Global Positioning System Clock Research Articles

Impact of incorporating Spire CubeSat GPS observations in a global GPS network solution

Commonly, Global Positioning System (GPS)-based precise orbit determination (POD) of a low Earth orbiting (LEO) satellite is conducted by introducing fixed GPS orbits and clock corrections that were derived in a previous independent network solution using ground-based GPS receivers only. There have been a number of studies showing that the integration of LEO GPS observations can be advantageous in global network solutions, particularly when it comes to estimating geodetic parameters, such as the Earth’s center-of-mass. There has been an increase in GPS data available from LEO CubeSats over the past few years, such as from the Spire satellites, which are equipped with dual-frequency GPS receivers. The goal of the present study is to determine how GPS observations collected by specific Spire satellites affect global network solutions when combined with GPS observations from ground stations of the International GNSS Service. To obtain a combined GPS-LEO solution, GPS observations collected from receivers of selected Spire satellites and additional scientific LEOs are used to obtain combined GPS-LEO solutions by performing one joint least-squares adjustment process. Using this method, LEO satellite orbit parameters are determined along with GPS orbit parameters and geodetic parameters, namely GNSS station coordinates, Earth rotation parameters (ERPs), and the Earth’s center-of-mass. The influence of the Spire LEOs is assessed by analyzing different solutions that use GPS observations from different Spire and LEO satellites. Besides comparing the different global network solutions with one another, a comparison to a solution using only terrestrial GPS data is performed as well. As part of the quality assessment of the GPS-LEO solution, quality characteristics are analyzed, which are the geodetic parameters, orbit overlaps at arc boundaries, and formal errors determined for the estimated parameters. The results show that the estimation of the Earth’s center-of-mass coordinates benefits from including GPS-LEO observations. The solution is more improved by integrating GPS-LEO observations from scientific LEOs than from GPS-Spire-included solutions. The analysis reveals that GPS orbits are improved using the LEO-integrated approach, while orbit misclosures of LEO and Spire trajectories currently indicate a decrease in LEO orbit precision due to LEO orbit modeling deficiencies.

Advanced architectures for high-performance quantum networking

As practical quantum networks prepare to serve an ever-expanding number of nodes, there has grown a need for advanced auxiliary classical systems that support the quantum protocols and maintain compatibility with the existing fiber-optic infrastructure. We propose and demonstrate a quantum local area network design that addresses current deployment limitations in timing and security in a scalable fashion using commercial off-the-shelf components. First, we employ White Rabbit switches to synchronize three remote nodes with ultra-low timing jitter, significantly increasing the fidelities of the distributed entangled states over previous work with Global Positioning System clocks. Second, using a parallel quantum key distribution channel, we secure the classical communications needed for instrument control and data management. In this way, the conventional network that manages our entanglement network is secured using keys generated via an underlying quantum key distribution layer, preserving the integrity of the supporting systems and the relevant data in a future-proof fashion.

Comparison of COSMIC and COSMIC-2 Radio Occultation Refractivity and Bending Angle Uncertainties in August 2006 and 2021

We compare the random error statistics (uncertainties) of COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate, C1) and COSMIC-2 (C2) radio occultation (RO) bending angles and refractivities for the months of August 2006 and 2021 over the tropics and subtropics using the three-cornered hat method. The uncertainty profiles are similar for the two RO missions in the troposphere. However, a higher percentage of C2 profiles reach close to the surface in the moisture-rich tropics, an advantage of the higher signal-to-noise ratio (SNR) in C2. C2 uses signals from both GPS (Global Positioning System) and GLONASS Global Navigation System Satellites (GNSS). The GPS occultations show smaller uncertainties in the stratosphere and lower mesosphere (30–60 km) than the GLONASS occultations, a result of more accurate GPS clocks. Therefore, C2 (GPS) uncertainties are smaller than C1 uncertainties between 30–60 km while the C2 (GLONASS) uncertainties are larger than those of C1. The uncertainty profiles vary with latitude at all levels. We find that horizontal gradients in temperature and water vapor, and therefore refractivity, are the major cause of uncertainties in the tropopause region and troposphere through the violation of the assumption of spherical symmetry in the retrieval of bending angles and refractivity.

