High-accuracy Global Positioning System Research Articles

Rapid Earthquake Source Description Using Variometric-Derived GPS Displacements toward Application to the 2019 Mw 7.1 Ridgecrest Earthquake

AbstractUsing near-field high-rate Global Positioning System (GPS) displacements to invert for earthquake fault slips in real time has the potential to improve the accuracy of earthquake early warning or tsunami early warning. For such applications, real-time retrieval of high-accuracy GPS displacements is essential. Here, we report on rapid modeling of the 2019 Mw 7.1 Ridgecrest earthquake with real-time GPS displacements derived from a variometric approach with readily available broadcast ephemeris. This method calculates station variations in real time by differencing continuous phase observations and does not rely on precise orbit and clock information. The phase ambiguity is also removed, and thus the method does not suffer from a relatively long convergence time. To improve the accuracy of variometric displacements, we use a local spatial filter to decrease the influence of residual errors that cannot be removed completely by the time difference. We invert for the centroid moment tensor, static fault slips, and fault rupture process from the derived displacements. Our results show that all inverted models are available within about 65 s after the origin time of the earthquake and are comparable with models inverted by real-time precise point positioning displacements. This study highlights the great value of variometric displacements for the rapid earthquake source description with only broadcast ephemeris.

A large dataset of detection and submeter-accurate 3-D trajectories of juvenile Chinook salmon

Acoustic telemetry has been used extensively to study the behavior of aquatic animals. The Juvenile Salmon Acoustic Telemetry System (JSATS) is one such system; it was developed for studying juvenile salmonids but has been used to study numerous species. A recent innovation of the JSATS system is an acoustic transmitter that is small enough to be implanted through injection or small incision that doesn’t require sutures. Use of the JSATS system involves deploying cabled acoustic receivers at hydroelectric dams, or other structures, and autonomous acoustic receivers in free-flowing sections of a river. The raw detections from acoustic-tagged fish are processed to remove potential false positives. The clean detections (5,147,996 total) are used to generate detection events and to compute 3-D trajectories (403,900 total), which are used to assign fish to a passage route through a dam. Controlled field testing involving a high-accuracy Global Positioning System receiver is done to validate the submeter accuracy of the trajectories. The JSATS dataset could be reused for expanding the understanding of near-dam fish behavior.

A High-Accuracy GPS-Aided Coarse Alignment Method for MEMS-Based SINS

In order to improve the computational efficiency and alignment accuracy of a microelectromechanical system (MEMS)-based strap-down inertial navigation system (SINS), this article proposes a high-accuracy global positioning system (GPS)-aided coarse alignment method. The attitude matrix between current and initial body frames and the unknown gyro bias, accelerometer bias, and lever arm are jointly estimated based on the proposed closed-loop approach, where the attitude error and unknown parameters are jointly inferred based on the derived linear state-space model using the Kalman filter. Simulation and experimental results illustrate that the proposed GPS-aided coarse alignment method can achieve better accuracy than existing state-of-the-art coarse alignment methods for MEMS-based SINS.

Extending multipath hemispherical model to account for time-varying receiver code biases

Multipath effects on code observables account for one of the major error sources in high-accuracy Global Positioning System (GPS)-based positioning, atmosphere sounding and timing applications. The multipath hemispherical model (MHM) represents one of the most widely used methods of mitigating code multipath effects by taking advantage of their spatial repeatability. The use of MHM usually assumes that the receiver code biases (RCBs) are time-invariant; however, this assumption is not always valid, as RCBs and linear combinations thereof (differential code biases, for instance) have long been found to be time-varying over a period of one day. In this contribution, we propose an extended multipath hemispherical model (EMHM) that is capable of mitigating the code multipath effects in the presence of time-varying RCBs. Consequently, the proposed EMHM has two advantages. First, the EMHM gives rise to code multipath corrections with improved reliability because it addresses the intraday variability of RCBs. Second, more interestingly, the EMHM allows easy and effective calibration of short-term temporal variations, if any, in the RCB on each frequency. These advantages are hopeful to benefit GPS code-related applications.

