Showing papers in "Gps Solutions in 2002"
TL;DR: The use of precise post-processed satellite orbits and satellite clock corrections in absolute positioning, using one GPS receiver only, has proven to be an accurate alternative to the more commonly used differential techniques for many applications in georeferencing.
Abstract: The use of precise post-processed satellite orbits and satellite clock corrections in absolute positioning, using one GPS receiver only, has proven to be an accurate alternative to the more commonly used differential techniques for many applications in georeferencing. The absolute approach is capable of centimeter accuracy when using state-of-the-art, dual-frequency GPS receivers. When using observations from single-frequency receivers, however, the accuracy, especially in height, decreases. The obvious reason for this degradation in accuracy is the effect of unmodeled ionospheric delay. This paper discusses the availability of some empirical ionospheric models that are publicly available and quantifies their usefulness for absolute positioning using single-frequency GPS receivers. The Global Ionospheric Model supplied by International GPS Service (IGS) is the most accurate one and is recommended for absolute positioning using single-frequency GPS receivers. Using high-quality single-frequency observations, a horizontal epoch-to-epoch accuracy of better than 1 m and a vertical accuracy of approximately 1 m is demonstrated. © 2002 Wiley Periodicals, Inc.
111 citations
TL;DR: In this article, the tracking performance of a phase lock loop (PLL) is affected by the influence of several error sources, such as thermal noise and dynamic stress error, oscillator phase noise can cause significant phase jitter which degrades tracking performance.
Abstract: The tracking performance of a Phase Lock Loop (PLL) is affected by the influence of several error sources. In addition to thermal noise and dynamic stress error, oscillator phase noise can cause significant phase jitter which degrades the tracking performance. Oscillator phase noise is usually caused by two different effects: Allan deviation phase noise is caused by frequency instabilities of the receiver's reference oscillator and the satellite's frequency standard. It can be termed as system-inherent phase noise and is relevant for both static and dynamic applications. “External” phase noise, however, is caused by vibration and is a major problem for dynamic applications. In the context of this paper, both types of phase noise will be modeled and the resulting integrals will be evaluated for PLLs up to the third order. Besides, phase jitter induced by thermal noise and signal dynamics will also be discussed, thus providing all necessary formulas for analyzing the performance of a phase lock loop in case of different forms of stress. Since the main focus is centered on the effects of oscillator phase noise, the overall PLL performance is graphically illustrated with and without consideration of oscillator phase noise. © 2002 Wiley Periodicals, Inc.
95 citations
TL;DR: Whether wireless mobile technologies are ready to support LBS, which mobile positioning technologies can be used and what are their shortcomings, and how GIS developers manipulate spatial information to generate appropriate map images on mobile devices are discussed.
Abstract: Geographical Information System (GIS) and Global Positioning System (GPS) technologies are expanding their traditional applications to embrace a stream of consumer-focused, location-based applications Through an integration with handheld devices capable of wireless communication and mobile computing, a wide range of what may be generically referred to as 'Location-Based Services' (LBS) may be offered to mobile users A location-based service is able to provide targeted spatial information to mobile workers and consumers These include utility location information, personal or asset tracking, concierge and route-guidance information, to name just a few of the possible LBS The technologies and applications of LBS will play an ever increasingly important role in the modern, mobile, always-connected society This paper endeavours to provide some background to the technology underlying location-based services, and to discuss some issues related to developing and launching LBS These include whether wireless mobile technologies are ready to support LBS, which mobile positioning technologies can be used and what are their shortcomings, and how GIS developers manipulate spatial information to generate appropriate map images on mobile devices (such as cell phones and PDAs) In addition, the authors discuss such issues as interoperability, privacy protection and the market demand for LBS
89 citations
TL;DR: The market for location-based services is overviewed and algorithmic innovations that address challenges posed by urban environments are discussed, including those that accommodate high degrees of signal attenuation and multipath effects unique to the "urban channel".
Abstract: The proliferation of mobile devices and the emergence of wireless location-based services has generated consumer demand for availability of GPS in urban and indoor environments. This demand calls for enhanced GPS algorithms that accommodate high degrees of signal attenuation and multipath effects unique to the "urban channel." This paper overviews the market for location-based services and discusses algorithmic innovations that address challenges posed by urban environments.
86 citations
TL;DR: In this paper, a high-sensitivity receiver which operates in unaided stand-alone mode is tested under a range of shaded and blocked signal environments, ranging from residential outdoor areas to urban canyons and residential houses.