Assessment of the BDS-3 on-board clocks and their impact on the PPP time transfer performance

The atomic clock is the heart of a satellite navigation system and limits the user positioning, navigation and timing (PNT) accuracy; hence, it is necessary to acquire more information about the BDS-3 on-board clocks. In this contribution, sixty days of BDS-3 clock results were analyzed in terms of their stability and prediction accuracy and compared with the results from BDS-2 (BDS: BeiDou Navigation Satellite System), GPS (Global Positioning System), GLONASS (Global Navigation Satellite System) and Galileo clocks. The results show that BDS-3 clocks have greater stability than BDS-2 and BDS-3 s clocks, GPS Block IIA, Block IIR, and Block IIR-M clocks, and GLONASS clocks, while their stability is comparable to that of Block IIF clocks and inferior to that of Galileo clocks. In comparison with the stability of BDS-2 clocks, the percentages of improvement in the stability of BDS-3 clocks range from 5% to 65%. Overall, the stabilities of BDS-3 clocks at 1000 s, 10000 s and 86400 s are nearly 3 ~ 6E-14, 2 ~ 8E-14 and 1 ~ 24E-14, respectively. Furthermore, the prediction accuracy of BDS-3 clocks is 0.24 ns, which is smaller than that of BDS-2 and GPS clocks. Finally, preliminary experiments were conducted with regard to precise point positioning (PPP) time transfer using BDS-3 signals. The four solutions, namely, the BDS-2, BDS-2 + BDS-3, BDS-3, and GPS solutions, were compared in terms of their frequency stability. As a slight imperfection, BDS-3 applications are restricted by the limited number of satellites. Nevertheless, the link results based on BDS-3 are more stable than those based on BDS-2 and BDS-2 + BDS-3. In particular, the BDS-3 results outperform the GPS results in their long-term stability. At typical averaging times, namely, 1000 s, 10,000 s, and 86,400 s, in comparison with the GPS results, the percentages of improvement in the link stability are 22.91, 38.40 and 19.03%, respectively, while those in comparison with the BDS-2 results are 40.08, 34.06 and 10.90%, respectively.

Assessment of L5 frequency on real-time GPS orbit and clock estimation

Real-time precise Global Navigation Satellite System (GNSS) satellite orbit and clock products are the prerequisite of real-time GNSS-based applications. With the modernization of Global Positioning System (GPS) constellation and construction of other GNSS constellations, the number of GNSS satellites transmitting multi-frequency signals is increasing rapidly. While the benefit of multi-frequency to Precise Point Positioning (PPP) and Precise Clock Estimation (PCE) has been evaluated by many researchers, the impact of multi-frequency on orbit and clock estimation is still limited, especially in real-time mode. Different from the dual-frequency model, the Inter-Frequency Clock Bias (IFCB) must be treated carefully in the triple-frequency model. In this contribution, the triple-frequency model for real-time GPS satellite orbit and clock estimation is derived and the influence of L5 frequency observations on real-time GPS satellite orbit and clock are analyzed. With observation data from globally distributed stations spanning January 1 to 14 of 2018, real-time GPS orbit and clock are estimated with both dual-frequency and triple-frequency observations. Numerical experiments indicate that, compared to the International GNSS Service (IGS) final orbit, the L5 frequency observations improve the consistency of the normal component with about 0.2 cm, however lead to a decrease of consistency in the radial component with about 0.6 cm; the influence on the tangential direction varies with different number of stations. The potential reason for this phenomenon may partly be attributed to the absence of accurate antenna information for L5 frequency. However, due to the tight constraint of dynamic force model on orbit, the overall differences for each component is less than 1.0 cm, which means the contribution of the third frequency observation on the orbit is limited.

