The lidar in-space technology experiment (LITE) is a three- wavelength backscatter lidar which was flown on space shuttle mission STS-64 in September 1994. Observations of clouds and aerosols were acquired between 57 degrees N and 57 degrees S on 10 days of the mission. LITE provides a dataset with which to explore the applications of space lidar and to begin to develop and demonstrate the retrieval algorithms required for future space lidars. Results from a number of science investigations using LITE data will be discussed to illustrate system performance characteristics and capabilities. A LITE level 1 data product is being created and will be made available through the NASA Langley Distributed Active Archive Center.

The clouds signals of the 'Balkan-1' space lidar, intended for cloud sounding, were instated using the Monte Carlo method. The calculations are carried out for the wavelength (lambda) equals 0.532 nm and specific parameters of orbit and the lidar. The clouds of homogeneous and inhomogeneous structure in the cloud layer (Delta) H equals 100 m is given depending on the opto-geometric experimental conditions. From the analysis of the result it follows that the main parameter, determining the signal structure, is the optical thickness of the sounded cloud layer. The viewing angle, the scattering phase function have no effect on the amplitude and the space-time signal characteristics. For inhomogeneous clouds the lidar signal repeats qualitatively the extinction coefficient profile if the layer optical thickness is less than 2.5-3.0.

Since its first successful demonstration in 1964, the precision of Satellite Laser Ranging (SLR) has improved by three orders of magnitude, i.e., from a few meters to a few mm. Each technological improvement has been rapidly followed by a new scientific capability. To date, centimeter accuracy SLR measurements to the passive LAGEOS satellites by a ground-based network of approximately 40 stations have; (1) helped to define a Terrestrial Reference Frame accurate to a centimeter globally; (2) measured the motions of tectonic plates and detected regional crustal deformations near the plate boundaries; (3) helped define the terrestrial gravity field; and (4) monitored variations in the Earth's gravity field, the orientation of the Earth's spin axis, and its rate of rotation and related them to angular momentum exchanges and/or large mass redistribution within the land, ocean, atmosphere system. By providing few centimeter precision orbits measurement to altimetric satellites such as ERS-1 and 2 and TOPEX/POSEIDON, SLR has enabled precise measurements of global ocean, circulation, wave heights, ice topography, and even mean sea level rises on the order to two mm/yr. In addition to providing useful test of general relativity, centimeter accuracy SLR measurements to five retroreflector packages placed on the lunar surface by US and Soviet landers have helped to define the planetary reference frame, provided ultraprecise lunar ephemerides, defined the lunar librations, and constrained models of the Moon's internal structure.

The SPAD has proven already its capability of timing single- photon events with picosecond accuracy; it does that also for multi-photon events, but introduces here a time walk effect: with received energies of 1000 photons and more, the measured epoch time is shifted 200 ps or more towards earlier times; although the specific SPAD type used shows the lowest time walk effect of all measured silicon avalanche diodes, this effect still might introduce range errors of up to 30 mm, when measuring distances to satellites. It has been shown that this time walk effect is connected with a very small change of the avalanche rise time; this effect has been successfully used to develop an electronic circuit which measures this rise time difference, and uses it to compensate automatically almost all of the time walk effect. Some prototypes have been built and tested successfully in the satellite laser ranging station Graz; improved versions of the circuit are operated or tested now successfully in other SLR stations. It has been shown that the time walk effect can be reduced to more or less zero, for a dynamical range from single photon up to more than 1000 photons. For best time walk compensation, the circuit is adjusted for a specific laser pulse length; it has been shown however, that this adjustment also gives good time walk compensation for other laser pulse lengths.

Satellite range measurements which include differential time of flight measurement between simultaneous doubled and tripled Nd:YAG laser pulses are being made at the NASA/Goddard Space Flight Center's 1.2m telescope tracking facility. A description of the streak camera-based range receiver is given along with the differential time of flight measurement. Typical streak camera return waveforms are displayed from satellite tracks which include: ADEOS/RIS, AJISAI, GFZ-1, STELLA, and TOPEX.

The first ground-to-satellite laser communication experiments were performed during December 1994 and July 1996 to demonstrate basic technologies for space laser communication systems. It used an optical communication package on-board the engineering test satellite VI and its companion ground optical terminals. A b-directional optical link over 40000 km was demonstrated along with precise transmission control of extremely narrow laser beams at both on-board and ground terminals. It is important to know characteristics of laser beam propagation through turbulent atmosphere and to accumulate experimental data. In the paper an overview of measurement result and discussion on laser beam propagation between the ground and the satellite are presented.

