Toshikasu Itabe

Development of coherent Doppler lidar at CRL

Global wind profiling with a space-borne Doppler lidar is expected to bring big progress in the studies on global climate change and Numerical Weather Prediction. A feasibility study has been done for an eye-safe 2micron coherent Doppler lidar aiming at demonstration of the technology onboard the Japanese Experiment module of the International Space Station. We are now developing an airborne coherent Doppler lidar system to measure wind profile under a jet plane for simulation of the Doppler lidar measurement in space. This system is also operated in the ground to develop algorithm of the wind measurements and the results of the wind profiles are compared with those derived from other instruments.

Plan for the experiment with the retroreflector in space (RIS) on ADEOS

Experiments on the earth-satellite-earth laser long-path absorption measurements of atmospheric trace species will be carried out with the retroreflector in space (RIS) for the Advanced Earth Observing Satellite (ADEOS). The RIS is a single-element hollow retroreflector with an effective diameter of 0.5 m, which was designed for spectroscopic measurement in the infrared region. The ground system for the experiment employs two single-longitudinal-mode pulsed CO2 lasers. High-resolution atmospheric absorption spectra are measured by using the Doppler shift of the return beam caused by the satellite movement. Vertical profiles of O3 and CH4, and column contents of CFC12, HNO3, CO, N2O, etc. will be obtained from the measured spectra.

NASDA ELISE (MDS-lidar) program

The experimental lidar in space equipment (ELISE), one of NASDAs lidar programs, means the two-wavelength backscatter lidar. It is planned to be loaded onto the mission demonstration satellite (MDS)-2 planned to be launched early in 2001. One of the special features of ELISE is to be developed in short period using two models called the Basic Test Model and the demonstration model (DM). Through this program, we try to demonstrate some key devices, such as a lightweight laser diode (LD)-pumped high power LASER, a large diameter telescope an a photon counting detector using silicon avalanche photo diode, which are required for future spaceborn lidars. The experimental data of key devices in the space environment will be obtained. Furthermore, ELISE will observe clouds in the high altitude, multi-layered clouds, aerosols and the atmospheric density through one year. This observation will reveal the scientific value and the availability of spaceborn lidars. The collection of the information on clouds, aerosols and the density will be a great help to the design of future spaceborn lidars.

Development of DIAL for CO 2 and CH 4 in the atmosphere

Development of a mid-infrared differential absorption LIDAR system around 2.0 and 2.3 tm wavelength is proposed for remote sensing of greenhouse-effect gases, such as CO2 and CH. A model calculation shows that the 1.6 to 2 ptm wavelength region is also suitable for the CO2 measurements. A widely tunable self—injection seeded Ti3: sapphire laser in combination with a Raman shifter is used for a tunable mid-infrared laser source. This combination could allow a wide tuning range between 1.6 and 2.3 .tm. The spectral width of the Ti3: sapphire laser was measured around 0.04 cnf 1 and stability of the laser source was also studied by a Fabry-Perot interferometer in order to determine theaccuracy of the DIAL measurement. Currently, output energy is limited to only a few mJ limiting the detection sensitivity. Nevertheless improved output characteristics should meet the requirements satisfactorily to monitor CO2 and CH gases in the atmosphere.

Seasonal variation of aerosol over Bandung, Indonesia

Variation of stratospheric aerosol affects atmospheric minor constituents and climate through changes in the radiation field as well as by dynamic and chemical processes. In order to estimate the impact quantitatively, it is very important to observe stratospheric aerosol vertical distribution and their lime variation To obtain stratospheric aerosol especially in equatorial region a lidar was installed in Bandung (6.9° S; 107.7° E) Indonesia under collaboration between National Institute of Aeronautic and Space (LAPAN), Communication Research Laboratory of Japan and Meteorological Research Institute of Japan. The Lidar transmitter system employ fundamental (1064 nm), second harmonic (532 nm) and third harmonic (355 am) wavelengths ofNd: YAG laser are transmitted. The second harmonic backscatter light and its N2 ranian backscatter light (607 am) are collected by a 28 cm and a 35 cm diameter telescope. In 35 cm telescope system, upper troposphere and stratosphere are observed by photon counting. 532 am light component polarized parallel and perpendicular to the laser light are separately observed to get information about shape of aerosol. Bandung is located at around 750 m above mean sea level and it is surrounded by mountains, therefore clouds are easily formed especially between 10 and 17 km height. Stratospheric aerosol over Bandung are spreaded between 18 and 35 km in altitude and form more than 1 sub layer, below 30 km and upper 30 km. The maximum aerosol concentration is obtained around 22.5 km height. To understand the seasonal variation of stratospheric aerosol we integrated the backscattering coefficients for altitude range 18-35 km and it was obtained that the stratospheric aerosol concentration in 1997 was higher than in 1999. The integrated backscattering coefficient in June was higher than in August. But seems the seasonal variation of integrated backscattering coefficient in Bandung does not so clear determined yet.