Abstract: The use of GPS for personal location using wireless devices requires satellite signal measurements both outdoors and indoors. In the outdoor environment propagation paths may range from line-of-sight to shaded or blocked. The indoor environment may range from single-floor wooden constructions to high-rise buildings and underground facilities. In this paper, a high-sensitivity receiver which operates in unaided stand-alone mode is tested under a range of shaded and blocked signal environments, ranging from residential outdoor areas to urban canyons and residential houses. The measurement analysis is performed in both the observation and position domains. The results show that the receiver tested is able to yield measurements with C/N0 degradations in excess of 20 dB-Hz, as compared to line-of-sight measurements. Position results are a function of the geometry of the remaining satellites, which in turn is a function of the environment.
77 citations
TL;DR: It is illustrated here how the algebraic techniques of Multipolynomial resultant and Groebner basis explicitly solve the nonlinear GPS pseudo-ranging four-point equations once they have been converted into algebraic (polynomial) form and reduced to linear equations.
Abstract: Several procedures for solving, in a closed form the GPS pseudo-ranging four-point problem P4P in matrix form already exist. We present here alternative algebraic procedures using Multipolynomial resultant and Groebner basis to solve the same problem. The advantage is that these algebraic algorithms have already been implemented in algebraic software such as “Mathematica” and “Maple”. The procedures are straightforward and simple to apply. We illustrate here how the algebraic techniques of Multipolynomial resultant and Groebner basis explicitly solve the nonlinear GPS pseudo-ranging four-point equations once they have been converted into algebraic (polynomial) form and reduced to linear equations. In particular, the algebraic tools of Multipolynomial resultant and Groebner basis provide symbolic solutions to the GPS four-point pseudo-ranging problem. The various forward and backward substitution steps inherent in the clasical closed form solutions of the problem are avoided. Similar to the Gauss elimination techniques in linear systems of equations, the Multipolynomial resultant and Groebner basis approaches eliminate several variables in a multivariate system of nonlinear equations in such a manner that the end product normally consists of univariate polynomial equations (in this case quadratic equations for the range bias expressed algebraically using the given quantities) whose roots can be determined by existing programs (e. g., the roots command in MATLAB). © 2002 Wiley Periodicals, Inc.
62 citations
TL;DR: A robust estimator that clearly identifies outlying observations caused by obstacles like diagonal cables, branches, or leaves is presented, which performs significantly better than standard LS and signal-to-noise ratio dependent weighting if unfavorable signal distortion occurs, and is equal to LS otherwise.
Abstract: Least-squares estimation (LS) yields results of low accuracy in the presentce of GPS phase-corrupting environmental conditions. We present a robust estimator that clearly identifies outlying observations caused by obstacles like diagonal cables, branches, or leaves. It performs significantly better than standard LS and signal-to-noise ratio dependent weighting if unfavorable signal distortion occurs, and is equal to LS otherwise. The estimator is realized by an iterated LS algorithm using an equivalent weight matrix. It is a generalization of the Danish Method to heterogeneous and correlated observations. The excellent peformance of the estimator for processing short static sessions is demonstrated using data obtained from an investigation of GPS signal obstruction. © 2002 Wiley Periodicals, Inc.
49 citations
TL;DR: The experimental results indicate that the accuracy of the height component can indeed be significantly improved – the RMS of the vertical component has been reduced by a factor of 4, to the same level as the horizontal components.
Abstract: In this article, three general classes of potential pseudolite applications for deformation monitoring are described. The first is GPS augmentation with pseudolite(s), which is suitable for circumstances such as urban canyons, or for monitoring in valleys and deep open-cut mines. The second is indoor applications of pseudolite deformation monitoring systems. Pseudolite arrays can, in principle, completely replace the GPS satellite constellation. This could extend the “satellite-based” deformation monitoring applications into tunnels or underground, where GPS satellite signals cannot be tracked. The last case is an inverted pseudolite-based deformation monitoring system, where a “constellation” of GPS receivers with precisely known “orbits” track a mobile pseudolite. The system consists of an array of GPS receivers, the base reference pseudolite, the mobile pseudolite, and a central processing system. However, in the case of such pseudolite-only or hybrid pseudolite-GPS deformation monitoring systems, some additional issues need to be addressed. These include multipath, atmospheric delay effects, and pseudolite location-dependent biases. To address deformation monitoring applications, some practical procedures to mitigate or eliminate their influence are suggested. Some experiments were carried out using NovAtel GPS receivers and IntegriNautics IN200CXL pseudolite instruments. The experimental results indicate that the accuracy of the height component can indeed be significantly improved – the RMS of the vertical component has been reduced by a factor of 4, to the same level as the horizontal components. Their performance will be demonstrated through case study example. © 2002 Wiley Periodicals, Inc.
