Advanced global navigation satellite system receiver design (797918), страница 5
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Theproblem of multi-peaks goes away and there is no loss of potential accuracy. Theinvention has been assigned to the University of Surrey, and a professionally draftedpatent application filed with the Patent Office from August 2006 with ourselvesnamed as joint inventors. All simulations, emulations and further practical work seemto show that the invention does indeed work and has no problems. There is noindication in any published work that others have perceived our way to the solution.This thesis describes in detail this invention and the first experimental results.
Whenin due course it is published our work hopefully will re-assure conservative technicaland political interests internationally, if such exist, that it is now safe to go ahead withBOC (and also AltBOC for the Europeans).19Introduction1.3 Outline of thesisThis thesis consists of 11 chapters with the following content.Chapter 2 describes the driving forces behind this project and the motivation forresearch in this area. The specific goals of the research are described, which satisfyboth the industrial and academic requirements of the research.
To emphasise the needfor the research examples of space applications to which this work may benefit aregiven.Chapter 3 gives an overview of the current and future GNSS signal characteristics.This is essential to provide background for the thesis, which is fundamentally aboutthe new techniques and approaches required to transmit and receive these signals.Details of the modulation, multiplexing and coding schemes are given along withmathematical representations of current and future GNSS signals.Chapter 4 provides an analysis of the advantages and disadvantages of BOCmodulation with comparison to the existing modulation techniques. The effects ofBOC on the transmitted frequency spectrum are described and detailed comparativeanalysis of the tracking and multipath performance of BOC modulation is given.Chapter 5 gives a detailed theoretical description and performance analysis of currentand future GNSS receiver search, acquisition and tracking techniques.
Firstly, currentGPS receiver techniques are described in depth covering both coherent and incoherentsystems. Following this, a comparison is given of the techniques currently proposedby the literature in order to successfully search for and track BOC signals. Eachscheme is compared with consideration of the receiver acquisition speed, trackingsensitivity and the hardware impact of the technique.Chapter 6 provides a detailed theoretical description and performance analysis of thenovel Double Estimating (DE) BOC receiver, which was developed during thisresearch.
A comprehensive description of DE technique is given, showing how it canbe applied to coherent and incoherent BOC systems and to the Alternate BOC(AltBOC) modulation. The technique is compared against the BOC tracking schemes20Introductionidentified in Chapter 5 and a number of advantages are found for both performanceand hardware considerations. The practical issue of distortions or asymmetry intransmitted BOC spectrums and its effect on receiver tracking techniques are alsoanalysed. Further advantages are identified for the DE BOC tracking technique whileoperating with distorted signals.Chapter 7 describes hardware implementations of GNSS signal generators.
Emphasisis given to the SSTL approach to implementing a Galileo signal generator on theGiove-A satellite, which this research has contributed to. This efficient and elegantapproach to generating GNSS signals is capable of producing all variations of signalsthat have been proposed for the future GNSS. Examples of signals from anIntermediate Frequency (IF) bench prototype signal generator and the flight moduleflown on Giove-A are given. The hardware implementation of a digital noisegenerator in the signal generator is also described.
This noise generator can be usedto emulate various levels of additive Gaussian noise for receiver bench testing.Chapter 8 provides a in-depth look at the SSTL Space GNSS Receiver (SGR) designand follows on to describe the development of a Prototype IF (PIF) receiver, designedto receiver future GNSS signals. Firstly, an overview of the SSTL SGR architectureis given to provide a background receiver design and identify specific areas that canbe improved by this research. Subsequently, the design of the PIF receiverarchitecture is then described, which is intended to replace and enhance many of thecore SGR components.
The PIF receiver platform also has been used to provide abench demonstration and evaluation of the DE BOC tracking technique detailed inChapter 6. The adaptations required to the correlator hardware elements, processorsoftware to implement the DE technique are presented.Chapter 9 described the testing of the PIF receiver using current and future GNSSsignals.
Methods for deriving measurement data from the receiver while operating theDE BOC tracking technique and evaluating the performance of the low-level receiverfunctions are given.21IntroductionChapter 10 shows the development and testing of a prototype receiver based on asingle FPGA. The receiver was tested with live signals from both the GPSconstellation and the Galileo E1 signal from Giove-A satellite.Chapter 11 describes the contributions made to the field of GNSS by this research.Each contribution is detailed with a description of its potential benefit to the GNSScommunity. Following this suggestions are made for future research into the areascovered by this research.
