Laser Ranging System Research Articles

Absolute Calibration of Jason-1 and TOPEX/Poseidon Altimeters in CorsicaSpecial Issue: Jason-1 Calibration/Validation

The double geodetic Corsica site, which includes Ajaccio-Aspretto and Cape Senetosa (40 km south Ajaccio) in the western Mediterranean area, has been chosen to permit the absolute calibration of radar altimeters. It has been developed since 1998 at Cape Senetosa and, in addition to the use of classical tide gauges, a GPS buoy is deployed every 10 days under the satellites ground track (10 km off shore) since 2000. The 2002 absolute calibration campaign made from January to September in Corsica revealed the necessity of deploying different geodetic techniques on a dedicated site to reach an accuracy level of a few mm: in particular, the French Transportable Laser Ranging System (FTLRS) for accurate orbit determination, and various geodetic equipment as well as a local marine geoid, for monitoring the local sea level and mean sea level. TOPEX/Poseidon altimeter calibration has been performed from cycle 208 to 365 using M-GDR products, whereas Jason-1 altimeter calibration used cycles from 1 to 45 using I-GDR products. For Jason-1, improved estimates of sea-state bias and columnar atmospheric wet path delay as well as the most precise orbits available have been used. The goal of this article is to give synthetic results of the analysis of the different error sources for the tandem phase and for the whole studied period, as geophysical corrections, orbits and reference frame, sea level, and finally altimeter biases. Results are at the millimeter level when considering one year of continuous monitoring; they show a great consistency between both satellites with biases of 6 ± 3 mm (ALT-B) and 120 ± 7 mm, respectively, for TOPEX/Poseidon and Jason-1.

Centimeter accuracy for the french transportable laser ranging station (FTLRS) through sub-system controls

The French Transportable Laser Ranging Station (FTLRS)is a highly mobile satellite laserranging (SLR) system dedicated to the trackingof geodetic satellites equipped withretroreflectors. This station weighs only 300kg witha 13-cm diameter telescope and is housedin eight containers. The reliability of such a station and its accuracy of 2 cmin real field experiment conditionswere demonstrated during a first field campaign carried outfrom October 1996 to February1997 near Ajaccio on Corsica Island, France. The results ofthis probatory experiment suggestedthat several technical improvements and some modificationswere necessary for JASON-1validation and calibration phase and for new applicationssuch as the Time Transfer by LaserLink (T2L2) experiment. A first change concerns theuse of a new laser wavelength (green instead ofinfrared) and of a new avalanche photodiode with atime walk compensation system. Anotherchange is the installation of a coaxial cabletransmitting directly the signal coming from thereturn detector. Finally, a new calibration systemwas developed with several other changes.A short description of the system is first given.Then, the major changes and the main resultsof ground accuracy tests are summarized and presented.

Development of the Active Laser Ranging System for Autonomous Underwater Vehicles

Development of the Active Laser Ranging System for Autonomous Underwater Vehicles

Optimization of Methods of Data Acquisition and Processing in Satellite Laser Ranging Systems

The opportunities for increasing the accuracy of measurement by satellite ranging systems operating in an analog mode of echo-signal reception are considered. In particular, a new method is proposed for including the weights of measurements when processing the results of satellite laser ranging.

Sub-Cm Subsidence Measurements With The Wide-Angle Airborne Laser Ranging System

The Wide-Angle Airborne Laser Ranging System (WA-ALRS) is aninstrument developed at IGN with the aim of measuring landsubsidence with sub-cm accuracy. The instrument provides rangemeasurements that are used to estimate the co-ordinates of aground-based network of cube-corner retroreflectors (CCRs).Millimeter accuracy on relative radial co-ordinates has beendemonstrated in a ground-based experiment and centimeter accuracyhas been obtained in a recent airborne experiment. The mainlimitation in the latter experiment came from the low link budgetat longer distances. In this paper we analyze the performanceexpected with two different detector modules (avalanchephotodiodes with amplifiers). Laboratory measurements are used tocharacterize detector parameters and estimate ranging accuracy.Numerical simulations predict a 2–3 mm accuracy in the verticalco-ordinate with these detectors combined to a 2-km aircraftaltitude, a 20 laser beam divergence, a 250-m CCR separation and∼ 103 measurements per CCR.

