The present paper reports on the design and performance of a scanning, photon-counting laser altimeter, capable of daylight operations from aircraft cruise altitudes. In test flights, the system has successfully recorded high repetition rate returns from clouds, soils, man-made objects, vegetation, and water surfaces under full solar illumination. Following the flights, the signal was reliably extracted from the solar noise background using a Post- Detection Poisson Filtering technique. The passively Q-switched microchip Nd:YAG laser measures only 2.25 mm in length and is pumped by a single 1.2 Watt GaAs laser diode. The output is frequency-doubled to take advantage of higher detector counting efficiencies and narrower spectral filters available at 532 nm. The transmitter produces several microjoules of green energy in a subnanosecond pulse at rates approaching 10 kHz. The illuminated ground area is imaged by a 14-cm diameter, diffraction-limited, off-axis telescope onto a segmented anode photomultiplier. Each anode segment is input to one channel of fine range receiver (5-cm resolution), which records the times-of-flight of individual photons. A parallel coarse receiver provides a lower resolution (greater than 75 cm) histogram of all scatterers between the aircraft and ground and centers the fine receiver gate on the last set of returns.

This contribution describes a ranging system based on a new kind of optical sensor, called the Photonic Mixer Device (PMD). This unit combines fast optical sensing and modulation of incoherent light signals in one component part by its unique and powerful principle of operation. The key feature of this new sensor is the direct detection of the correlation value between the light and an electrical reference signal as an analog voltage value. The realized ranging system is based on the following concept: a laser diode or LED is modulated with a PN-code sequence and the reference signal consists of the same PN-code sequence. With a delay unit the phase of the reference signal is shifted until the PMD-sensor returns an expected correlation value. The delay between the reference and the reflected light signal implies information concerning the distance to the reflecting object. An array of these sensors provides a 3D- ranging system that could be used for, e.g., collision avoidance in new traffic control systems or in product monitoring systems. Fast modulation of the laser and the PMD provides a high distance resolution. Nowadays PMD-realizations are based on standard CMOS technology, but we also keep investigation in a new kind of PMD based on MSM technology (metal- semiconductor-metal).

Although developments of Remotely Operated Vehicles have been rapid, the underwater robot vision is still regarded as a major tough undertaking when being maneuvered by a remote pilot, particularly in turbid water condition. Sonar imaging offers a large range for detection but often falls short in achieving the required resolution for identification. Normal optical image method such as underwater camera offers inherently high resolution capability, but the absorption and scattering characteristic of marine medium severely limits range performance, e.g. the visible range in Singapore coastline area is 2-3 meter. Currently, an experimental underwater laser gated-ranging system for underwater robot vision has been finished and precisely demonstrated ns-level gated images in highly turbid waters. We are first to study in such a 3-m short water tank with highly turbid waters successfully, and show the range-gated phenomenon much more clearly based on our newly-designed series targets. The target-set is a series of three bar test targets at intervals of 22.5 cm since the light speed in water is 22.5 cm/ns. We synchronize to range-gate on the targets which we want to have its image, and to gate out the targets which we don't want its image. The attenuation coefficient in the turbid water tank is 1.0/m and 2.3/m separately.

Using the inverse filtering of the outgoing laser pulse and of the return signal, one can increase the range accuracy of a laser radar for a single pulse. The method was tested experimentally and in the virtual world. A virtual lidar instrument was developed for further studies. The pulse length can vary from 10 ns up to 500 ns for hard targets ranges to remain within 50-cm accuracy. For a gain switched pulse like from a CO2 Doppler lidar with 3 ms pulse duration, the hard target return can be determined with 1-m resolution. For a hard target with an aspect angle to the laser radar, one can determine this angle with sufficient accuracy. The method will be described in detail.

We perform maximum range calculations for surface sensing direct-detection mid-infrared Random-Modulation Continuous- Wave (RM-CW) lidar, and propose a system's figure of merit. In this type of lidar, noise can be additive and determined by detector's noise given by its specific detectivity D, area, and integration time. Fundamental limits of the sensing range are then imposed by diffraction, provided that the Background-Limited Infrared Photodetection (BLIP) limit (controlled by the field-of-view) is not exceeded. There is no dependence on a specific pseudorandom code (PRC) used since noise spectral density is assumed to be independent of frequency. Under most favorable conditions, including no losses due to optics or atmospheric attenuation, an object of angularly uniform unity reflectance and size not less than the laser beam can be detected within 1s with a 20-cm diameter telescope and a signal-to-noise ratio of one from a distance of about 200 km if a cooled detector and a laser emitting 500 mW cw at 5 micrometers are used. For an integration time of 0.1 ms (which is well below a typical atmospheric correlation time), or a room-temperature detector, the maximum range reduces to about 20 km. As the technology of Quantum-Cascade lasers (QCLs) advances, the maximum range of several tens of kilometers can be expected in this type of lidar with all-thermoelectrically-cooled semiconductor components, and, perhaps after some modifications in the technique, about 1 km with all-room-temperature components.

