Showing papers in "Gps Solutions in 1999"

TEQC: The Multi-Purpose Toolkit for GPS/GLONASS Data

Abstract: For many common GPS/GLONASS native receiver formats, a single freeware program called TEQC now allows the user to translate from the binary receiver format to the standard Receiver Independent Exchange (RINEX) format, to edit existing RINEX files, and to quality-check the data before postprocessing. TEQC is 100% noninteractive and has a command line interface modeled after common UNIX commands. This combined with TEQC's extensive documentation makes it simple to use for new and experienced users and in automated processing scripts. © 1999 John Wiley & Sons, Inc.

GPS Antenna Calibration at the National Geodetic Survey

Abstract: The precise point whose position is being measured when a GPS baseline is determined is generally assumed to be the phase center of the GPS antenna. However, the phase center of a GPS antenna is neither a physical point nor a stable point. For any given GPS antenna, the phase center will change with the changing direction of the signal from a satellite. Ideally, most of this phase center variation depends on satellite elevation. Azimuthal effects are only introduced by the local environment around each individual antenna site. These phase center variations affect the antenna offsets that are needed to connect GPS measurements to physical monuments. Ignoring these phase center variations can lead to serious (up to 10 cm) vertical errors. This article will describe the procedure by which the National Geodetic Survey is calibrating GPS antennas and how this information may be obtained and used to avoid problems from these antenna variations. © 1999 John Wiley & Sons, Inc.

A New Solar Radiation Pressure Model for GPS Satellites

Abstract: The largest error in currently used GPS orbit models is due to the effect of solar radiation pressure. Over the last few years many improvements were made in modeling the orbits of GPS satellites within the International GPS Service (IGS). Howeer, most improvements were achieved by increasing the number of estimated orbit and/or solar radiation pressure parameters. This increase in the number of estimated satellite parameters weakens the solutions of all estimated parameters (not only orbit parameters). Because of correlations the additional orbit parameters may introduce biases in other estimated quantities, for example the length of day. We present a recently developed solar radiation pressure model for the GPS satellites. This model is based on experiences and results gained at the Center for Orbit Determination in Europe (CODE) in the context of its IGS activities since June 1992. The performance of the new model is almost an order of magnitude better than that of the existing ROCK models. It also allows a reduction of the number of orbit parameters that have to be estimated. © 1999 John Wiley & Sons, Inc.

Variances of GPS Phase Observations: The SIGMA-ɛ Model

Abstract: The noise term of GPS phase data can be calculated from the measured carrier-to-noise power density ratios (C/N0). The C/N0 values are used in the proposed SIGMA-ɛ model to calculate the variance matrix of double-differenced GPS phase data. Examples show the capability of this model to yield higher accuracies for GPS surveys than the use of the standard weighting scheme. Most importantly, the SIGMA-ɛ model allows the use of noisier phase data from very low elevation satellites to overcome poor satellite geometry problems. © 1999 John Wiley & Sons, Inc.

Differentiation for High-Precision GPS Velocity and Acceleration Determination

Abstract: Accurate estimates of the velocity and acceleration of a platform are often needed in high dynamic positioning, airborne gravimetry, and geophysics. In turn, differentiation of GPS signals is a crucial process for obtaining these estimates. It is important in the measurement domain where, for example, the phase measurements are used along with their instantaneous derivative (Doppler) to estimate position and velocity. It is also important in postprocessing, where acceleration is usually estimated by differentiating estimates of position and velocity. Various methods of differentiating a signal can have very different effects on the resulting derivative, and their suitability varies from situation to situation. These comments set the stage for the investigations in this article. The objective is twofold: (1) to carry out a comprehensive study of possible differentiation methods, characterizing each in the frequency domain; and (2) to use real data to demonstrate each of these methods in both of the measurement and position domains, in conditions of variable, high, or unknown dynamics. Examples are given using real GPS data in both the measurement domain and in the position and velocity domain. The appropriate differentiator is used in several cases of varying dynamics to derive a Doppler signal from carrier phase measurements (rather than using the raw Doppler generated by the receiver). In the statistic case, it is seen that the accuracy of velocity estimates can be improved from 4.0 mm/s to 0.7 mm/s by using the correct filter. In conditions of medium dynamics experienced in an airborne gravity survey, it is demonstrated that accelerations as the 2–4 mGal level (1 mGal = 0.00001 m/s2) can be obtained at the required filtering periods. Finally, a precision motion table is used to show that when using the correct filter, velocity estimates under high dynamics can be improved by an order of magnitude to 27.0 mm/s. © 1999 John Wiley & Sons, Inc.

