The near-earth asteroid rendezvous (NEAR) mission is the first of the NASA discovery programs. Discovery-class programs emphasize small, low-cost, quick turnaround space missions that provide significant science returns. The NEAR spacecraft and ground control system are currently being developed and tested at the Applied Physics Laboratory (APL). The NEAR spacecraft will orbit, 433 Eros, possibly the most studied of the near-Earth asteroids. Subsequent to a 3-year cruise, the NEAR spacecraft is inserted into a 50-km-altitude orbit about Eros for 1 year to permit data collection in the infrared, visible, x-ray and gamma-ray regions. One instrument, the NEAR laser rangefinder (NLR), will provide altimetry data useful in characterizing the geophysical nature of Eros. In addition, ranging data from the NLR will support navigation functions associated with spacecraft station-keeping and orbit maintenance. The NLR instrument uniquely applies several technologies for use in space. Our configuration uses a direct-detection, bistatic design employing a gallium arsenide (GaAs) diode-pumped Cr:Nd:YAG laser for the 1.064-micrometer transmitter and an enhanced-silicon avalanche-photodiode (APD) detector for the receiver. Transmitter pulse energy provides the required signal-to-noise power ratio, SNR_p, for reliable operation at 50 km. The selected APD exhibited low noise, setting the level achievable for noise equivalent power, NEP, by the receiver. The lithium-niobate (LiNbO₃) Q-switched transmitter emits 12-ns pulses at 15.3 mJ/pulse, permitting reliable NLR operation beyond the required 50-km altitude. Cavity aperturing and a 9.3X Galilean telescope reduce beam divergence for high spatial sampling of Eros's surface. Our receiver design is an f/3.4 Dall-Kirkham Cassegrain with a 7.62-cm clear aperture -- we emphasized receiver aperture area, A_rx, over transmitter power, P_t, in our design based on the range advantage attainable according to the simplified range equation, R_max equals [(P_t(rho) _BA_rx)/(SNR_p NEP)]^1/2. Asteroid reflectivity, (rho) _B, is estimated to be 0.05 at our wavelength. A reasonable power signal- to-noise ratio for reliable operation, SNR_p, was assumed. To minimize our noise equivalent power, NEP, we carefully designed and selected the receiver components. The receiver circuit uses leading-edge detection of the laser backscatter. Our detector circuit is an enhanced-silicon APD hybrid using a video amplifier, an integrating Bessel filter, and a high- speed programmable threshold comparator. We accomplish time-of-flight (TOF) measurements digitally with an APL-designed GaAs application-specific integrated circuit. A radiation-hardened FORTH microprocessor controls range gating, data collection and formatting, and operational modes. Implementation of control and data communications between the spacecraft and rangefinder uses the MIL-STD 1553-bus architecture. Functional testing and calibration indicate exceptional performance; return power levels were reliably detected over several thresholds with 71-dB attenuation, while observed range jitter was equivalent to the resolution determined by the TOF GaAs chip (31.5 cm). This paper discusses NLR performance requirements, design implementation, and qualification testing. It also provides preliminary results from calibration and performance testing.

This paper outlines the operation of a synchro scan streak camera in conjunction with a phase coupled Nd:YAG laser in order to perform range measurements to artificial satellites at two wavelengths simultaneously. The driving frequency for both, the laser optoacoustic modelocker and the streak camera, is obtained from a stable frequency standard. The synchronization of transmitter and receiver permits the designation of phases to the laser pulses detected by the streak camera relative to the transmitted ones. Therefore ranges can be determined in modulo parts of the driving frequency. In order to calibrate the device, ranging to a local ground target was carried out. It is shown that a standard deviation of about 10 ps can be reached operating the device in multi photon mode, despite the fact that the laser is operated with 200 ps FWHM pulses. The analysis of the satellite laser ranging experiments show that this high precision can be reached as well in the earth to space propagation channel.

The investigation of the atmospheric path delay with a dual color satellite laser ranging experiment indicated that the modeling of the water vapor content of the atmosphere may not be sufficient. The need for a measurement of this observable along the line of sight parallel to the satellite ranging operation led to the construction of a Raman scattering experiment. Starting from the existing setup of the Wettzell Laser Ranging System (WLRS) a statistical approach for the performed measurements was chosen. This paper describes the applied technique and compares the obtained results to the theoretical expectations.

Airborne laser fluorosensor measurements of hydrographic parameters have been performed in the German Bight and in the Baltic Sea. The results are compared with data measured with a luminescence spectrometer on board research vessels. Fluorescence spectra excited at different excitation wavelengths (on board the ship) give us information on fluorescent gelbstoff components in natural water. These ground truth data have proved the remotely detected data of gelbstoff in coastal waters. Recent measurements made in the Canary Islands region demonstrate that gelbstoff fluorescence can be registered also in clear open ocean waters with low concentrations of this material.