We are presenting a new satellite laser ranging technique. The Differential Satellite Laser Ranging (DSLR) is a technique providing highly accurate short ground baselines and relative station heights. Two satellite laser ranging systems are sharing one common laser transmitter. Recording the echo pulses arrival times at different locations, the corresponding baseline and relative station height may be evaluated. Due to the differential approach, most of the error contributors present in the conventional satellite laser ranging are minimized or not existing at all: system internal delay, ground survey, satellite orbit modeling, atmospheric correction and satellite signature. Due to this fact the baseline may be determined with high accuracy on the basis of several satellite passes, only. The presented technique may be used as a powerful tool for satellite laser ranging systems diagnostics. The DSLR opens new perspectives - in collocation experiment it permits to check the system calibration independently on the ground target ranging. For the laser ranging systems equipped with several independent receivers the DSLR enables to double - check the system internal delay - the calibration value, once the ground target ranging is inaccurate or even impossible.

The French Transportable Laser Ranging Station (FTLRS) is a highly mobile laser ranging station dedicated to geodetic and oceanographic satellite tracking. Its performances were estimated during a probatory experiment in Corsica (from October 1996 to February 1997). The results showed that the 1-sigma rms was close to 2 cm for 80% of the passes. But, about 20% of the passes exhibited significant variations in the signal quality (as much as 10 cm) and a large bias. The FTLRS has been considerably improved to reach the 1 cm accuracy. That is the level necessary for JASON-1 validation and calibration for which the FTLRS is a main component. The instrument evolutions concern all the FTLRS fundamental components: laser, detection devices, timing chain, and calibration process. Thanks to laboratory measurements and to some comparisons with the Grasse laser ranging fixed station with similar characteristics, the FTLRS new performances are satisfactory: the ranging accuracy may be of about 11 mm, with 9 mm due to the station. The first results of this new collocation experiment are presented.

We designed the arrangement of the corner cube reflectors to be carried on the H2A-LRE satellite. There are 126 reflectors on its quasi-spherical surface; 66 are made of fused silica and the others are made of BK7 glass. Fused silica remains active for more than a few decades, while BK7 degrades in a short time. The reflectors are arranged on the body faces so that the satellite can respond most angles of incidence and so that the optical response does not change significantly after the BK7 reflectors degrade. The optical response from the H2A-LRE satellite was simulated based on the measured position of all its reflectors. The centre-of-mass corrections for multi-photon systems were calculated at 215.3 mm while the BK7 was active and at 210.5 mm after it totally degraded. For single-photon systems, the corrections were 208.5 mm before degradation and 204.7 mm after degradation.

The precision and accuracy of the Satellite Laser Ranging (SLR) is one of the biggest issues for the entire network to perform as a calibrator of the other space-born geodetic systems and to establish millimeter level accurate terrestrial reference frame. The principal idea behind the Portable Calibration Standard (PCS) is the high degree of redundancy in measuring hardware, data analysis software and operational procedures. The most rigorous calibration would involve a complete reference SLR system, which would collocate with the system under test, a more economical approach allows the laser, telescope and optical detectors of the system under test to be shared. In this configuration, the output timing signals from the optical detectors are processed in an independent device - Portable Calibration Standard. It consists of the Pico Event Timer, GPS time and frequency receiver, meteorological sensor and a personal computer with the software package. The main parameters of the PCS based on a Pico Event Timer are: single shot precision 3 psec rms, timing linearity 2.5 psec, stability +/- 0.5 psec/hour, drift below 0.1 psec/K, the accuracy traceable down to US National Standards.

A Frequency Modulation Continuous Wave (FMCW) laser range- finder has been designed for distances from 1 m to 20 m with cooperative or diffusing targets. This range-finder is compared to a phase shift laser range-finder. A heterodyne photoreception method is studied. In order to reduce noise, the nonlinear current-voltage characteristic of the APD is used to obtain an optoelectronic mixer. The photocurrent gain is modulated by the VCO signal working as a local oscillator (LO). The optical and electrical LO injection methods are studied. The theoretical analysis using the Miller model and thermal effects are compared with the first experimental results. The electrical LO injection gives a better ratio-to-noise than the optical LO injection. For electrical injection, it is shown that the heterodyne conversion gain can reach the maximal theoretical value 0.5.