Single-Epoch Ambiguity Resolution for Real-Time GPS Attitude Determination with the Aid of One-Dimensional Optical Fiber Gyro

Abstract: High-accuracy real-time GPS-based attitude determination requires that integer ambiguities be resolved very quickly so that the attitude angles can be output with minimum delay. This article describes an attitude determination algorithm that can resolve integer ambiguities instantaneously, relative to one antenna of a multi-antenna array configuration. The carrier phase and pseudorange observations are used with fixed baseline length constraints and fiberoptic gyro data. Real-time stochastic model improvement using empirical elevation-dependent standard deviation function and an estimated scale factor are a feature of this algorithm. Integer ambiguity search using the LAMBDA method, sophisticated validation criteria, and an adaptive procedure has also been implemented within the software. An experiment was carried out using four Leica dual-frequency GPS receivers (but only the L1 carrier phase and pseudorange data were used) and a low-cost fiberoptic on a car. The results indicate that integer ambiguities can be resolved on a single-epoch basis with a 98.9% success rate. © 1999 John Wiley & Sons, Inc.

Eye on the Ionosphere: Potential Solar Radio Burst Effects on GPS Signal to Noise

Abstract: A. J. VanDierendonck joins the regular contributors to this column to discuss potential solar radio burst effects on GPS. While this topic does not spedivically involve ionospheric effects on GPS, it falls under the general area of environmental concerns that are of importance to the GPS community. © 1999 John Wiley & Sons, Inc.

IGS Earth Rotation Parameters

Abstract: Since its official start in January 1994, the International GPS Service (IGS) has been distributing, as part of its product combination, two distinct Earth rotation parameter (ERP) series: the IGS Rapid series and the IGS Final series. Initially, the IGS Rapid ERP values were interpolations of the International Earth Rotation Service (IERS) Bulletin A, whereas the IGS Final ERP series was based on the IERS Bulletin B. Since June 1996, the IGS has been generating its own Final ERP series consistent with the IGS combined orbit products and based on weighted means of individual IGS analysis center (AC) solutions. At first, only the polar motion (PM) coordinates and their rates were combined. Length of Day (LOD) and Universal Time (UT) solutions, also based on separate weighted mean combinations, followed in March 1997. Currently, the IGS Rapid and Final combinations are produced and made available within 17 hours and 11 days, respectively, after the last observation. Both IGS and the best AC series are consistent and precise at the 0.1-milliarcsecond (mas) level for PM and at about 30 μs for LOD. Biases in some AC solutions may exceed these consistency levels. Comparisons of both IGS ERP series with external standards, such as the IERS multitechnique Bulletins and atmospheric angular momentum series, confirm the estimated precisions. © 1999 John Wiley & Sons, Inc.

The reliability of GPS ambiguity resolution

Abstract: GPS ambiguity resolution is the process of resolving the unknown cycle ambiguities of double-difference (DD) carrier-phase data as integers. It is the key to fast and high-precision relative GPS positioning. Critical in the application of ambiguity resolution is its reliability. Unsuccessful ambiguity resolution, when passed unnoticed, will too often lead to unacceptable errors in the positioning results. High success rates are required for ambiguity resolution to be reliable. In this contribution we will introduce and evaluate such diagnostic measures. They complement existing methods of ambiguity resolution and allow the user and/or analyst to infer their reliability. © 1999 John Wiley & Sons, Inc.

