Showing papers on "GNSS augmentation published in 1998"

A software radio approach to global navigation satellite system receiver design

Abstract: The software radio has been described as the most significant evolution in receiver design since the development of the superheterodyne concept in 1918. The software radio design philosophy is to position an analog-to-digital converter (ADC) as close to the antenna as possible and then process the samples using a combination of software and a programmable microprocessor. There are a number of important advantages to be gained through full exploitation of the software radio concept. The most notable include: (1) The removal of analog signal processing components and their associated nonlinear, temperature-based, and age-based performance characteristics. (2) A single antenna/front-end configuration can be used to receive and demodulate a variety of radio frequency (RF) transmissions. (3) The software radio provides the ultimate simulation/testing environment. Global Navigation Satellite Systems (GNSSs) are the latest and most complex radionavigation systems in widespread use. The United States' Global Positioning System (GPS) and, to a lesser extent, the Russian Global Orbiting Navigation Satellite System (GLONASS) are being targeted for use as next generation aviation navigation systems. As a result, it is critical that a GNSS achieve the reliability and integrity necessary for use within the aerospace system. The receiver design is a key element in achieving the high standards required. This work presents the complete development of a GNSS software radio. A GNSS receiver front end has been constructed, based on the software radio design goals, and has been evaluated against the traditional design. Trade-offs associated with each implementation are presented along with experimental results. Novel bandpass sampling front end designs have been proposed, implemented and tested for the processing of multiple GNSS transmissions. Finally, every aspect of GNSS signal processing has been implemented in software from the necessary spread spectrum acquisition algorithms to those required for a position solution. The GNSS software radio is the first of its kind and will thus bring all the assets associated with the concept into GNSS receiver research. Not only does the work describe the multiple benefits available through a GNSS software radio implementation, but it also establishes the feasibility of such through actual hardware design and experimental results.

Gps augmentation system

Abstract: A system providing information that augments the navigational data transmitted by GPS satellites by providing fast and long-term corrections, a user differential range error, a grid ionospheric vertical error, and ionospheric corrections. The augmentation system includes one or more reference stations (2) having two or more reference receivers that independently receive signals from the GPS satellites and transmit the received information to at least one master station (8). Each master station has a set of processors, designated as correction and verification processors, each configured to receive the output signal from one of the reference receivers and calculate a set of augmentation data from that output signal. The augmentation system compares the augmentation data produced by the correction processor to the augmentation data produced by the verification processor to validate operation of augmentation system and transmits the validated augmentation data to users of the system.

EUROFIX: definition and current status

Abstract: The existing Loran-C infrastructure may with some minor changes become a very powerful augmentation system for GNSS. By additional modulation of the Loran-C pulses a long-range data channel can be established which enables broadcasting of DGNSS correction data and integrity information to GNSS users. Unlike other DGNSS services Eurofix offers a full navigation back-lip system in case GNSS fails. The applied additional three-level modulation is fully balanced so it has negligible influence on the basic Loran-C positioning accuracy. Since 1989 Delft University has been working on this system called Eurofix. The user may expect DGNSS accuracies of better than 5 meters (95%) over the full coverage area that eventually will cover entire Europe. During normal operational conditions the Loran-C ASF model imperfections stored in the receiver are continuously calibrated by the found precise DGNSS positions thereby introducing an efficient zero-baseline DLoran-C system. So, during periods of poor GNSS reception, highly calibrated Loran-C may take over. This way, Eurofix offers the user highly accurate DGNSS data, as well as external GNSS integrity and improved radionavigation availability. The German DOT in close cooperation with NELS started experimental Eurofix transmissions in February 1997. The continuing successful DGPS performance tested in France, Germany, the Netherlands and Switzerland mane NELS decide to extent Eurofix in 1998 to another three Loran-C stations at Lessay (France), Voerlandet (South Norway) and Bo (north Norway). This will make Eurofix available in all NELS countries ranging from the North Cape down to the Pyreneans. These four stations also allow extensive testing of the Regional Area Augmentation System capability of Eurofix. As RAAS may significantly reduce temporal and spatial correction data decorrelation further improvements of the system are anticipated.

GPS/INS integration in a severe urban environment

Abstract: GPS satellite visibility data collected over a ground track in the "deep urban canyons" of lower Manhattan indicated that there were usually too few satellites being tracked to form a position solution. The present study investigates the augmentation of such limited GPS data with a low cost, low accuracy INS. The navigation system is assumed to be self-contained and unobtrusively attached to the host vehicle. Initial covariance matrix analyses produced errors far in excess of that deemed necessary for a remotely sited operator to locate the vehicle on the correct street, when the navigation data is projected onto a map display. A series of software upgrades were then shown to produce significant performance improvements. In the final phase of this effort, a simplified simulation program was used to display navigation data on a grid map of the area in question. It was then found that a revised navigation algorithm produced data that appears to greatly enhance the ability of an operator to correlate the navigation data with the correct streets on the map.