Observed geocenter motion from precise orbit determination of GRACE satellites using GPS tracking and accelerometer data

We present a method to estimate geocenter motion through single low-earth orbiter (LEO) precise orbit determination (POD) using global positioning system (GPS) tracking data and accelerometer data from the GRACE satellites. We fix the values of the GPS ephemerides and time-varying clock offsets to precise estimates from the definitive constellation product produced by the Jet Propulsion Laboratory. As part of the POD process of the LEOs, we estimate a translation of the reference coordinate system realized by the GPS orbit and clock product. Doing so accounts for the inconsistency between the Earth’s center of mass coordinate system used for the orbit integration of the LEOs, and the terrestrial reference frame produced by the GPS orbit and clock product. The resulting translation parameters estimated separately from the GRACE-A and GRACE-B satellites show very similar variations from day to day. They represent the geocenter motion, as realized from the difference between the origin of the terrestrial reference frame represented by the GPS orbit and clock product and the Earth’s instantaneous center of mass defined by the GRACE satellite dynamical orbital motion. Comparisons with geocenter motion observations from other techniques show that our daily estimates of geocenter motion agree well, when smoothed, in both the amplitude and phase of the annual signal. This validates both the high sensitivity of the GRACE GPS measurement type to geocenter motion and the high precision of the GRACE force model enabled by accelerometer measurements, the two essential components for estimation of geocenter motion with this technique.

Evaluation of Orbit, Clock and Ionospheric Corrections from Five Currently Available SBAS L1 Services: Methodology and Analysis

To meet the demands of civil aviation and other precise navigation applications, several satellite-based augmentation systems (SBASs) have been developed around the world, such as the Wide Area Augmentation System (WAAS) for North America, the European Geostationary Navigation Overlay Service (EGNOS) for Europe, the Multi-functional Satellite Augmentation System (MSAS) for Japan, the GPS (Global Positioning System) Aided GEO Augmented Navigation (GAGAN) for India, and the System for Differential Corrections and Monitoring (SDCM) for Russia. The SBASs broadcast messages to correct satellite orbit, clock, and ionosphere errors to augment the GPS positioning performance. In this paper, SBAS orbit, clock and ionospheric corrections are evaluated. Specifically, the orbit, clock and ionospheric corrections derived from SBAS messages are comprehensively evaluated using data collected from the above mentioned systems over 181 consective days. The evaluation indicates that the EGNOS outperforms other systems with signal-in-space range error (SISRE) at 0.645 m and ionospheric correction accuracy at 0.491 m, respectively. Meanwhile, the accuracy of SDCM is comparable to EGNOS with SISRE of 0.650 m and ionospheric correction accuracy of 0.523 m. For WAAS, the SISRE is 0.954 m and the accuracy of ionospheric correction is 0.505 m. The accuracies of the SBAS corrections from the MSAS and GAGAN systems, however, are significantly worse than those of others. The SISREs are 1.931 and 1.325 m and the accuracies of ionospheric corrections are 0.795 and 0.858 m, for MSAS and GAGAN, respectively. At the same time, GPS broadcast orbit, clock and ionospheric corrections are also evaluated. The results show that there are no significant improvements in the SISRE of the broadcast navigation data by applying SBAS corrections. On the other hand, the accuracy of SBAS ionospheric corrections is still much better than GPS broadcast ionospheric corrections, which could still be beneficial for single-frequency users.

A Tool to Characterize Delays and Packet Losses in Power Systems With Synchrophasor Data

This paper describes the implementation of a tool to estimate latencies and data dropouts in communication networks transferring synchrophasor data defined by the C37.118 standard. The tool assigns a time tag to synchrophasor packets at the time it receives them according to a global positioning system clock and with this information is able to determine the time those packets took to reach the tool. The tool is able to connect simultaneously to multiple phasor measurement units (PMUs) sending packets at different reporting rates with different transport protocols such as user datagram protocol or transmission control protocol. The tool is capable of redistributing every packet it receives to a different device while recording the exact time this information is re-sent into the network. The results of measuring delays from a PMU using this tool are presented and compared with those of a conventional network analyzer. The results show that the tool presented in this paper measures delays more accurately and precisely than the conventional network analyzer.

