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No description available.

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (p. 170-178).

The Portable Satellite Laser Ranger (PSLR) is a light weight, highly portable satellite laser ranging system which employs many of the techniques and equipment types of larger fixed systems. It has a primary telescope aperture of 62 cm and uses a 150 ps pulse of 130 mJ at the second harmonic wavelength of 532 nm. The system is designed to use as little ancillary equipment as possible and only requires one small instrument rack and one PC based control computer. All of the control features of the system are based or installed in the control computer.The PSLR project at Curtin University was concerned with repairing and u p grading the PSLR to return the system to operational capacity. This involved the replacement of missing control components, repair of some hardware, modifications to the control program, and several calibration and operational tests. These tests showed that the PSLR system was capable of a ranging accuracy to fixed targets of 8.5 mm with an average accuracy of 23 mm. They showed that the PSLR was capable, in selective conditions, to track star ephemerides to less than 54. The mount error (standard deviation) over several orientations was shown to be 0.253° in elevation and 0.337° in azimuth.This dissertation will discuss; i) the operation of the equipment used with the PSLR and similar systems, the steps taken to repair or replace the necessary equipment, ii) the tests required to calibrate or evaluate various sub-systems of the PSLR and, iii) the results and conclusions drawn as a result.

One of the applications of the Satellite Laser Ranging (SLR) technique is the derivation of gravity field models; these models have various geophysical and geodynamical applications. Gravity field modelling has reached a new era where the latest satellite missions (CHAMP, GRACE and GOCE) are thought to provide significant improvement of global gravity field information in terms of quality and spatial resolution. In particular, the recent satellite missions carry on-board Global Navigation Satellite System (GNSS) receivers, accelerometers, K/Kaband microwave system (e.g. in GRACE) and gradiometers (e.g. in GOCE) allowing measurements of gravity field with unprecedented accuracy in contrast to the unsteady and fragmented orbit tracking by unevenly distributed SLR ground stations. Numerous gravity field models have been derived based on the newly available data sets by various research groups globally. Due to the availability of high quality SLR and satellite data, some of the older gravity field models are being updated as new models with higher degree and order are developed. Notwithstanding the significant progress in gravity field modelling, research focusing on assessing the accuracy and precision of the existing gravity field models has largely remained insufficient. The difference between the observed and computed satellite orbit (which is often expressed as the O-C range residuals) is used as a parameter for Precise Orbit Determination (POD) of satellites. Furthermore, O-C range residuals computed during SLR analysis are used as proxy parameters for evaluating the accuracy of gravity field models. The work presented in this thesis firstly reviewed and evaluated the accuracy of gravity field models released between 1990 and 2008. The accuracy of the gravity field models was examined by analysing the O-C residuals computed from LAGEOS 1 and 2 data analysis based on a set of twelve gravity field models. The results demonstrated that in general, there has been an improvement in the accuracy of gravity field models released between 1990 and 2008 by a factor of 2 based on improvements in the O-C residuals. Additionally, the influence of SLR tide parameterization (the IERS 2010 solid Earth and pole tide models) on the O-C residuals across five gravity field models has been assessed and results illustrate that the solid Earth and pole tides parameterization influence on the O-C residuals is dependent on the type of gravity field model. In order to ascertain the significance of mean differences in the Standard Deviations (SD) of O-C residuals based on the tide parameterization options, the student’s t-test was used. Results suggest that in general the O-C residuals derived from SLR LAGEOS 1 data have insignificant mean SD differences across the tide parameterizations. On the other hand analysis of SLR observations of LAGEOS 2 resulted in statistically significant mean SD differences in the O-C based on EIGEN-CG03C, EGM2008 and AIUB-GRACE01S gravity field models. The J2 coefficient forms part of the SLR Data Analysis Software (SDAS) package output products and was investigated in this thesis due to its role in understanding mass-redistribution within the Earth system (i.e. the equatorial bulge due to centrifugal force and rotation). In particular, the J 2 coefficient computed from SLR analysis of LAGEOS 1 and 2 data sets and based on the four selected gravity field models were compared with a priori J2 coefficients from the four models and those published in the literature. The results indicated that the J2 coefficients computed from the SDAS package were in agreement with the published coefficients. For geophysical applications, the relationship between the J2 parameter and LOD and AAM was investigated by use of data adaptive analysis methodology (the empirical mode decomposition). The results demonstrated that some degree of synchronization exists between the signal components of J2 and LOD and J2 and AAM. / Thesis (PhD)--University of Pretoria, 2013. / Geography, Geoinformatics and Meteorology / Unrestricted

