Performance analysis of low earth orbit (LEO) satellite link in the presence of elevation angle, fading and shadowing

This thesis report is submitted in partial fulfillment of the requirements for the degree of Bachelor of Science in Computer Science and Engineering, 2009.Cataloged from PDF version of thesis report.Includes bibliographical references (page 65).Tanjila FarahPrianka RoyB. Computer Science and Engineerin

Global Reach: A View of International Cooperation in NASA's Earth Science Enterprise

Improving life on Earth and understanding and protecting our home planet are foremost in the Vision and Mission of the National Aeronautics and Space Administration (NASA). NASA's Earth Science Enterprise end eavors to use the unique vantage point of space to study the Earth sy stem and improve the prediction of Earth system change. NASA and its international partners study Earth's land, atmosphere, ice, oceans, a nd biota and seek to provide objective scientific knowledge to decisi onmakers and scientists worldwide. This book describes NASA's extensi ve cooperation with its international partners

Modelling the temporal variation of the ionosphere in a network-RTK environment

The Global Positioning System (GPS) has been widely used for precise positioning applications throughout the world. However, there are still some limiting factors that affect the performance of satellite-based positioning techniques, including the ionosphere. The GPS Network-RTK (NRTK) concept has been developed in an attempt to remove the ionospheric bias from user observations within the network. This technique involves the establishment of a series of GNSS reference stations, spread over a wide geographical region. Real time data from each reference station is collected and transferred to a computing facility where the various spatial and temporal errors affecting the GNSS satellite observations are estimated. These corrections are then transmitted to users observations in the field. As part of a Victorian state government initiative to implement a cm-level real time position ing service state-wide, GPSnet is undergoing extensive infrastructure upgrades to meet high user demand. Due to the sparse (+100km) configuration of GPSnet's reference stations, the precise modelling of Victoria's ionosphere will play a key role in providing this service. This thesis aims is to develop a temporal model for the ionospheric bias within a Victorian NRTK scenario. This research has analysed the temporal variability of the ionosphere over Victoria. It is important to quantify the variability of the ionosphere as it is essential that NRTK corrections are delivered sufficiently often with a small enough latency so that they adequately model variations in the ionospheric bias. This will promote the efficient transmission of correctional data to the rover whilst still achieving cm-level accuracy. Temporal analysis of the ionosphere revealed that, during stable ionospheric conditions, Victoria's double differenced ionospheric (DDI) bias remains correlated to within +5cm out to approximately two minutes over baselines of approximately 100km. However, the data revealed that during more disturbed ionospheric conditions this may decrease to one minute. As a preliminary investigation, four global empirical ionospheric models were tested to assess their ability to estimate the DDI bias. Further, three temporal predictive modelling schemes were tested to assess their suitability for providing ionospheric corrections in a NRTK environment. The analysis took place over four seasonal periods during the previous solar maximum in 2001 and 2002. It was found that due to the global nature of their coefficients, the four global empirical models were unable to provide ionospheric corrections to a level sufficient for precise ambiguity resolution within a NRTK environment. Three temporal ionospheric predictive schemes were developed and tested. These included a moving average model, a linear model and an ARIMA (Auto-Regressive Integrated Moving Average) time series analysis. The moving average and ARIMA approaches gave similar performance and out-performed the linear modelling scheme. Both of these approaches were able to predict the DDI to +5cm within a 99% confidence interval, out to an average of approximately two minutes, on average 90% of the time when compared to the actual decorrelation rates of the ionosphere. These results suggest that the moving average scheme, could enhance the implementation of next generation NRTK systems by predicting the DDI bias to latencies that would enable cm-level positioning

Radio occultation measurements from the Australian microsatellite FedSat

The Australian Low Earth Orbit (LEO) microsatellite, FedSat (named to commemorate the centenary of the Australian Federation in 2001), was launched into orbit on December 14, 2002 from the Tanegashima Space Centre, Japan. A Global Positioning System (GPS) receiver was one of the instruments onboard. The received GPS signals can be used to investigate the ionospheric electron density and the atmosphere below FedSat's orbiting altitude, using radio occultation (RO) techniques. The RO technique developed involves a simplified form of the Abel transform using the slant total electron content (STEC) determined from radio signals that traverse below FedSat's orbiting altitude. Electron density profiles from the GPS RO data, recorded by the GPS receiver onboard the FedSat satellite, are determined for the first time. The technique combined with simultaneous occultation density profile extraction from different LEO satellites and satellite navigation systems has the potential to image near real-time 3-D structures of the ionospheric electron density