Antarctic ice velocities from GPS locations logged by seismic stations

AbstractIn 2007–08, seismologists began deploying passive seismic stations over much of the Antarctic ice sheet. These stations routinely log their position by navigation-grade global positioning system (GPS) receivers. This location data can be used to track the stations situated on moving ice. For stations along the traverse from Zhongshan station to Dome A in East Antarctica and at the West Antarctic Ice Sheet divide the estimated velocities of the ice surface based on positions recorded by navigation-grade GPS are consistent with those obtained by high-accuracy geodetic GPS. Most of the estimated velocities have an angle difference of <28° with the steepest downhill vector of the ice surface slope at the stations. These results indicate that navigation-grade GPS measurements over several months provide reliable information on ice sheet movement of ≥1 m yr-1. With an uncertainty of ~0.3–1 m yr-1, this method is able to resolve both very slow ice velocities near Dome A and velocities of >100 m yr-1 on Thwaites Glacier. Information on ice velocity at three locations for which no data from satellite-based interferometric synthetic aperture radar are available have also been provided using this method.

Accuracy assessment of a mobile terrestrial lidar survey at Padre Island National Seashore

The higher point density and mobility of terrestrial laser scanning (light detection and ranging (lidar)) is desired when extremely detailed elevation data are needed for mapping vertically orientated complex features such as levees, dunes, and cliffs, or when highly accurate data are needed for monitoring geomorphic changes. Mobile terrestrial lidar scanners have the capability for rapid data collection on a larger spatial scale compared with tripod-based terrestrial lidar, but few studies have examined the accuracy of this relatively new mapping technology. For this reason, we conducted a field test at Padre Island National Seashore of a mobile lidar scanner mounted on a sport utility vehicle and integrated with a position and orientation system. The purpose of the study was to assess the vertical and horizontal accuracy of data collected by the mobile terrestrial lidar system, which is georeferenced to the Universal Transverse Mercator coordinate system and the North American Vertical Datum of 1988. To accomplish the study objectives, independent elevation data were collected by conducting a high-accuracy global positioning system survey to establish the coordinates and elevations of 12 targets spaced throughout the 12 km transect. These independent ground control data were compared to the lidar scanner-derived elevations to quantify the accuracy of the mobile lidar system. The performance of the mobile lidar system was also tested at various vehicle speeds and scan density settings (e.g. field of view and linear point spacing) to estimate the optimal parameters for desired point density. After adjustment of the lever arm parameters, the final point cloud accuracy was 0.060 m (east), 0.095 m (north), and 0.053 m (height). The very high density of the resulting point cloud was sufficient to map fine-scale topographic features, such as the complex shape of the sand dunes.

Teaching High-Accuracy Global Positioning System to Undergraduates Using Online Processing Services

ABSTRACTHigh-accuracy Global Positioning System (GPS) has become an important geoscientific tool used to measure ground motions associated with plate movements, glacial movements, volcanoes, active faults, landslides, subsidence, slow earthquake events, as well as large earthquakes. Complex calculations are required in order to achieve high-precision positions and thereby high-accuracy displacement measurements. It is difficult to familiarize undergraduates with the complex data processing within a period of one semester. Several national organizations offer free online GPS processing services. Using these online services, a GPS beginner can bypass the complex aspect of data processing and focus on applications of the high-accuracy GPS technology. This paper introduces the author's experience using Online Positioning User Service, provided by the National Geodetic Survey (NGS) and Automatic Precise Positioning Service, provided by the Jet Propulsion Laboratory (JPL) in teaching two undergraduate courses, Applications of GPS in Geosciences and Geological Hazards, at the University of Puerto Rico at Mayaguez. Two class projects, “Where is My House?” and “GPS Landslide Monitoring,” were designed to practice GPS data collection, processing, and analysis. The enrollments of the two courses were about 10 senior undergraduate students. Online GPS data processing helped both the instructor and students in teaching and learning the intricacies of GPS data processing, understanding different reference frames and coordinate systems, and familiarizing local permanent reference stations. Students who had taken the GPS classes often help professors in the geology department and other departments to survey field sites with centimeter-level accuracy.