Automated GPS Data Analysis Service

Abstract: Automatic analysis of geodetic-quality GPS data is available with the use of e-mail and ftp (file transfer program) as an interface to a computer at the Jet Propulsion Laboratory (JPL), where precise transmitter parameters – GPS ephemerides and clock errors – are computed regularly. The interface is such that e-mail from an external user causes the JPL computer to fetch the user's data. The computer than analyzes the data, and places the results in an area accessible to the user. An e-mail to the user gives information on the location of the analysis results, which the user can subsequently fetch. Operations on the JPL computer are entirely automatic, and require essentially no labor. © 1999 John Wiley & Sons, Inc.

Vulnerability of the GPS Signal to Jamming

Abstract: The Global Positioning System (GPS) and Wide Area Augmentation System (WAAS) provide lower power signals that are susceptible to interference. The potential exists for a person or organization to jam GPS or WAAS signals, causing a loss of service over a large area. Accidental interence by extraneous radio transmissions on the GPS frequency also can cause loss of service. Jamming has become a serious issue for GPS, and the U. S. military is making a significant effort to mitigate jamming effects. While jamming is a major concern of the military, interference is of a no lesser concern to the civilian sector. In fact, with varying emphasis both jamming and interference are concerns of the entire GPS user community. This article explores jamming and interference threats to the GPS. We provide a general overview of the GPS signal structure, discuss jamming effects on the GPS, and consider mitigating options. © 1999 John Wiley & Sons, Inc.

Time and Frequency Transfer Using GPS Codes and Carrier Phases: Onsite Experiments

Abstract: Recent studies have shown the capabilities of Global Positioning System (GPS) carrier phases for frequency transfer based on the observations from geodetic GPS receivers driven by stable atomic clocks. This kind of receiver configuration is the kind primarily used within the framework of the International GPS Service (IGS). The International GPS Service/Bureau International des Poids et Mesures (IGS/BIPM) pilot project aims at taking advantage of these GPS receivers to enlarge the network of Time Laboratories contributing to the realization of the International Atomic Time (TAI). In this article, we outline the theory necessary to describe the abilities and limitations of time and frequency transfer using the GPS code and carrier phase observations. We report on several onsite tests and evaluate the present setup of our 12-channel IGS receiver (BRUS), which uses a hydrogen maser as an external frequency reference, to contribute to the IGS/BIPM pilot project. In the initial experimental setup, the receivers had a common external frequency reference; in the second setup, separate external frequency references were used. Independent external clock monitoring provided the necessary information to validate the results. Using two receivers with a common frequency reference and connected to the same antenna, a zero baseline, we were able to use the carrier phase data to derive a frequency stability of 6 × 10−16 for averaging times of one day. The main limitation in the technique originates from small ambient temperature variations of a few degrees Celsius. While these temperature variations have no effect on the functioning of the GPS receiver within the IGS network, they reduce the capacities of the frequency transfer results based on the carrier phase data. We demonstrate that the synchronization offset at the initial measurement epoch can be estimated from a combined use of the code and carrier phase observations. In our test, the discontinuity between two consecutive days was about 140 ps. © 1999 John Wiley & Sons, Inc.

Eye on the Ionosphere: Measuring Ionospheric Scintillation Effects from GPS Signals

Abstract: From time to time, this column will include short contributions from invited guest contributors on specialized subjects pertaining to inonospheric effects on GPS signals. In this issue, Dr. A. J. Van Dierendonck discusses the required specifications of a civilian GPS receiver specially designed to make quantitative measurements of both ionospheric amplitude and carrier phase scintillation effects from GPS signals. © 1999 John Wiley & Sons, Inc.

Hybrid GPS + GLONASS

Abstract: The hybridization of GPS with GLONASS has formed a first stage in GNSS development. We examine the performance of the hybrid system in the position domain for both code and carrier phase cases. Several major differences exist between GPS and GLONASS; most significant is GLONASS's signal frequency diversity, which can lead to measurement bias, particularly so when a pair of receivers are operating at different temperatures. Unless signal frequency diversity is addressed either on-receiver or at the data processing stage, positioning errors can occur at the centimeter level. We outline the difficulties of combining observations from the two systems and discuss how these may be overcome. © 1999 John Wiley & Sons, Inc.