Towards sub-microarsecond rigid Earth nutation series in the Hamiltonian theory

AbstractThe nonrigid Earth nutation series adopted by the IAU (International Astronomical Union) are based on the works of Kinoshita (1977) and Wahr (1979). In the first one, the rigid Earth nutation series were calculated by the application of the Hamiltonian canonical equations to the rotation of the rigid and elliptical Earth. In the second one, the transfer function for the nutations of an elliptical, elastic and oceanless Earth with fluid core and a solid inner core was obtained. The nonrigid Earth nutation coefficients were derived from the convolution between Wahr’s transfer function and Kinoshita’s rigid Earth nutation series.The improvement in the accuracy of the techniques as a Very Long Baseline (VLBI), Lunar Laser Ranging (LLR) and Global Positioning System (GPS) has led in this decade to the extension of Kinoshita’s theory and more precise determination of Wahr’s transfer function. In the present paper and starting from Kinoshita’s work (1977), we present the different steps carried out, during this last decade, to obtain the sub-microarcsecond rigid Earth nutation series REN 2000 from the Hamiltonian study of the rotation of a rigid Earth (Souchay et al., 1999).

Investigation on the combination of space geodetic techniques

The Space Geodesy Center (CGS) of the Italian Space Agency (ASI), located near Matera (Italy), hosts permanent Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Global Positioning System (GPS) systems and manages the other receivers of the Italian GPS Fiducial Network. The presence of these collocated space geodetic systems in the area and the data analysis activities usually carried out at the CGS for each of the three techniques makes the construction of fully integrated solutions a natural objective. This paper describes the preliminary work carried out to assess the consistency of the various geodetic solutions and to explore their compatibility in order to combine the information content from the different techniques. The consistency of the CGS geodetic global solutions providing site positions and velocities is assessed, with particular attention to the Mediterranean and European areas. In addition a direct comparison between baselines and baseline rates for the collocated european GPS and VLBI sites is made.

Retroreflector Target for Co-aligning a Pair of Counter-propagating Optical Beams: Or How to Find a Star While Looking Down a Very Long Pipe

The use of a retroreflector is described for co-alignment of two optical beams that are collimated but propagating in opposite directions. This method is applicable to any optical system that requires precise pointing, such as laser-ranging and tracking systems. The application illustrated here is star acquisition in long-baseline stellar interferometry.

Laser-ranging scanning system to observe topographical deformations of volcanoes

We have developed a laser-ranging system to observe the topographical structure of volcanoes. This system can be used to measure the distance to a target by a laser and shows the three-dimensional topographical structure of a volcano with an accuracy of 30 cm. This accuracy is greater than that of a typical laser-ranging system that uses a corner-cube reflector as a target because the reflected light jitters as a result of inclination and unevenness of the target ground surface. However, this laser-ranging system is useful for detecting deformations of topographical features in which placement of a reflector is difficult, such as in volcanic regions.

Insights on the Fort Davis ? Mc Donald site stability: a GPS footprint

In june 1990, NASA's Crustal Dynamics Project (CDP) has established a geodetic footprint for the Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) systems located near Fort Davis, Texas using GPS measurements. The purpose of such a footprint is to assess both the tectonic and physical stability of the main observing monuments. Included in the survey were four new monuments forming a 20 to 30 km quadrilateral, as well as four existing monuments near the McDonald Laser Ranging System (MLRS), the Harvard Radio Astronomy (HRA) 85' antenna, and the new 85' antenna of the Very Long Baseline Array (VLBA). This network was partly reoccupied by the University of Texas Austin in may 1992. Three additional sites not observed in 1990 were occupied, two of them being old monuments in the observatory area (Harvard-RM4 and VLBA-SLR), and a new monument 80 km away across the closest active fault near Valentine, Texas. Thanks to existing ties between nearby monuments in the observatory area, the comparison between the two epochs was made possible. Significant motion of more than 1 cm of any of the implied sites over a two year period can be ruled out. Despite this quite reassuring results, smaller displacements cannot be detected mainly because of the poor quality of the 1990 data. Therefore it is strongly advised that a third occupation of the footprint sites takes place in the close future. Such a survey, occuring five years after the first one and three years after the second one, should definitely answer the questions on the Fort Davis area stability at the millimeter per year level.

