Dual-frequency GPS Research Articles

Multi-wavelength coordinated observations of ionospheric irregularity structures from an anomaly crest location during unusual solar minimum of the 24th cycle

The present paper reports coordinated ionospheric irregularity measurements at optical as well as GPS wavelengths. Optical measurements were obtained from Tiny Ionospheric Photometer (TIP) sensors installed onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites. GPS radio signals were obtained from a dual frequency GPS receiver operational at Calcutta (22.58°N, 88.38°E geographic; geomagnetic dip: 32.96°; 13.00°N, 161.63°E geomagnetic) under the SCIntillation Network Decision Aid (SCINDA) program. Calcutta is located near the northern crest of Equatorial Ionization Anomaly (EIA) in the Indian longitude sector. The observations were conducted during the unusually low and prolonged solar minima period of 2008–2010. During this period, four cases of post-sunset GPS scintillation were observed from Calcutta. Among those cases, simultaneous fluctuations in GPS Carrier-to-Noise ratios (C/No) and measured radiances from TIP over a common ionospheric volume were observed only on February 2, 2008 and September 25, 2008. Fluctuations observed in measured radiances (maximum 0.95 Rayleigh) from TIP due to ionospheric irregularities were found to correspond well with C/N0 fluctuations on the GPS links observed from Calcutta, such effects being noted even during late evening hours of 21:00–22:00 LT from locations around 40° magnetic dip. These measurements indicate the existence of electron density irregularities of scale sizes varying over several decades from 135.6 nm to 300–400 m well beyond the northern crest of the EIA in the Indian longitude sector during late evening hours even in the unusually low solar activity conditions.

Research on Construction Control Network Technology of Hydropower Project in Steep Mountainous Area

Due to the limitation of natural geographical conditions, many hydropower hub projects are built in steep mountainous areas, which leads to difficulties in fabric control network, excessive deformation of side length projection length, long error transmission route, and low precision of control network. The accuracy of the entire control network is difficult to control. This paper discusses the establishment of a planar control network using a high-precision dual-frequency GPS receiver instrument. The elevation control network is built using precision leveling plus triangular elevation measurements. It is proved by examples that the control network established by this method can not only ensure the overall accuracy and overall consistency of the control network, but also achieve economic, rapid and efficient effects.

Estimability analysis of differential inter-system biases and differential inter-frequency biases for dual-frequency GPS and BDS combined RTK

Inter-system biases (ISBs) and inter-frequency biases (IFBs) are critical for the data processing of multi-frequency and multi-constellation global navigation satellite systems (GNSS). Calibration of the ISB and IFB makes it possible to enhance the accuracy, integrity, and availability of multi-GNSS positioning, navigation, and timing. In this study, we investigate the magnitude and stability of the code and carrier phase differential inter-system biases (DISBs) between the GPS/BeiDou navigation satellite system (BDS) L1–B1/L2–B2 and differential inter-frequency biases (DIFBs) between L1–L2/B1–B2 using zero and short baseline data collected using both identical and different receiver types in real-time kinematic (RTK) positioning. We verify that the code and carrier phase DISBs of GPS/BDS L1–B1/L2–B2 and DIFBs of L1–L2/B1–B2 are both approximately constant on the timescale of a consecutive five-day observation period from a statistical perspective, and can thus be corrected by a priori DISB and DIFB, respectively. Moreover, the performance of the a priori model correction and real-time estimation strategies for the DISB and DIFB are compared. Both can precisely correct the DISB and DIFB and achieve significantly improved performance for multi-GNSS inter-system and inter-frequency combined RTK positioning compared to the classical intra-system and intra-frequency combined RTK approach. The DISB and DIFB using the a priori model correction and real-time estimation effectively improve the positioning model strength, thereby enabling superior performance for multi-GNSS positioning.

