Use of Two-Way Time Transfer Measurements to Improve Geostationary Satellite Navigation

<p>An emerging use of GPS is to provide accurate navigation information for satellites in orbit. The GPS satellites are designed to provide service to terrestrial users so the antenna array points directly towards the Earth and uses a narrow primary beamwidth. Because GEO altitudes are well above the GPS constellation the Earth occludes most of the GPS signals to the satellite. Decreased satellite visibility is debilitating as GPS navigation requires at least four visible satellites to determine position. To assist with the visibility problem the receiver can look at the GPS satellite transmit antenna side lobes but this does not entirely solve the navigation problem. GPS measurements are inherently bound by receiver clock errors. The clock error must be known or estimated in order to obtain meaningful ranging information. To obtain threedimensional positioning at least four satellites must be tracked to solve for three dimensions of position plus the receiver clock error. A new method for improving geostationary navigation accuracy using GPS is to correct the time error by including Two-Way Time Transfer (TWTT) measurements. TWTT is a technique in which signals are simultaneously exchanged between two clocks and is one of the most accurate methods of comparing clocks. By effectively removing the clock error between the GPS satellite and the GPS receiver TWTT allows meaningful information to be gathered when less than four GPS satellites are available. The results show a 21-38% improvement in the 3-D RMS position accuracy while using TWTT between the GEO satellite and an atomic clock on the ground. There was a 60-70% improvement when the clock on the ground was synchronized to GPS time.</p><p>This work has been selected by scholars as being culturally important and is part of the knowledge base of civilization as we know it. This work was reproduced from the original artifact and remains as true to the original work as possible. Therefore you will see the original copyright references library stamps (as most of these works have been housed in our most important libraries around the world) and other notations in the work.</p><p>This work is in the public domain in the United States of America and possibly other nations. Within the United States you may freely copy and distribute this work as no entity (individual or corporate) has a copyright on the body of the work.</p><p>As a reproduction of a historical artifact this work may contain missing or blurred pages poor pictures errant marks etc. Scholars believe and we concur that this work is important enough to be preserved reproduced and made generally available to the public. We appreciate your support of the preservation process and thank you for being an important part of keeping this knowledge alive and relevant.</p>