These include the exploitation of a patent filled for thenovel BOC tracking scheme developed during this research.222Background, motivation and goalsIn order to judge the merit of this research it is important to understand how theproject came into being and its potential impact to GNSS academia and industry.In this chapter the driving forces behind this research and the history of its conceptionare discussed. To provide a context for the research, examples of GNSS receiverapplications in space are given, with emphasis on the research activities at SSC andSSTL.
Following this, the aims of the research are described, which, given the natureof the CASE studentship, contain a strong industrial emphasis to reflect the researchinvestment by SSTL.2.1SSC, SSTL, satellites and GPSIn 1993 SSTL and SSC pioneered the use of GNSS in space through the firstoperation of a GPS receiver on a micro-satellite in low-earth orbit (LEO). Thereceiver was used to provide the satellite with position, velocity and time (PVT)information [Unwin 1995], which enabled determination of the satellite’s orbit. Thefirst receiver flown by SSTL in orbit was manufactured and adapted for operation inspace by the GNSS receiver manufacturer Trimble. Since then SSTL have producedtheir own space GPS receivers (SGR).
The SGR receivers are operated on SSTLsown missions and have also been supplied to ESA, NASA and the US Air-Force.As the SGR designs have progressed they have enabled a variety of applications. In1999 the 24-channel SGR-20 was flown on the Uosat-12 satellite. This receiver canprocess signals on four separate antennas with six channels devoted to each antenna.This enabled a demonstration of attitude determination using GPS by comparing thesignals across the antennas [Purivigraipong et al 2000]. Further research has takenplace to provide a GPS attitude system which is more robust to multipath errors, againmaking use of real data and demonstrations using the SSTL hardware [Duncan et al2006].On-board the UK-DMC micro-satellite SSTL have an adapted version of the SGR-10,which is pioneering research into the use of sea-state monitoring with GNSS signals23Background, motivation and goals[Gleason et al 2005].
The experimental receiver is specially designed to receive GPSsignals reflected back to the satellite from the ocean surface. Again this is supportedby research from SSC into modelling and simulating the scatter of the signal with theultimate aim of determining quantities such as ocean roughness, wave height andwave direction. The acquisition of real data from the satellite has successfullydemonstrated the credibility and validity of this technology and fuelled research intothis area.Another experimental SSTL receiver has been flown on the Giove-A satellite,specially designed to operate in a high-earth orbit (HEO) with the potential ofproviding a low-cost receiver for geostationary earth-orbits (GEO) in the future[Steenwijk et al 2006].
This receiver is adapted to acquire and track extremely weakGPS signals in order to obtain PVT information while in an orbit outside thetransmitting GPS constellation. Again complementing SSTL’s advances in receiverdesigns this application provides a number of research challenges currently underinvestigation at SSC, including weak signal acquisition, weak signal tracking andorbit estimation.Each of the above applications can potentially benefit through advances in GNSSreceiver technology provided by this research.
Specific aims to benefit GNSSattitude, GNSS remote sensing and GEO GNSS receivers are given in the followingsection of this chapter.In 2003 SSTL was awarded the contract to design and manufacture the first Galileotest satellite, Giove-A.
This satellite’s mission was to claim the frequency bandsallocated to the European Community by transmitting representative signals in thosebands. Also, the satellite provides a demonstration of a number of key technologiesrequired by the Galileo programme. In December 2005, Giove-A was launched andhas successfully claimed the Galileo frequency bands.The contract to manufacture Giove-A has provided SSTL and SSC with a uniqueopportunity to study, analyse and evaluate the impact of the next generation of GNSS(GPS modernisation and Galileo) signals on receiver design.
Under the Giove-Acontract SSTL has developed a Galileo transmitter capable of producing24Background, motivation and goalsrepresentative Galileo signals, which this research has contributed to (see Chapter 7).This access to the early Galileo specification, representative signal generators and realsignals from space has been an enabler and a driver of this research.The working relationship between SSC and SSTL has already provided manyvaluable contributions to the world of GNSS. The partnership strikes a rare balancewhich many organisations aspire to; between the industrial ‘market pull’ of acommercial company and the ‘technology push’ driven by academia.
Thispartnership is entirely in tune with the sprit of the EPSRC CASE studentship bringingindustry and academia together through research with mutual goals.2.2Research objectives and goalsThe aim of this research was to analyse the impact of the next generation in GNSS(GPS modernisation and Galileo) and determine the advances required by receiverarchitectures to take advantage of these new systems, providing novel approacheswhere possible. Emphasis was given to developing receiver architectures for smallsatellite applications, although the work also provides significant technical value tothe terrestrial applications of GNSS.SSTL have developed a range of space GPS receivers [SSTL 2007] and havepioneered a number of novel space applications.
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