The Bundesamt fuer Kartographie und Geodaesie (BKG) will provide a major contribution to the improvement and maintenance of the global reference frames: ICRF (International Celestial Reference Frame), ITRF (International Terrestrial Reference Frame) with the operation of TIGO (Transportable Integrated Geodetic Observatory). TIGO is designed as a transportable geodetic observatory which consists of all relevant geodetic space techniques for a fundamental station (including VLBI, SLR, GPS). The transportability of the observatory enables to fill up gaps in the International Space Geodetic Network and to optimize the contribution to the global reference frames. TIGO should operate for a period of 2 to 3 years (at minimum) at one location. BKG is looking for a cooperation with countries willing to contribute to the ITRF and to support the operation of TIGO.
The lunar laser ranging (LLR) has been dependent on the retro-reflectors placed on the Moon since the past three decades before. In spite of the technical improvement, regular observations are performed by a limited number of stations due to the weak echo. To allow more opportunities for observations, it is most effective to place modern device on the Moon. Since a Japanese second lunar probe is planned to be launched in 2006, the ideas are collected for scientific purposes. It is described here that an optical transponder on the Moon would enhance the possibility of LLR observations to a large degree. It is also proposed to have three radio sources on the Moon to determine the angular component perpendicular to the Earth-Moon vector and librations. One of the most important scientific targets for the proposed mission is a relativistic experiment which was not attained by a conventional LLR.
Space geodetic techniques of artificial satellites tracking have been greatly improved since this last decade. Particularly, the successful development of radio tracking systems like GPS, DORIS, and PRARE has to be emphasized, concerning precise positioning, orbit determination, and gravity field determination. All these techniques are able to operate under all weather conditions and are of very easy use. In fact, all the complexity of systems is to be found in space on board the satellites. To some extent, for laser satellite tracking, the situation is the opposite. It is weather dependent and comparatively complex and costly at the ground level. However, this technique is the most straightforward in concept for accurate measuring of the Earth-satellite distance. But, it is true, provided that instrumental biases can be well understood and precisely estimated. As a result, the role of laser tracking has to be reassessed and the control of range biases is nowadays of the utmost importance. Therefore, the international organization for laser cooperation has been reconsidered and the purpose of this paper is to present the effort made in this field at the Grasse, France observatory.
We have devised a new method to identify a timer's behavior at frequencies of tens of megahertz which corresponds to the basic vernier frequency of the device. Our idea is that spectral analysis should be applied to the post-fit residual of full-rate satellite tracking data with respect to the two-way range. To attain tens of megahertz resolution and one ps sensitivity, we needed to collect nearly one million shots of full-rate data of low-orbit satellites, and we got them from Herstmonceux, UK, and Orroral, Australia. The results of the spectral analysis showed two clear peaks of a few ps exactly at 22.5 and 45 MHz in Herstmonceux data, and a peak with a similar size exactly at 1 MHz in Orroral data. We conclude this effect is very small for these two stations. However, in general, we have shown that this kind of analysis can detect such a timer-related problem which will cause an offset bias due to the fixed-range nature of ground target calibration.
To contribute to future laser ranging we have developed the Pico Event Timer (PET), consisting of two, or more, Event Timing Modules, Clock Generator Module, Input Trigger Module, Control Unit, Range Gate Generator, Microprocessor Module and the Software Package. The Pico Event Timer is designed and constructed entirely for the purpose of millimeter precision laser ranging to Satellites and the Moon. Timing Modules provide the picosecond event timing, Clock Generator Module supplies the precise clock frequency for timing, the Gate Generator Module maintains the smart arming and gating of the entire timing system. The Microprocessor Module equipped with a powerful operating system dedicated for parallel processing provides a real time control of the device, evaluates in a real time the gating epochs, identifies the measurements pairs and controls the data flow between the PET device and the host computer. The PET has been employed at five laser ranging stations for millimeter precision, two wavelength satellite laser ranging and for calibration reference purposes.