We are presenting the concept, design and construction of an all solid state photon counting detector package for the laser transponder operating on the surface of the planet Mars. The Earth-Mars laser transponder experiment is proposed by NASA to provide range measurements to the Mars planet with decimeter accuracy. To detect the optical pulses at the Mars side of the transponder link, the photon counting detector with nanosecond timing resolution is required. The solid state photon counter based on silicon avalanche photodiode has been selected for its unique properties just suitable for the Mars surface environment: low operating voltage, low power consumption, low sensitivity to radiation. The concept of the detector enables to avoid any analog signal processing - amplification and discrimination, which are usually both temperature dependent and power consuming. The detector is based on the space qualified components.

Precise orbit determinations in SLR and LLR results require corrections for the contribution of the atmosphere to the effective index of refraction. Despite the fact that the range measurements are obtained consistently, there is a discrepancy of approximately 4 cm reported in the comparison between optical and microwave based range measurements for the same GPS satellite target. As a conclusion from earlier work, the knowledge of the water vapour content of the atmosphere at least for some selections of the operation wavelength seems important. To overcome the lack of sensitivity for the measurement of water vapour profiles in the Earth's troposphere by means of remote sensing, a biaxial Rayleigh- and Raman-LIDAR has been set up on the Fundamentalstation Wettzell, making use of the still existing equipment of a former SLR facility. By operating the station as a Rayleigh-LIDAR, its geometrical characteristics and the overall sensitivity of this setup as a LIDAR system could be established and optimised. Observations in the spectral range of 607 nm and 660 nm are probing the troposphere for nitrogen and water molecules. By combining this information water vapour profiles of the lower troposphere were obtained.

LaSen, Inc. is continuing development of compact and rugged lidar systems for ground-based and airborne detection of volatile organic components (VOC) in the mid-infrared spectral region. The first-generation ground-based sensor (Lidar I) measures 0.19 m³ and weighs 55 kg. Throughout 2001, Lidar I is undergoing a series of field tests in south-central New Mexico, USA. The tests completed to date include measuring methane level at rural dumpsites and monitoring emissions from contaminated soil at a diesel fuel spill site. To support the lidar programs, LaSen developed a compact tunable mid-infrared laser source based on an intracavity optical parametric oscillator. The laser source delivers up to 1.3 mJ of mid-infrared energy per pulse and is angular tuned in the 3-5 micrometers band through a servo- controlled loop. A piezo-driven tuner for fast switching between on and off lines is available as an option. Lidar I performance characteristics relevant to detection of various VOC are discussed. The second-generation sensor (Lidar II) is undergoing development and is scheduled for tests in the fourth quarter of 2001. Lidar II is a re-packaged version of Lidar I, with a significant decrease in both weight and size. Although not specifically designed for an airborne platform, Lidar II will be capable of demonstration flights on board a helicopter operating in a look-down topographic mode.

The design of a ground based and transportable combined Raman elastic-backscatter lidar for the remote sensing of lower tropospheric water vapor and nitrogen concentration is described. This lidar is intended to be used for an external calibration of the wet path delay of GPS signals. A description of the method used to derive water vapor and nitrogen profiles in the lower troposphere is given. The instrument has been tested during the ESCOMPTE campaign in June 2001 and first measurements are presented.

Data obtained from the USU incoherent backscatter lidar at Logan Utah during the interval 1994 through 1998 has been analyzed in terms of the climatology of stratospheric and lower mesospheric mean density and density variance (delta) (rho) /(rho) )² vs. altitude. The total database consists of over 322 nights of cloud-free data over the altitude range, 40 to 90 km. We selected a subset of 142 days (886 hours) of the highest signal to noise ratio data for the analysis. The temporal and spatial resolutions were 2 minutes and 112.5 meters respectively. We compared the mean density profiles, normalized to MSISe90 on a monthly and seasonal basis to MSISe90. The mean density profiles compared well with the MSISe90 model with the following exceptions. The measured densities were less than MSISe90 predictions in the 45 to 80 km range by up to 10% in the 1st calendar quarter, and greater than MSISe90 density predictions in the last calendar quarter, by about 8 to 10 %, but over a narrower altitude range. The density variance profiles (delta) (rho) /(rho) )² are proportional to the mean free energy of the atmospheric gravity wave and turbulent structure.