40 citations
TL;DR: A calibration procedure has been developed by Geo++ and the Institut für Erdmessung, which directly determines absolute phase center variations (PCVs) without any multipath influence by field measurements, which allows the determination of reliable azimuthal variations.
Abstract: Phase variations of GPS receiving antennas are a significant error component in precise GPS applications. A calibration procedure has been developed by Geo++ and the Institut fur Erdmessung, which directly determines absolute phase center variations (PCVs) without any multipath influence by field measurements. The precision and resolution of the procedure allows the determination of reliable azimuthal variations. PCV may affect long-term static GPS differently than real-time GPS, depending on the applications. At the same time, different antenna types are involved. Less investigations have been done on absolute PCV of rover antennas than on geodetic antennas which, however, becomes more important due to the mixed antenna situation in GPS reference networks and RTK networks. The concepts of the absolute PCV field calibration are summarized and emphasis is placed on a variety of absolute PCV patterns of geodetic and rover antennas.
39 citations
TL;DR: In this paper, the problem of finding the critical configuration manifold (Determinantal Loci, Inverse Function Theorem, Jacobi map) where no solution of P4P exists is solved in a closed form.
Abstract: P4P is the pseudo-ranging 4-point problem as it appears as the basic configuration of satellite positioning with pseudo-ranges as observables. In order to determine the ground receiver/satellite receiver (LEO networks) position from four positions of satellite transmitters given, a system of four nonlinear (algebraic) equations has to be solved. The solution point is the intersection of four spherical cones if the ground receiver/satellite receiver clock bias is implemented as an unknown. Here we determine the critical configuration manifold (Determinantal Loci, Inverse Function Theorem, Jacobi map) where no solution of P4P exists. Four examples demonstrate the critical linear manifold. The algorithm GS solves in a closed form P4P in a manner similar to Groebner bases: The algebraic nonlinear observational equations are reduced in the forward step to one quadratic equation in the clock bias unknown. In the backward step two solutions of the position unknowns are generated in closed form. Prior information in P4P has to be implemented in order to decide which solution is acceptable. Finally, the main target of our contribution is formulated: Can we identify a special configuration of satellite transmitters and ground receiver/satellite receiver where the two solutions are reduced to one. A special geometric analysis of the discriminant solves this problem. © 2002 Wiley Periodicals, Inc.
38 citations
TL;DR: In this paper, a linear fit to the full span of data shows agreement between the two frames at the level of 1 −1 and 0 −0.1 −ppb/year.
Abstract: The Global Positioning System is a constellation of 24–28 satellites, which can be used to define a global terrestrial reference frame. Daily offsets between a GPS defined frame and ITRF2000 have been estimated using more than a decade of GPS observations from 1990–2001. A linear fit to the full span of data shows agreement between the two frames at the level of –1 ppb and –0.1 ppb/year for scale, 5 mm and 0 mm/year for the X component of center of mass, –2 mm and –3 mm/year for the Y component, and 4 mm and 6 mm/year for the Z component. GPS is a viable tool for defining the global reference frame either alone, or in combination with other geodetic techniques.
TL;DR: A study to assess the RAIM capability of the Galileo system when used alone and when combined with the Global Positioning System (GPS).
Abstract: Integrity relates to the trust that can be placed in the correctness of information supplied by a navigation system. It includes the ability of the navigation system to provide timely warning to users when the system fails to meet its stated accuracy. Specifically, a navigation system is required to deliver a warning (alarm) when the error in the derived user position solution exceeds an allowable level (alarm limit). This warning must be issued to the user within a given period of time (time-to-alarm) and with a given probability (integrity risk). The two main approaches to monitoring the integrity of satellite navigation systems are Receiver Autonomous Integrity Monitoring (RAIM), and monitoring based on an independent network of integrity monitoring stations and a dedicated Ground Integrity Channel (GIC). More recently Satellite Autonomous Integrity Monitoring (SAIM) methods have also been investigated. This article presents the results of a study to assess the RAIM capability of the Galileo system when used alone and when combined with the Global Positioning System (GPS). The assessment was based on the Marginally Detectable Error (MDE) algorithm. The results show a significant improvement in the capability to perform RAIM using a combined Galileo/GPS system compared to the performance using the Galileo system alone. This study was supported by Alcatel Space and was a contribution to the Galileo definition studies carried out for the European Community under the GALA project. © 2002 Wiley Periodicals, Inc.