Evaluation of the SSN and F10.7 indices as a cause for performance variation of the IRI 2016 and IRI-Plas 2017 models

This paper mainly focuses on the evaluation of efficiency of the Sunspot Number (SSN) and the 10.7 cm Solar Radio Flux (F10.7) indices as a cause for the variation of the performance of the latest versions of the International Reference Ionosphere model (IRI 2016 with NeQuick, IRI01-corr and IRI2001 options for the topside electron density) and the IRI Extended to the Plasmasphere (IRI-Plas 2017) for the modeling of the Total Electron Content (TEC). The Global Positioning System (GPS)-derived TEC data obtained from the dual frequency GPS receivers located at Observation Rock, OBSR (geog 46.90°N, 238.18°W, Geom. 52.46°N) and Husband, HUSB (44.12°N, 238.15°W, Geom. 49.73°N) in the West Pacific region during the recent solar maximum (2012-2014) years have been considered for the validation of the performance of the models. The results show that both the GPS-derived TEC (GPS VTEC) and modelled (IRI 2016 and IRI-Plas 2017 VTEC) seasonal diurnal values tend to peak at 00:00 UT (16:00 LT) and 20:00 (12:00 LT) with the highest being observed mostly at 20:00 (12:00 LT); while, their minima are mostly observed at about 13:00 UT (05:00 LT). In addition, in utilizing the SSN, the best performance is generally observed in the June solstice months, especially by the IRI-Plas 2017 model. However, for the equinoctial and December solstice months, the best performance is generally observed by the IRI 2016 model with NeQuick and IRI01 options. It has also been shown that the root-mean-square deviations between the GPS-derived and modelled VTEC diurnal variation in using the F10.7 index are generally less than those of the SSN option in all months, revealing that both the IRI 2016 and IRI-Plas 2017 models generally show better performance using F10.7 index than the SSN. Hence, the F10.7 option is recommended for better TEC modeling employing the IRI 2016 and IRI-Plas 2017 models during the recent solar maximum years over the West Pacific region. In addition, both models cannot effectively estimate the geomagnetic storm time TEC variation.

Ionosphere modeling in the context of Algerian Satellite-based Augmentation System

To guarantee safety-critical implementation of Algerian Satellite-based Augmentation Systems (SBAS), a comprehensive study of the impact of space weather on Global Navigation Satellite Systems (GNSS) over this region is necessary. In this work, three typical ionospheric algorithms: inverse distance weighting (IDW), kriging, and planar-fit were applied on GPS dual-frequency data from seven GPS stations spread over Algerian territory during three major geomagnetic storms of different intensity in addition to a quiet geomagnetic period. The performance of these ionospheric algorithms was investigated based on the broadcast ionospheric vertical delay (Grid Ionospheric Vertical Delay, GIVD) and its corresponding confidence bound error (Grid Ionospheric Vertical Error, GIVE) at two Ionospheric Grid Points (IGPs): (35°N, 5°E) and (30°N, 5°E), located in the Algerian mid-latitudes. The obtained GIVDs were compared to their corresponding delays estimated at nearby user stations as a first reference and to EGNOS messages as a second reference. In addition, the Global Ionospheric Maps (GIMs) from the International GNSS Service (IGS) was incorporated in the analysis. Our results indicate that the studied models provided equivalent accuracy even though kriging slightly outperformed the other models in most of the cases of study. The EGNOS model showed less agreement with the ground truth data during 2013 and 2015, while in 2017 became more accurate after the upgrade. The analysis of estimated GIVEs shows the ability of the three ionospheric models to protect and to safely bound GIVD errors while kriging is intended to provide better availability of Algerian SBAS. In addition, EGNOS GIVEs exhibit effective protection of users at the edge of its service area.