OPUS for Horizontal Subcentimeter-Accuracy Landslide Monitoring: Case Study in the Puerto Rico and Virgin Islands Region

This study demonstrates the usefulness of an approach based on the Online Positioning User Service (OPUS) provided by the National Geodetic Survey (NGS) of the National Oceanic and Atmospheric Administration (NOAA) to process Global Positioning System (GPS) data and conduct long-term landslide monitoring in the Puerto Rico and Virgin Islands region. Continuous GPS data collected at a creeping landslide site during 2 years were used to evaluate different scenarios for landslide surveying: continuous or campaign, long duration or short duration, morning or afternoon (during different weather conditions). OPUS uses the Continuously Operating Reference Station (CORS) network managed by the NGS as control points and user-collected data to solve for the position of the occupied station (rover). In July 2011, there were 19 NGS CORS sites in the Puerto Rico and Virgin Islands region. This dense GPS network provided a precise and reliable reference frame for subcentimeter-accuracy landslide monitoring in this region. OPUS static solutions (OPUS-S) for sessions as short as 4 h, and OPUS rapid static solutions (OPUS-RS) for sessions as short as 15 min, can achieve subcentimeter horizontal accuracy if the collection of data during extreme weather conditions is avoided. The uncertainty (peak-to-peak error) reported by a single OPUS-S solution differs from the “true” accuracy by a factor of 1.7 for the horizontal components and 1.3 for the vertical component. The uncertainty reported by a single OPUS-RS solution differs from the accuracy by a factor of 1.4 for horizontal components, while the uncertainty of vertical component statistically agrees with the vertical accuracy. This study also indicates that rainfall events can seriously degrade the performance of high-accuracy GPS. Field GPS landslide surveying should avoid rainfall episodes when accompanied by thunderstorms and the passage of detrimental weather fronts. Once appropriate precautions are taken, the results of this investigation show that OPUS-S and OPUS-RS are ideal alternative tools for subcentimeter-accuracy landslide monitoring.

A Digital Spot-mapping Method for Avian Field Studies

Abstract Avian mapping, also known as spot mapping or territory mapping, is a breeding season bird-survey technique that traditionally uses paper maps on which locations of birds are recorded. This method is often considered the most accurate in yielding a density, but has been criticized as being inefficient, time consuming, and inexact. We describe a novel digital-mapping method, incorporating a hand-held computer and high-accuracy global positioning system receiver (GPS), used in an ongoing boreal birds study. Digital mapping surpassed our expectations as to efficiency, flexibility, and work flow. We expect this method will become increasingly useful in many types of field studies, especially as costs decrease (currently ∼$2,100 for the field receiver used).

Analysis and Evaluation of Various Tropospheric Modeling Approaches for High-Precision GPS Kinematic Positioning over Medium Ranges and at High Altitude: Case Study

In global positioning system (GPS) positioning, the tropospheric delay is a systematic error. Mismodeling of the tropospheric delay results in a degradation of the estimated height component, and thus constitutes a limitation to high-accuracy GPS applications. As such, it is obvious that the tropospheric delay should be modeled as accurately as possible. Modeling the tropospheric delay for some applications, such as precise airborne kinematic differential positioning, is further complicated due to the large altitude difference between the ground-based receiver and the airborne roving receiver. This study tests and analyzes three methods for modeling the tropospheric delay, in an attempt to improve the accuracy of the height component for airborne GPS kinematic positioning. As there are several other error sources in GPS, the test and evaluation have to be carefully designed, as any improvements in accuracy due to the use of any tropospheric modeling approaches could be masked by other effects, such as residual orbit errors and ionospheric delays. The analysis is performed for real airborne GPS data and data from a multiple-base station network, and model performance evaluation is based on an independently well-determined aircraft trajectory. Test results show that the estimation of residual tropospheric zenith delay simultaneously with the position parameters after applying the tropospheric delay corrections derived from a network of GPS receivers is seen as the best option with an additional 62% improvement for the height component in terms of standard deviation when compared with results from using a UNB3 tropospheric model only.