Crustal deformations in the mediterranean area computed from SLR and GPS observations

Space geodesy provides an efficient technique to determine contemporary crustal deformations. This paper discusses the computation of tectonic motions in the Mediterranean area from observations acquired by Satellite Laser Ranging (SLR) and Global Positioning System (GPS) instruments. The SLR contribution is based on measurements on LAGEOS-1, taken in the period September 1983 to December 1992. For this analysis, the period was divided into 38 consecutive intervals. An independent station coordinates solution was computed for each subinterval. During this 9.3 year interval, the WEGENER project organized four mobile SLR observation campaigns in the Mediterranean region, resulting in three or four independent position solutions for the majority of the sites in this area. The GPS contribution is based on observations taken during the Euref 89 campaign, which took place from May 16 to 28, 1989, and during the WEGENER/GPS-92 campaign, which took place from July 29 to August 3, 1992, as a sub-campaign for the IGS Epoch'92 campaign. Most of the space-geodetic reference points in the Mediterranean area were occupied with GPS receivers during both campaigns, providing two valuable additional position solutions. The time-series of position solutions for each station are converted into individual station motion vectors. The horizontal components of the latter reveal a clear and consistent picture of the deformations taking place in the area: the northward motion of Arabia, the lateral escape to the west of Anatolia, the NE-SW expansion in the Aegean Basin and the northward motion of Africa being transduced into the central part of the Mediterranean. The results are in agreement with the qualitative kinematical models for this region, derived from geophysical evidence. The vertical station motion solutions are generally very small and insignificant from a statistical point of view. The internal and mutual consistency of the independent SLR and GPS network solutions is also investigated. The agreement is shown to be at the level of 5–20 mm.

Laser Ranging System to Observe Deformation of Shape of Volcano

Laser Ranging System to Observe Deformation of Shape of Volcano

Introduction of JD 19: Nutation

Due to both precise time measurements and precise geodetic positioning methods (like Very Long Baseline Interferometry (VLBI), Lunar Laser Ranging (LLR), Satellite Laser Ranging (SLR) and Global Positioning System (GPS)), the position of the instantaneous axis of the Earth’s rotation in space is measured with a precision better than a tenth of milliarcsecond. Simultaneously the amplitudes of the nutations of the Celestial Ephemeris Pole (CEP) deduced from the observations, i.e. the periodic motions in space of the CEP due to the luni-solar attraction or to other planetary attractions, have also been improved. However, these observed nutation amplitudes differ with respect to the computated ones based on an elliptical, uniformly rotating and deformable Earth responding to the lunar and solar attractions, as adopted by the IAU in 1980. The first session on “Observations and data reduction” dealt with Earth’s orientation observations and data analysis for deriving precession and nutations, as well as the associated residuals with respect to the adopted precession constant and nutation series. Comparisons between the different results have been presented including in-phase and out-of-phase components of the prograde and retrograde nutations or of nutations in longitude and in obliquity (see Session 1 of our JD: Newhall et al., McCarthy and Luzum, Herring, and Session 2: Gross). These differences “observed - adopted” nutations achieve several milliarcseconds and exhibit periodic as well as secular characteristics.

Geoscience laser ranging system: Estimated accuracy of geodetic parameters and their dependence on system characteristics

Covariance analysis of the performance of the Geoscience Laser Ranging System (GLRS) indicates that three‐dimensional relative positions can be recovered to an accuracy of several millimeters over spatial scales from a few kilometers to several hundred kilometers and over temporal scales as short as several days. The key factors influencing the accuracy are range noise, number of targets and their locations, system pointing capability, dwell time on the targets, orbital geometry, and gravity field uncertainties. Based on the present trade‐off studies, GLRS is designed to provide range measurements with 10 mm or better accuracy, fire at a rate of 40 pulses‐per‐seeond, point over a cone extending to 50° from nadir, and operate with a dwell time on individual targets of 2 s or less. Given a strain rate of 10−14/s, estimated GLRS accuracy parameters suggest that the deformation can be detected in less than a month if it extends over 100 km and in less than 6 months if it extends over 10 km.

