpose. Some of recent airborne measurements using sinusoidal modulation and ground-based measurements using
chirp modulation in progress will be shown in this meeting.
For the application to the global CO2 monitoring from the space-borne active sensor have been studied. We have
developed the Laser Absorption Sensor (LAS) system for ground-based CO2 monitoring using the wavelength of 1.6
micron. Furthermore, we have also reported about measurement result with short time fluctuation corresponding to the
concentration of 4 ppm (rms) in 32 s intervals and 1 km path. In this paper, we discuss how to achieve this performance.
A 1.57-µm laser remote sensor using differential absorption spectrometry is being developed as a candidate for the next space-based mission to observe atmospheric CO2 and/or other trace gases. The performance of the newly-developed active remote sensor has been evaluated for horizontal measurements and initial vertical measurements have been demonstrated. This study shows the results of in-house and field measurements to evaluate column-averaged CO2 mixing ratios. The in-house measurements demonstrated the instrumental response showing agreement within a correlation coefficient of 0.998 for a known CO2 density. Field measurements to evaluate horizontal and vertical column-averaged CO2 mixing ratio were made with a measured precision of 0.49% and 1.7%, respectively. The horizontal integration range was 2.1 km and the vertical range extended from the surface up to the cloud base at ~3 km with corresponding accumulation time of 25 min. Complementary measurements with a multi-positioned in-situ sensor along the observation path demonstrated that the mean horizontal column-averaged CO2 density agreed within the difference of 2.8 ppm of the atmospheric CO2 density.
The Green-house gas Observation SATellite (GOSAT) was launched to determine the continental CO2 inventories. Its
sensor is based on a passive remote sensing technique developed to achieve less than 1% relative accuracy for
atmospheric CO2 measurements. Meanwhile, a laser remote sensor with the differential absorption spectrometry has been
developed for a candidate of a future space-based mission to observe the atmospheric CO2 or other trace gases. A
prototype of the newly developed active remote sensor has been performed to demonstrate a properly validated
performance for ground-based and airborne systems. This study shows the results of the in-house and field
measurements. The in-house measurement demonstrated the linearity with the correlation coefficient of over 0.99
between the instrumental response and the known CO2 density in the cell. The diurnal variation obtained from our
system is consistent (correlation coefficient of 0.95) with that of multi-positioned in situ sensors, indicates the spatial
responsibility of the atmospheric CO2 obtained from our remote sensor with two ~3-km observation paths.
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