Kinematic Terrestrial Light-Detection and Ranging System for Scanning

Highway corridor surveys are becoming more difficult and expensive to carry out because of the need to minimize lane closures, traffic disruptions, and safety hazards posed to the surveyors and public. Roadway surveyors are at risk when working in the traffic corridor if they are unable to move with the flow of traffic. Safety for the traveling public is also a concern when lanes are closed or blocked by slow-moving vehicles. Costs for the placement of safety features to protect a survey crew can in some instances be greater than the cost of the survey itself. To mitigate these problems, a survey data acquisition tool is needed that can collect topographic and infrastructure information without disrupting traffic flow. Terrapoint has developed a novel kinematic laser scanning system that can be deployed on a passenger vehicle or small watercraft. Light-detection and ranging (LIDAR) digital imagery and video are collected from the survey platform while it is moving at traffic speeds. The system is georeferenced with a high-accuracy Global Positioning System–inertial measurement unit. Terrapoint's mobile LIDAR scanner has successfully surveyed existing highway corridors all over North America. It has also proven sufficiently accurate for scanning airport runway surfaces so as to predict areas where water will pool. Moving with the traffic flow and not requiring an escort, the system scans a 360° swath that includes the pavement surface and objects to the sides and above the survey vehicle.

Efficiency and Reliability of Ambiguity Resolution in Network-Based Real-Time Kinematic GPS

Fast and reliable ambiguity resolution (AR) is particularly challenging in long-range real-time kinematic (RTK) global positioning system (GPS), since the atmospheric errors decorrelate with the increasing base-rover separation, effectively reducing the success rate of integer fixing. In order to improve the speed and the success rate of AR, external atmospheric corrections are required. In this paper, four different methods of ionosphere modeling are used as a source of external information, and their impact on the speed and reliability of AR and the rover positioning accuracy is discussed. An example data set, collected by the Ohio Continuously Operating Reference Stations on August 31, 2003, is analyzed, with special emphasis on varying ionospheric conditions during the course of the day in order to study the applicability of these ionospheric models to high-accuracy RTK GPS. In particular, the time-to-fix, the level of AR success, and the accuracy of the resulting rover coordinates are analyzed. Each ...

Morphology parameters of ephemeral gully in characteristics hillslopes on the Loess Plateau of China

The Loess Plateau of North China has one of the highest erosion rates in the world. Ephemeral gullies are an important erosion type found on hill slopes of upland areas, and soil loss due to these gullies equals 30–70% of total soil loss. In this study, we used a high-accuracy global positioning system (GPS) to measure the morphology of characteristic ephemeral gullies near Suide, Shaanxi Province. We discuss the geomorphological characteristics of the gullies (their length, the distance between the head of ephemeral gully and the watershed divide, the distance between two neighboring hill slope ephemeral gullies, the critical slope gradient, the upslope drainage area, and the flow erosivity). The length and average distance between gullies both followed a Pearson IV distribution function, but the function parameters differed. The product of upslope drainage area and the square of the critical slope gradient ranged from 4.74 to 892.66 m 2 and can be used to determine locations where gullies are likely to begin in a digital elevation model with a 2 m × 2 m grid. The relationship between the upslope drainage area ( A) and the critical slope gradient ( S) could be expressed as S = 0.058 A −0.3. For the first time, this paper determined the relationship between gully length ( L) and flow erosivity ( E): L = 0.0305 E + 18.185 ( R 2 = 0.71). These results improve our understanding of the erosion process in ephemeral gullies.