Discussion of GPS Anti-Jam Technology

Abstract: The Global Positioning System (GPS) satellita navigation aiding system, sometimes called NAVSTAR, has become a utility to the military and many civilian areas. GPS, currently consisting of 24 satellites, is used by the military for navigation, precision weapons delivery, and the future digital battlefield. In the civilian sector, GPS is widely used as the primary or secondary aid for land, water, and air navigation; as a surveying aid; as a vehicle location system; and as a precision time standard for cellular and ATM sites. In the aviation community, GPS is becoming an integral part of the WAAS (Wide Area Augmentation System) and the LAAS (Local Area Augmentation System) for en route navigation in North America and Category II and III precision approach, and for surface navigation. The vulnerability of GPS have become the vulnerabilities of WAAS and LAAS, and require consideration of interference mittigation techniques. © 1999 John Wiley & Sons, Inc.

Testing JPL's PPP Service

Abstract: The Jet Propulsion Laboratory's (JPL's) automated GPS data analysis service, which is available via the Internet, has been tested. This service is attractive not only because data submission and retrieval are entirely automated on the Internet, but also because it makes centimeter positioning possible for individual dual-frequency receivers. To learn more about this service, the repeatability of position solutions were investigated by means of two simple experiments. The positions of a continuously operating reference station have been computed repeatedly for observation times ranging from 1 hour to 24 hours. In addition, short baseline repeatability and accuracy were tested. Centimeter-level results were obtained. The difference in the vertical definition of the electronic center for antennas of different types could readily be identified. The JPL service is easy to use, even in its present form (which one might label version 1.0). © 1999 John Wiley & Sons, Inc.

IGS/BIPM Time Transfer Pilot Project

Abstract: INTRODUCTION he IGS/BIPM Pilot Project to Study Accurate Time and Frequency Comparisons Using GPS Phase and Code Measurements was authorized in December 1997 jointly by the International GPS Service (IGS) and the Bureau International des Poids et Mesures (BIPM). A Call for Participation was issued shortly afterwards, with responses received from about 35 groups. The respondents have formed a working group co-chaired by C. Thomas, BIPM, and J. Ray, U.S. Naval Observatory (USNO). A number of groups have been working for several years to develop the capability of using geodetic GPS techniques for accurate time and frequency transfer. A variety of convincing demonstrations has already been performed showing the potential for determining clock differences at the level of a few hundred picoseconds. The current state of maturity of both the global tracking network and data analysis techniques now allows practical applications to be considered. The central goal of this Pilot Project is to investigate and develop operational strategies to exploit GPS measurements for improved availability of accurate time and frequency comparisons worldwide. This will become especially significant for maintaining the international UTC time scale as a new generation of frequency standards emerges with an accuracy of 10 ~ or better. The Project is expected to run until January 2000. By that time, those aspects that are suitable for integration into the operational activities and official products of the IGS or BIPM should be under way. To the extent

Monitoring Isostatic Rebound in Antarctica with the Use of Continuous Remote GPS Observations

Abstract: A solar-powered GPS receiver has been installed near Beaver Lake, Antarctica, to monitor postglacial isostatic rebound that may be occurring as a result of ice thinning near the Lambert Glacier since the last glacial maximum. The equipment is 400 km from the nearest Australian Antarctic base and is completely automated. It is expected that there will be sufficient solar power to operate the equipment from January 1998 to May 1998, but the data will not be recovered until the following summer season. The scatter in height computed from the first 25 days of data is ± 7.5 mm. If such precision is representative of the accuracy of the height estimates, isostatic rebound of < 1 mm/yr will be able to be detected after a few years of observations at the site. © 1999 John Wiley & Sons, Inc.

Timing and Positioning with GLONASS and GPS

Abstract: Considering GLONASS as one of the pillars of the international Global Navigation Satellite System (GNSS), the Russian Federation works toward the integration of GLONASS with other navigation systems, cooperates with the internal user community, and contributes to the development and coordination of standards concerning GLONASS and the combined use of GLONASS and the global positioning system (GPS). This work is pursued in conformity with recommendations of respective international organizations. Most users recognize that the GLONASS/GPS combination has better characteristics in terms of availability, accuracy, integrity, and so on. However, the combined use of these satellite systems raises problems that must be addressed. This article reviews problems encountered when using two different navigation systems. Solutions developed thus far are outlined. The potential of GLONASS and approaches for high accuracy UTC time transfer are discussed. The transformation between the WGS 84 and PZ 90 reference frames and their conformity with the international terrestrial reference frame (ITRF) is considered. Various solutions are viewed in connections with recommendations made by the International Civil Aviation Organization (ICAO), the Global Navigation Satellite System Panel (GNSP), and the Consultative Committee for Definition of the Second (CCDS) concerning the desirability of using either or both systems interchangeably. © 1999 John Wiley & Sons, Inc.

