Showing papers in "Gps Solutions in 2003"

A general map matching algorithm for transport telematics applications

Abstract: This paper describes a map-matching algorithm designed to support the navigational functions of a real-time vehicle performance and emissions monitoring system currently under development, and other transport telematics applications. The algorithm is used together with the outputs of an extended Kalman filter formulation for the integration of GPS and dead reckoning data, and a spatial digital database of the road network, to provide continuous, accurate and reliable vehicle location on a given road segment. This is irrespective of the constraints of the operational environment, thus alleviating outage and accuracy problems associated with the use of stand-alone location sensors. The map-matching algorithm has been tested using real field data and has been found to be superior to existing algorithms, particularly in how it performs at road intersections.

MATLAB Tools for viewing GPS velocities and time series

Abstract: Over the past decade, many Global Positioning System (GPS) networks have been installed to monitor tectonic motions around the world Some of these networks contain hundreds of sites spread across active tectonic margins where the differences in velocities across the network can be 50–100 mm/year For networks that have been running for a number of years, the uncertainty in the velocity estimates can be less than 1 mm/year In some cases the vertical motions can also be significant and of importance Often, the time series of the motions of the GPS sites show complex non-linear behavior, and in all cases the statistical model of the time series is more complex than simple white noise In this article, we describe a set of Matlab tools developed for use with the GAMIT/GLOBK GPS data analysis system (King 2002; King and Herring 2002) that allow interactive viewing and manipulation of GPS velocities and time series with a Matlab-based graphical user interface (GUI) The formats of the data files used by the tools are specific to GAMIT/GLOBK, but they are simple ASCII files that can be generated from other file formats The tools are referred to as GGMatlab

In-flight performance analysis of the CHAMP BlackJack GPS Receiver

Abstract: JPL's BlackJack receiver currently represents the most widely used geodetic grade GPS receiver for space applications. Using data from the CHAMP science mission, the in-flight performance of the BlackJack receiver has been assessed and the impact of various software updates performed during the 2.5 years since launch is described. Key aspects of the study comprise the channel allocation, anomalous data points, and the noise level of the code and carrier data. In addition, it has been demonstrated that the code measurements collected onboard the CHAMP satellite are notably affected by multipath errors in the aft-looking hemisphere, which can be attributed to cross-talk between the occultation antenna string and the primary precise orbit determination antenna. For carrier smoothed 10 s normal points, the code noise itself varies between a minimum of 5 cm at high elevations and 0.5 m (C/A) to 1.0 m (P1, P2) at 10° elevation. Carrier-phase data exhibit representative errors of 0.2 to 2.5 mm. The results of the CHAMP GPS data analysis contribute to a better understanding and possible improvement of the BlackJack receiver and support the design of optimal data editing and weighting strategies in precise orbit determination applications.

A brief review of basic GPS orbit interpolation strategies

Abstract: The basic test cycle consisted of interpolating a standard source ephemeris with a 15-min data interval to produce satellite coordinates at 5-min intervals for the entire 24 h of the day. The interpolator output was then compared to a control ephemeris also with a 5-min tabular interval. How closely the interpolator output matched the control ephemeris at all epochs over the full 24 h of the day was the measure of success of the interpolator. The source ephemeris was structured like a typical International GPS Service (IGS) rapid or final ephemeris which implies an SP3 file format (Spofford and Remondi 1999) with a 15-min table interval spanning 00:00:00 through 23:45:00 GPS Time (GPST) for a single day. The control ephemeris was also in the SP3 format but with a 5-min interval spanning the entire day plus several hours from both the preceding and subsequent days. The source and control ephemerides were created from the IGS rapid ephemeris for 2002-01-01, igr11472.sp3. The IGS ephemeris was used as input data for an orbit adjustment solution using the PAGES software (Schenewerk et al. 2002). Once the IGS ephemeris was successfully recreated from the orbit adjustment process, a second copy was created from the same adjustment but covering the longer time span with a 5-min interval. These copies became the source and control ephemerides for the interpolator tests. Note that when processing data taken between 23 and 24 h GPST of a day, the source ephemeris must be extrapolated for the last 15 min of the day or a combination with the next day’s ephemeris must be created. Both options involve some risk: problems with ‘‘ringing’’ of the interpolated values near the time limits of the file for the former, and omitted satellites and discontinuities between files in the latter for example. For this reason, special emphasis in the evaluation of the interpolators was placed on the last hours of the comparison to the control ephemeris. Also note that the claimed 3D accuracy of the IGS rapid and final products is 5 cm or better (IGS 2001). Thus, 5 cm became a useful benchmark in the evaluation because the interpolator output must exceed this accuracy in representing the ephemeris to avoid significantly degrading the original information. A common and completely reasonable response in selecting an interpolation strategy is to use

