The Global Positioning System (GPS) has become a powerful tool for ionospheric studies. In addition, ionospheric corrections are necessary for the augmentation systems required for Global Navigation Satellite Systems (GNSS) use. Dual-frequency carrier-phase and code-delay GPS observations are combined to obtain ionospheric observables related to the slant total electron content (sTEC) along the satellite-receiver line-of-sight (LoS). This observable is affected by inter-frequency biases [IFB; often called differential code biases (DCB)] due to the transmitting and the receiving hardware. These biases must be estimated and eliminated from the data in order to calibrate the experimental sTEC obtained from GPS observations. Based on the analysis of single differences of the ionospheric observations obtained from pairs of co-located dual-frequency GPS receivers, this research addresses two major issues: (1) assessing the errors translated from the code-delay to the carrier-phase ionospheric observable by the so-called levelling process, applied to reduce carrier-phase ambiguities from the data; and (2) assessing the short-term stability of receiver IFB. The conclusions achieved are: (1) the levelled carrier-phase ionospheric observable is affected by a systematic error, produced by code-delay multi-path through the levelling procedure; and (2) receiver IFB may experience significant changes during 1 day. The magnitude of both effects depends on the receiver/antenna configuration. Levelling errors found in this research vary from 1.4 total electron content units (TECU) to 5.3 TECU. In addition, intra-day vaiations of code-delay receiver IFB ranging from 1.4 to 8.8 TECU were detected.

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Calibration errors on experimental slant total electron content (TEC) determined with GPS

The international reference ionosphere (IRI) is the internationally recognized and recommended standard for the specification of plasma parameters in Earth’s ionosphere. It describes monthly averages of electron density, electron temperature, ion temperature, ion composition, and several additional parameters in the altitude range from 60 to 1,500 km. A joint working group of the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI) is in charge of developing and improving the IRI model. As requested by COSPAR and URSI, IRI is an empirical model being based on most of the available and reliable data sources for the ionospheric plasma. The paper describes the latest version of the model and reviews efforts towards future improvements, including the development of new global models for the F2 peak density and height, and a new approach to describe the electron density in the topside and plasmasphere. Our emphasis will be on the electron density because it is the IRI parameter most relevant to geodetic techniques and studies. Annual IRI meetings are the main venue for the discussion of IRI activities, future improvements, and additions to the model. A new special IRI task force activity is focusing on the development of a real-time IRI (RT-IRI) by combining data assimilation techniques with the IRI model. A first RT-IRI task force meeting was held in 2009 in Colorado Springs. We will review the outcome of this meeting and the plans for the future. The IRI homepage is at http://www.IRI.gsfc.nasa.gov.

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The international reference ionosphere today and in the future

[1] With the current data availability from both ground- and space-based sources, the network of ground-based Global Positioning System (GPS) receivers, GPS occultation receivers, in situ electron density sensors, and dual-frequency beacon transmitters, the time is right for a comprehensive review of the history, current state, and future directions of ionospheric imaging. A brief introduction and history of ionospheric imaging is presented, beginning with computerized ionospheric tomography. Then, a comprehensive review of the current state of ionospheric imaging is presented. The ability of imaging algorithms to ingest multiple types of data and use advanced inverse techniques borrowed from meteorological data assimilation to produce four-dimensional images of electron density is discussed. Particular emphasis is given to the mathematical basis for the different methods. The science that ionospheric imaging addresses is discussed, and the scientific contributions that ionospheric imaging has made are described. Finally, future directions for this research area are outlined.

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History, current state, and future directions of ionospheric imaging

The main goal of this paper is to provide a summary of our current knowledge of the ionosphere as it relates to space geodetic techniques, especially the most informative technology, global navigation satellite systems (GNSS), specifically the fully deployed and operational global positioning system (GPS). As such, the main relevant modeling points are discussed, and the corresponding results of ionospheric monitoring are related, which were mostly computed using GPS data and based on the direct experience of the authors. We address various phenomena such as horizontal and vertical ionospheric morphology in quiet conditions, traveling ionospheric disturbances, solar flares, ionospheric storms and scintillation. Finally, we also tackle the question of how improved knowledge of ionospheric conditions, especially in terms of an accurate understanding of the distribution of free electrons, can improve space geodetic techniques at different levels, such as higher-order ionospheric effects, precise GNSS navigation, single-antenna GNSS orientation and real-time GNSS meteorology.