Evaluation of single-frequency receivers for studying crustal deformation at the longitudinal Valley fault, eastern Taiwan

Applications of low-cost single-frequency continuous GPS receivers for monitoring volcano and landslide activities as well as to complement dual-frequency receivers have been demonstrated to produce stable and accurate positioning. In studies of crustal deformation, the relative distance between monitoring stations may vary from several kilometers to tens of kilometers, hence the differential single-frequency observations cannot model the ionospheric delay or other distance dependent errors. The 55 low-cost single-frequency continuous stations have been deployed together with 52 continuous dual-frequency stations in southeastern Taiwan since 2008. All of the single-frequency stations have applied corrections using dual-frequency stations to eliminate the distance dependent errors. Comparing velocity estimates from 8 co-located, the differences in horizontal and vertical components are less than 3 mm/yr and 6 mm/yr, respectively. Our study shows that the combination of single- and dual-frequency GPS data can provide robust results to study the fault slip behavior on the Longitudinal Valley fault.

Performance evaluation of the IRI 2016 and IRI-Plas 2017 models over central Asian mid-latitude regions in the descending phase of solar cycle 24

This paper scrutinizes the performance of the latest versions of the IRI model (IRI 2016 with NeQuick, IRI01-corr and IRI2001 options for the topside electron density) and the IRI extended to the plasmasphere (IRI-Plas 2017) models in the estimation of the Vertical TEC (VTEC) variation over the central Asian mid-latitude regions in the descending phase of solar cycle 24 (2014–2016). The GPS dual frequency receivers installed at Kurchatov, KRTV (geog 50.71°N, 78.62°E, Geom. 41.84°N), Khantau, SUMK (44.21°N, 74.00°E, Geom. 35.73°N), Talas, TALA (42.45°N, 72.21°E, Geom.34.13°N) and Kazarman, KAZA (41.38°N, 73.94°E, Geom.32.92°N) have been used to derive the estimate of the vertical TEC (GPS-VTEC) for the comparison of the monthly and seasonal performance of the models. The modelled VTEC values generally tend to be larger than the GPS-VTEC values during periods of high solar irradiance (daytime hours), with the highest overestimation being observed by IRI-Plas 2017 model followed by IRI 2016 model with the IRI2001 topside option. However, the differences between the models and between the models and the GPS-VTEC values become diminished as the solar irradiance decreases, with the highest underestimation being observed by the IRI 2016 model with NeQuick topside option. It has also been shown that the smallest root-mean-square deviations between the GPS-VTEC and modelled VTEC are observed generally in the June solstice months, showing that the models perform best during local summer. On the contrary, the largest root-mean-square deviations between the modelled VTEC and GPS-VTEC are observed during high solar irradiance on the surface of the Earth (especially in the time interval between 05:00 and 10:00 UT which corresponds to the daytime hours 10:00 and 15:00 LT), showing that the models perform poorly during high solar irradiance. In addition, both the IRI 2016 and IRI-Plas 2017 models show a progressive decline in VTEC during a negative storm, but do not adequately estimate the storm time VTEC variation.

A low-cost autonomous rover for polar science

Abstract. We present the developmental considerations, design, and deployment of an autonomous modular terrestrial rover for ice-sheet exploration that is inexpensive, easy to construct, and allows for instrumentation customization. The total construction cost for this rover is less than USD 3000, approximately one-tenth the cost of existing platforms, and it can be built using facilities frequently available at academic institutions (machine shop, 3-D printer, open-source hardware and software). Instrumentation deployed on this rover can be customized; the rover presented in this study was equipped with a dual-frequency GPS receiver and a digital SLR camera for constructing digital elevation models using structure-from-motion (SfM) photogrammetry. We deployed this prototype rover on the Northeast Greenland Ice Stream to map local variations in snow accumulation and surface topography. The rover conducted four autonomous missions based out of the East Greenland Ice-Core Project (EastGRIP) camp during July 2017, measuring surface elevation transects across the hazardous ice-stream shear margins. During these missions, the rover proved capable of driving over 20 km on a single charge with a drawbar pull of 250 N, sufficient to tow instrumentation of up to 100 kg. The rover also acquired photographs that were subsequently used to construct digital elevation models of a site monitored for spatiotemporal variability in snow accumulation, demonstrating adequate stability for high-resolution imaging applications. Due to its low cost, low-power requirements, and simple modular design, mass deployments of this rover design are practicable. Operation of the rover in hazardous areas circumvents the substantial expense and risk to personnel associated with conventional, crewed deployments. Thus, this rover is an investigatory platform that enables direct exploration of polar environments considered too hazardous for conventional field expeditions.