TL;DR: It is demonstrated how integer carrier phase cycle ambiguity resolution will perform in near future, when the US GPS gets modernized and the European Galileo becomes operational.
Abstract: In this short contribution it is demonstrated how integer carrier phase cycle ambiguity resolution will perform in near future, when the US GPS gets modernized and the European Galileo becomes operational. The capability of ambiguity resolution is analyzed in the context of precise differential positioning over short, medium and long distances. Starting from dual-frequency operation with GPS at present, particularly augmenting the number of satellites turns out to have beneficial consequences on the capability of correctly resolving the ambiguities. With a 'double' constellation, on short baselines, the confidence of the integer ambiguity solution increases to a level of 0.99999999 or beyond.
TL;DR: A methodology to monitor dynamic vertical sub-centimeter displacement, of a GPS antenna is described by choosing the appropriate reference satellite for measurement differencing and by applying a FFT filter on the double-difference phase residuals.
Abstract: This article describes a methodology to monitor dynamic vertical sub-centimeter displacement, of a GPS antenna. The dynamic movement of an antenna is determined by choosing the appropriate reference satellite for measurement differencing and by applying a FFT filter on the double-difference phase residuals. The validity of the method depends on the time variations of the GPS residuals and errors, such as, receiver noise, atmospheric contribution, multipath effects, and the antenna movement. This research is under development and results for simulated motion are presented here. © 2002 Wiley Periodicals, Inc.
TL;DR: The application of GPS in collecting travel time, speed, and delay information of 64 major roads in the state of Delaware is presented and GPS data proved to be at least as accurate as the data collected by the conventional method, and it was 50% more efficient in terms of manpower.
Abstract: Integrated Traffic Management Systems (ITMS) need reliable, accurate, and real-time data. Travel time, speed, and delay are three of the most important factors used in ITMS for monitoring, quantifying, and controlling congestion. GPS has recently become available for civil applications. Because it provides real-time spatial and time measurements, it has an increasing use in conducting different transportation studies. This article presents the application of GPS in collecting travel time, speed, and delay information of 64 major roads in the state of Delaware. A comparative statistical analysis was performed on data collected by GPS, with data collected simultaneously by the conventional method. The GPS data proved to be at least as accurate as the data collected by the conventional method, and it was 50% more efficient in terms of manpower. Moreover, the sample-size requirement was determined to maintain 95% confidence level throughout the controlled test. Benefiting from the Geographic Information System's dynamic segmentation tool, our travel time, delay, and speed information were integrated with other relevant traffic data. This was presented graphically on the Internet for public use. Statistical trend analysis for the data collected in 1997, 1998, 1999, and 2000 are also presented and applications on the overall ITMS are discussed. © 2002 Wiley Periodicals, Inc.
TL;DR: In this article, the relativistic time transformations of GPS satellite frequency and clocks are critically reviewed, taking into account all the contributions larger than the 10−18 (or 0.001 ns).
Abstract: Since Selective Availability was permanently switched off on 7 May 2000, most of the GPS satellite clocks have been well behaved. During a 24-h period precise satellite clock solutions, corrected for GPS conventional relativistic corrections, follow straight lines within a few nanoseconds. The linear clock fit RMS for the best satellite clocks are well below the 1-ns level, which is consistent with the nominal stability of the GPS frequency standards. Typically, the GPS satellite clocks show an Allan variance at or below one part in 1011/100 s for the Cesium frequency standards and a few parts in 1012/100 s for the Rubidium frequency standards. These results correspond to clock RMSs for 15-min sampling at or below 3 and 0.3 ns, respectively. This already confirms experimentally that the conventional periodic relativity correction of the GPS system, also adopted for all the IGS clock solution products, is precise and correct to 0.6 ns or better. To establish the precision limits of the GPS conventional relativity treatment, the relativistic time transformations of GPS satellite frequency and clocks are critically reviewed, taking into account all the contributions larger than the 10−18 (or 0.001 ns). The conventional GPS relativity treatment was found to be accurate, i. e., correctly modeling the actual relativistic frequency (clock rate) effects of GPS satellites at about the 10−14 level. However, it is also affected by small periodic errors of the same magnitude. The integration of these small periodic frequency relativistic errors gives the approximation errors of the conventional periodic relativistic clock correction with amplitudes of about 0.1 ns and a predominant period equal to a half of the orbital period (∼ 6 h). These approximation errors of the conventional GPS relativistic clock correction are at about the same level as the current precision of the IGS clock solutions. © 2002 Wiley Periodicals, Inc.