Distributed Estimation of Power System Oscillation Modes Under Attacks on GPS Clocks

Phasor measurement units (PMUs) are playing an increasingly important role in wide-area monitoring and the control of power systems. PMUs allow synchronous real-time measurements of voltage, phase angle, and frequency from multiple remote locations in the grid, enabled by their ability to align to global positioning system (GPS) clocks. Given that this ability is vulnerable to GPS spoofing attacks, which have been confirmed easy to launch, in this paper, we propose a distributed real-time wide-area oscillation estimation approach that is robust to GPS spoofing on PMUs and their associated phasor data concentrators. The approach employs the idea of checking update consistency with histories and across distributed nodes and can tolerate up to one third of compromised nodes. It can be implemented in a completely decentralized architecture and in a completely asynchronous way. The effectiveness of the approach is confirmed by numerical simulations of the IEEE 68-bus power system models.

Search for transient ultralight dark matter signatures with networks of precision measurement devices using a Bayesian statistics method

We analyze the prospects of employing a distributed global network of precision measurement devices as a dark matter and exotic physics observatory. In particular, we consider the atomic clocks of the Global Positioning System (GPS), consisting of a constellation of 32 medium-Earth orbit satellites equipped with either Cs or Rb microwave clocks and a number of Earth-based receiver stations, some of which employ highly-stable H-maser atomic clocks. High-accuracy timing data is available for almost two decades. By analyzing the satellite and terrestrial atomic clock data, it is possible to search for transient signatures of exotic physics, such as "clumpy" dark matter and dark energy, effectively transforming the GPS constellation into a 50,000km aperture sensor array. Here we characterize the noise of the GPS satellite atomic clocks, describe the search method based on Bayesian statistics, and test the method using simulated clock data. We present the projected discovery reach using our method, and demonstrate that it can surpass the existing constrains by several order of magnitude for certain models. Our method is not limited in scope to GPS or atomic clock networks, and can also be applied to other networks of precision measurement devices.

Determination of near real-time GNSS satellite clocks for the FORMOSAT-7/COSMIC-2 satellite mission

In this study, we determine the near real-time (NRT) clocks of the Global Positioning System (GPS) and Globalnaya Navigatsionnaya Sputnikovaya Sistema (GLONASS) satellites in the Taiwan RO Process System (TROPS), which is mainly designed for the data processing in both the FORMOSAT-3/COSMIC (F3C) and FORMOSAT-7/COSMIC-2 (F7C2) satellite missions. The accuracy of GNSS clocks defines the quality of the atmospheric excess phase, which is used for the retrieval of bending angle profiles in GNSS radio occultation (RO) observations. The accuracy of the NRT GNSS clocks is assessed by comparing the clock rate, clock stability and clock-induced positioning error on receivers with the final solutions given by the European Space Agency (ESA). Overall, the standard deviations of the clock rates from TROPS agree with those from ESA within 0.05 mm/s over 2304 clock solutions. Additionally, we find that the clock stability of the GPS Block IIF type (3 × 10−13) is an order of magnitude better than that of IIR Block types (3 × 10−12) over a time interval of 30 s. In comparison, the stabilities of GLONASS clocks are approximately 3 × 10−12. We quantify the NRT clock error on the receiver positioning by using the precise point positioning technique obtained from the Bernese GNSS software. The 3-dimensional clock-induced positioning error is approximately 3.3, 3.2 and 0.9 cm for station AUCK and 6.9, 6.3 and 3.1 cm for station NRC1 for the GPS-only, GLONASS-only and GPS + GLONASS cases, respectively. For GNSS-RO applications, the bending angle profiles derived using TROPS GPS clocks agree with the COSMIC Data Analysis and Archive Center products to within 0.01–1.00 μrad. However, this is not the case for the GLONASS clock, because the GLONASS clock-induced errors on the RO profile are 10–100 times greater than those induced by the GPS clock. This suggests that different weightings should be used for RO applications, such as data assimilation, when different satellite clocks are involved in GNSS-RO retrievals. This study serves as a reference for assessing the impact of GNSS clocks on both GNSS-POD (precise orbit determination) and GNSS-RO in preparation for the F7C2 satellite mission.