Accuracy of an acoustic location system for monitoring the position of duetting songbirds in tropical forest

A field test was conducted on the accuracy of an eight-microphone acoustic location system designed to triangulate the position of duetting rufous-and-white wrens (Thryothorus rufalbus) in Costa Rica's humid evergreen forest. Eight microphones were set up in the breeding territories of 20 pairs of wrens, with an average intermicrophone distance of 75.2+/-2.6 m. The array of microphones was used to record antiphonal duets broadcast through stereo loudspeakers. The positions of the loudspeakers were then estimated by evaluating the delay with which the eight microphones recorded the broadcast sounds. Position estimates were compared to coordinates surveyed with a global-positioning system (GPS). The acoustic location system estimated the position of loudspeakers with an error of 2.82+/-0.26 m and calculated the distance between the "male" and "female" loudspeakers with an error of 2.12+/-0.42 m. Given the large range of distances between duetting birds, this relatively low level of error demonstrates that the acoustic location system is a useful tool for studying avian duets. Location error was influenced partly by the difficulties inherent in collecting high accuracy GPS coordinates of microphone positions underneath a lush tropical canopy and partly by the complicating influence of irregular topography and thick vegetation on sound transmission.

Challenges of Integrating Geospatial Technologies Into Rangeland Research and Management

With the development and commercial availability of submeter spatial resolution satellite imagery, geospatial tools can better accommodate the needs of range professionals than ever before. However, with these new tools comes a new set of challenges. Range managers and range scientists must now 1) better understand and take advantage of the geotechnical tools at their disposal, 2) collect field observations/measurements in ways that act synergistically with these tools, and 3) utilize high-accuracy global positioning system (GPS) receivers. To produce reliable rangeland models it is important to collect field data that correspond with what the satellite “sees.” Further, it is frequently necessary to use high-resolution imagery, which subsequently necessitates the use of high-accuracy GPS receivers to ensure field data are recorded in the correct pixel and properly coregistered. This paper describes the results of research and experimentation that have led to the development of techniques to improve geospatial rangeland applications. For optimal classification accuracy, field data collected for use in remote sensing applications should estimate/measure ground cover using general vegetation community types and must never exceed 100%. Further, the field sample sites used for classification must be located using a GPS receiver with accuracy ≤ 50% of the size of satellite imagery pixels (e.g., if Landsat imagery is used—with 28.5-m pixels—the GPS receiver must be able to achieve ± 14 m accuracy with 95% confidence). Finally, a series of best practices are suggested to help range managers and range scientists better understand and implement geospatial technologies.

Challenges of Integrating Geospatial Technologies Into Rangeland Research and Management

With the development and commercial availability of submeter spatial resolution satellite imagery, geospatial tools can better accommodate the needs of range professionals than ever before. However, with these new tools comes a new set of challenges. Range managers and range scientists must now 1) better understand and take advantage of the geotechnical tools at their disposal, 2) collect field observations/measurements in ways that act synergistically with these tools, and 3) utilize high-accuracy global positioning system (GPS) receivers. To produce reliable rangeland models it is important to collect field data that correspond with what the satellite ‘‘sees.’’ Further, it is frequently necessary to use high-resolution imagery, which subsequently necessitates the use of high-accuracy GPS receivers to ensure field data are recorded in the correct pixel and properly coregistered. This paper describes the results of research and experimentation that have led to the development of techniques to improve geospatial rangeland applications. For optimal classification accuracy, field data collected for use in remote sensing applications should estimate/measure ground cover using general vegetation community types and must never exceed 100%. Further, the field sample sites used for classification must be located using a GPS receiver with accuracy 50% of the size of satellite imagery pixels (e.g., if Landsat imagery is used—with 28.5-m pixels—the GPS receiver must be able to achieve +/- 14 m accuracy with 95% confidence). Finally, a series of best practices are suggested to help range managers and range scientists better understand and implement geospatial technologies.