TL;DR: In this article, the Fast Nonlinear Adjustment Algorithm (FNon Ad Al) has been already proposed by Gauss whose work was published posthumously and Jacobi (1841), which solves the over-determined GPS pseudo-ranging problem without reverting to iterative or linearization procedure except for the second moment (Variance-Covariance propagation).
Abstract: The nonlinear adjustment of GPS observations of type pseudo-ranges is performed in two steps. In step one a combinatorial minimal subset of observations is constructed which is rigorously converted into station coordinates by means of Groebner basis algorithm or the multipolynomial resultant algorithm. The combinatorial solution points in a polyhedron are reduced to their barycentric in step two by means of their weighted mean. Such a weighted mean of the polyhedron points in ℝ3 is generated via the Error Propagation law/variance-covariance propagation. The Fast Nonlinear Adjustment Algorithm (FNon Ad Al) has been already proposed by Gauss whose work was published posthumously and Jacobi (1841). The algorithm, here referred to as the Gauss-Jacobi Combinatorial algorithm, solves the over-determined GPS pseudo-ranging problem without reverting to iterative or linearization procedure except for the second moment (Variance-Covariance propagation). The results compared well with the solutions obtained using the linearized least squares approach giving legitimacy to the Gauss-Jacobi combinatorial procedure. © 2002 Wiley Periodicals, Inc.
TL;DR: In this article, a unified GPS data processing method based on equivalent observation equations is proposed, where the unknowns to be eliminated as vector of zero, i.e., the satellite clock errors, all clock errors and ambiguity parameters or user-defined unknowns, can be formed.
Abstract: A unified GPS data processing method based on equivalent observation equations is proposed. The theoretic background of this method is given in detail. By selecting the unknowns to be eliminated as vector of zeros, i.e. the satellite clock errors, all clock errors, the clock and ambiguity parameters or user-defined unknowns, the equivalent observation equations can be formed. These equations are equivalent to the zero-, single-, double-, triple- or user-defined differenced equations. The advantages of the method are that the various GPS data processing methods are unified to a unique one, whereas the observation vector remains the original one and the weight matrix keeps the un-correlated diagonal form. Using this equivalent method one may selectively reduce the number of unknowns without having to deal with the correlations. Several special cases of single, double and triple difference are discussed in detail to illustrate the theory.
TL;DR: In this paper, the authors introduce rigorous matrix equations to compute variance-covariance matrices for transformed coordinates at any epoch t based on a stochastic model that takes into consideration all a priori in-formation of the parameters involved at epoch t, and the coordinates and velocities at the reference frame initial epoch t0.
Abstract: With the advances in the field of GPS positioning and the global densification of perma- nent GPS tracking stations, it is now possible to determine at the highest level of accuracy the transformation parameters connecting various international terrestrial reference frame (ITRF) realizations. As a by-product of these refinements, not only the seven usual parameters of the similarity transformations between frames are available, but also their rates, all given at some epoch tk. This paper introduces rigorous matrix equations to esti- mate variance-covariance matrices for transformed coordinates at any epoch t based on a stochastic model that takes into consideration all a priori in- formation of the parameters involved at epoch tk, and the coordinates and velocities at the reference frame initial epoch t0. The results of this investiga- tion suggest that in order to attain maximum accu- racy, the agencies determining the 14-parameter transformations between reference frames should also publish their full variance-covariance matrix.
TL;DR: In this article, the temporal variations of the broadcast satellite clock model and orbit parameters, as well as ionospheric errors are analyzed independently, and the effect of each error in the position domain is also investigated in addition to the combined effect.
Abstract: This paper is aimed at investigating the stability of point positions over time in support of applications that require high position stability when differential GPS is not feasible. One such application is the use of a P3-Orion aircraft offshore for magnetic measurement in support of submarine detection. Temporal changes in several GPS errors lead to variability in the computed positions, so it is not the absolute errors, but rather their temporal variations that are of importance. Furthermore, the temporal variability of the different error sources may dictate a certain algorithm approach and processing strategy. This paper analyzes the temporal variations of the broadcast satellite clock model and orbit parameters, as well as ionospheric errors, because these will typically be the dominant errors for real-time point positioning. These three errors are analyzed independently. A tropospheric correction is applied when computing all of the position results, so the tropospheric error itself is not investigated. Satellite clock and orbit errors are analyzed by comparing broadcast and precise post-mission SV clock corrections and orbits. For the ionosphere, the effect is separated using dual-frequency data. The analysis comprises primarily of assessing error behaviors and magnitudes through time and frequency analyses. In this way, the differences in variability of the errors are easily determined. The effect of each error in the position domain is also investigated in addition to the combined effect. Results show that, on a typical day when single frequency data are processed with broadcast orbit and clock data, the root mean square (RMS) of the changes in the position errors over a 50-s interval is about 5.8 cm in northing, 4.0 in easting, and 11.0 cm in height. When using precise orbits and clocks, in addition to dual frequency data, these values improve by 46–56% to 2.7 cm in northing, 2.2 cm in easting, and 4.9 cm in height. Under severe ionospheric activity, the RMS of the errors decrease from 8.1 to 3.3 cm in northing, 5.7 to 2.6 cm in easting, and 17.0 to 4.9 cm in height, which are improvements of 54–71%.
