Electronic Version

This paper reviews the development, status and current capabilities of GPS. The modernization improvements planned for GPS are then discussed and summarized, including brief descriptions of the additional features planned for the spacecraft, the control segment and the user equipment. A discussion is presented of the impact of the system modernization plans and activities in improving the performance of the four principal operating modes of GPS. The implications of the GPS modernization and enhancement activities and their relationship to the analogous European Galileo program activities and other GNSS efforts are covered. Several technical, policy and implementation concerns relating to the timely deployment of the improvements to GPS are briefly addressed.

The ionosphere affects the electromagnetic waves that pass through it by inducing an additional transmission time delay. The ionosphere influence has now become the largest error source in GPS positioning and navigation after the turn-off of the Selective Availability (SA). In this paper, methods of 2D gridbased and 3D tomography-based ionospheric modeling are developed based on regional GPS reference networks. Performance analysis was conducted using data from two different regional GPS reference networks. The modeling accuracy of the vertical TEC (VTEC) is at the level of several TECU for 2D ionospheric modeling and about one TECU for 3D tomographic modeling after a comparison to independent ionospheric map data or directly measured ionospsheric TEC values. The data analysis has indicated that the modeling accuracy based on the 3D tomography method is much higher than the 2D grid-based approach.

GPS carrier phase multipath with varying amplitudes of up to several centimetres and periods of couple of minutes is a major error source, which affects the correct interpretation of bridge deformation. In this paper, a recursive adaptive filtering (AF) algorithm has been employed to mitigate multipath signature in the coordinate time series of GPS solutions. In order to maximise the suppression of the multipath signature, exact alignment of the input time series into the AF system is crucial. An algorithm using the crosscorrelation coefficient of day-to-day GPS coordinate time series is presented to align GPS coordinates. To isolate multipath from the contaminated GPS coordinate time series, relative displacements calculated from the accelerations sensed simultaneously with GPS receiver by a triaxial accelerometer housed in a specially designed cage is used as the reference signal sequence within the AF system. Associated algorithm for the relative velocity and displacement calculation is also introduced in the paper. It demonstrates that it is possible to achieve millimetre positioning accuracy by the AF approach and an integrated sensor system of GPS receiver and triaxial accelerometer.

A 3D multi-static SAR imaging system which utilises reflected GPS signals from objects on the Earth's surface is described in this paper. The principle of bistatic radar is used to detect movement of, or changes to, the imaged object. The indirect GPS signals are processed by a match filter with the aim of improving the spatial resolution of detection. The measure of spatial resolution of this imaging system is derived, and is confirmed by MATLAB simulation. Several scenarios are considered, for the visible satellite at a given receiver and object location. The scenarios for different satellites are: a) static receiver with two targets which move with the same speed; and b) moving receiver with one static target and one moving target. Simulation results show that the spatial resolution of detection depends on the relative positions of the GPS satellites, the imaged objects and the GPS receiver, as well as their respective velocities.

Recent developments in differential GPS (DGPS) services have concentrated mainly on the reduction of the number of permanent reference stations required to cover a certain area and the extension of the possible ranges between reference and rover stations. Starting from networked DGPS stations where all stations are linked to a central control station for data correction and modeling, the most advanced technique nowadays is based on the virtual reference station (VRS) network concept. In this case, observation data for a non-existing ‘virtual’ station are generated at the control center and transmitted to the rover. This leads to a significant improvement in positioning accuracy over longer distances compared to conventional DGPS networks. This paper summarizes the various DGPS architectures and the corresponding accuracy. This is followed by a description of the models and algorithms used for the VRS station concept. Practical examples of correction data services in Europe are given to highlight the achievable positioning accuracy. The results of an analysis of test data in a virtual reference station network in southern Germany show that always a horizontal positioning accuracy in the order of ± 5 cm can be achieved for baselines with a length up to 35 km.

Global navigation satellite systems have been revolutionising surveying, geodesy, navigation and other position/location sensitive disciplines. However, there are two intrinsic shortcomings in such satellite-based positioning systems: signal attenuation and dependence on the geometric distribution of the satellites. Consequently, the system performance can decrease significantly under some harsh observing conditions. To tackle this problem, some new concepts of positioning with the use of pseudo-satellites have been developed and tested. Pseudo-satellites, also called pseudolites, are ground-based transmitters that can be easily installed wherever they are needed. They therefore offer great flexibility in positioning and navigation applications. Although some initial experimental results are encouraging, there are still some challenging issues that need to be addressed. This paper reviews the historical pseudolite hardware developments and recent progress in pseudolite-based positioning, and discusses the current technical issues.

Biography:Thomas P. Yunck, holds a bachelor's degree in electrical engineering from Princeton University and a Ph.D. in systems and information science from Yale University. Since 1978 he has been with the Jet Propulsion Laboratory, California Institute of Technology, where he currently manages the GPS Observatories Office. At JPL, Dr. Yunck has been involved in the development of radio metric techniques for spacecraft navigation and for a variety of related science pursuits. For the past 15 years he has managed the development of technologies to employ the signals from GPS for high precision Earth science and remote sensing. His current work focuses on the development of spaceborne GPS systems for applications in geodesy, atmospheric sounding, and ionospheric imaging.

Biography:Christoph Reigber, Prof. Dr.-Ing. Dr.-Ing. E.h, Director of Division 1 'Kinematics and Dynamics of the Earth' of GeoForschungsZentrum Potsdam, is in particular engaged in all aspects of satellite geodesy and its relation to geotectonics, Earth rotation, Earth gravity field, oceanography and atmosphere/ionosphere.He is Director of the CHAMP mission, Co-Principal Investigator of the GRACE mission and Chair of the Coverning Board of the international GPS Service.Peter Schwintzer, Dr.-Ing., Head of Section 1.3, 'Gravity Field and Figure of the Earth' within Division 1 of GeoForschungsZentrum Potsdam, is engaged in global gravity field modelling from space and its geophysical application.He is the Science Data System manager of the CHAMP mission.

Biography:Cinzia Zuffada, Senior Member Technical Staff in the Earth Orbiter Systems Group, Jet Propulsion Laboratory (JPL). Her work at JPL has been focused on the use of GPS for remote sensing of the atmosphere and the ocean. She has been managing the research in GPS altimetry at JPL for the past three years.

Biography:E. A. Essex, is a senior lecturer in the Department of Physics at La Trobe University. She obtained a PhD in Space Physics from the University of New England. Since obtaining her doctorate, she has worked overseas at the University of the West Indies, and also at the Air Force Geophysics Laboratory, Massachusetts. Currently she is the leader of the GPS Space Science project for the Australian satellite FedSat.