Propagating PZ 90 to WGS 84 Transformation Parameters

Abstract: The 29th Research Institute recently published its latest transformation parameters for WGS and PZ 90. Because these tranformation parameter estimates were derived from stations located within Russia, variance-covariance propagation was carried out to study the benefits of a better global distribution of stations. An example that includes two stations in North America is presented. The transformation improves significantly. © 1999 John Wiley & Sons, Inc.

The Influence of Code Multipath on the Estimated Parameters of the Geometry-Free GPS Model

Abstract: In this article, the influence of biases in GPS code observations on the estimated parameters of the geometry-free model is investigated. This is done for undifferenced as well as double-differenced data from short baselines, that is, baselines for which ionospheric effects may be assumed absent. It is shown how introducing a linear model for code multipath affects the original model parameters. The performance of the original and extended model is illustrated by analyzing data from a single receiver and a short baseline. © 1999 John Wiley & Sons, Inc.

Centimeter-Level Positioning of a U. S. Coast Guard Buoy Tender

Abstract: With the availability of high-accuracy, differential global positioning system (GPS) results in real-time, there is a new opportunity to use GPS to accurately measure a marine vessel's dynamic draft (settlement and squat) and 3D attitude (roll, pitch, and heading). The National Geodetic Survey (NGS) and the Coast Survey (CS), offices of the National Ocean Service (NOS), National Oceanic and Atmospheric Administration (NOAA), propose to transfer this technology to the shipping industry. The overall goal of this project is to provide the position of a vessel's keel in real time to within 10 cm (about 4 inches) relative to the bottom of the shipping channel. In support of this phase of the project, there were three meetings hosted by the Port of Oakland, California and NOS to discuss the real-time positioning of vessels project. On December 3 and 4, 1996, CS, NGS, Trimble Navigation Ltd., and the U. S. Coast Guard (USCG) performed GPS tests on a USCG buoy-tender ship. GPS data were used to compute the vessel's dynamic draft and 3D attitude. During the test, five receivers continually collected data; one receiver was located at a base station on the USCG pier on Yerba Buena Island, and four were on the ship: two on the stern and two on the bow. CS installed a TSS-335B vertical reference unit (to measure heave, pitch, and roll) in the engine room of the ship. NOS processed the GPS data and computed the vessel's dynamic draft and 3D attitude. The results indicate that the linear equivalent to the vessel's dynamic draft and 3D attitude were accurate to the 10-cm level using GPS. It was also demonstrated how a ship can be used to measure local water-level changes and actual water-level values everywhere it travels. © 1999 John Wiley & Sons, Inc.

Seasonal Motion in the Annapolis, Maryland GPS Monument

Abstract: The permanent GPS tracking site at Annapolis, MD shows a 7-mm seasonal signal primarily in its horizontal position. It is suggested that thermal expansion of the pier on which the antenna rests is the source of this motion. A simple numerical model of the pier reproduces the observed motion of the GPS antenna, lending credence to this hypothesis. Although adding an additional level of complexity, this motion is predictable and the site retains its value for high precision monitoring. Although the arrangement of this GPS site is somewhat uncommon, these results emphasize the importance of the underlying antenna monumentation when measuring crustal motions. © 1999 John Wiley & Sons, Inc.

A Modular Approach to Precise GPS Positioning

Abstract: A modular approach consisting of three steps is presented for precise positioning with GPS: integrity, monitoring (data validation), ambiguity resolution, and positioning. Results of one step are used as input for the next. After a brief description of reliability and testing and techniques for GPS ambiguity resolution, the observation equations for the geometry-free GPS model are given, used for integrity monitoring, followed by those for single-difference relative positioning with the double-difference ambiguities removed. © 1999 John Wiley & Sons, Inc.