This paper outlines the results of a streak-camera based timing device applicated to satellite laser ranging. By means of synchronizing the deflection voltage of the streak camera with the mode locking frequency of the Nd:YAG laser and locking both to a stable frequency standard, the streak camera represents a complete timing system allowing simultaneous measurements of absolute ranges in the IR and green wavelength domain. Furthermore it is possible to obtain high resolution differential range information from quasi simultaneous echoes from the two used laser frequencies. In addition to that, the laser can be operated in semi train mode compensating the shortcomings of the relative insensitive S1-Photocathode partly by integration of succeeding pulses. Operating the device in multi-photon- mode, yields a standard deviation of about 6ps for the centroid detection of gaussian shaped laser pulse. The analysis of the satellite laser ranging experiments show that this high precision can be reached in the earth to space propagation channel. Receiving the backscattered intensity distribution over time, the incoherent optical transfer function of the satellite retroreflector array can be reconstructed. This allows the comparison of the optical transfer functions of different satellites and an evaluation of target induced point spread functions in concern with the satellite and an evaluation of target induced point spread functions in concern with the satellite geometry. Moreover, looking at the shot by shot detected signal strength in both wavelengths, the correlation and coherence functions of the log amplitude fluctuations, which determine the satellite irradiance, can be measured.

We discuss the design of next generation hollow satellite retroreflectors for satellite laser ranging and atmospheric measurements. We also review a method for controlling the beam pattern of the reflection using curved mirrors in the retroreflector, and this method is applied to the design of both single-element and four-element satellite retroreflectors. The concept of the acute angle retroreflector and possible applications to a satellite retroreflector are also discussed.

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 satellite's 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.

Residuals of AJISAI ranging data are investigated for single-photon detection and multi-photon detection by comparing our simulation model with actual range data. The simulated distribution histogram for a single-photon system based on our model agrees well with that obtained by the Herstmonceux station. We also found the variations in the AJISAI's center-of-mass correction for multi-photon systems was linked to the satellite attitude as a result of doing a spectral analysis of the Orroral station's data.

The advanced earth observing satellite (ADEOS) was launched on August 17th, 1996. The ADEOS carries a large aperture laser-reflector, referred as Retroreflector in Space (RIS). In order to hit laser properly onto the RIS, we need a trajectory prediction with accuracy of about 100 m. The flight-dynamics team at the NASDA ordinarily derives a satellite trajectory with range and range rate (RARR) measurements using S-band radio wave. However, the trajectory prediction is expected to be only as accurate as 1 km for ADEOS. This uncertainty is not acceptable for the RIS. Our main goal is to provide a trajectory prediction valid for three days with uncertainty of 100 meters or smaller with satellite laser ranging (SLR) method. This subject is being studied with the ADEOS/RIS experiment. As a result, our accuracy of a position prediction with the SLR method is about 80 meters or better, and therefore we believe performance of the SLR method will supersede our RARR method with at least a ten-fold improvement on its accuracy. In the future missions of the NASDA, a spacecraft needs higher accuracy in trajectory determination and prediction. A brief discussion on the post-ADEOS mission plan will be found in this paper as well.

SLR2000 is an autonomous and eyesafesatellite laser ranging station with an expected single shot range precision of about one centimeter and a normal point precision better than 3 mm. The system will provide continuous 24 hour tracking coverage. Replication costs are expected to be roughly an order of magnitude less than current operational systems, and the system will be about 75% less expensive to operate and maintain relative to the manned systems. Computer simulations have predicted a daylight tracking capability to GPS and lower satellites with telescope apertures of 40 cm and have demonstrated the ability of our current autotracking algorithm to extract mean signal strengths as small as 0.000 I photoelectrons per pulse from background noise. The dominant cost driver in present SLR systems is the onsite and central infrastructure manpower required to operate the system, to service and maintain the complex subsystems (most notably the laser and high precision timing electronics), and to ensure that the transmitted laser beam is not a hazard to onsite personnel or to overflying aircraft. In designing the SLR2000 system, preference was given to simple hardware over complex, to commercially available hardware over custom, and to passive techniques over active resulting in the prototype design described here. This general approach should allow long intervals between maintenance visits and the "outsourcing" of key central engineering functions on an "as needed" basis. As a result, many of the signal extraction techniques and engineering designs employed here may have application in remotely operated or even spacebome lidar applications. SLR2000 consists of seven major subsystems: (1) Time and Frequency Reference Unit; (2) Optical Subsystem; (3) Tracking Mount; (4) Correlation Range Receiver; (5) Meteorological Station; (6) Environmental Shelter with Azimuth Tracking Dome; and (7) System Controller. The Optical Subsystem in tum consists of a 40 cm aperture telescope and associated transmit/receive optics, a passively Q-switched microlaser operating at 2 KHz with a transmitted single pulse energy of 135 μJ, a start detector, a quadrant stop detector for simultaneous ranging and subarcsecond angle tracking, a CCD camera for automated star calibrations, and spectral and spatial filters to reduce the daylight background noise. The meteorological station includes sensors for surface pressure, temperature, relative humidity, wind speed and direction, precipitation type and accumulation, visibility, and cloud cover. The system operator is replaced by a software package called the "pseudooperator" which, using a variety of sensor inputs, makes all of the critical operational decisions formerly made by onsite personnel. Keywords: laser ranging, microlasers, laser altimetry, lidar, single photon detection, meteorological instrumentation, satellites, autonomous instruments