Modern space geodetic techniques are required to provide measurements of millimetre-level accuracy. A new fundamental space geodetic observatory for South Africa has been proposed. It will house state-of-the-art equipment in a location that guarantees optimal scientific output. Lunar Laser Ranging (LLR) is one of the space geodetic techniques to be hosted on-site. This technique requires optical (or so-called astronomical) seeing conditions, which allow for the propagation of a laser beam through the atmosphere without excessive beam degradation. The seeing must be at ~ 1 arc second resolution level for LLR to deliver usable ranging data. To establish the LLR system at the most suitable site and most suitable on-site location, site characterisation should include a description of the optical seeing conditions. Atmospheric turbulence in the planetary boundary layer (PBL) contributes significantly to the degradation of optical seeing quality. To evaluate astronomical seeing conditions at a site, a two-sided approach is considered – on the one hand, the use of a turbulence-resolving numerical model, the Large Eddy Simulation NERSC (Nansen Environmental and Remote Sensing Centre) Improved Code (LESNIC) to simulate seeing results, while, on the other hand, obtaining quantitative seeing measurements with a seeing monitor that has been developed in-house. / Dissertation (MSc)--University of Pretoria, 2012. / Geography, Geoinformatics and Meteorology / MSc / Unrestricted

Satellite laser ranging provides an opportunity to track spacecraft trajectory fluctuationsas well as contributes to study of Earth’s tectonic motions and parameters ofMoon motions. Routine operations of laser ranging are carried out during night-timewhen noise probability is low. Classical approach for distance estimation includesdistance prediction thus reflected signal is expected to arrive during known time intervalcalled gate, however this is not possible when targeted object trajectory is notknown. In these cases gateless registration mode can be applied, but this mode greatlyreduces signal-to-noise ratio. In this thesis filter for noise event reduction during gatelessor full event registration was successfully developed. The filter is applied duringthe post-processing stage to improve signal-to-noise ratio of gathered data from laserranging. Not only the application of filter resulted in reduction of noise events, butalso it provides an insight whether there is an actual useful signal being emitted fromspacecraft. Investigation of event loss during full registration showed loss probabilityof 7.6% for calculated noise rate of 19 events per 0.01 millisecond. The developedfilter in combination with gateless registration mode opens the door for precipitationaltitude determination.

An increasing number of GPS applications require a high level of accuracy. To reduce the error contributed by the GPS ephemerides, an accurate modeling of the forces acting on GPS satellites is necessary. These forces can be categorized into gravitational and non-gravitational forces. The non-gravitational forces are a significant contribution to the total force on a GPS satellite but they are still not fully understood whereas the gravitational forces are well modeled. This study focuses on two non-gravitational forces: Solar Radiation Pressure (SRP) and the y-bias force. Different SRP models are available in the University of Texas Multi-Satellite Orbit Determination Program (MSODP). The recently developed University College London model was implemented for the purpose of this study. Several techniques to compute parameters associated with SRP models and the y-bias force during an orbit prediction were examined. Using the International GNSS Service (IGS) precise ephemerides as a reference, five different models were compared in the study. Satellite Laser Ranging (SLR) residuals were also studied to validate the approach. Results showed that the analytical UCL model performed as well as a purely empirical model such as the Extended CODE model. This is important since analytical models attempt to represent the physical phenomena and thus might be better suited to separate SRP from other forces. The y-bias force was then shown to have a once per revolution effect. The time evolution of the y-bias was found to be dependent on the SRP model used, the satellite Block type, the orbital plane, and the attitude of the satellite which suggests that estimates of y-bias contain errors from other sources, particularly the SRP models. The dependency of the y-bias evolution on the orbital plane suggests that the orientation of the plane towards the Sun is important. / text

This paper attempts to use image processing methods to reduce the influences of ambient light and scattering effect on the performance of an underwater range finder. The Taguchi method, as well, is employed to increase the repeatability of underwater range finding. In this study, the image processing methods of the least-squares approximation, brightness and contrast adjustment, and primary color processing are presented. The illumination center is also used to estimate the position of the laser spot in the image. In addition, a bandpass optical filter at the receiving end is used to investigate the effects of filters on the quality of range finding. To verify the effectiveness of the proposed image processing methods, a series of DOE process runs are carried out to study effects of the design parameters on quality of range finding. For each image processing method, its corresponding control factors and levels are assigned to an inner orthogonal array. To make the proposed image processing methods robust against noises, both environmental illumination and turbidity are forced into the experiments by utilizing an outer orthogonal array. Images for processing are then captured under different noise conditions in accordance with the allocation of the outer noise array. And, according to the layout of the inner array, the S/N ratio of each treatment combination is calculated. After that, the optimum combination of control factors is predicted through the analysis of variance. Then, the confirmation experiments are carried out to verify that the combination of control factors at the perceived best levels is valid. Based on the results of experiments and analyses, it is found that the least-squares approximation is better than other proposed image processing methods for increasing the quality of range finding. Moreover, the effect of increasing quality of range finding by using the least-squares approximation is superior to that of using a bandpass optical filter. Even though a range finding system has incorporated a bandpass optical filter for filtering out unwanted noises, the quality of range finding can still be increased distinctly while the algorithm of the least-squares approximation is employed. As well, the least-squares approximation is feasible to reduce the scattering effects in the laser images if the size of the sparse backscattering light spot is smaller than that of the target light spot.