Receiver Architectures for Improved Carrier Phase Tracking in Atenuation, Blockage, and Interference

Abstract: A significant impediment to real-time centimeter survey and vehicle guidance in difficult reception environments is the underlying signal tracking performance of the receiver. A number of researchers have investigated receiver signal tracking enhancements that take advantage of intersatellite path correlations, of particular benefit in environments with multiple satellite attenuations and blockages. This article surveys work in the area, with a focus upon an optimal estimation scheme known as integrated demodulation/navigation (IDN). By contrast with other techniques, IDN employs a high update rate extended Kalman filter to combine and process raw correlator data across satellites. Nonlinear loop simulations of real time kinematic (RTK) tracking during high-g aircraft maneuvers with vehicle-induced blockages are used to illustrate the filtering method and it s robustness. © 1999 John Wiley & Sons, Inc.

United States' Nationwide Differential Global Positioning System

Abstract: This article describes the Nationwide Differential Global Positioning System (NDGPS) service being developed in the United States and the enormous benefits to federal agencies, state governments, and the general public. © 1999 John Wiley & Sons, Inc.

IGEX-98: International GLONASS Experiment

Abstract: GLONASS is a Russian military navigation satellite system. Even if the constellation is far from being operational, a large community is potentially interested in using its signal in combination with GPS to improve availability, integrity, or precision. The goal of this article is to present the international GLONASS campaign, named IGEX-98, which is really the first attempt to obtain precise GLONASS orbits for geodetic applications. A world-wide network of GLONASS receivers has been deployed and is still operational. Several analysis centers process the GLONASS data on a regular basis in the same way it is done for GPS within the International GPS Service. © 1999 John Wiley & Sons, Inc.

A Regional GPS Network Solution for Monitoring Deformations of the Southeastern Eurasian Plata

Abstract: Investigations of deformation styles in Asia have already led to new kinematic models that predict the spatial and temporal evolution of deformations in the region. Testing these models is now within realm of current GPS technology. In this study, GPS data during 4 months from six stations in Asia, including a new station from Hong Kong whose data were not in public domain, were analyzed for generating kinematic geophysical constraints for the deformations at the regional scale. Processing of 4 months' continuous GPS data from all stations shows small relative station velocities (Wuhan, Shanghai, Xian, and Hong Kong stations in China), except the Lhasa station, which has a northeasterly velocity of about 3 cm/year with respect to the fixed GPS station operating in Taiwan. © 1999 John Wiley & Sons, Inc.

The Discription and Performance of a Multisensor Precision Pointing System

Abstract: The purpose of this article is to report on an integrated system that uses GPS and other low cost sensors for azimuth and pitch determination. The ability of the integrated system to maintain a solution over periods of induced GPS outage is also demonstrated. © 1999 John Wiley & Sons, Inc.

Assessment of the LandStar Real-Time DGPS Service under Several Operational Conditions

Abstract: LandStar is a differential global positioning service (DGPS) that provides 24-h real-time positioning for various applications on land, water, and air in North America, Australia, New Zealand, Europe, and Africa. Its focus is on real-time applications requiring a submeter positioning capability such as agriculture, forestry, Geospatial Information Systems (GIS), survey/mapping, and land/vehicular navigation. LandStar uses a Wide Area Network of reference stations to derive DGPS corrections to model the variation of GPS error sources over a large area. These model parameters are used by the Virtual Reference Station processors to calculate standard corrections that are available for all predefined locations in the network. The corrections are transmitted to the user by L-band satellite communication in the standard RTCM SC104 DGPS correction format. This article investigates the performance of the LandStar Mk III system under various operational conditions and assesses its performance in both static and kinematic modes. Four field tests were conducted during 12 months that tested the sysem in clear static and kinematic conditions as well as suboptimal environments associated with low and heavy foliage conditions. Both the accuracy and availability of the system under these conditions is investigated, with an emphasis on whether the above variables are caused by the LandStar system differential corrections, the GPS measurements, or a combination of both. © 1999 John Wiley & Sons, Inc.