A start has been made on practical implementation of spaceborne lidar systems for scientific research and solution of numerous ecological, meteorological, and other problems. In this report, we present results of theoretical investigations on the feasibility of lidar system application to monitoring of optical properties of a subsurface oceanic layer and to bathymetry.

The Lidar In-space Technology Experiment (LITE) is a three-wavelength backscatter lidar developed by NASA Langley Research Center to fly on the Space Shuttle. LITE flew on Discovery in September 1994 as part of the STS-64 mission. The LITE mission presented an opportunity to explore the applications of space lidar and to gain operational experience which will benefit the development of future systems on free-flying satellite platforms. The performance of the LITE instrument was excellent, resulting in the collection of over 40 Gbytes of data. These data present us with our first highly detailed global view of the vertical structure of cloud and aerosol, from the Earth's surface through the middle stratosphere. These preliminary results highlight the benefits to be obtained from long duration satellite lidars.

First, we present some examples of calculations of polarized multiply scattered return signals of a space-based multichannel LIDAR (such as the LITE) and compare them to return signals of a ground-based LIDAR. These methods are variance reduction Monte Carlo algorithms allowing for controlling the empirical variance. Second, we recall the method of random search briefly. Random search procedures are Monte Carlo algorithms which are designed to solve deterministic or stochastic optimization problems and, hence, also deterministic or stochastic equations. Third, we show how variance reduction Monte Carlo methods may be combined with properly chosen random search procedures to retrieve one or more environmental parameters from a given (measured or calculated) LIDAR return signal. Such a procedure is time consuming, of course, but it will allow for the retrieval of several parameters simultaneously and for a sensitivity analysis. Hence, a retrieval procedure based on the combination of variance reduction Monte Carlo methods and random search including such a sensitivity analysis gives much more information than the usual inversion procedures (e.g. based on integral equations and often uncheckable environmental assumptions). Fourth, we present some examples of the (simultaneous) retrieval of the extinction coefficient and the particle size distribution of a cloud from the multiply scattered return signal of a space-based LIDAR and a sensitivity analysis of this retrieval.

The benefit of a spaceborne backscatter lidar for studies on clouds and aerosols is discussed. Based on the technical and orbital parameters as planned for the European atmospheric lidar ('ATLID') the potential to detect optically thin clouds and aerosol layers, to contribute to cloud climatologies and to derive optical properties, e.g. the extinction coefficient, was investigated. It was found out that under favorable conditions even very thin cirrus clouds (extinction coefficient alpha^p approximately equal to 0.05 km ^-1) can be identified by ATLID. Radiatively relevant ice clouds can be detected in any case. Caused by multiple scattering the detection of clouds below a cirrus layer is also possible. As a consequence, and according to the good spatial sampling, it is expected that ATLID will provide useful data for cloud climatologies. The determination of optical depths however seems to be difficult. Improvements of the present aerosol database can be anticipated at least in case of stratospheric layers and medium turbid planetary boundary layers. It is recommended to further investigate the benefit gained from the joint exploitation of different sensors such as a backscatter lidar, a cloud radar and imagers.

The paper presents some results of stratospheric aerosol sounding obtained during ground- based correlative measurements in the framework of the NASA LITE program at the Siberian Lidar Station (Tomsk) in September 1994. The results are compared with the data of our previous studies and data of spaceborne sounding.

The discriminants in a backscatter lidar operated from space like different wavelengths, Raman scattering, polarization, multiple scattering or scanning are reduced to polarization, scanning and multiple scattering caused by the limited space and power available in space. Polarization and scanning influences the information content of backscattering from non- spherical particles. A solution for the arising problems cannot be proposed. Multiple scattering is the only discriminant which can be applied from space.

In this paper we present a description of an instrumentation complex developed for ground support and testing of the spaceborne lidars of 'BALKAN-1' type. The whole cycle of ground tests includes final technological tests prior to formal acceptance of the device from a manufacturer, input control preceding its assembling onboard the spaceborne module 'SPEKTR' of the orbiting station 'MIR', as well as its final tests before launch at a launch site.

An aerosol Raman lidar with a frequency doubled and tripled Nd:YAG laser and a 1140 mm diameter receiving telescope is under development for measurements of aerosol backscatter, extinction coefficients and depolarization in the altitude range 5 to 50 km. The light received by the telescope is split according to polarization and fed via glass fibers to a filter polychromator and to photon-counting detectors. The lidar is mounted on a removable carrier inside an air-conditioned 20 ft standard container and can be operated anywhere in the world, even at sea and under extreme climatic conditions (Antarctica, Tropics). Tests are carried out during a 1996 cruise of the German research vessel Polarstern from Europe to Antarctica.