Results from scanning lidar measurements obtained during a large field study entitled Physical and Chemical Evolution of Aerosols in Smelter and Power Plant Plumes will be presented. The Meteorological Service of Canada has recently developed a mobile scanning lidar facility capable of fast azimuth and elevation scanning profiles of the lower troposphere. The lidar was located downwind of the stack source and the scanning speeds were adjusted to allow a complete scanning profile to be collected within 30-60 seconds. The lidar portion of the study was divided into four three- week parts. Ontario Power Generation-Nanticoke is a coal- fired generating station on the shores of Lake Erie. The lidar ground location selected for both the summer and winter study was approximately 8.5 km east of the stacks. The other location was the Noranda-Horne Smelter in Northern Quebec. The lidar was located approximately 4.6 km SE of the smelter during the winter and approximately 2.5 km NNE during the summer study. Preliminary lidar results will be shown for both Nanticoke and Noranda with a focus on the differences between winter and summer periods when the humidity and boundary layer dynamics were very different. New algorithms are being developed to extract boundary layer heights and plume heights for comparison with model predictions.

The development of the techniques for spaceborne detecting gas anomalies in the lower atmosphere is very important. The results of the Kioto protocol, an extended use of hydrocarbon raw material, the occurrence of new chemical emissions call for further realization of global control over gaseous contaminations in the atmosphere. A spaceborne location of sensors is very promising for solving this problem. In this case the light automatic satellites, oriented to a limited area of application, are the most promising. As a rule, for such satellites the orbits at 500 km altitude and more are selected. This altitude and small mass of a satellite impose severe requirements on the efficiency of the techniques and apparatus realizations. Taking account of the last-named fact, the paper describes the use of the differential absorption method with a reflection from the Earth's surface for global monitoring of gaseous contaminations. The experiments were performed to assess lidar detection of ground anomalies of hydrocarbons in the 3-5 micrometers transmittance window. It is shown that, as applied to a spaceborne platform MKA- 200, this technique provides for localization of the background concentration excess of gases of hydrocarbon cycle with an error from 15% to 25%.

Estimations are carried out on creation possibilities of all solid state laser sources capable significantly or fully to solve the problem of the universal Aerosol-Gas Lidar System design. Best existing Ho²⁺:ILF and Nd:YAG lasers supplied with LBO, KTA, KTP, BBO, CLBO, DLAP; GaSe, GaSe:In, AgGa_xIn_1-xSe₂, LiInS₂, LiInSe₂, AgGaS₂, AgGa_xIn_1-xS₂, and HgGa₂S₄ frequency converters are considered. The investigation results show development of UV to FIR laser source is really to carry out with efficiencies from one-two up to several tens of percent in several ways.

The goal of the GroundWinds program is to develop and demonstrate technology for a future space-based LIDAR system that will measure the vertical structure of global wind profiles (Nardell and Hayes 2000). The only current source of such data is from balloonborne radiosondes, which are common over the U.S. mainland but rare over the oceans. A satellite system would provide wind measurements distributed all over the globe. Because the air over the Pacific is largely free of aerosols, the NOAA observatory on Mauna Loa at 3-km altitude is an ideal site for testing the optics, hardware, and detectors needed for the satellite system. The GroundWinds Hawaii system is scheduled to begin operations in fall 2001 and can operate day or night. Thereafter, LIDAR team plans continued operations to provide calibration support for the satellite system when it flies.

Possibility of sounding of the clear-air turbulence by airplane coherent Doppler lidar is analyzed based on the numerical simulation. It is shown that the accuracy of estimation of the strength of turbulence from the lidar data depends on the intensity of turbulence, the signal-to-noise ratio, and number of sounding pulses. The conditions are determined for which detecting of the clear-air turbulence zones with the coherent Doppler lidar is possible.

Lunar Laser Ranging (LLR) data can be used to determine parameters of the Earth-Moon system (e.g. lunar gravity, tidal parameters) and relativistic effects in the solar system. Moreover, LLR contributes to the realization of the International Terrestrial Reference Frame (ITRF) and provides a set of Earth Orientation Parameters (EOP) like UT0 or nutation coefficients. A prerequisite for a good analysis is the availability of sufficient observations with high accuracy from globally well distributed sites on the Earth to the reflectors on the Moon. However, the measurement of the Earth-Moon distance is difficult and much more challenging for the laser system than satellite laser ranging (SLR). The signal to noise ratio of the received signal is low because a lot of power is lost on the round trip from the Earth to the Moon. Therefore special observation strategies and processing algorithms are required. To improve the visibility of the real lunar returns in the noise, a semi-pulse pattern is incorporated in the analysis of the raw data by applying a correlation procedure. Based upon these results and after subtracting the noise, the final normal point is computed. Here, all relevant processing steps are explained, where the big advantage of the new processing method will be shown by examples.