To maintain high accuracy for the Herstmonceux SLR system, we have carried out a detailed study of the potential for observational bias from two key elements of the system: range-dependent bias induced by the time-of-flight measurement process; and satellite signature effects which are ranging-system dependent. We have carried out tests on our cluster of four Standard SR620 computers and find that range-dependent bias introduced by any one of the timers can approach a level of up to one centimeter at the distance of the prime geodetic satellite Lageos. At the single-photon level of return the characteristics of the satellites' retroreflector arrays affect the distribution of the observations, and consequently corrections to be applied to the data to refer them to the center of mass of the satellites depend upon the processing technique.
Avalanche photo diodes operated in the Geiger mode in an laser ranging application can be considered almost as digital detectors. The output signal amplitude does not show any dependence on the input light level. On the other hand the incoming light intensity accounts for the characteristic delay in the response time of this semiconductor detector. Once a break down of the bias voltage occurs, the amplitude steeply increases to a preset voltage level until the avalanche is being quenched by the electronic circuit. However the evaluation of the signal risetime makes a correction of the observed delay caused by a variant transit time inside the avalanche diode possible. A simple simulation model was made and matched to experimentally obtained measurements in order to understand the basic mechanism of the observed timewalk. A linear relationship between the characteristic delay of the detector response and the measurable output pulse risetime can be exploited for a correction to the measured range between a laser ranging tracking station and the target satellite. An electronic circuit was designed for an automatic shot by shot correction of the SLR-measurements, where large fluctuations in the receive signal intensity are commonplace because of the turbulent atmospheric propagation path. This unit has been evaluated under laboratory conditions and in the satellite ranging application.
SLR2000 is an autonomous and eyesafe single photon-counting satellite 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 that of current manned systems, and the system will be about 75% less expensive to operate and maintain relative to the current manned systems. Computer simulations have predicted a daylight tracking capability to GPS and lower satellites. Computer and hardware simulations have demonstrated the ability of our current correlation range receiver and autotracking algorithms to extract mean signal strengths as small as 0.0001 photoelectrons per pulse from solar background noise during daylight tracking. The initial SLR2000 system concept was developed in 1994 , and the technical approach was refined in later years . However, significant funding for the project was not provided by NASA until August 1997. During the first year of funding, prototypes of several "enabling' components, without which the system is not feasible, were successfully developed. These include: (1) a sensitive, high speed, quadrant microchannel plate photomultiplier; (2) a moderate power microlaser transmitter; (3) a "smart" meteorological station; (4) a high speed range gate generator; and (5) a high speed, high resolution event timer. Once the key specifications on these advanced components were largely met and system feasibility had been established, attention then turned to the detailed engineering design and procurement of more conventional elements of the system such as the shelter and protective dome, arcsecond precision tracking mount, telescope, and optical transceiver. The principal challenge during this second phase was to keep prototype fabrication and replication costs as low as possible to meet our cost goals. Prototypes of the various SLR2000 components and subsystems have either been developed or are well into the detailed design! build phase. The system is scheduled to conduct field tests in the 2000-2001 time frame. The primary driver for schedule is a fixed level of funding available each year to support SLR2000 development. A fairly detailed engineering overview of the SLR2000 system was presented approximately one year ago at the 1 1th International Workshop on Laser Ranging in Deggendorf, Germany, and has recently been published in the Workshop Proceedings [31. In addition, the SLR2000 project maintains a web site at the following URL address: http://cddisa.gsfc.nasa.gov/920_3/slr2000/slr2000.html Thus, only a brief overview of engineering status and a summary of recent developments (i.e. within the past year) on the various subsystems will be given here. The reader is referred to earlier publications [1-3] for more detail on the overall system.