Real Time Quality Assessment for CORS Networks

The growing use of real time high accuracy Global Positioning System (GPS) techniques has resulted in an increase in the number of critical decisions made on the basis of a GPS derived position. When making these decisions mobile users require assurance that the GPS position quality meets their requirements. Providers of Continually Operating Reference Stations (CORS), whom mobile users are generally reliant upon, must also be able to assure users that their data meets agreed quality standards. Unfortunately, the realistic and reliable description of position and data quality is an area in which GPS has traditionally been weak. Research being undertaken as part of the Cooperative Research Centre for Spatial Information (CRC-SI) is attempting to address this problem by assessing and reporting on the quality of raw GPS observations in real time. This paper examines a number of existing approaches to assessing the quality of raw GPS observations and presents a conceptual architecture for the development of a real time quality control system.

Filtering GPS time-series using a Vondrak filter and cross-validation

Multipath disturbance is one of the most important error sources in high-accuracy global positioning system (GPS) positioning and navigation. A new data filtering method, based on the Vondrak filter and the technique of cross-validation, is developed for separating signals from noise in data series, and applied to mitigate GPS multipath effects in applications such as deformation monitoring. Both simulated data series and real GPS observations are used to test the proposed method. It is shown that the method can be used to successfully separate signals from noise at different noise levels, and for varying signal frequencies as long as the noise level is lower than the magnitude of the signals. A multipath model can be derived, based on the current-day GPS observations, with the proposed method and used to remove multipath errors in subsequent days of GPS observations when taking advantage of the sidereal day-to-day repeating characteristics of GPS multipath signals. Tests have shown that the reduction in the root mean square (RMS) values of the GPS errors ranges from 20% to 40% when the method is applied.

A High-Performance, High-Accuracy RTK GPS Machine Guiadance System

As technology matures, Real Time Kinematic (RTK) global positioning system (GPS) receivers are providing increased performance in accuracy, on-the-fly (OTF) initialization times, measurement speed, and latency. Leica's Dozer 2000 machine guidance system combines an MC1000 GPS sensor, graphic intensive guidance software, a digital terrain model, and a digital site plan. Figure 1 shows the operator's view of the system. The end results is a highly accurate, extremely responsive local navigation system. Engineers can easily upload design information onto a touch-screen machine guidance personal computer (PC) and greatly reduce earthmoving costs while simultaneously recording “as-built” inspection data. The Leica MC1000 machine control sensor is a true 10-Hz, centimeter-level-accuracy, open-architecture GPS sensor specifically designed for machine guidance and control. At the heart of the MC1000's performance are a very stable oven-controlled oscillator (3 × 10−11/s short-term stability), a low-power Intel 486-DX4-80 processor, and four user-configurable input/output (I7O) ports. Special algorithms mitigate the effects of multipath and Selective Availability (SA) while providing centimeter accuracy up to 10 times per second with only 30 milliseconds' latency. The high update rate and low latency are essential for machine guidance and control. The MC1000 GPS reference station is capable of outputting RTK and Differential GPS (DGPS) data simultaneously, providing and entire site with GPS information for machine guidance and control, tracking and dispatching of vehicles, and support of survey crews. This article describes how recent advances in technology have combined to produce the Leica Dozer 2000 – a machine guidance system that is ideal for a wide range of earthmoving and inspection applications. © 2000 John Wiley & Sons, Inc.

Performance analysis of GPS carrier phase observable

The accuracy analysis of Global Positioning System (GPS) carrier phase observable measured by a digital GPS receiver is presented. A digital phase-locked loop (DPLL) is modeled to extract the carrier phase of the received signal after a pseudorandom noise (PRN) code synchronization system despreads the received PRN coded signal. Based on phase noise characteristics of the input signal, the following performance of the first, second, and third-order DPLLs is analyzed mathematically: (1) loop stability and transient process; (2) steady-state probability density function (pdf), mean and variance of phase tracking error; (3) carrier phase acquisition performance; and (4) mean time to the first cycle-slipping. The theoretical analysis is verified by Monte Carlo computer simulations. The analysis of the dependency of the phase input noise and receiver design parameters provides with an important reference in designing the carrier phase synchronization system for high accuracy GPS positioning.