The contribution of multiply scattered photons to signals from a spaceborne backscatter signal is investigated. Monte Carlo calculations are performed for different atmospheric conditions and for different lidar configurations. It is found out that multiple scattering signals in most cases can accurately be described by a very simple approximation. It is concluded that the process of multiple scattering will be no drawback for the application of spaceborne lidars. In contrary, the transparency of clouds is enhanced so that lidar signals can fully penetrate cirrus clouds, even cirrostratus layers. As a consequence, clouds below a cirrus can be observed which would be obscured if only single scattering would occur.

In this paper we present analytical formulas for the brightness of images of a lidar scattering volume formed by two cross polarized components of the lidar backscatter derived for the case of a spherical polidispersion irradiated with a narrow laser beam. The calculational results we compare with the experimental ones obtained with a DLR lidar having two fields of view.

For future spaceborne water vapor DIAL systems, we started a laser design study in 1994. New laser materials such as Cr:LiSAF are very attractive, but at present there are no high power diode lasers for direct pumping those materials. Therefore we determined to develop a high power diode-pumped Nd:YLF laser and Ti:sapphire laser for water vapor DIAL. The output energy of Nd:YLF laser is expected to be 550 mJ at 1053 nm and 400 mJ at 527 nm with a maximum repetition rate of 150 Hz. A Ti:sapphire laser will be pumped by the SHG of the Nd:YLF laser. Tuning of the Ti:sapphire laser to a strong absorption line (ON1), a weak absorption line (ON2) of water vapor and an off line (OFF) is made by an injection seeder which consists of two single longitudinal mode laser diode modules. Two on-line laser diodes are locked to water vapor absorption lines using an absorption cell or a photo-acoustic cell. These three laser lines (ON1, OFF and ON2) are transmitted into the atmosphere with a triple pulse technique for measurements of water vapor profiles from the ground up to 10 km. The laser spectral width of the on line is expected to be 0.5 pm with a stability of 0.05 pm. The output energy of each laser line is to be more than 100 mJ. This laser system will be developed within three years, and then incorporated as an airborne water DIAL.

A spaceborne Doppler wind lidar (DWL) is a unique instrument for probing atmospheric wind fields on a global scale with high lateral and vertical coverage and resolution. A promising and mature technology is a pulsed CO₂-laser based instrument employing heterodyne reception of the Doppler shifted laser light scattered back from atmospheric aerosols. The main features of the proposed instrument concept of ALADIN are a fixed nadir-oriented telescope with an oversized primary mirror and an array of off-axis secondary mirrors, addressing shot azimuth positions on a 30 degree nadir-angle cone. Step scanning is performed by a small focal plane stepper, avoiding lag-angle and torque compensation problems encountered in scanning telescope concepts. Hence, the concept is insensitive to misalignment and employs a minimum of internal alignment control at full scanning capability. It is applicable for an autonomous mission as well as for a mission on the space station.

AEROSPATIALE, leading a European team, has just conducted a successful study, under ESA contract, to demonstrate the feasibility of a spaceborne Doppler wind lidar instrument meeting the scientific requirements of wind velocity measurements from space with high spatial resolution. A first parametric investigation, based upon the initial set of mission requirements, and supported by dedicated models and detailed trade-off studies, took account of capabilities of the most promising signal processing algorithms and calibration/validation constrains: it yielded a large conically scanned instrument deemed technologically risky. A risk analysis was then carried out to propose a less challenging instrument meeting most key mission requirements. The fixed line-of-sight concept with return signal accumulation appeared as most attractive. A second set of requirements agreed upon by scientific users was therefore issued, with relaxed constraints mainly on horizontal resolution, keeping roughly the same level of wind velocity measurement accuracy. A second instrument and subsystem trade- off was then performed to eventually produce an attractive instrument concept based upon a pair of small diameter telescopes each one associated to one scanning mirror rotating stepwise around the telescope axis, which drastically reduces the detection bandwidth. Following the main contract, studies of accommodation on the International Space Station have been performed, confirming the interest of such an instrument for wind measurements from space.

Two consortia have undertaken parallel pre-phase A studies funded by ESA to determine a preferred concept for ALADIN (atmospheric laser Doppler instrument). This paper reports the results of the laser transmitter study forming part of the Dornier led instrument study. The requirements were for a high energy, frequency-stable, pulsed CO₂ laser, operating at 9.11 micrometer with a high efficiency and long lifetime. The optimum discharge technology, resonator design, pulse energy and pulse length were determined and numerical frequency chirp modeling carried out. A preliminary design was developed for a compact, lightweight device and critical technology areas identified.