Precise orbit determination of the Sentinel-3A altimetry satellite using ambiguity-fixed GPS carrier phase observations

The Sentinel-3 mission takes routine measurements of sea surface heights and depends crucially on accurate and precise knowledge of the spacecraft. Orbit determination with a targeted uncertainty of less than 2 cm in radial direction is supported through an onboard Global Positioning System (GPS) receiver, a Doppler Orbitography and Radiopositioning Integrated by Satellite instrument, and a complementary laser retroreflector for satellite laser ranging. Within this study, the potential of ambiguity fixing for GPS-only precise orbit determination (POD) of the Sentinel-3 spacecraft is assessed. A refined strategy for carrier phase generation out of low-level measurements is employed to cope with half-cycle ambiguities in the tracking of the Sentinel-3 GPS receiver that have so far inhibited ambiguity-fixed POD solutions. Rather than explicitly fixing double-difference phase ambiguities with respect to a network of terrestrial reference stations, a single-receiver ambiguity resolution concept is employed that builds on dedicated GPS orbit, clock, and wide-lane bias products provided by the CNES/CLS (Centre National d’Etudes Spatiales/Collecte Localisation Satellites) analysis center of the International GNSS Service. Compared to float ambiguity solutions, a notably improved precision can be inferred from laser ranging residuals. These decrease from roughly 9 mm down to 5 mm standard deviation for high-grade stations on average over low and high elevations. Furthermore, the ambiguity-fixed orbits offer a substantially improved cross-track accuracy and help to identify lateral offsets in the GPS antenna or center-of-mass (CoM) location. With respect to altimetry, the improved orbit precision also benefits the global consistency of sea surface measurements. However, modeling of the absolute height continues to rely on proper dynamical models for the spacecraft motion as well as ground calibrations for the relative position of the altimeter reference point and the CoM.

Precise real-time navigation of LEO satellites using a single-frequency GPS receiver and ultra-rapid ephemerides

Precise (sub-meter level) real-time navigation using a space-capable single-frequency global positioning system (GPS) receiver and ultra-rapid (real-time) ephemerides from the international global navigation satellite systems service is proposed for low-Earth-orbiting (LEO) satellites. The C/A code and L1 carrier phase measurements are combined and single-differenced to eliminate first-order ionospheric effects and receiver clock offsets. A random-walk process is employed to model the phase ambiguities in order to absorb the time-varying and satellite-specific higher-order measurement errors as well as the GPS clock correction errors. A sequential Kalman filter which incorporates the known orbital dynamic model is developed to estimate orbital states and phase ambiguities without matrix inversion. Real flight data from the single-frequency GPS receiver onboard China's SJ-9A small satellite are processed to evaluate the orbit determination accuracy. Statistics from internal orbit assessments indicate that three-dimensional accuracies better than 0.50 m and 0.55 mm/s are achieved for position and velocity, respectively.

Development of a Communication Aid App with iOS Devices to Support Children/Persons with Speech Disabilities Improvement in Obtaining Positioning Information with iBeacon as Near Field Radio Communication Technology

Communication has such a vital function as the foundation of human social life that speech disabilities can drastically deteriorate people’s quality of life. The author has developed a voice output communication aid (VOCA) to enable children/persons with speech disabilities to communicate with other people according to conversation situations with iOS devices with global positioning system (GPS), positional information acquiring technology (iBeacon), and a clock function installed. The developed technology is an application compatible with iOS (hereinafter, app) that displays a list of messages (hereinafter, VOCA interface) if any of them correspond to the information on the user’s location and time of use as acquired by users with a portable information device with a GPS function, iBeacon function, and clock function installed. Users can communicate by selecting the icons of required messages from VOCA interfaces to reproduce registered voice data instead of their own voices. We have named the developed technology “Friendly VOCA” because its intentions are that: (1) users can communicate with other people with as few operations involved as possible; (2) it should be easy for users to learn and to operate. This paper reports the development processes of the Friendly VOCA and defines the ratios of preregistered VOCA interfaces displayed (hereinafter, correct answer rates) in acquiring positional information with GPS and with iBeacon. In Research Task 1, we have conducted experiments to verify the correct answer rates of the GPS system in outdoor use of the Friendly VOCA and in Research Task 2 we have conducted experiments to verify the correct answer rates of the iBeacon system in indoor use of the Friendly VOCA. The research results show that both the GPS system and iBeacon system can specify user’s spatial information and display appropriate lists of messages. However, the experiments found that iBeacon signal transmitters can transmit signals over a distance of 50 m or more: to improve the correct answer rates of the iBeacon system, we may need to increase the distance between iBeacon signal transmitters or select devices with a shorter range of transmission.