Clouds and aerosols affect the magnitude of radiative fluxes at the Earth's surface and within the atmosphere through the scattering and absorption of incoming solar and outgoing thermal radiation. Through these mechanisms they have important influences ont he climate and remote sensing from space is required to assess their effects on a global scale. Current capabilities to observe clouds and aerosols using passive satellite sensors are limited, however. The lidar in-space technology experiment, flown on the space shuttle in September 1994, demonstrated the application of space lidar to the study of clouds and aerosols. Lidar technology has now matured to a point where satellite lidars with on- orbit lifetimes of several years are feasible. Space lidar will provide vertically resolved measurements of the distribution of clouds and aerosols as well as optical and microphysical properties, allowing improved characterization of the role of aerosol and cloud in global climate. This paper will discuss the application of cloud/aerosol satellite lidars to the climate problem.

We consider here some peculiarities in ranging with a space- based lidar 'Balkan-1' of the ocean surface for two modes of the space station 'Mir' orientation used during lidar measurement sessions. To demonstrate the peculiarities we have analyzed regular and random fluctuations of the lidar optical axis position in space that occurred in the measured sessions conducted. Analysis made allows us to discuss possible causes of the systematic difference between the ballistic calculated data on range and the values of slant range measured with the lidar in both modes of the station orientation.

This paper presents a CO₂ heterodyne remote sensing system that will be implemented in a Doppler LIDAR system. The system uses the 'chirp' technique in order to investigate the Doppler effect and has the advantage that is quite simple, easy to be adjusted and has a very good sensitivity.

Lidar, and in particular, differential absorption and scattering lidar or DIAL have today reached a high degree of maturity. It now appears appropriate that efforts be taken in the direction of standardization of the technique and assurance and control of the quality of its results. To this end the German Commission on Air Pollution of VDI and DIN established a working group whose task was to prepare a set of recommendations for the use and operation of lidar systems. This group now completed, as a first result, a guideline for the use of differential absorption and scattering lidar for gas concentration measurements. Peculiarities associated with such a task are presented, and the contents of the draft of the resulting guideline VDI 4210 Part 1 are discussed.

Wind over sea can be determined with sufficient accuracy by SAR in global scale. Wind profiles can be measured by Doppler lidar. A first comparison in a few levels was performed.

For the airborne measurement of the wind field, exact data of velocity and orientation of the platform are mandatory. Any small deviation in these parameter will cause a systematic error in the estimation of the wind field. In practice all navigation systems like inertial reference systems (IRS) and also GPS have some errors in their data which may lead to a significant error in the estimated wind field. However a conical scanning Doppler lidar like ADOLAR will also offer some information from the ground return which can be used to select and correct most of the housekeeping data from the other systems. This paper describes briefly the Doppler lidar used for the wind measurements and then extends on the different processing steps necessary to retrieve the 3D wind field with an accuracy better than 1 m/s. The campaign with the airborne Doppler Lidar ADOLAR on board the DLR research aircraft Falcon F20 at November 96 will be used to illustrate this topic. At this campaign three flights were performed at Bavaria, the North sea , and the Baltic Sea. Especially at the first flight in Bavaria approximately one . hour of lidar data with ground return have been obtained. This allows an extended comparison of the different sources of housekeeping data like IRS, GPS, and the Doppler Udar itself.

From the analysis of dual color range measurements to satellites it was found that the correlation between the locally measured humidity and the obtained differences between the two simultaneously carried out range measurements at (lambda) equals 1.06 micrometers and (lambda) equals 0.53 micrometers was relatively high, while the more obvious parameter atmospheric pressure did not correlate well. A model for the refractive index of the atmosphere along the line of sight is used to reduce the measurements to real ranges. However, these corrections are based on assumptions for the profiles of pressure temperature and humidity, starting from measured values at the ground near the ranging station. Since the refractive index is strongly dependent on the density of the atmosphere the profile of the atmospheric pressure is represented extremely well in this model. However this is not necessarily the case for the water vapor content and the atmospheric correction model is not sensitive to this parameter. It is to be expected, that most of the influence of the water vapor is coming from the lower troposphere, so remote sensing techniques promise to fill a gap here. By using the slr-station in a way, that permits the detection of Raman backscattering, water vapor profiles of the lower troposphere were obtained and compared to the atmospheric correction model.

The experimental lidar in space equipment (ELISE), one of NASDA's 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.

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.