The Role of Time and Frequency in the MTSAT Satellite-Based Augmentation System (MSAS)

Abstract: : The MTSAT Satellite-based Augmentation System (MSAS) is a satellite-based augmentation system (SBAS) for navigation. The MSAS is one of several applications that shares the use of the Multifunctional Transport Satellite (MTSAT), a geostationary earth orbiting (GEO) satellite, which will be launched by the Japanese Ministry of Transport in August 1999. The MSAS design is based on the development of the US Wide Area Augmentation System (WAAS) and therefore may be familiar to those who are acquainted with the WAAS. Time and frequency play a critical role in four areas within MSAS, and these areas are the focus of this paper. Firstly, MSAS performs several functions. One - it collects data from the NA VSTAR Global Positioning System (GPS); two - it provides an independent ranging signal from MTSAT to supplement the navigation signals from the GPS constellotion; three - it provides differential corrections to improve the navigation capability of the GPS; four - it provides the offset of MSAS Network Time (MNT) from Coordinated Universal Time (UTC); andpve - it provides system integrity information to the user. With these functions, the MSAS can become the primary means of time distribution and synchronization within Japan and other areas under the footprint of the MTSAT. The accuracy and precision attainable by MSAS is critically dependent on the performance of the time and frequency subsystems in the four areas of discussion. The first area of discussion is the manner in which the data recording system tags its observations. Accurate recording of the time of observations is essential in order to measure the timeliness (latency) of all transmitted information. This information is based on algorithm that derive their input from data collected from a network of eight data collection sites,including four Ground Monitoring Stations (GMS) and four Monitor and Ranging Stations (MRS). Each GMS/MRS has three independent, free-running cesium-beam frequency standards.

Development of icao standards for the global navigation satellite system is moving ahead

Abstract: SUBTITLE: DEMANDS FOR SARPS ARE GROWING, AND IT IS ONLY A MATTER OF TIME BEFORE GNSS PRECISION APPROACHES WILL BE IMPLEMENTED: THE PRIORITY FOR NOW, HOWEVER, IS TO INTRODUCE EN-ROUTE AND TERMINAL AREA APPLICATIONS THAT CURRENT NAVIGATION AIDS CANNOT SUPPORT ADEQUATELY.

GNSS navigation system accuracy requirement for vehicle driving automation

Abstract: Vehicle driving automation can be realized with a high precision navigation system The major concern of such automation system is the safety According to the tunnel concept and Required Navigation Performance (RNP), the safety risk is sum of accuracy, integrity and continuity risks of all components of the whole system A vehicle automation system using high precision differential GNSS navigation system is subject to continuity and integrity risks caused by so called tunnel penetration events This paper analyses navigation accuracy requirement that makes these tunnel event related integrity and continuity risks less than preset values The parameters affecting these continuity and integrity risks are tunnel dimension, the vehicle driving control system error (CSE) and the navigation system error (NSE) The integrity and continuity risk results based on probability density functions of CSE and NSE show that there exists a combination of CSE and NSE accuracy which can guarantee tunnel event related integrity and continuity risks less than preset values

A dual-frequency GNSS sensor for space applications

Abstract: High-performance laboratory breadboards of a dual-frequency global navigation satellite system (GNSS) receiver have been developed at the Institute of Satellite Navigation at the University of Leeds (ISN) for a number of years under a European Space Agency (ESA) programme. The main objective of these breadboards is to serve as development tools for receiver signal-processing research in view of scientific applications of GNSSs. Some of these applications will be realized by means of receivers embarked on satellites in low earth orbit (LEO) as part of on-going and future ESA programmes and will include precise orbit determination, radio occultation measurements and measurements of the parameters characterizing GNSS signals reflected at the sea surface. The principal application for which the signal-processing developments described in this paper have been targeted is radio occultation. During an occultation event the GNSS signal encounters a challenging environment. Substantial Doppler dynamics occur during a measurement, due to the effect of sharp gradients of the refraction index. Signal attenuation and deep fading caused by multipath propagation are also encountered. The areas of concern are the acquisition and tracking of signals in high dynamics and at low carrier-to-noise ratios (C/N0), and also the performance of tracking the GPS P code without full knowledge of the spreading code, as dual-frequency atmospheric measurements are crucial to this application. © 1998 John Wiley & Sons, Ltd.

Development of a dual-frequency GPS/GLONASS receiver for space application

Abstract: Laboratory breadboards of a high precision dual-frequency Global Navigation Satellite System (GNSS) receiver have been developed at the Institute of Satellite Navigation at the University of Leeds (ISN) under a European Space Agency (ESA) programme. The main objective of these breadboards is to serve as development tools for receiver signal processing research in view of the scientific applications of GNSS. Some of these applications will be realised by means of receivers embarked on satellites in low Earth orbit (LEO) as part of on-going and future ESA programmes and will include precise orbit determination, radio occultation measurements and measurements of the parameters characterising GNSS signals reflected at the sea surface. During an occultation measurement the GNSS signal propagation encounters a challenging environment. Not only substantial Doppler dynamics occur during a measurement due to the effect of gradients of the refraction index, but also attenuation and deep fading caused by multipath propagation. The signal tracking requirements for space applications are analysed and applied to the hardware and software tasks. Resultant enhancements to the receiver digital processing are presented along with their impact on the intended application. The areas of concern are the acquisition and tracking of signals in high dynamics and at low carrier-to-noise ratios (CNR), and on improving the performance of tracking the GPS P code without full knowledge of the spreading code.