Three methods to retrieve slant total electron content measurements from ground‐based GPS receivers and performance assessment

AbstractThe high sampling rate along with the global coverage of ground‐based receivers makes Global Positioning System (GPS) data particularly ideal for sensing the Earth's ionosphere. Retrieval of slant total electron content measurements (TECMs) constitutes a key first step toward extracting various ionospheric parameters from GPS data. Within the ionospheric community, the interpretation of TECM is widely recognized as the slant total electron content along the satellite receiver line of sight, biased by satellite and receiver differential code biases (DCBs). The Carrier‐to‐Code Leveling (CCL) has long been used as a geometry‐free method for retrieving TECM, mainly because of its simplicity and effectiveness. In fact, however, the CCL has proven inaccurate as it may give rise to TECM very susceptible to so‐called leveling errors. With the goal of attaining more accurate TECM retrieval, we report in this contribution two other methods than the CCL, namely, the Precise Point Positioning (PPP) and the Array‐aided PPP (A‐PPP). The PPP further exploits the International GPS Service (IGS) orbit and clock products and turns out to be a geometry‐based method. The A‐PPP is designed to retrieve TECM from an array of colocated receivers, taking advantage of the broadcast orbit and clock products. Moreover, A‐PPP also takes into account the fact that the ionospheric effects measured from one satellite to all colocated receivers ought to be the same, thus leading to the estimability of interreceiver DCB. We perform a comparative study of the formal precision and the empirical accuracy of the TECM that are retrieved, respectively, by three methods from the same set of GPS data. Results of such a study can be used to assess the actual performance of the three methods. In addition, we check the temporal stability in A‐PPP‐derived interreceiver DCB estimates over time periods ranging from 1 to 3 days.

Nanosecond-level time synchronization of autonomous radio detector stations for extensive air showers

To exploit the full potential of radio measurements of cosmic-ray air showers at MHz frequencies, a detector timing synchronization within 1 ns is needed. Large distributed radio detector arrays such as the Auger Engineering Radio Array (AERA) rely on timing via the Global Positioning System (GPS) for the synchronization of individual detector station clocks. Unfortunately, GPS timing is expected to have an accuracy no better than about 5 ns. In practice, in particular in AERA, the GPS clocks exhibit drifts on the order of tens of ns. We developed a technique to correct for the GPS drifts, and an independent method is used to cross-check that indeed we reach a nanosecond-scale timing accuracy by this correction. First, we operate a ``beacon transmitter'' which emits defined sine waves detected by AERA antennas recorded within the physics data. The relative phasing of these sine waves can be used to correct for GPS clock drifts. In addition to this, we observe radio pulses emitted by commercial airplanes, the position of which we determine in real time from Automatic Dependent Surveillance Broadcasts intercepted with a software-defined radio. From the known source location and the measured arrival times of the pulses we determine relative timing offsets between radio detector stations. We demonstrate with a combined analysis that the two methods give a consistent timing calibration with an accuracy of 2 ns or better. Consequently, the beacon method alone can be used in the future to continuously determine and correct for GPS clock drifts in each individual event measured by AERA.