Assessment of the Single Frequency Low Cost GPS RTK Positioning

The Global Navigation Satellite System (GNSS), nowadays, is to be an important tool for determining positions and times on the real-time and post-processing applications. This study aims to assess the performance of the single frequency L1-GPS using low cost receivers. The performance will be evaluated by comparing with the geodetic L1+L2 GPS receiver to positioning performance. The low cost receivers used were Ublox NEO-M8T and Allystar HD9100. While, the dual frequency GPS TOPCON was used as reference receiver. The results show, RTK mode can improve the accuracy compare with single positioning up to centimetres. In this experiment, the Allystar HD9100 gives a better result than Ublox Neo M8T. However, the result discrepancies may be caused by the simple antenna splitter.

An online error calibration method for spaceflight TT&C systems based on LEO-ground DDGPS

To overcome the shortcomings of the traditional measurement error calibration methods for spaceflight telemetry, tracking and command (TT&C) systems, an online error calibration method based on low Earth orbit satellite-to-ground doubledifferential GPS (LEO-ground DDGPS) is proposed in this study. A fixed-interval smoother combined with a pair of forward and backward adaptive robust Kalman filters (ARKFs) is adopted to solve the LEO-ground baseline, and the ant colony optimization (ACO) algorithm is used to deal with the ambiguity resolution problem. The precise baseline solution of DDGPS is then used as a comparative reference to calibrate the systematic errors in the TT&C measurements, in which the parameters of the range error model are solved by a batch least squares algorithm. To validate the performance of the new online error calibration method, a hardware-in-the-loop simulation platform is constructed with independently developed spaceborne dual-frequency GPS receivers and a Spirent GPS signal generator. The simulation results show that with the fixed-interval smoother, a baseline estimation accuracy (RMS, single axis) of better than 10 cm is achieved. Using this DDGPS solution as the reference, the systematic error of the TT&C ranging system is effectively calibrated, and the residual systematic error is less than 5 cm.

Equatorial and low-latitude positive ionospheric phases due to moderate geomagnetic storm during high solar activity in January 2013

The day-to-day variability of the equatorial and low-latitude ionosphere during quiet and disturbed periods is one of the ionospheric highlighted Space Weather research topics, particularly the ionospheric electrodynamics during geomagnetic storms. This study investigates the response of ionospheric F-region from the equatorial region to beyond the Equatorial Ionization Anomaly (EIA) crest during moderate geomagnetic storm (minimum Dst = −53 nT) that took place on January 17 to 18, 2013, during the high solar activity period of solar cycle 24. The Total Electron Content (TEC) obtained through a network of 82 dual frequency GPS receivers, spanning over an area of 30° × 30° in latitude and longitude are used. Also the F-layer virtual height (h’F) and critical frequency (foF2) observations from 3 ionosondes, in the South American sector are used. Specifically, these GPS-TEC receivers and ionosondes are used to investigate how the F-layer was disturbed by two positive ionospheric phases occurred during the aforementioned disturbed period. The first positive ionospheric phase was probably due to a travelling ionospheric disturbance (TID). When this TID reached the Brazilian coast at low-latitude, the EIA crest was in the growth phase and makes it challenging to separate the spatial-temporal evolution of both phenomena. The second positive ionospheric phase was caused by an anomalous nighttime equatorial positive ionospheric cloud travelling from the east sector towards the west sector. In addition, how the EIA was disturbed by these two positive ionospheric phases in the eastern and western Brazilian sectors is also investigated.