TL;DR: It has been established that the GPS/PL/INS and PL/INS integration schemes would make it possible to ensure centimeter-level positioning accuracy even if the number of GPS signals is insufficient, or completely unavailable.
Abstract: This paper discusses the introduction of pseudolites (ground-based GPS-like signal transmitters) into existing integrated GPS/INS systems in order to provide higher availability, integrity, and accuracy in a local area. Even though integrated GPS/INS systems can overcome inherent drawbacks of each component system (line-of-sight requirement for GPS, and INS errors that grow with time), performance is nevertheless degraded under adverse operational circumstances. Some typical examples are when the duration of satellite signal blockage exceeds an INS bridging level, resulting in large accumulated INS errors that cannot be calibrated by GPS. Such a scenario, unfortunately, is a common occurrence for certain kinematic applications. To address such shortcomings, both pseudolite/INS and GPS/pseudolite/INS integration schemes are proposed here. Typically, the former is applicable for indoor positioning where the GPS signal is unavailable for use. The latter would be appropriate for system augmentation when the number and geometry of visible satellites is not sufficient for accurate positioning or attitude determination. In this paper, some technical issues concerned with implementing these two integration schemes are described, including the measurement model, and the appropriate integration filter for INS error estimation and correction through GPS and pseudolite (PL) carrier phase measurements. In addition, the results from the processing of simulated measurements, as well as field experiments, are presented in order to characterize the system performance. As a result, it has been established that the GPS/PL/INS and PL/INS integration schemes would make it possible to ensure centimeter-level positioning accuracy even if the number of GPS signals is insufficient, or completely unavailable.
TL;DR: Windows-based C/C++ software for generating the azimuths and elevation angles, as well as software for viewing and printing the skyplots are described, which are freely available on the Internet.
Abstract: One of the best tools for diagnosing problems encountered when processing GPS data is the skyplot, a simple illustration of GPS satellite trajectories over a given ground site These plots, also called satellite visibility diagrams, provide an intuitive feel for satellite geometry and they reveal the impact of obstructions on satellite visibility Most commercial GPS software is capable of creating skyplots; however, freely available skyplot software may be difficult to find Thanks to freely available mapping software such as the Generic Mapping Tools (GMT) and postscript viewing software such as Ghostscript, GPS users can generate skyplots without concerning themselves with the intricacies of computer graphics or with printer drivers Instead, skyplots can be generated by providing azimuth and elevation-angle input data which can then be converted into x, y coordinates for input into the existing mapping software This article describes Windows-based C/C++ software for generating the azimuths and elevation angles, as well as software for viewing and printing the skyplots All the software is freely available on the Internet
TL;DR: It is demonstrated that GPS positioning precision for ambiguity fixed solutions reduces when the latitude increases, and it is quantified that the positioning precision in the north-south direction is worse than that in the east-west direction at all latitudes.
Abstract: We analyze the dependency of the GPS positioning precision on station location in an idealized positioning environment. Using a weighting function to account for an averaged number of satellites at different latitudes, we demonstrate that GPS positioning precision for ambiguity fixed solutions reduces when the latitude increases. We also quantify that the positioning precision in the north-south direction is worse than that in the east-west direction at all latitudes using the same approach.
TL;DR: In this article, a residual-based adaptive Kalman filter is proposed to improve network correction availability, and the VRS concept is used to transmit corrections to the user for RTK positioning.