Multisensor integration using neuron computing for land-vehicle navigation

Abstract: Most of the present navigation sensor integration techniques are based on Kalman-filtering estimation procedures. Although Kalman filtering represents one of the best solutions for multisensor integration, it still has some drawbacks in terms of stability, computation load, immunity to noise effects and observability. Furthermore, Kalman filters perform adequately only under certain predefined dynamic models. Neuron computing, a technology of artificial neural network (ANN), is a powerful tool for solving nonlinear problems that involve mapping input data to output data without having any prior knowledge about the mathematical process involved. This article suggests a multisensor integration approach for fusing data from an inertial navigation system (INS) and differential global positioning system (DGPS) hardware utilizing multilayer feed-forward neural networks with a back propagation learning algorithm. In addition, it addresses the impact of neural network (NN) parameters and random noise on positioning accuracy.

Virtual reference stations (VRS)

Abstract: This column provides the web-based GPS resources and their technical background information. Its purpose is to inform the reader about the data, software, and electronic documents that are available on-line. This column is coordinated by Dr. Jinling Wang, The University of New South Wales, Sydney. Comments and suggestions are appreciated (jinling.wang@unsw.edu.au). In this issue, the column provides a brief introduction to differential GPS based on virtual reference station (VRS) techniques and presents addresses of several websites containing information on either software to generate VRS observations or networks providing VRS positioning. The sources are selected by Dr. Lambert Wanninger, who is a software developer and consultant for precise GPS applications. He proposed the VRS technique for network RTK in 1997. Since then he has published several papers on this topic.

The GPS Easy Suite–Matlab code for the GPS newcomer

Abstract: The Matlab computing environment has become a popular way to perform complex matrix calculations, and to produce sophisticated graphics output in a relatively easy manner. Large collections of Matlab scripts are now available for a wide variety of applications and are often used for university courses. The GPS Easy Suite is a collection of ten Matlab scripts, or M-files, which can be used by those just beginning to learn about GPS. The first few scripts perform basic GPS calculations such as converting GPS Time in year/month/day/hour/minute/second format to GPS week/seconds of week, computing the position of a satellite using a broadcast ephemeris, and computing the coordinates of a single point using pseudorange observations. The latter scripts can perform calculations such as computing baseline components using either traditional least-squares or a Kalman filter, fixing cycle slips and millisecond clock jumps, and computing ionospheric delay using carrier phase observations. I describe the purpose of each M-file and give graphical results based on real data. The Matlab code and the sample datasets are available from my website. I have also included additional text files (in pdf format) to discuss the various Time Systems and Coordinate Systems used in GPS computations, and to show the equations used for computing the position of a satellite using the ephemeris information broadcast from the satellites.

Local deformation monitoring using GPS in an open pit mine: initial study

Abstract: High-performance GPS RTK software has been developed within the Geodetic Research Laboratory (GRL) at the University of New Brunswick (UNB). This software was initially designed for gantry crane auto-steering. Due to limitations with classical geodetic deformation monitoring techniques, the Canadian Centre for Geodetic Engineering (CCGE) at UNB has decided to augment its fully automated deformation monitoring system with GPS. As a result, the GRL and CCGE have combined efforts to achieve the required precision. As a first step, tests of the GPS real-time kinematic (RTK) software have been carried out at Highland Valley Copper Mine in British Columbia, Canada. An open-pit mine environment places certain constraints on the achievable accuracies attainable with GPS. Consequently, the software has been modified to meet the needs of this particular project and data have been post-processed for analysis. This paper describes the approach taken at UNB to address high precision requirements in a constrained signal availability environment. Technical and scientific aspects of the UNB software, especially in handling two predominant errors (residual tropospheric zenith delay and multipath) at the mine, are discussed. Results of tests that have been carried out at the mine are presented.