Directional and photometric observations of artificial earth satellites using the U.K. satellite laser ranging system

The recent use of the U.K. Satellite Laser Ranging System in non-ranging mode to obtain astrometric and photometric observations of artificial satellites is described. It is found that the system is capable of obtaining a series of directional normal points of accuracy about 5″ throughout a pass of a close-Earth satellite. Simultaneous with the directional observations photometric observations were obtained with a time resolution of 0.2 s. The directional and photometric observations obtained from a pass of the discarded rocket body of Cosmos 1844 (1987-41B) have been analysed as a demonstration of the system's potential to yield possible solutions for the behaviour of the spin vector during the pass. It is found to precess on a cone of semi-angle 39°±3° with precession period 8.7±0.7 min.

A Portable Lidar Boundary-Layer Wind Profiling System

A rugged portable laser-ranging pilot balloon tracking system was developed to measure wind profiles in the boundary layer. The portable laser-ranging system provides digital output and can be operated by one relatively unskilled person. Additional personnel may be needed to launch the pilot balloons. Wind speeds were measured at 50-m intervals with an accuracy of 0.25 m s−1. Under clear sky conditions, the nominal range of the system is 3000 m.

Computer applications to lunar and artificial satellite laser ranging

The University of Texas McDonald Observatory has long been a pioneer in acquiring laser ranging data, a data type which has substantially improved our knowledge of the dynamics of the earth-moon system as well as various aspects of geophysics and general relativity. (See Mulholland, 1980; Shelus, 1985; Shelus, 1987.) The McDonald Laser Ranging System (MLRS) is one of only 2 laser ranging stations world-wide having the capability of routine data acquisition on both lunar and artificial satellite targets (Shelus, IEEE, 1985). In this paper we discuss the current applications of modern computer technology to the problems of acquiring and reducing that ranging data. As technology continues to improve, the logical upgrade is the replacement of obsolescent station minicomputers with the resource-rich environment of micro-computers. The goal is to allow the automation of many station ranging functions as well as the enhancement of on-site data quality control, filtering, and analysis. Plans for such upgrades and their implications for dynamical astronomy are discussed.

An analysis of MLRS Near-Real-Time Earth orientation results

In this paper a comparison is presented between near-real-time earth orientation parameters, produced on-site by the McDonald Laser Ranging System (MLRS) at McDonald Observatory, using observations to the Apollo 15 lunar retroreflector, and those results which are obtained after the fact at the University of Texas at Austin and elsewhere, as well as the results obtained from other techniques. The MLRS data set which is included in this study spans the interval from the commencement of on-site earth orientation solutions at MLRS in February 1985, through the present time, September 1986. This research is supported by the National Aeronautics and Space Administration under Grant NAG5-754 and Contract NAS5-29404 to McDonald Observatory and the University of Texas at Austin from the Goddard Space Flight Center in Greenbelt, Maryland.

The mobile laser-ranging system as a comparative tool for monitoring the terrestrial reference frame

The proposed earth rotation service and the related efforts to monitor the terrestral reference frame will employ instrumental and computational techniques of widely different kinds. It has already been demonstrated that the results produced by these techniques are sensitive to systematic errors occurring at the level of a few millimeters. As a result it will be extremely important to verify the performance of the different systems by making comparative observations at the same sites. Only mobile laser-ranging systems are currently in a position to perform this kind of service. To demonstrate the status of laser ranging during co-location, this paper presents a proposal for a controlled collocation experiment involving U.S. and European stations of the IRIS network.

Launch and Observation Program of the Experimental Geodetic Satellite of Japan

The Experimental Geodetic Satellite (EGS) of Japan has been developed by the National Space Development Agency of Japan and was completed in mid-1985. The functions of the EGS are 1) to reflect input laser light back toward the ground for precise ranging and 2) to reflect solar light to determine the direction to the satellite from an observation site. The satellite is a hollow sphere, 2.15 m in diameter, and weighs 685 kg. The surface is covered with corner cube reflectors and separate solar light reflectors. The EGS was launched on August 12, 1986. The launch orbit is circular with an inclination of 50 degrees and altitude of 1500 km. Tracking observation was started by Japanese and cooperative international organizations using Satellite Laser Ranging (SLR) systems and optical cameras just after the launch. The range precision given by some NASA SLR systems reached 1 cm. The Hydrographic Department of Japan will obtain and use positioning data in the construction of a marine geodetic control network. In addition to observations by fixed cameras and the SLR system, observations in isolated islands will be made using a transportable laser ranging station (TLRS). This system will have a range accuracy of 5 cm.