Abstract: Conventional RTK positioning is usable, but requires the use of a local base station. It is also restricted by the effects of the de-correlate atmospheric refraction on the GPS signal, which limits the use of the RTK positioning up to distances of 10–15 km from the reference station to the user. With a multiple reference station network approach, precise RTK positioning capability may be extended for a much larger area. The Singapore Integrated Multiple Reference Station Network (SIMRSN) has been established for this purpose. Using an existing method termed linear combination method, the multiple reference station network corrections are generated for the user on an epoch by epoch, satellite-by-satellite basis. A residual-based adaptive Kalman filter is proposed to improve network correction availability. The VRS concept is used to transmit corrections to the user for RTK positioning. Field tests were conducted to demonstrate the general performance of SIMRSN. The tests confirmed that RTK positioning within a multiple reference station network can provide the user with better than 3 cm in horizontal position, the height accuracy is in the range of 1–7 cm, and the average TTF (Time To Fix) are 46 and 76 s during GLC station and LPR station tests, respectively. With this highly efficient survey technique, the user needs only to be equipped with a single GPS receiver. This reduces equipment and manpower costs compared with traditional RTK positioning using a specially set up reference station. The multiple reference station framework also paves the way for various other applications beyond the traditional surveying profession.
TL;DR: In this article, a simplified two-dimensional form of the radio-holographic equations which are developed under an assumption of local spherical symmetry can be used to obtain 2D radio images of the atmosphere and terrestrial surface.
Abstract: GPS radio occultation (RO) signals are highly coherent and precise, and thus sufficient for holographic investigation of the atmosphere, ionosphere, and the Earth's surface from space. In principle, three-dimensional radio-holographic remote sensing is possible by using new radio holographic equations to retrieve the radio field within the atmosphere from a radio field known at some interface outside the atmosphere. A simplified two-dimensional form of the radio-holographic equations which are developed under an assumption of local spherical symmetry can be used to obtain two-dimensional radio images of the atmosphere and terrestrial surface. To achieve this, radio holograms recorded by a GPS receiver onboard a low earth orbit (LEO) satellite at two GPS frequencies can be used and focused synthetic aperture principle applied. Analysis of GPS/MET RO data is presented to show the effectiveness of a radio-holographic approach. It is shown that the amplitude of GPS radio signals (in addition to phase data) can be used to obtain detailed altitude profiles of the vertical gradient of refractivity in the atmosphere and electron density in the mesosphere. The results demonstrate the applicability of GPS radio holography for a detailed global study of the natural processes in the atmosphere and mesosphere.
TL;DR: It is shown, for example, that the time of day in which the survey is carried out and the dynamic characteristics of the aircraft being used are two of the most crucial parameters for very high-resolution gravity field estimation.
Abstract: Although airborne gravimetry is now considered a fully operational technique, errors due to motion compensation using differential GPS (DGPS) continue to influence both its accuracy and the range of applications in which it can be used. In typical medium-resolution applications such as airborne geoid mapping, errors due to DGPS contribute considerably to the error budget of an airborne gravity system. At the same time, efforts to increase the resolution of such systems for demanding applications such as resource exploration remain impedded by errors in DGPS. This article has three objectives. The first one is to compare eight industrially relevant DGPS software packages for the determination of aircraft acceleration. The second objective is to analyze and quantify the effect that each relevant portion of the DGPS error budget has on the determination of acceleration. Using data sets that represent a wide range of operational conditions, this is done in the frequency domain over a range of frequencies corresponding to spatial resolution as high as 450 m. The third objective is to use that information to recommend and demonstrate approaches that optimize the estimation of aircraft acceleration for determining the geoid and for resource exploration. It is shown, for example, that the time of day in which the survey is carried out and the dynamic characteristics of the aircraft being used are two of the most crucial parameters for very high-resolution gravity field estimation. It is demonstrated that when following the above-mentioned recommendations, agreements with ground daa of better than 1.5 and 2.5 mGal can be achieved for spatial resolutions (half-wavelengths) of 2.0 and 1.4 km, respectively. © 2002 Wiley Periodicals, Inc.
TL;DR: A new C/C++ program is described which gives Windows-based TEQC users increased flexibility for viewing and printingTEQC plot files, which can then be viewed or printed.
Abstract: In a 1999 issue of GPS Solutions (Vol. 3, No. 1) Estey and Meertens describe a multi-purpose software toolkit for GPS and GLONASS data called TEQC (pronounced "tek"). The program's name reflects its three main functions: translating, editing, and quality checking. TEQC can be used on many different computer platforms, including Windows-based personal computers. This amazingly capable program can be used for converting native binary files from a wide variety of GPS receivers into the standard Receiver Independent Exchange Format (RINEX). It can also be used for editing and quality-controlling existing RINEX files. And it can be used for generating plot files of several different quantities normally associated with satellite observations. This article describes a new C/C++ program which gives Windows-based TEQC users increased flexibility for viewing and printing TEQC plot files. The user can now specify a start and stop time for each plot, a subset of satellites to be plotted, labels for the x- and y-axes, and a title to appear at the top of each plot. The new software converts the original "Compact Format" plot files output by TEQC into PostScript files, which can then be viewed or printed. An additional benefit of PostScript files is that they can be used to provide high-quality, high-resolution graphics for document publishing.