The atmospheric laser Doppler instrument ALADIN will provide information about the wind velocity using the motion of small particles and the resulting Doppler shift between the transmitted and received laser frequency. The accuracy of the frequency estimation depends mainly on the signal-to-noise ratio (SNR) of the received signal. An improvement of the signal quality i.e. SNR can be done by reducing the bandwidth and by averaging over some shots. The bandwidth reduction can be performed either analogue or digital. The analogue technique includes a lot of risks and error sources, the digital demodulation and filtering shows only one critical component, the analogue to digital converter. The shot averaging can also be performed by two different techniques: coherent or incoherent accumulation. Both, the bandwidth reduction as well as the averaging techniques are presented.

A study was finished in 1994 to identify a payload package for the originally planned Russian MIR-2 station. Part of this study now can be used for an ALADIN type of instrument. Installation on a Russian module of the International Space Station Alpha gives no problems with solar panels. The 8-step version of the Daimler Benz group can be realized. Footprint structure is analyzed for a pre-operational use of the data. Additional information on the backscatter mode of an ALADIN is given.

For the measurement of the wind field from a moving platform one big challenge is the elimination of the Doppler shift due to platform motion. If the housekeeping data (platform velocity and attitude) are not accurate enough, a calibration to the ground return of the lidar is a possible solution. This presentation deals with the advantages and problems of such a procedure at the example of measurements obtained with the airborne Doppler lidar ADOLAR. Here a rectangular flight pattern from October 13, 1994 over the Nordsee is discussed together with the possibility of using this technique at a satellite.

The effect of dynamic turbulence of the atmosphere on the statistical uncertainty of wind velocity vector components measured by cw coherent lidar using sector conical scanning method has been studied. The calculations of the variance of wind velocity estimates for different scan sector angles and number of scan repetitions (rotations) have been carried out. The results obtained theoretically are compared to experimental data. It is shown that the increase of both the ratio of scan cone foundation radius to outer scale of turbulence and the number of rotations cannot considerably decrease significant random errors arising in lidar wind velocity estimates at small scan sector angles. The spatial averaging of wind velocity fluctuations over lidar sounding volume and corresponding decrease of measurement errors takes place only when scan sector angle exceeds 270 degrees.

Along with measurements of mean wind fields, Doppler lidars are used for estimation of the turbulence parameters. In particular, the attempts to use the Doppler lidars for measurement of the dissipation rate of the turbulent kinetic energy (epsilon) _T and the wind field structure constant from estimations of mean spectrum width of Doppler signal are discussed. This approach is true for small sizes of lidar probe volume. If longitudinal size of probe volume (Delta) _Z is comparable or exceeds the outer scale of turbulence L_V this method cannot be used. We discuss the feasibility of estimation of dissipation rate from Doppler lidar data at arbitrary size of lidar probe volume.

This paper presents the results of computer simulation associated with differential absorption lidar (DIAL) measurements of atmospheric water vapor profiles from spaceborne for the low- latitude (15N), for summer and winter in the mid-latitude (45N), and for summer and winter in the high-latitude (60N), respectively. We think that a Nd:YAG laser pumped Ti:sapphire laser is available to use with the spaceborne water vapor lidar at present. Then the water vapor absorption lines used in these calculations are in the 820 nm spectral region. The analysis suggests that the spaceborne DIAL system needs 2 or 3 pairs of the on (strong absorption) - off (weak absorption) laser lines for less than 10% profile measurements accuracy from 0 km altitude to about 10 km altitude in the mid-latitude.

The possibility of direct detection of tropospheric wind speed Doppler shift with an ultraviolet laser is considered. The use of the UV eliminates all practical concerns of eye safety, permits the use of uncooled detectors, and yields enhanced aerosol and Rayleigh backscatter signals. The Rayleigh signal, which in the free troposphere can exceed the aerosol signal by three orders of magnitude, is itself a candidate for wind speed measurement, despite the Doppler broadening of this signal. The basis of this approach is a diode-pumped, frequency-doubled alexandrite laser, which offers very high electrical to optical energy efficiency, an estimated 9%, in generating UV output. Efficiency is critical for a satellite based lidar system due to the size, cost, and mass of solar power generation and waste heat disposal subsystems. Pumping of alexandrite with 680 nm laser diodes has been demonstrated. Narrow linewidth, high spectral purity, and high frequency stability have been obtained with laser diode injection seeding of a ring alexandrite laser. The tunable diode laser control allows tuning of the laser for spacecraft velocity compensation. The potential performance of a wind sounding lidar scaled to match the 300 W power capability of a mid-sized satellite is evaluated for the extremely weak aerosol conditions of the southern hemisphere oceans. A 20 W output laser system, with 1 m aperture telescope, at 350 km altitude, may yield measurement precisions better than plus or minus 3 m/s through most of the troposphere, deteriorating to plus or minus 10 m/s under extreme conditions. A Rayleigh backscatter system will yield plus or minus 3 m/s precision to 8 km altitude, plus or minus 5 m/s at 15 km, even with zero aerosol content.