Ionospheric Rayleigh Wave Disturbances Following the 2018 Alaska Earthquake from GPS Observations

Big earthquakes often excite the acoustic resonance between the earth’s surface and the lower atmosphere. The perturbations can propagate upward into the ionosphere and trigger ionospheric anomalies detected by dual-frequency GPS observations, but coseismic ionospheric disturbance (CID) directivity and mechanism are not clear. In this paper, the ionospheric response to the Mw = 7.9 Alaska earthquake on 23 January 2018 is investigated from about 100 continuous GPS stations near the epicenter. The fourth-order zero-phase Butterworth band-pass filter with cutoffs of 2.2 mHz and 8 mHz is applied to obtain the ionospheric disturbances. Results show that the CIDs with an amplitude of up to 0.06 total electron content units (TECU) are detected about 10 min after the Alaska earthquake. The CIDs are as a result of the upward propagation acoustic waves triggered by the Rayleigh wave. The propagation velocities of TEC disturbances are around 2.6 km/s, which agree well with the wave propagation speed of 2.7 km/s detected by the bottom pressure records. Furthermore, the ionospheric disturbances following the 2018 Mw = 7.9 Alaska earthquake are inhomogeneous and directional which is rarely discussed. The magnitude of ionospheric disturbances in the western part of the epicenter is more obvious than in the eastern part. This phenomenon also corresponds to the data obtained from the seismographs and bottom pressure records (BPRs) at the eastern and western side of the epicenter.

Assessment of NeQuick and IRI-2016 models during different geomagnetic activities in global scale: Comparison with GPS-TEC, dSTEC, Jason-TEC and GIM

The range delay induced by the Earth’s ionosphere is one of main errors affecting the positioning performance of Global Navigation Satellite System (GNSS). It is very significant to analyze the performance of the three-dimensional ionospheric models under different geomagnetic conditions using different methods. With the dual-frequency GPS measurements obtained from 83 globally distributed receivers of the International GNSS Service (IGS), we analyzed the performance of different three-dimensional ionospheric models, i.e. the original NeQuick2, the re-estimated NeQuickC and IRI-2016, under different ionospheric conditions. The absolute GPS-derived total electron contents (TECs), the difference of slant TEC observed in independent ground reference stations (dSTEC), the TEC derived from Jason-2 and the absolute TECs calculated from Global Ionospheric Map (GIM) are selected as reference. The validations against GPS-TEC and GIM-TEC indicate that NeQuickC can mitigate ionospheric delay by 70–76% under different geomagnetic activity levels on a global scale, which is 7–16% higher than that of NeQuick2 model, while IRI-2016 can only correct about 50% of the ionospheric delay. The validations against GPS-dSTEC presented that the NeQuickC model can make a good representation of the subtle variation in the ionospheric TEC. The validations against Jason TEC indicate that the relative error from the NeQuickC model is about 10% smaller than that from other two models over oceanic regions. The results show that the NeQuickC presents the best performance during different ionospheric activities, and the performance of those models during perturbed period is approximately 2–3 times worse than that during the quiet period.

Mitigation of ionospheric signatures in Swarm GPS gravity field estimation using weighting strategies

Abstract. Even though ESA's three-satellite low-earth orbit (LEO) mission Swarm is primarily a magnetic field mission, it can also serve as a gravity field mission. Located in a near-polar orbit with initial altitudes of 480 km for Swarm A and Swarm C and 530 km for Swarm B and equipped with geodetic-type dual frequency Global Positioning System (GPS) receivers, it is suitable for gravity field computation. Of course, the Swarm GPS-only gravity fields cannot compete with the gravity fields derived from the ultra-precise Gravity Recovery And Climate Experiment (GRACE) K-band measurements. But for various reasons like the end of the GRACE mission in October 2017, data gaps in the previous months due to battery aging, and the gap between GRACE and the recently launched GRACE Follow-On mission, Swarm gravity fields became important to maintain a continuous time series and to bridge the gap between the two dedicated gravity missions. By comparing the gravity fields derived from Swarm kinematic positions to the GRACE gravity fields, systematic errors have been observed in the Swarm results, especially around the geomagnetic equator. These errors are already visible in the kinematic positions as spikes up to a few centimeters, from where they propagate into the gravity field solutions. We investigate these systematic errors by analyzing the geometry-free linear combination of the GPS carrier-phase observations and its time derivatives using a combination of a Gaussian filter and a Savitzky–Golay filter and the Rate of Total Electron Content (TEC) Index (ROTI). Based on this, we present different weighting schemes and investigate their impact on the gravity field solutions in order to assess the success of different mitigation strategies. We will show that a combination of a derivative-based weighting approach with a ROTI-based weighting approach is capable of reducing the geoid rms from 21.6 to 12.0 mm for a heavily affected month and that almost 10 % more kinematic positions can be preserved compared to a derivative-based screening.