The ionosphere: effects, GPS modeling and the benefits for space geodetic techniques

The dual frequency radio signals of the Global Positioning System (GPS) allow measurements of the total number of electrons, called total electron content (TEC), along a ray path from GPS satellite to receiver. We have developed a new technique to construct two-dimensional maps of absolute TEC over Japan by using GPS data from more than 1000 GPS receivers. A least squares fitting procedure is used to remove instrumental biases inherent in the GPS satellite and receiver. Two-dimensional maps of absolute vertical TEC are derived with time resolution of 30 seconds and spatial resolution of 0.15° × 0.15° in latitude and longitude. Our method is validated in two ways. First, TECs along ray paths from the GPS satellites are simulated using a model for electron contents based on the IRI-95 model. It is found that TEC from our method is underestimated by less than 3 TECU. Then, estimated vertical GPS TEC is compared with ionospheric TEC that is calculated from simultaneous electron density profile obtained with the MU radar. Diurnal and day-to-day variation of the GPS TEC follows the TEC behavior derived from MU radar observation but the GPS TEC is 2 TECU larger than the MU radar TEC on average. This difference can be attributed to the plasmaspheric electron content along the GPS ray path. This method is also applied to GPS data during a magnetic storm of September 25, 1998. An intense TEC enhancement, probably caused by a northward expansion of the equatorial anomaly, was observed in the southern part of Japan in the evening during the main phase of the storm.

A new technique for mapping of total electron content using GPS network in Japan

[1] The aim of this work is the analysis of the annual, semiannual, and seasonal effects in the total electron content (TEC) of the terrestrial atmosphere during low solar activity. Spatial and temporal ionospheric variability are investigated from global International Global Navigation Satellite System Service (IGS) VTEC maps during low solar activity in 2006. Two different techniques, principal component analysis (PCA) and Fourier analysis, are applied to the data set. Applying the PCA technique on a time series of global IGS VTEC maps gives us a method to analyze the main ionospheric anomalies on a global scale. The Fourier series provide us a comparison with the results obtained with PCA. The behavior of VTEC variations at 2 h periods centered at 1200 and 2200 local time (LT) are analyzed. Particular characteristics associated with each period and with the geomagnetic region are highlighted. All the stations show an annual behavior, which means that the maxima variations of the VTEC occur in summer while minimum variations are in winter, except in the stations located at the Northern Hemisphere at noon. Some regions show patterns of the semiannual anomaly during local noon, and it is also possible to see a higher peak of VTEC during spring rather than autumn in the Northern Hemisphere. However, if we analyze the pattern in the Southern Hemisphere, both peaks in equinox are of the same magnitude. Results obtained with Fourier series are comparable with the ones mentioned above.

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Annual and semiannual VTEC effects at low solar activity based on GPS observations at different geomagnetic latitudes

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A Multi‐GNSS, Multifrequency, and Near‐Real‐Time Ionospheric TEC Monitoring System for South America

Evaluating the accuracy of ionospheric range delay corrections for navigation at low latitude

La Plata Ionospheric Model (LPIM) corrections in ionospheric range delay can be studied by analysing their effect on geodetic positioning. To evaluate these corrections, a scheme similar to that used in Satellite-Based Augmentation System (SBAS) is employed in this work. So, in this context an appropriate model for computing the ionospheric range delay and its error is required. By means of numerical simulation, the accuracy of LPIM for computing the ionospheric range delay correction at low latitude is evaluated. The slant range delays for the observing network are simulated using the NeQuick ionospheric model. The simulated ionospheric range delay correction applied to geodetic positioning is analysed at different levels of ionospheric activity as well as to solstices and equinoxes. The large errors of the ionospheric correction on geodetic positioning are obtained in high solar activity during the equinox. This work shows that the accuracy achieved using simulated data at low latitude is similar to the accuracy reported using real data at mid latitude.

Amalia Meza

Papers

Annual and semiannual VTEC effects at low solar activity based on GPS observations at different geomagnetic latitudes

A Multi‐GNSS, Multifrequency, and Near‐Real‐Time Ionospheric TEC Monitoring System for South America

Evaluating the accuracy of ionospheric range delay corrections for navigation at low latitude

Evaluating the accuracy of ionospheric range delay corrections for navigation at low latitude

Global behaviour of the ionosphere electron density using GPS observations