Suppression of multipath and jamming signals by digital beamforming for GPS/Galileo applications

Abstract: Digital beamforming (DBF) has been studied to obtain automatic beam steering towards desired signals and simultaneous elimination of multipath and jamming signals at GNSS receivers, which is made possible by spatial and temporal digital signal processing. In this paper, the limitations of conventional multipath and jamming suppression techniques, which have been proven and widely used in GPS, are investigated. Different DBF algorithms suitable for GNSS applications are investigated theoretically. New ideas for future development of DBF are presented. The implementation of digital beamforming in FPGA/DSP for practical application environments is also discussed.

The activities of the Ionosphere Working Group of the International GPS Service (IGS)

Abstract: This article is based on a position paper presented at the IGS Network, Data and Analysis Center Workshop 2002 in Ottawa, Canada, 8–11 April 2002, and introduces the IGS Ionosphere Working Group (Iono_WG). Detailed information about the IGS in general can be found on the IGS Central Bureau Web page: http://igscb.jpl.nasa.gov. The Iono_WG commenced working in June 1998. The working group's main activity currently is the routine production of ionosphere Total Electron Content (TEC) maps with a 2-h time resolution and daily sets of GPS satellite and receiver hardware differential code bias (DCB) values. The TEC maps and DCB sets are derived from GPS dual-frequency tracking data recorded with the global IGS tracking network.

Degradation of GPS performance in geomagnetically disturbed conditions

Abstract: During geomagnetically disturbed conditions, the accuracy and quality of GPS performance is impaired. Unlike geomagnetically quiet conditions, magnetic storm conditions are accompanied by an increase in the spherical standard deviation of the position determination for all types of GPS receivers. In magnetic storm conditions there is an increase in the number of slips of the one-frequency mode of coordinate determination. We have identified an unambiguous tendency of the slip density of the one-frequency mode to increase for the ASHTECH and TRIMBLE GPS receivers. For the AOA receivers this tendency was not observed in any of the cases considered. The slip density in the two-frequency mode of coordinate determination in magnetic storm conditions increases most dramatically for the ASHTECH receivers. Regarding the TRIMBLE receivers, there is a similar, but less clearly pronounced, picture. No slips of the two-frequency mode were detected for the AOA receivers in the cases under consideration.

Integration of GPS and dead reckoning for real-time vehicle performance and emissions monitoring

Abstract: Transport-related environmental problems continue to constitute a major challenge to policy makers at all levels. A key feature of these problems is that they arise from the interaction of human behavioral systems and physical systems. Thus, to improve our understanding of environmental and health problems associated with vehicle emissions it is necessary to combine data on both travel and traffic behavior with environmental data linked to the corresponding spatial and temporal variables. There are currently no such databases available. A new low-cost real-time device is currently under development utilizing the latest developments in environmental monitoring, navigation, communications, data mining and warehousing to capture spatio-temporally referenced data on vehicle and driver performance and the level of emissions and concentrations. Because of the need to acquire data in all environments, there are potential limitations in using a global satellite navigation system such as GPS to determine the spatial and temporal data in built-up areas. Therefore, an augmentation strategy involving differential GPS and new low-cost dead reckoning sensors utilizing micro-electro-mechanical systems technology has been explored. This paper presents a high-level description of the real-time vehicle performance and emissions monitoring system, and details the results of a study carried out to characterize the performance of stand-alone and augmented GPS, and assess whether the required navigation performance is achievable. The study characterized the performance of different types of stand-alone GPS receivers and GPS augmented with differential infrastructure and low-cost dead reckoning sensors. The performance indicators used were satellite visibility, coverage, accuracy and integrity. The results highlight the weaknesses and differences in performance, depending on the type of GPS receiver used and shows that, unlike GPS alone, an integrated system employing GPS and low-cost dead reckoning sensors is capable of meeting the required navigation performance in built-up areas. Furthermore, no significant difference in accuracy between stand-alone GPS and differential GPS has been seen.