Lidar and Optical Particle Counter (OPC) measurements of polar and tropical stratospheric aerosols

Lidar and Optical Particle Counter (OPC) measurements were performed in the Canadian Arctic and in the Indonesian Tropical region. The observations yielded very interesting and important results about the features of the latitudinal difference in the stratospheric aerosols. Besides the latitudinal difference, the aerosol distributions and their time variation showed unique characteristics in each of the regions. In Arctic winter, the aerosol concentration varies frequently day-to-day. In the Tropical region, the aerosol distribution and the vertical transport is, probably, controlled by the variation of the circulation pattern in the lower stratosphere related to the QBO in the tropical stratosphere. Based on the results of the simultaneous measurements by lidar and OPCs, we estimated surface area density, volume density, S-parameter (extinction to backscatter ratio), backscatter to surface area conversion factor, and backscatter to volume conversion factor of the stratospheric aerosols at 20km-altitude in the Arctic and Tropical regions.

Estimation of orbital Doppler shift change due to nutation of attitude for 2-μm coherent Doppler lidar on ISS-JEM (International Space Station-Japanese Experimental Module)

Coherent Doppler Lidar (CDL), which has a capability of 3-D wind velocity measurements, can realize a global measurement of the wind profile in the troposphere from space. ISS(International Space Station) has been constructing from last year, and JEM(Japanese Experimental Module) attached to ISS is scheduled to be provided as a laboratory in space. We have been making a feasibility study for ISS-JEM/2-micrometer CDL. We expect that the ISS might give some technical problems because of a large scale and a man attended station. On the other hand, a measurement accuracy of 1m/s is required from the atmospheric science. We have to analyze the systematic error with considering the error factors to meet this requirement. There are two factors of the wind measurement errors caused by the nutation of ISS?fs attitude. One of them is a change of receiving power, since it should change distances between CDL and a observing point. In this case, we have already shown that the wind error estimated was only ?}0.05m/s (assumed width of pitching angle ;?}3deg). The other factor is a change of orbital Doppler shift. The accuracy of 1m/s is corresopnding to 1MHz. We calculated a frequency of orbital Doppler shift in case of non conical scanning one when ISS has the nutation of attitude with ?}3deg,as a pitching angle and a rolling angle,respectively. The results obtained in this syudy are very useful to design a frequency agile CW laser as a local oscillator.In this paper, we describe more details for our analysis.

Lidar observations of the stratospheric aerosols at Bandung, Indonesia

We installed a lidar system for observations of the stratospheric aerosols at Bandung, Indonesia on November 1996. The system employed the second harmonic wavelength (532 nm) of Nd:YAG laser. We can measure scattering ratio and depolarization of 532 nm, and Raman scattering of N2 (607 nm). The system works well and the stratospheric aerosols were detected between 18 km and about 35 km. Cirrus clouds are always observed between 10 km and tropopause and area around tropopause is clear except for cloud-like structures. Integrated backscattering coefficient (IBC) of the stratospheric aerosols in 1997 is about 6 X 10-5sr-1 level and smaller than the value observed in mid-latitude, corresponding to the higher tropopause in the equatorial region. Variation of IBC at Bandung seems to be small. It is yet not clear whether current aerosol load is background level or not. We need more long period observations to discuss about seasonal, QBO, and long term variation of aerosol load.

Laser long-path absorption experiment using the Retroreflector in Space (RIS) on the ADEOS satellite

We conducted an earth-satellite laser long-path absorption experiment using the Retroreflector in Space on the advanced earth observing satellite, which was launched on August 17, 1996 by the National Space Development Agency of Japan. Two single-longitudinal-mode transverse, electric, atmospheric (TEA) CO2 lasers were used in the spectroscopic measurement of atmospheric trace species. One TEA laser was used to measure the absorption of the ozone, and the other to record a reference signal. We measured the absorption spectrum of the atmosphere using the Doppler shift of reflected beam caused by the satellites movement. The ozone spectrum was successfully measured using the 10R(24) line of 13CO2 and the 9P(24) line of 12CO2. We studied the measurement error and reduced it by adding a spatial filter to the transmitter optics.

Optical characteristics of the Retroreflector in Space (RIS) on the ADEOS satellite

This paper reports the optical characteristics of the Retroreflector in Space (RIS) on the advanced earth observing satellite (ADEOS) in orbit. The RIS is a 0.5 m diameter single-element hollow cube-corner retroreflector with a unique design which uses a curved mirror to correct the velocity aberration caused by the satellite movement. We used a Nd:YAG laser to test the efficiency of the reflection at 532 nm. The ADEOS was actively tracked with a 1.5 m diameter tracking telescope using the image of the RIS lit by the Nd:YAG laser. We measured the intensity of return light with an image-intensified CCD camera on the guiding telescope with a diameter of 20 cm. The intensity of the return was quantified by comparing it with images of stars. We compared the result with the theoretical reflectance of the RIS, and confirmed that the reflectance of the RIS agreed with the designed value. The return from the RIS was comparable to a stellar magnitude of 2 to 3, depending on the elevation angle when lit by a 0.3 W laser with a beam divergence of 0.5 mrad.