Mes travaux de thèse ont pour finalité la réalisation d’un transfert de temps intercontinentald’une stabilité meilleure que 2 ns sur 10 000 s, entre les observatoires géodésiquesde l’International Laser Ranging Service (ILRS). Ce transfert de temps est effectué à l’aide deliens spatiaux optiques obtenus par l’expérience de Transfert de Temps par Lien Laser (T2L2).T2L2 a été lancée le 20 Juin 2008 à 1336 km d’altitude à bord du satellite océanographiqueJason-2. Le principe de cette expérience est basé sur la datation, au sol dans les stations laseret à bord du satellite, d’impulsions laser très courtes (30 - 100 ps) aller - retour (2 voies) venantde 25 stations laser réparties sur le Globe. La performance du lien sol-bord (dont la stabilitéatteint des valeurs inférieures à 10 ps sur 100 s), procurée par la technologie laser d’une partet par la qualité de l’instrument spatial d’autre part permet de lire précisément les variationsde l’oscillateur bord (à quartz) développé pour le système d’orbitographie et de localisationDoppler Orbitography and Radiopositionning Integrated by Satellite (DORIS) du Centre Nationald’Études Spatiales (CNES). Nous montrons qu’il est possible de construire un modèle defréquence déterministe, à court terme (10 j) et moyen terme (plusieurs mois) d’évolution de lafréquence de l’oscillateur bord DORIS, avec une précision relative de 3 - 5·10≠13. Les variationsde fréquence sont induites par un environnement spatial complexe, où les radiations, la températureet le vieillissement du résonateur dégradent l’oscillateur. L’intégration de ce modèlenous permet la réalisation d’un temps à bord pour synchroniser le réseau ILRS complet et ainsiestimer les biais en temps des stations laser par rapport à l’Universal Time Coordinate (UTC).L’effet des biais en temps, estimé à l’aide de T2L2 sur l’orbite ainsi que sur les coordonnées desstations laser de l’International Terrestrial Reference Frame (ITRF) est déterminé précisémentau niveau de quelques millimètres. Enfin T2L2 étant également capable de dater le Pulse ParSeconde (PPS) du système Global Positionning System (GPS), nous étudions l’évolution sur lelong terme (plusieurs années) des horloges utilisées dans les stations laser et nous montrons lesproblèmes insoupçonnés de l’instabilité de leur système de temps/fréquence au sol. / The purpose of my Ph.D. works is the realization of a intercontinental time transfer,with a stability better than 2 ns over 10,000 s, between the International Laser RangingService (ILRS) geodetics observatories. This time transfer is performed thanks to optical spacelinks and the Time Transfer by Laser Link (T2L2) experiment. T2L2 is a passenger on-board theoceanographic satellite Jason-2, which was launched the 20th June 2008, at 1336 km of altitude.The principle of this experiment is based on the (two-way) short laser pulses (30 - 100 ps) timetagging, in laser station on ground and on-board the satellite, which come from 25 worldwidelaser stations. The high performance of the ground-to-space link (where the stability reach valuesbetter than 10 ps over 100 s), given by the laser technology in one hand, and the quality of thespace instrument in the other hand, allows to precisely read the frequency variations of the onboardoscillators (quartz) built for the orbitography and localization Doppler Orbitography andRadiopositionning Integrated by Satellite (DORIS) French system. We demonstrate the possibilityto build a deterministic frequency model, on the short term (10 days), mid-term (severalsmonth) for the on-board DORIS oscillator frequency evolution, with a relative precision at 3- 5·10≠13. Frequency variations are caused by a complex space environment, where radiations,temperature and device aging damage the oscillator. The integration of this model allows us tobuild an "on-orbit" time realization to synchronize the whole ILRS network and thus, estimatelaser station time biases in regard to the Universal Time Coordinate (UTC). The time bias effects,estimate thanks to T2L2, on the International Terrestrial Reference Frame (ITRF) stationcoordinates, is precisely determined at the level of a few millimeters. Finally, T2L2 is able totime tagged the Global Positionning System (GPS) Pulse Per Second (PPS) signal in order tostudy the clock long term (several years) behavior in laser station and we show the unexpectedproblems due to the instability of the station time and frequency system.