Characterization of the effects of high multipath phase rates in GPS

Abstract: GPS multipath has been studied since the early 1970s. Prior to the investigation described in this paper, however, the effects due to the relative Doppler shift between the direct and multipath signal components have received scant attention. The single previous study that did address the issue indicated coherent receivers had significant performance advantages over noncoherent receivers. Specifically, it was stated that under the condition of fast-fading multipath, noncoherent receivers would yield a bias error, whereas coherent receivers would not. After reviewing the background theory, this paper describes a revised model of the phenomenon and shows the results of hardware simulations which validate the existence of the bias in both receiver types and offer support for the new model. A case study of a specific commercial receiver is presented.

A note on frame transformations with applications to geodetic datums

Abstract: Rigorous equations in compact symbolic matrix notation are introduced to transform coordinates and velocities between ITRF frames and modern GPS-based geocentric geodetic datums. The theory is general but, after neglecting higher than second-order terms, it is shown that the equations revert to the formulation currently applied in most major continental datums. We discuss several examples: the North American Datum of 1983 (NAD83), the European Terrestrial Reference System of 1989 (ETRS89), the Geodetic Datum of Australia of 1994 (GDA94), and the South American Geocentric Reference System (SIRGAS).

Access to the EGNOS signal in space over mobile-IP

Abstract: The EGNOS service will provide better positioning availability and accuracy than that from the standalone GPS solutions. However, in order to access the EGNOS service, the end user needs to access the corresponding GEO satellites that broadcast the augmentation information for the region. This is not a problem normally for aviation and maritime applications because an open sky is always available for such applications. However, an open sky is not always available for land applications because of the obstacles in the vicinity of the end users, for example, in the city canyons. The situation gets worse for the regions at high latitudes because the elevation angles to the GEO satellites are rather low (e.g. 4–22° in Finland). This article describes briefly the SISNeT technology, designed and developed by the European Space Agency, which allows accessing the EGNOS SIS via the Internet. It will describe in detail the handheld SISNeT receiver, designed and developed by the Finnish Geodetic Institute under ESA contract. The SISNeT data server is an IP-based server that acquires the EGNOS messages from an EGNOS receiver, and broadcasts them over the Internet in real-time. The handheld receiver consists of a GPS PC-card receiver, a GPRS (or GSM) card phone, and a pocket PC as the host platform. The receiver software is a Windows CE-based package with a multi-process and multi-thread architecture. It simultaneously receives: (1) the EGNOS SIS over a GPRS wireless connection and the Internet and (2) the NMEA messages from a serial connection to a GPS receiver. It decompresses and decodes the EGNOS messages, and utilizes the information in the messages to estimate the EGNOS-corrected coordinates, which are finally delivered to the end user via a virtual COM port. The virtual COM port has been implemented as a stream interface driver in the Pocket PC. It can be accessed in the same way as the physical COM port in a GPS receiver is accessed. Therefore, it is easy to interface to any third-party applications. The test results show that the handheld SISNeT receiver can provide a positioning accuracy of about 1–2 m for the horizontal components, and 2–3 m for the vertical component in real time. Due to the poor performance of the wireless connection, 10–30% of the EGNOS messages can be lost depending on the services provided by the wireless network operators. The impact of the messages lost on the positioning accuracy is about 0.5 m in both the horizontal and vertical components.

Comparative range delay and variability of the earth's troposphere and the ionosphere

Abstract: The relative magnitudes of the range delays encountered by signals from GPS satellites due to the earth’s ionosphere and the troposphere can be comparable at times. Of course, there are major differences in the variability of these two components of range delay, as well as different methods of correcting for them. In this short paper, the differences between ionospheric and tropospheric range delay are briefly described, and suggestions are made on how best to correct for each of these components of delay for an individual user station. The range delay of the troposphere and the ionosphere differs in several important aspects. The range delay of the earth’s troposphere is not dispersive; that is, it is not a function of frequency, at least not over the normal radio frequency range used for ranging to artificial earth satellites. The range delay of the ionosphere, on the other hand, is dispersive; that is, it varies inversely with frequency in the following manner:

LEO GPS Attitude Determination Algorithm for a micro-satellite using boom-arm deployed antennas

Abstract: This paper describes a low earth orbiter micro-satellite attitude determination algorithm using GPS phase and pseudorange data as the only observables. It is designed to run in real-time, at a rate of 10 Hz, on-board the spacecraft, using minimal chip and memory resources. The spacecraft design includes four GPS antennas deployed on boom arms to improve the antenna separations. The boom arms feature smart sensors, from which time-varying deformation data are used to calculate changes in the body-fixed system (BFS) co-ordinates of the attitude antennas. These data are used as input to the attitude algorithm to improve the accuracy of the output.

Three-dimensional outlier detection for GPS networks and their densification via the BLIMPBE approach

Abstract: This article discusses outlier detection based on Baarda's theory, but applied to three-dimensional GPS baseline vectors, as compared to the traditional approach of investigating the vector components individually (one-dimensional). In addition, Schaffrin's recently proposed estimator of type BLIMPBE is discussed and contrasted with a minimum constrained least-squares adjustment, specifically Partial-MINOLESS. A more detailed discussion of these topics, which includes additional formulas and derivations, can be found in the first author's Master of Science thesis, hereinafter referred to as Report 465.

Fast direct GPS P-Code acquisition

Abstract: GPS P-Code has a higher chipping rate, better accuracy, and anti-jamming property than C/A code. Traditionally, GPS P-Code acquisition depends on handover from C/A code. This potentially needs long acquisition time. Moreover, when C/A code is not available, it is no longer possible to acquire GPS P-Code through handover from C/A code. The purpose of this paper is to describe a new overlap average method to facilitate hardware design of fast direct P-Code acquisition. It allows the rapid code phase search to acquire GPS P-Code signals, and also decreases the hardware resource requirement. The small size FFT in the proposed methods is very promising for fast FPGA hardware system design using FFT cores. The simulation results and theoretical analysis are included demonstrating the overall performance of the proposed method.

A performance comparison of single and dual frequency GPS ambiguity resolution strategies

Abstract: The impact of observation selection, observation combination and model parameterization on GPS carrier phase ambiguity resolution and position accuracy under operational conditions is investigated. The impact of an ionospheric bias for a generic linear combination of L1 and L2 measurements is assessed and the results are used to clearly outline the desirable characteristics for improving ambiguity resolution versus positioning accuracy performance. Ambiguity resolution performance and position accuracy are shown for widelane (WL), L1-only, and ionospheric-free (IF) combinations. Several techniques for dealing with the ionospheric bias are also presented and compared, including stochastic ionospheric modelling. Multiple carrier phase combination solutions estimated in the same filter are also compared. The concept of an optimal processing strategy—in terms of both reliable ambiguity resolution and high accuracy positions—is presented. In total, eight strategies, which vary in observables and parameters, are tested on several datasets ranging from 13 km to 43 km.

GPS/GNSS Current Bibliography

Abstract: This quick bibliography is a compilation of recently published peer-reviewed articles describing original research with citations that may be of interest to other investigators working in any of the GPS/GNSS fields. Titles of new books and conference proceedings that include a significant number of chapters and/ or articles dealing with GPS/GNSS are also listed in italics in this bibliography. Articles, primarily appearing in trade journals, containing interspersed advertisements are only identified by their first page number followed by the symbol +. Contributions with more than five authors will be referenced under the first author name and the extension et al.

A strategy for GPS data processing in a precise local network during high solar activity

Abstract: This paper presents the analyses connected with reduction of errors from ionospheric refraction using GPS data from local satellite networks. This is particularly essential during rising solar activity. The Bernese GPS Software v. 4.2 was used, as an analytical tool. The test data included measurements from a geodynamic network SUDETES situated in the Sudety Mountains across the border between the Czech Republic and Poland. A local ionospheric activity model developed from a regional model augmented with data from a local network has been tested at three levels. The criteria included comparison with a global model, the success rate of ambiguity determination using the quasi-ionosphere free and wide-lane/narrow-lane strategies, and in the position domain through analysis of residuals. The results show that the local model increases the success rate for ambiguity determination for the wide-lane/narrow-lane strategy and is available sooner than the global models. The output of the SUDETES network processing including the models of local ionospheric and tropospheric activity have been used to process data from a number of relatively small networks situated in the Sudety mountains.