For the SLR system to be described, it has been decided to use the bistatic approach. The two separate telescopes are spaced by about 2 meters and are using identical azimuthal mounts. A 130 mm diam. lens is used for transmitting the laser beam and a 450 mm parabolic mirror for the receiver. Each telescope is enclosed by an independently driven housing isolating it from wind forces as well as environmental contamination thus enabling even a use without any dome. The telescopes are driven by direct on-axis motors which are controlled by built-in digital servo systems. Moving cables are avoided by sliding contacts allowing a continuous rotation in both axes without limitation. The focal units of each telescope are mounted in separate boxes below the platform of the observation building. The transmitter box contains the variable beam expander and the receiver box contains the filters (spatial and spectral) as well as the photoelectric receivers.
Atmospheric turbulence is, next to the limited size of transmit apertures and the degree of collimation of the transmit laserbeam, the most influent effect limiting the maximum possible link budget in Satellite Laser Ranging (SLR) systems. A low order adaptive optics system for SLR stations is discussed and evaluated in terms of the usually rugged seeing conditions at SLR-sites. In contrast to astronomical adaptive optics applications, this system uses sunlit geodetic satellites or lunar craters as point source reference objects in order to impose a precorrection on the transmit metrology laser beam wavefront.
The aim of the project `Tectonica-A' is the development of the technique and the creation of a spaceborne lidar for determining the sites of enhanced concentration of surface aerosols. The lidar data combined with the data of different sensors (spaceborne and ground-based) will be used for the short-term forecast of the earthquakes. Using the requirements for the lidar `Tectonica-A' the computer simulation was made of the operation of the elastic scattering multifrequency lidar in orbit at 600 km altitude. The need for selection of no less than three sounding wavelengths in the range of 300 - 1000 nm is shown. The lidar potential is justified to meet the demand for the necessary space resolution. Besides, the program of physical modeling of the lidar `Tectonica' was developed for the study of dynamics of the aerosol concentration in the atmospheric boundary layer in the seismic active area of Russia (the lake Baikal). The optical observations are combined with the simultaneous measurements of the electromagnetic radiation intensity of the Earth's crust.
We have measured and simulated the evolution of the diurnal water vapor profile over the Crati valley during a clear sky summer day. A twin TEA CO2 laser based dial (Differential Absorption Lidar) system has been developed to measure the water vapor profiles. The acquired data shows the vertical daily trend at a range up to 1.5 km. These measurements are compared with modeled profiles. The simulations have been performed using the CSU-RAMS model. Preliminary results show that the orography plays a fundamental role in determining the vertical distribution of water vapor and in the development of the Planetary Boundary Layer (PBL). The diurnal evolution of the PBL can be inferred from the measured profiles and, as a consequence, they can be used to improve the model. The measured and modeled vertical profiles show reasonable agreement.
An airborne coherent Doppler Lidar to retrieve mesoscale wind fields has been developed in the frame of the Franco- German WIND project. The instrument is based on a pulsed CO2 laser transmitter, heterodyne detection and wedge scanner. The performance of the instrument operating on the ground and in the aircraft is reported.
A novel imaging laser radar system using a Tm, Ho:YLF laser, applicable for multi-function such as obstacle warning avoidance, range finding, range mapping and target profiling was fabricated. The maximum field of view of the laser radar image produced with Palmer scanning is 12 degree(s) by 48 degree(s). A tower at 2,500 m was observed, demonstrating its capability for range finding function. Obstacle warning avoidance function and target profiling function was tested by taking the laser radar image of the wires and a car, respectively.
