Incorporation of GNSS multipath to improve autonomous rendezvous, docking and proximity operations in space

Automated rendezvous and docking (AR&D;) operations are important for many future space missions, such as the resupply of space stations, repair and refueling of large satellites, and active removal of orbital debris. These operations depend critically on accurate, real-time knowledge of the relative position and velocity between two space vehicles. Unfortunately, Global Navigation Satellite System (GNSS) capabilities remain severely limited in close proximity to large space structures due to significant multipath effects and signal blockage. Although GNSS is used for the initial stages of approach, other instruments such as laser, radar and vision-based systems, are required to augment GNSS during AR&D; over the last few hundred meters. This dissertation evaluates the feasibility of GNSS multipath-based relative space navigation. Methods for separating and interpreting reflected signals are demonstrated using GNSS data collected during Hubble Servicing Mission 4 (HSM4), a model of the mission geometry, electromagnetic (EM) ray tracing, and a custom GNSS software receiver. EM ray tracing is used to show that a number of signals sufficient for ranging are reflected by the Hubble Space Telescope (HST) during HSM4, and the properties of these reflections are used to generate simulated GNSS data. The impact of reflected signals on code correlation shape, code tracking error, and pseudorange measurement is demonstrated using the simulated and experimental data. Relative navigation is demonstrated using simulated reflected signal measurements and the dependence of relative navigation on the reflecting object’s scattering properties is illustrated. From the tracking of data from two oppositely polarized antennas, both simulated and experimental, it is determined that multipath measurements are limited by system properties such as antenna polarization quality and front end bandwidth. Design considerations involved in optimizing a receiver to measure reflected signals are discussed

Präzise Orbitbestimmung des globalen Navigationssatellitensystems der 2. Generation

GNSS-2 (Global Navigation Satellite System of Second Generation) is a new generation of satellite-based navigation system. The primary goal is to improve the existing satellite systems such as GPS and GLONASS. Europes contribution to a new navigation satellite system of GNSS-2 is called Galileo. The typical space segment of a GNSS-2 system is composed of inclined geosynchronous (IGSO), geostationary (GEO) and Medium Earth Orbit (MEO) satellites. The space segment of the Galileo system is now only composed of MEO satellites. With this new satellite navigation system precise navigation and positioning with accuracy of at least 10 meters without differential techniques may be achieved. Therefore high precision orbit determination is required for successful applications of GNSS-2/Galileo systems with this accuracy level. The precise orbit determinations of IGSO, GEO and MEO satellites are discussed using dynamic and kinematic methods in this dissertation. The effort is focused, however, on IGSO and GEO satellites on ground tracking stations. In Chapter 1, GNSS-2/Galileo development plan and phase are presented. In Chapter 2, the basic observations of orbit determination are discussed, in Chapter 3 current systems used for various orbit determination applications are evaluated, in Chapter 4 major sources of observation errors are analyzed, in Chapter 5 perturbations on IGSO, GEO and MEO are modeled and estimated, in Chapter 6 major algorithms of orbit determination of IGSO, GEO and MEO, for examples, dynamic, reduced dynamic and kinematic methods are developed and discussed, in Chapter 7 high accuracy of IGSO and GEO orbit determination using carrier phase observation are discussed, in Chapter 8 a serious problem of GEO orbit determination during satellite maneuvers is presented and solved, and finally in Chapter 9 the simulation results of a possible satellite tracking system of GNSS-2/Galileo are presented

Reusable Reentry Satellite (RRS) system design study

The Reusable Reentry Satellite (RRS) is intended to provide investigators in several biological disciplines with a relatively inexpensive method to access space for up to 60 days with eventual recovery on Earth. The RRS will permit totally intact, relatively soft, recovery of the vehicle, system refurbishment, and reflight with new and varied payloads. The RRS is to be capable of three reflights per year over a 10-year program lifetime. The RRS vehicle will have a large and readily accessible volume near the vehicle center of gravity for the Payload Module (PM) containing the experiment hardware. The vehicle is configured to permit the experimenter late access to the PM prior to launch and rapid access following recovery. The RRS will operate in one of two modes: (1) as a free-flying spacecraft in orbit, and will be allowed to drift in attitude to provide an acceleration environment of less than 10(exp -5) g. the acceleration environment during orbital trim maneuvers will be less than 10(exp -3) g; and (2) as an artificial gravity system which spins at controlled rates to provide an artificial gravity of up to 1.5 Earth g. The RRS system will be designed to be rugged, easily maintained, and economically refurbishable for the next flight. Some systems may be designed to be replaced rather than refurbished, if cost effective and capable of meeting the specified turnaround time. The minimum time between recovery and reflight will be approximately 60 days. The PMs will be designed to be relatively autonomous, with experiments that require few commands and limited telemetry. Mass data storage will be accommodated in the PM. The hardware development and implementation phase is currently expected to start in 1991 with a first launch in late 1993