Vertical ionospheric delay estimation for single-receiver operation

Abstract An apparent delay is occurred in GPS signal due to both refraction and diffraction caused by the atmosphere. The second region of the atmosphere is the ionosphere. The ionosphere is significantly related to GPS and the refraction it causes in GPS signal is considered one of the main source of errors which must be eliminated to determine accurate positions. GPS receiver networks have been used for monitoring the ionosphere for a long time. The ionospheric delay is the most predominant of all the error sources. This delay is a function of the total electron content (TEC). Because of the dispersive nature of the ionosphere, one can estimate the ionospheric delay using the dual frequency GPS. In the current research our primary goal is applying Precise Point Positioning (PPP) observation for accurate ionosphere error modeling, by estimating Ionosphere delay using carrier phase observations from dual frequency GPS receiver. The proposed algorithm was written using MATLAB and was named VIDE program. The proposed Algorithm depends on the geometry-free carrier-phase observations after detecting cycle slip to estimates the ionospheric delay using a spherical ionospheric shell model, in which the vertical delays are described by means of a zenith delay at the station position and latitudinal and longitudinal gradients. Geometry-free carrier-phase observations were applied to avoid unwanted effects of pseudorange measurements, such as code multipath. The ionospheric estimation in this algorithm is performed by means of sequential least-squares adjustment. Finally, an adaptable user interface MATLAB software are capable of estimating ionosphere delay, ambiguity term and ionosphere gradient accurately.

Analysis of Total Electron Content (TEC) Near Real Time Using Dual Frequency GPS Data (Study Case: Surabaya)

Ionosphere is part of the atmospheric layer located between 50 to 1000 km above the earth's surface which consists of electrons that can influence the propagation of electromagnetic waves in the form of additional time in signal propagation, this depends on Total Electron Content (TEC) in the ionosphere and frequency GPS signal. In high positioning precision with GPS, the effect of the ionosphere must be estimated so that ionospheric correction can be determined to eliminate the influence of the ionosphere on GPS observation. Determination of ionospheric correction can be done by calculating the TEC value using dual frequency GPS data from reference stations or models. In making the TEC model, a polynomial function is used for certain hours. The processing results show that the maximum TEC value occurs at noon at 2:00 p.m. WIB for February 13, 2018 with a value of 35,510 TECU and the minimum TEC value occurs in the morning at 05.00 WIB for February 7, 2018 with a value of 2,138 TECU. The TEC model spatially shows the red color in the area of Surabaya and its surroundings for the highest TEC values during the day around 13.00 WIB to 16.00 WIB.

Seven days in Chile: Impact of the 2011 Puyehue-Cordon Caulle volcanic eruption on GPS ionospheric delay

Satellite navigation is a global utility and an essential component of national infrastructure. Disruptions of GNSS PNT services may be considered a threat to society and civilisation in general. Natural hazards may cause the conditions that disrupt or temporarily deny GNSS PNT services. As a contributor to ionospheric dynamics, volcanic activity is considered a source of GNSS positioning performance degradation. Here we studied the 2011 Puyehue-Cordon Caulle event, the largest 21st century volcanic eruption so far, in terms of its contribution to formation of Total Electron Content (TEC), the source of ionosphere-caused GNSS positioning error, and the effects the event made on GPS positioning accuracy. TEC values were derived from dual-frequency GPS observations collected experimentally at the International GNSS Service Network reference stations in Santiago, central Chile closest to the Puyehue-Cordon Caulle volcano. We identified considerable anomalous behaviour of TEC dynamics prior to, during and after the volcanic eruption, and examined the extent to which it affected GPS positioning accuracy. The research presented here will continue with the aim of characterisation of TEC anomalous dynamics around the eruption, and its effects on GNSS positioning performance.