Lidar is a well established remote sensing method. One gets range resolved information from remote location. Ranging, cloud ceiling, aerosol layer identification are a few examples. Using a lidar from satellites gives topographic maps of the Earth's surface (Degnan 1997). For environmental purposes the aerosol smoke stack emission canbe monitored (Measures 1983, Klein und Werner 1993). The measuring principle is based onthe time measurementof a laser pulse reflected froma target. Targets canbe hard targets like the Earth's surface or diffuse targets like clouds. The backscattered signal contains the information on the density characteristics of diffuse targets or the reflection characteristics of the hard target. The range resolution depends onthe pulse characteristics transmitted. In the lidar application the range resolution Lx is proportional to the laser pulse length zt: 2 Therefore, for satellite laser ranging short pulses are used to get resolution in the order of centimeters (for example t =200 ps gives 3 cm resolution for a single event). Can one get the same resolution with a longer pulse? -is one of the questions this paper focusses on. The second question is the identification of aerosol concentrations with high range resolution.
In the marine boundary layer over coastal areas aerosol size distribution and aerosol concentration strongly depend on wind speed, direction and duration. Therefore, measurements have been taken from seven stations on the Polish coast of the Baltic Sea by means of the lidar system FLS-12. The results obtained provide valuable inputs for investigations of the physical processes, as well as an important an data set for use in the development of aerosol type modeling and aerosol dynamics in the coastal areas of the southern Baltic Sea.
Since multiple scattering (`MS') makes it more difficult to interpret lidar returns in terms of characteristics and parameters of a sounded medium, since 1987 there have been a series of workshops aimed at comparing research dealing with MS and lidar technique.
Supercooled cloud and precipitation water droplets constitute a safety hazard to aviation. Concerned agencies are funding research on methods of remotely sensing the icing potential of such clouds and precipitation. The parameters needed are the temperature, the liquid water content (LWC) and the average droplet size. We report here on preliminary results obtained with a multiple-field-of- view (MFOV) lidar in an experimental program carried out at Mount Washington during April 1999. The MFOV technique consists in the measurement of the forward multiple scattering contributions to lidar returns coupled with a solution method that makes use of this additional information to calculate the extinction coefficient and the effective droplet diameter as the dependent functions, and the LWC as a by-product. The paper describes the MFOV retrieval method and gives sample results obtained in the Mount Washington experiment. The retrieved solutions demonstrate the lidar capability of remotely sensing droplet size and LWC profiles of clouds and precipitation. Solution accuracy is determined to be 30 - 40% but the analysis of the data from all fielded sensors will be needed to validate these numbers.
We present results from a study investigating the influence of multiple scattering effects on measurements with ELISE, a new spaceborne lidar to be launched in 2002. The influence of the multiple scattering portion of the total detected signal on measurements of aerosols in the Planetary Boundary Layer is discussed.
Refined data of spaceborne sensing of the Earth's surface with the BALKAN lidar in the spring of 1996 are presented. The lidar operated in the mode of orientation of the orbital coordinate system (calculated), in which the lidar optical axis was oriented near the nadir. Measured, Dm(f), and calculated, Dc(t), distances of the ocean surface are compared for sensing sessions examined. In each session the difference (Delta) D(t) equals Dm(t) - Dc(t) had different signs and slopes with respect to the zero line, different systematic biases, and weak oscillations. With consideration of telemetry report on the oscillations of the axes of the fixed station coordinate system about the reference orbital coordinate system, several possible reasons for the oscillations of the difference (Delta) D(t) are analyzed. In addition to the oscillations of the axes of the fixed station coordinate system, the effect of wind- driven ocean waves is evaluated.
The first aircraft experiment with the Wide-Angle Airborne Laser Ranging System has been conducted in May 1998 over an air base in France equipped with a network of 64 cub-corner retroreflectors. The ranging system was operated from the Avion de Recherche Atmospherique et de Teledetection of CNES/IGN/INSU. Data have been collected during two 4-hour flights. The paper describes the data processing methods and presents the first experimental results. The precision is of 2 cm on the difference of vertical coordinates from two sets of 3 X 103 distance measurements, which is consistent with simulations and a posteriori covariance. The precision is mainly limited by the smallness of the number of efficient measurements remaining after a drastic data sorting for outliers. Higher precision is expected for future experiments after some instrumental improvements (achieving higher link budget) and measurement of aircraft attitude during the flight.