Synchronization of weak indoor GPS signals with doppler frequency offset using a segmented matched filter and accumulation

Recent government regulations for Enhanced 911 locating of wireless handsets require accuracy to within 50 and 300 meters. Two technologies under consideration are triangulation using existing wireless base stations and location using global positioning satellites (GPS). Satellite positioning is the leading candidate, however, reception of GPS signals within large buildings is difficult and considerable research is devoted to this topic. Conventional GPS receivers require line of sight to at least four satellites and, under outdoor conditions, the expected signal level is about -160 dBW. Within large buildings, detection is very difficult because there is high thermal noise and some satellite signals can be attenuated to less than -185 dBW while others can suffer little attenuation. In order to construct the pseudo-ranges necessary for position finding, the receiver must synchronize to the incoming codephase of each satellite and must operate with substantial Doppler frequency offset caused by satellite motion. This thesis investigates the application of a parallel non-coherent spread spectrum synchronizer previously implemented as a very-large-scale integration (VLSI) circuit. The circuit processes one millisecond of incoming signal and uses a segmented matched filter (SMF) by which the segmentation provides some tolerance to Doppler shift. The thesis presents simulation results of averaging for tens of seconds. Through simulation, the SMF is compared with a transversal matched filter (TMF) under conditions of no Doppler shift; coherent and non-coherent integration are discussed. The simulation is conducted at 290 K (17°C) such that the Boltzmann noise is -204 dBW/Hz, with a GPS signal bandwidth of 2 MHz and signal level of -185 dBW, and the receiver input signal-to-noise ratio (SNR) is -44 dB. The SMF is applied using differing segment lengths to high-sensitivity GPS data from indoor and urban simulated GPS data. The results demonstrate the SMF’s ability to tolerate Doppler frequency offsets while allowing for long integration times to detect the weak GPS signals

Satellite applications to marine geodesy

Potential use of satellites for enhancing positioning capabilities and for marine geodetic contro

Fractal patterns in fractionated spacecraft

Multi spacecraft architectures have been addressed in response to the demand for flexible architectures with high reliability and reduced costs compared to traditional monolithic spacecraft. A task that can be easily carried out by this kind of systems is the deployment of distributed antennas; these are composed of, typically, receiving elements carried on-board multiple spacecraft in precise formation. In this paper decentralised control means, based on artificial potential functions, together with a fractal-like connection network, are used to produce the autonomous and verifiable deployment and formation control of a swarm of spacecraft into a fractal-like pattern. The effect of using fractal-like routing of control data within the spacecraft generates complex formation shape patterns, while simultaneously reducing the amount of control information required to form such complex formation shapes. Furthermore, the techniques used ensure against swarm fragmentation, while exploiting communication channels anyway needed in a fractionated architecture. In particular, the superposition of potential functions operating at multiple levels (single agents, subgroups of agents, groups of agents) according to a self-similar adjacency matrix produces a fractal-like final deployment with the same stability property on each scale. Considering future high-precision formation flying and control capabilities, this paper considers, for the first time and as an example of a fractionated spacecraft, a decentralised multi-spacecraft fractal shaped antenna. A fractal antenna pattern provides multiple resonance peaks, directly related to the ratios of its characteristic physical lengths. Such a scenario would significantly improve the level of functionality of any multi-spacecraft synthetic aperture antenna system. Furthermore, multi-spacecraft architecture exploiting particular inter agent spacing can be considered to investigate multi-scale phenomena in areas such as cosmic radiation and space plasma physics. Both numerical simulations and analytic treatment are carried out demonstrating the feasibility of deploying and controlling a fractionated fractal antenna in space through autonomous decentralised means

Initial Determination of Low Earth Orbits Using Commercial Telescopes

Within the last decade, many new technologies have significantly changed the face of private astronomy. Developments such as inexpensive but high-quality sensors, rapid personal computing, and easy networking inspire a reexamination of an old problem: how practical is it to develop initial orbit estimates for Low Earth Orbiting (LEO) satellites using optical tracking? This paper documents the design and implementation of a commercial telescope system used to answer precisely that question. This analysis deter- mined there are some challenging barriers to successful single-site orbit determination, but it is possible given the right conditions. Considering the low cost and small sup- port footprint of such systems, they could provide excellent support to Space Situational Awareness (SSA) missions or satellite tracking operations in general

A fractally fractionated spacecraft

The advantages of decentralised multi-spacecraft architectures for many space applications are well understood. Distributed antennas represent popularly envisaged applications of such an architecture; these are composed of, typically, receiving elements carried on-board multiple spacecraft in precise formation. In this paper decentralised control, based on artificial potential functions, together with a fractal-like connection network, is used to produce autonomous and verifiable deployment and formation control of a swarm of spacecraft into a fractal-like pattern. The effect of using fractal-like routing of control data within the spacecraft generates complex formation shape patterns, while simultaneously reducing the amount of control information required to form such complex formation shapes. Furthermore, the techniques used ensures against swarm fragmentation, which can otherwise be a consequence of the non-uniform connectivity of the communication graph. In particular, the superposition of potential functions operating at multiple levels (single agents, subgroups of agents, groups of agents) according to a self-similar adjacency matrix produces a fractal-like final deployment with the same stability property on each scale. Results from the investigations carried out indicate the approach is feasible, whilst outlining its robustness characteristics, and versatility in formation deployment and control. Considering future high-precision formation flying and control capabilities, this paper considers, for the first time and as an example of a fractally fractionated spacecraft, a decentralised multi-spacecraft fractal shaped antenna. Furthermore, multi-spacecraft architecture exploiting fractal-like formations can be considered to investigate multi-scale phenomena in areas such as cosmic radiation and space plasma physics. Both numerical simulations and analytic treatment are presented, demonstrating the feasibility of deploying and controlling a fractionated fractal antenna in space through autonomous decentralised means. This work frames the problem of architecture and tackles the one of control, whilst not neglecting actuation

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

Geodetic, teleseismic, and strong motion constraints on slip from recent southern Peru subduction zone earthquakes

We use seismic and geodetic data both jointly and separately to constrain coseismic slip from the 12 November 1996 M_w 7.7 and 23 June 2001 M_w 8.5 southern Peru subduction zone earthquakes, as well as two large aftershocks following the 2001 earthquake on 26 June and 7 July 2001. We use all available data in our inversions: GPS, interferometric synthetic aperture radar (InSAR) from the ERS-1, ERS-2, JERS, and RADARSAT-1 satellites, and seismic data from teleseismic and strong motion stations. Our two-dimensional slip models derived from only teleseismic body waves from South American subduction zone earthquakes with M_w > 7.5 do not reliably predict available geodetic data. In particular, we find significant differences in the distribution of slip for the 2001 earthquake from models that use only seismic (teleseismic and two strong motion stations) or geodetic (InSAR and GPS) data. The differences might be related to postseismic deformation or, more likely, the different sensitivities of the teleseismic and geodetic data to coseismic rupture properties. The earthquakes studied here follow the pattern of earthquake directivity along the coast of western South America, north of 5°S, earthquakes rupture to the north; south of about 12°S, directivity is southerly; and in between, earthquakes are bilateral. The predicted deformation at the Arequipa GPS station from the seismic-only slip model for the 7 July 2001 aftershock is not consistent with significant preseismic motion