TL;DR: The GPS Toolbox is dedicated to highlighting algorithms and source code utilized by GPS engineers and scientists, and to comment on any of the source code discussed here, or to leave a request for a piece of source code you may be looking for.
Abstract: The GPS Toolbox is dedicated to highlighting algorithms and source code utilized by GPS engineers and scientists. If you have an interesting subroutine or program you would like to share with our readers, please pass it along so that we might continue to bring you this column; e-mail it to us at gps-toolbox@ngs.noaa.gov. To comment on any of the source code discussed here, or to leave a request for a piece of source code you may be looking for, visit our web site at http:/www.ngs.noaa.gov/gps-toolbox. This column is edited by Stephen Hilla, National Geodetic Survey, NOAA, Silver Spring, Maryland, and Mike Craymer, Geodetic Survey Division, Natural Resources Canada, Ottawa, Ontario, Canada. © 2002 Wiley Periodicals, Inc.
TL;DR: The use of the Internet as the communication link between the base and rover stations for the development of an Internet-based Real-Time Kinematic (RTK) system has many advantages if compared to current radio-based RTK systems.
Abstract: This article investigates the use of the Internet as the communication link between the base and rover stations for the development of an Internet-based Real-Time Kinematic (RTK) system. An Internet-based RTK system has many advantages if compared to current radio-based RTK systems. To validate the concept, a prototype system has been developed and tested in both static and kinematic modes. The results indicated that the base differential data latency is in the range of about 1 second and the RTK positioning accuracy is at the centimeterl level. © 2002 Wiley Periodicals, Inc.
TL;DR: The paper describes the patented signal processing scheme, how ambiguity resolution and time synchronization are achieved, the wireless assistance technique, the acquisition strategy and the use of scanning channels, which provides sensitivity down to –185 dBW, which is near the practical limit of sensitivity with acceptable acquisition times and dynamic capability.
Abstract: A key requirement for emergency call location (e.g. E911), for robust operation of location-based m-commerce systems and for telematics systems is that the location technology be able to operate in urban canyons and inside buildings. We start from a definition of the target environments, which includes multi-level parking garages, office buildings and homes, but not underground parking garages or tunnels. Based on experience in these target environments and understanding of typical applications we derive specific requirements for sensitivity and acquisition speed. The primary problems associated with weak signal operation are as follows. (1) In conventional GPS receivers sampling at the correlator output typically occurs at a sampling interval of the order of 1 ms. With weak signals, however, the signal-to-noise ratio of these samples is too low to support lock-in of a phase-locked or frequency-locked loop. (2) With weak signals, the signal-to-noise ratio is too low to support the extraction of the 50BPS navigation message from the signal. Therefore, aiding data is required from an external source. (3) Because the data cannot be extracted, it is not possible for the receiver to synchronize to the incoming bits, words or subframes. Therefore, it is not possible to construct pseudoranges without prior information. (4) The paper describes Sigtec Navigation's subATTO technology. This technology provides sensitivity down to –185 dBW (19 dBHz assuming NF of 1.5 dB and no other implementation loss). This is 5 dB below an attoWatt (10–18 W) and has been shown to provide reliable positioning inside buildings, multi-level parking garages and in urban canyons without any aiding at all. The paper describes the patented signal processing scheme, how ambiguity resolution and time synchronization are achieved, the wireless assistance technique, the acquisition strategy and the use of scanning channels. Results are presented from trials in a multi-level parking garage. The results obtained in most parking garages are similar to these in terms of availability of fixes, signal strengths received and location accuracy achieved. The performance achieved in multi-level parking garages is rarely worse than this. One of the major impediments to practical application of weak signal-processing schemes is the limited dynamic range imposed by the GPS C/A code signal structure. This problem is discussed along with the problems of multipath distortion in the context of telematics operation in urban canyons. A realistic urban accuracy goal of 20 m for 95% of fixes is proposed based on experience with GPS and dead reckoning. Enhancements under development will provide sensitivity of –188 dBW, which will provide continuous availability within a broader range of indoor environments. For practical applications, this will require the use of modern 'search engine' hardware for acceptable acquisition speed. As the paper shows, this sensitivity is near the practical limit of sensitivity with acceptable acquisition times and dynamic capability.