Diver towed GPS to estimate densities of a critically endangered fish

Spotted handfish are a critically endangered anglerfish. Monitoring of the species has, in the past, been undertaken via underwater visual census (UVC) parameterised with fixed length 100m×3m strip transects measured with reel lines. However, the sparse distribution and cryptic nature of handfish resulted in low statistical power to track populations through time. To achieve reasonable confidence of detecting change ~40 transects are required for density estimates, making the logistical constraints of monitoring onerous. We trialled a new survey method for UVC at one study site with a known local population of fish, replacing reel transects with variable length strip transects parameterised with a diver towed Global Positioning System (GPS) float. From 18 transects we determined a density of 24 (±3.5) handfish per hectare, but bootstrapping suggested little improvement in precision occurred after 8 transects. Modelling these results to previous estimates at the site indicated that our GPS float approach required many fewer transects and dives to determine 50% increases or decreases in fish densities. This improvement was partly due to the better sampling efficiency; with 3 times the average search area per transect but was also a result of longer transects being a better solution for monitoring a sparse population. Our results returned a normal sample distribution of fish counts with few zeros and many multiple observations, which differed to the original approach which was heavily skewed towards zero observations per transect. However, at very low population densities even this more robust solution will require many samples to detect change. Our GPS parameterized surveys also improved the functionality of observing fish behaviour as we could accurately plot fish allowing for detailed investigation of distributions. When fish were observed they were also bi-laterally digitally photographed to record their unique natural markings. By synchronising the photographs timestamp with the GPS clock, we were able to geo-locate individual recaptures and our preliminary results suggests net movements were limited. This new approach increases the logistical feasibility for monitoring spotted handfish and may have applications for other sparsely distributed benthic species.

Hardware Implementation of an Automatic Adaptive Centralized Underfrequency Load Shedding Scheme

The underfrequency load shedding (UFLS) mostly used in industry is a decentralized deterministic scheme designed to shed a prespecified amount of load after a predetermined time delay. It sheds the same amount of load from the same location irrespective of how fast the frequency drops and without consideration of the disturbance location or dip in bus voltage. Recent studies focused on adaptive UFLS, but these studies are still based on software simulation. This study presents an implementation of a real-time centralized adaptive UFLS scheme using industry-grade hardware. It estimates the amount of load to be shed based on the rate of frequency decline and distributes the load to be shed among the load buses based on the voltage dip at these buses. The UFLS in this study is implemented using a real-time digital simulator, phasor measurement units embedded in the relays, a global positioning system clock, and a synchrophasor vector processor. The load is modelled as a mixture of dynamic and static load. The implemented scheme restored the system frequency and voltage. The results emphasize its adaptability and suitability for implementation in industry.

A new wavelet based fault detection, classification and location in transmission lines

This paper deals with the application of wavelet transforms for the detection, classification and location of faults on transmission lines. A Global Positioning System clock is used to synchronize sampling of voltage and current signals at both the ends of the transmission line. The detail coefficients of current signals of both the ends are utilized to calculate fault indices. These fault indices are compared with threshold values to detect and classify the faults. Artificial Neural Networks are employed to locate the fault, which make use of approximate decompositions of the voltages and currents of local end. The proposed algorithm is tested successfully for different locations and types of faults.

Evaluation of different approaches to modeling the second‐order ionospheric delay on GPS measurements

This work evaluates various approaches to compute the second order ionospheric correction (SOIC) to Global Positioning System (GPS) measurements. When estimating the reference frame using GPS, applying this correction is known to primarily affect the realization of the origin of the Earth's reference frame along the spin axis (Z coordinate). Therefore, the Z translation relative to the International Terrestrial Reference Frame 2008 is used as the metric to evaluate various published approaches to determining the slant total electron content (TEC) for the SOIC: getting the slant TEC from GPS measurements, and using the vertical total electron content (TEC) given by a Global Ionospheric Model (GIM) to transform it to slant TEC via a mapping function. All of these approaches agree to 1 mm if the ionospheric shell height needed in GIM‐based approaches is set to 600 km. The commonly used shell height of 450 km introduces an offset of 1 to 2 mm. When the SOIC is not applied, the Z axis translation can be reasonably modeled with a ratio of +0.23 mm/TEC units of the daily median GIM vertical TEC. Also, precise point positioning (PPP) solutions (positions and clocks) determined with and without SOIC differ by less than 1 mm only if they are based upon GPS orbit and clock solutions that have consistently applied or not applied the correction, respectively. Otherwise, deviations of few millimeters in the north component of the PPP solutions can arise due to inconsistencies with the satellite orbit and clock products, and those deviations exhibit a dependency on solar cycle conditions.