Ionospheric TEC Response to Geomagnetic Storms Occurred on 15-20 March 2013 and 2015 over the Eastern Africa Region

In this paper we present characteristics of the ionosphere over eastern Africa region in response to two geomagnetic storms of March 2013 and March 2015 that occurred at the same date and time but with different intensities. Ionospheric TEC has been derived from the IGS network of ground based dual-frequency GPS receivers from four stations, namely Eldoret (0.29°N, 35.29°E), Dodoma (6.19°S, 35.75°E), Malindi (2.99°S, 40.19°E) and Mtwara (10.26°S, 40.17°E) within the Eastern Africa region. The interplanetary magnetic field (IMF) Bz and corresponding Dst index, solar wind speed (Vsw) and density (Np) were used to represent the evolution of the storm events. Our results showed that, the behavior of the ionosphere over eastern Africa region during the two geomagnetic storms was similar. During both storms the main phases occurred on 17th of March. However, the main phase of the storm in 2015 had more intensity than that of 2013 but the same duration, and they were followed by a long-duration slow recovery with values that did not return to levels prior to the onset of the storm. The enhancement and reduction of TECv were observed at almost all stations at different times. The results from both storms also show that, there was a pre-reversal enhancement especially at stations that lie close to the equatorial region compared to the stations far from the equatorial region.

Comments on the percentage of occurrence methodology used in “a study of L band scintillations during the initial phase of rising solar activity at an Indian low latitude station” by H J Tanna, S P Karia and K N Pathak

Following Tanna et al. (2013), we computed the percentage of occurrence of S4 index for the period of 2012–2015 using the data of the dual frequency GPS receiver at the Tripura University, Agartala station (23.76°N, 91.26°E) situated at the northern crest of the equatorial ionization anomaly (EIA) region of the Indian Subcontinent. We have observed discrepancy in the results contradicting the actual scintillation occurrence. The distinctly noticeable discrepancy is that the maximum occurrence month is shifted to April 2013 instead of March 2014. The problem arises due to the denominator term used in the percentage of occurrence ratio i.e. the total number of days of observed scintillation activity during the complete period under consideration. But the conventional percentage of occurrence methodology uses the number of days of observation (the total number of days for which data is available) during each month in the denominator. It correctly assigns the maximum occurrence to March 2014 instead of April 2013 and the obtained monthly statistics follow the solar activity during this period.

PPP-RTK based on undifferenced and uncombined observations: theoretical and practical aspects

A synthesis of two prevailing global navigation satellite system positioning technologies, namely the precise point positioning and the network-based real-time kinematic, results in the emergence of the PPP-RTK, enabling single-receiver users to achieve high positioning accuracy with reasonable timeliness through integer ambiguity resolution. The realization of PPP-RTK needs to accomplish two sequential tasks. The first task is to determine a class of corrections including, among others, the satellite phase biases (SPBs) at the network level. With these corrections, the second task, then, is to solve for the ambiguity-fixed, absolute position at the user level. In this contribution, we revisit three variants (geometry-free, geometry-fixed and geometry-plus-satellite-clock-fixed) of the undifferenced and uncombined PPP-RTK network model and then point out their implications for practical use. We also carry out a case study using multi-day, dual-frequency global positioning system data from the crustal movement observation network of China stations, aiming to figure out what are the most appropriate linear combinations of the SPBs to be transmitted to the users from the viewpoint of decorrelation, and to assess the static and kinematic positioning performance.