Daisuke Sakaizawa

Development of 1.6 μm continuous-wave modulation hard-target differential absorption lidar system for CO 2 sensing

We have demonstrated the 1.6 mum cw modulation hard-target differential absorption lidar system for CO(2) sensing. In this system, ON and OFF wavelength laser lights are intensity modulated with cw signals. Received lights of the two wavelengths from the hard target are discriminated by modulation frequencies in the electrical signal domain. The optical circuit is fiber based, and this makes the system compact and reliable. It is shown that a stable CO(2) concentration measurement corresponding to a fluctuation of 4 ppm (rms) (ppm is parts per million) has been achieved in 32 s measurement intervals and the 1 km path.

Feasibility study on 1.6 μm continuous-wave modulation laser absorption spectrometer system for measurement of global CO 2 concentration from a satellite

A feasibility study is carried out on a 1.6 μm continuous-wave modulation laser absorption spectrometer system for measurement of global CO(2)concentration from a satellite. The studies are performed for wavelength selection and both systematic and random error analyses. The systematic error in the differential absorption optical depth (DAOD) is mainly caused by the temperature estimation error, surface pressure estimation error, altitude estimation error, and ON wavelength instability. The systematic errors caused by unwanted backscattering from background aerosols and dust aerosols can be reduced to less than 0.26% by using a modulation frequency of around 200 kHz, when backscatter coefficients of these unwanted backscattering have a simple profile on altitude. The influence of backscattering from cirrus clouds is much larger than that of dust aerosols. The transmission power required to reduce the random error in the DAOD to 0.26% is determined by the signal-to-noise ratio and the carrier-to-noise ratio calculations. For a satellite altitude of 400 km and receiving aperture diameter of 1 m, the required transmission power is approximately 18 W and 70 W when albedo is 0.31 and 0.08, respectively; the total measurement time in this case is 4 s, which corresponds to a horizontal resolution of 28 km.

Ground-based demonstration of a CO2 remote sensor using a 1.57μm differential laser absorption spectrometer with direct detection

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 CO 2 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 CO 2 mixing ratios. The in-house measurements demonstrated the instrumental response showing agreement within a correlation coefficient of 0.998 for a known CO 2 density. Field measurements to evaluate horizontal and vertical column-averaged CO 2 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 CO 2 density agreed within the difference of 2.8 ppm of the atmospheric CO 2 density.

Performance improvement and analysis of a 1.6 μm continuous-wave modulation laser absorption spectrometer system for CO 2 sensing

In a previous study, we developed a 1.6 μm continuous-wave (cw) modulation laser absorption spectrometer system for CO(2) sensing and demonstrated the measurement of small fluctuations in CO(2) corresponding to a precision of 4 parts per million (ppm) with a measurement interval of 32 s. In this paper, we present the process to achieve this highly specific measurement by introducing important points, which have not been shown in the previous study. Following the results of preliminary experiments, we added a function for speckle averaging on the optical antenna unit. We additionally came up with some ideas to avoid the influences of etalon effects and polarization dependence in optical components. Because of the new functions, we realized a calibration precision of 0.006 dB (rms), which corresponds to a CO(2) concentration precision of less than 1 ppm for a 2 km path. We also analyzed the CO(2) sensing performance after the improvements described above. The measured short time fluctuation of the differential absorption optical depth was reasonably close to that calculated using the carrier-to-noise ratio of the received signal.

Laser absorption spectrometer using frequency chirped intensity modulation at 1.57 μm wavelength for CO 2 measurement

We have demonstrated the laser-absorption spectrometer system using frequency chirped intensity modulation at 1.57 μm wavelength for measurement of CO(2) concentration. Using this technique, backscattered laser radiation from different ranges can be discriminated in the frequency domain of the electrical signal. We have reported the discrimination of two signals from the targets with different ranges. It is shown that stable measurements with short time fluctuation corresponding to 4 ppm (rms) were obtained with 32 s measurement intervals. Furthermore, there is qualitative good agreement on, at least, the diurnal changes between the results of the laser absorption spectrometer system and the in-situCO(2) sensor.

Overview of Japan's spaceborne vegetation lidar mission

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.

Path-averaged atmospheric CO2 measurement using a 1.57 μm active remote sensor compared with multi-positioned in situ sensors

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.

Improvement of the 1.57-micron laser absorption sensor with chirp modulation to evaluate spatial averaging carbon dioxide density

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.

Performance analysis on 1.6 micron CW modulation laser absorption spectrometer for CO2 sensing

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.

Overview of vegetation Lidar “MOLI”

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.