SNSS - Simulated Navigation Satellite System; Simulating a Generic GNSS Receiver in Virtual Environments

Abstract: In order to estimate the influence of a change in one component of the GNSS (e.g. a different carrier frequency or narrow correlating in a receiver) it is necessary to model the whole system (satellite constellation, signal transmission, receiver, environment etc.), not just parts of it. In view of future GNSS's (e.g. ENSS, GNSS-2), their design and development, it is very important to estimate their performance, before they are built. When designing a receiver for an existing GNSS (GPS and/or GLONASS), it may also be helpful to “try it out” in a simulation before any hardware is implemented. Thus an end-to-end simulation of a generic GNSS is of great importance for existing and future GNSS's. SNSS is a software tool for simulating GNSS systems (satellite orbits, signal structure etc.) with the corresponding receiver (user segment) in a virtual environment (buildings). SNSS is based on the Purpos software (Eissfeller & Winkel, 1996). The main components of SNSS are · Satellite orbit generator and editor · Environment and way point editors · Reflection model · Ray-tracer for geometric multipath analysis · Receiver · Position processor In this paper we present results from our simulations done with various GNSS Systems: GPS and a hypothetical GNSS-2 constellation with three different pulse-shaping schemes. This is done for different receiver characteristics, in several virtual urban areas.

Navigation systems integration

Abstract: Principles of integrated navigation and optimal estimation theory are discussed. The complexity of navigation system requirements is described, and a comprehensive system design approach is presented. The paper concludes by looking at specific issues which arise in testing integrated navigation systems. The paper is biased towards military vehicle navigation. However the potential for application in commercial vehicles will be evident.

Simulation of the Effects of the Low Latitude Ionosphere on the Performance of a Satellite-Based Augmentation System

Abstract: Ionospheric effects exhibit extreme variability in space and time, significantly degrading the performance of SpaceBased Augmentation Systems (SBASs). This is particularly true at equatorial anomaly latitudes, where intense gradients in the vertical TEC (total electron content) and field-aligned plasma bubbles can be observed, depending on the existing geophysical conditions. Brazil is one of the countries in the world that are affected by both peaks of the equatorial anomaly. Therefore, designing a SBAS to serve its large area will be a challenging task and planners would benefit from the availability of simulation tools. This contribution will present results from a simulation model of the combined effects of the equatorial anomaly and of plasma bubbles on the performance of a SBAS in the Brazilian sector. This model uses the Parameterized Ionospheric Model (PIM 1.7) [1],[2], driven by realistic input values, to represent the “quiet” ionosphere. The vertical TEC values provided by the model will be compared with those in the Global Ionospheric Maps produced by the Center for Orbit Determination in Europe (CODE) [3] to assess the suitability of PIM 1.7 to the present application. A statistical submodel of the effects of plasma bubbles on TEC will also be described. This submodel has been developed from the processing of data recorded by the Rede Brasileira de Monitoramento Continuo of the Instituto Brasileiro de Geografia e Estatistica (RBMC/IBGE), a Brazilian network of GPS stations for geodetic applications [4], as well as from other experimental results. PIM 1.7 is coupled to the statistical submodel of the effects of plasma bubbles on TEC to provide a more realistic representation of the ionosphere in the Brazilian sector. The simulation model also implements the basic procedures described in the RTCA MOPS Document [5]. That is, TEC values along slant paths between GPS satellites and Reference Stations are determined and used to estimate the vertical TEC at the corresponding 350-km pierce points. These results are next used to generate vertical TEC values at the vertices of a 5 o x 5 o reference grid. The model is able to employ a number of pre-selected linear or nonlinear interpolation schemes to do this. The vertical TEC values at the vertices of the reference grid are collected by the Master Station and broadcast through geostationary satellites. Finally, each user employs this information to correct the pseudorange to each visible GPS satellite. This is done by bilinear interpolation based on the vertical TEC values at appropriate vertices of the reference grid, followed by estimation of TEC along the path between the user and the corresponding GPS satellite [5]. The above model relies on a piecewise-planar representation of the variations of vertical TEC, which performs quite well in mid-latitude regions. However, its performance in the low latitude region may not be as satisfactory in the presence of the equatorial anomaly and of plasma bubbles. On the other hand, it should be noted that the performance of any model for the variations of vertical TEC with position, among many factors, also depends on the number of Reference Stations, as well as on their locations. This is a fundamental issue in the design of a Brazilian SBAS. It will be analyzed on the basis of the effects of two uniformly-deployed networks of Reference Stations on the cumulative distribution functions: (1) of the difference between vertical TEC values at the reference grid points either directly calculated by the representation of the perturbed ionosphere in the Brazilian sector or estimated by interpolation using TEC values at the pierce points; and (2) of the errors in the horizontal and vertical positions of the users.