The role of Global Navigation Satellite System (GNSS) software radios in embedded systems

Abstract: This paper outlines the motivation for Global Navigation Satellite System (GNSS) software receivers. Features of traditional and software-based GNSS receiver architectures are highlighted and compared, focusing on the advantages of the software design. The choice of which architecture is advantageous, particular in the case of embedded systems, is present along with design criteria—both for the current environment as well as what can be expected in the future.

Definition of a Reconfigurable and Modular Multi-standard Navigation Receiver

Abstract: The growth of innovative positioning-related services and applications within the wireless mass-market, and the green light for the development and modernization of Global Navigation Satellite Systems (GNSS) catalyze the research activity in the navigation field both at the system and user levels. In addition, different government institutions are working toward the definition of navigation plans and regulations which will require accurate locations of mobile users in case of emergency (US E-911 law and the European E-112 directive). From the technical standpoint, within every navigation/positioning system which will be available in the near future, the user terminal will play a central role. In fact, if navigation signals from different sources are available, the unique possibility to obtain the best navigation performance from the user perspective will be the employment of enhanced smart receivers able to fuse different data. With this aim, the Software Defined Radio (SDR) technology can be successfully employed for the design of innovative navigation receivers. The main goal of this paper is to present the overall SDR receiver architecture, focusing attention on reconfigurability and flexibility issues which are guaranteed by the use of reprogrammable high-speed hardware (FPGA–Field Programmable Gate Array and DSP–Digital Signal Processor). The direct benefit of such an implementation is the possibility to obtain a deep integration at the raw signal level between GPS and the future Galileo; the interoperability issue among different systems is then solved at the receiver level.

Optimizing a network-based RTK method for OTF positioning

Abstract: Differential GPS is able to provide cm-level positioning accuracies, as long as the carrier phase ambiguities are resolved to integer values. Classical methods are based on the use of a single reference station located in the vicinity of the rover. Due to the spatial decorrelation of the errors, the distance between the reference station and the user is generally limited to within 20–30 km or even less, mainly due to the ionosphere. The MultiRef method, developed at the University of Calgary, uses a network of reference stations to generate regional code and carrier phase corrections, which can be transmitted to users in order to increase the distance over which integer ambiguity resolution is possible. In the original method, the correlated errors, due to the satellite orbits, troposphere, and ionosphere are modeled together using the L1 and wide-lane observables. In this paper, extensive efforts were carried out towards optimizing the MultiRef method with the objective of maximizing its performance. Data collected in southeastern Brazil was used in this research. At first, the impact of using covariance functions calculated with different data sets was assessed, showing improvement variations of up to 14% in the observation domain compared to using no network corrections, with the exact improvement depending on the data set used in the computation. A new approach, also using least-squares prediction (collocation), was then proposed to separately model the correlated errors. An additional effort was carried out in terms of modeling the ionosphere into directional components. Results of the enhanced method showed the same level of improvement as those obtained using the original covariance functions. However, this new approach has advantages with respect to the transmission of the corrections. Finally, an additional step was taken in terms of applying a Kalman filter to the corrections in order to improve their quality. For cases when the corresponding satellite was setting at low elevations, the filter approach improved results up to 44%. A study on the impact of the various covariance functions on the estimated accuracy of the corrections is also included.

An Artificial Intelligent design for GPS Surveying Networks

Abstract: Metaheuristic techniques, which are based on ideas of Artificial Intelligence, are among the best methods for solving computationally the GPS surveying network problem. In this paper, the ant colony optimization metaheuristic, which is inspired by the behavior of real ant colonies, is developed to efficiently provide a general framework for optimizing GPS surveying networks. In this framework, a set of ants co-operate together using an indirect communication procedure to find good GPS observation schedules. A GPS surveying network can be defined as a set of stations, co-ordinated by a series of sessions formed by placing receivers on the stations. The problem is to search for the best order in which to observe these sessions to give the best schedule at minimum cost. Computational results obtained by applying the proposed technique on several networks, with known and unknown optimal schedules, prove the effectiveness of the proposed metaheuristic technique to solve the GPS surveying network problem.

A velocity field estimation of the Brazilian portion of the SOAM plate

Abstract: With the proposition for the adoption of Geocentric Reference System for the Americas (SIRGAS) as a terrestrial reference frame for South America, the need for temporal monitoring of station coordinates used in its materialization has become apparent. This would provide a dynamic characterization of the frame. The Brazilian Network for Continuous Monitoring of GPS (RBMC) has collected high accuracy GPS measurements since 1996. The Brazilian Institute of Geography and Statistics (IBGE) maintains this network in collaboration with several universities and organizations. Most of the stations are also part of the SIRGAS network. The RBMC also contributes data to the International Terrestrial Reference System (ITRS) to densify the global frame. Two of the RBMC stations are also part of the International GPS Service (IGS). This paper reports initial results from these stations. To estimate the velocity field defined by these stations, ten IGS stations located on the border of the South American plate and in adjacent plates, along with nine RBMC stations, were used. Observations covering five groups of 15 days each were used. These groups of observations were at epochs 1997.3, 1997.9, 1998.3, 1998.9 and 1999.2. Seven IGS stations were chosen to have their coordinates constrained to those epochs. IGS products (precise ephemeris and clocks) were used to process the daily solutions, which were carried out with Bernese software. Carrier phase double differences were formed using the ionospheric-delay free observable. The troposphere was modeled using a combination of the Saastamoinen model and the Niell mapping function. A tropospheric parameter was estimated every two hours. The results of the daily baseline solutions were combined using the summation of normal equations technique, in which the final coordinates and velocities were estimated. The results were compared with various models, such as the NNR-NUVEL1 and the APKIM8.80. Velocity vectors estimated for the RBMC stations show good agreement with those two models, with rates approximately equal to 2 cm/year.

Integration of GPS and radar interferometry

Abstract: Synthetic Aperture Radar Interferometry (or InSAR) is a technique first suggested in the early 1970s. The technique produces an ‘interferogram’ from the phase difference between two SAR images acquired over the same region. If the two SAR images are acquired from two repeated SAR coverages of the same region, the technique is referred to as ‘repeat-pass InSAR’. Radar satellites acquire images at frequencies in the C-, Lor X-microwave bands. Historical satellites such as Seasat, ERS-1 and JERS-1 have played an important role in promoting InSAR technique. The satellites which currently acquire SAR images are ERS-2 and RADARSAT-1, both in the C-band. ENVISAT was launched in March 2002 (also C-band), and new radar satellites are planned for launch over the next few years (including in the L-band). In recent years, InSAR has been increasingly used, for example, to generate DEMs, to monitor volcanoes, earthquakes, landslides, and ground subsidence. Studies have shown that the two techniques GPS and InSAR are very complementary (Ge 2001). In the following it has been briefly introduced how GPS can be integrated with InSAR, as seen from information available on the web. GPS as a navigation tool on the SAR platform

Propagation algorithms for attitude-related ambiguity resolution

Abstract: A new approach for ambiguity resolution is presented and tested by means of Monte-Carlo simulations. This approach applies to both single and double phase differences sampled at a given instant, and to any number of baselines and lines of sight, provided the number of lines of sight exceeds a certain threshold. The algorithms are based on the combined global geometry of the baselines and lines of sight and thereby consider all geometric constraints that are inherent in the problem. The algorithms have been programmed and applied to a large number of different configurations of GPS sight lines, and limited to baselines of approximately 1 m in length and the GPS L1 frequency only. The cause of the occurrence of multiple solutions has been investigated. To check the adequacy and limitations of the new algorithms for practical application, it was assessed in which proportion the algorithms either failed to provide a result, provided a false result, or provided a good result as a function of number of baselines, lines of sight and accuracy of the ambiguous phase difference measurements.