A simple way of improving the Global Navigation Satellite Systems (GNSS) slant ionospheric correction from Vertical Total Electron Content (VTEC) models is presented. In many GNSS applications, a mapping function is required to convert from VTEC, which may be provided in Global Ionospheric Maps (GIMs), to Slant TEC (STEC). Typical approaches assume a single ionospheric shell with constant height, which is unrealistic, especially for low-elevation signals. To reduce the associated conversion error, we propose the Barcelona Ionospheric Mapping Function and its first implementation at northern mid-latitudes (BIMF-nml). BIMF is based on a climatic prediction of the distribution of the topside vertical electron content fraction of VTEC (hereinafter µ2). BIMF is convenient to be applied since no external data are required in practice. To evaluate its performance, we use as independent reference the STEC difference (so-called dSTEC) values directly measured from mid-latitude dual-frequency Global Positioning System (GPS) receivers that have not been used in the computation of the VTEC GIMs under assessment. It is shown that the use of BIMF improves the GIM STEC estimation compared to the single-layer assumptions. This is the case for the mapping functions used by the International GNSS Service (IGS) and Satellite-Based Augmentation Systems (SBAS). This improvement is valid not only for the UPC GIMs, up to 15% for the year 2014, but especially for the GIMs of other analysis centers, such as those produced by CODE and JPL, up to 32 and 29%, respectively.

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The Barcelona ionospheric mapping function (BIMF) and its application to northern mid-latitudes

A novel ionospheric mapping function modeling at regional scale using empirical orthogonal functions and GNSS data

The ionosphere in low-latitude regions is a major challenge for a satellite-based augmentation system (SBAS). The intense dynamics of this layer of the atmosphere in this part of the globe is one of the main reasons why Brazil discarded years ago the idea of deploying its own SBAS. In this article, we re-examine the ionospheric error bounds for a high-density ground station network in Brazil by updating previous work that assessed specific parameters for low latitudes, especially the ones related to the generation of the ionospheric error bounds expressed by the grid ionospheric vertical error (GIVE). Ionospheric delay maps and their respective GIVE values were generated by varying the functional model used in the interpolation of the grid points and the decorrelation parameter of the measurements for quiet and active days of the ionosphere. The resulting GIVE maps show values significantly higher than the bounds found in mid-latitudes and represent one of the main contributions of this article. An availability assessment for APV-I shows that the planar fit model with a spatial decorrelation standard deviation of 1 m provides the best availability for quiet days, while a quadratic fit with a decorrelation parameter of 2 m results in better availability for active days. Even though it would be possible to provide some service in quiet days within the area with a higher density of reference stations, the resulting availability is not ideal for a real SBAS when the ionosphere is more active.

Reexamining Low-Latitude Ionospheric Error Bounds: An SBAS Approach for Brazil

Satellite-based Augmentation Systems (SBAS) such as the European Geostationary Navigation Overlay Service (EGNOS) augment GPS and GLONASS, providing and guaranteeing the availability of navigation signals for aeronautical, maritime and land mobile applications. The ionospheric information provided by the augmentation system is a key issue for accuracy and integrity. However, the actual SBAS standards for broadcasting ionospheric information in L1 band might be insufficient during high solar activity or storm periods, specially in equatorial regions. The availability of a new frequency band (L5) opens the possibility for broadcasting enhanced ionospheric information. This document presents a review of concepts that improve the available ionospheric broadcast information in SBAS. Those concepts might be considered as candidates for standardization in SBAS L5. They can be classified by the kind of improvement to the system: increase of robustness, spatial resolution or temporal resolution; reduction of ionospheric mismodelling and efficient use of bandwidth. The application of compression techniques were studied for improving the bandwidth use or equivalently for the broadcasting of more detailed information using the same bandwidth. Different algorithms were designed including data compression techniques based on a semi-predictive Huffman encoding and image compression techniques using the Discrete Wavelet Transform (DWT), considering the Vertical Total Electron Content as pixels of the image. Compression ratios ranging from 1.2 to 3 were achieved for different scenarios, techniques, power/memory requirements, and residual errors.

Ionospheric information broadcasting methods for standardization in SBAS L5

In differential GPS or DGPS, it is generally considered that the ionospheric correction is the same for both reference and unknown target (or mobile or user) receivers. However, it is clear that the ray paths from the satellite to the two receivers will generally traverse the ionosphere at slightly different elevations. Any horizontal ionospheric gradient present would also introduce errors into the DGPS correction. These two effects have been investigated in the present work in order to obtain a more accurate ionospheric correction for DGPS and have been found to be roughly comparable showing that they are both important. To accomplish this, the differential delay between the two paths for different elevations and azimuths has been done using an analytical ray tracing program. By performing ray-tracing calculations with and without a linear horizontal ionosphere gradient, the effects of elevation angle and horizontal gradient have been separated and a ratio of these effects has been determined for different elevations and horizontal ionosphere gradients. Empirical models have been introduced to model these variations based on the ray-tracing results.

Analytical approximation to differenciate between the effect of ionospheric horizontal gradient and elevation angle in dgps

The performance of Satellite-Based Augmentation System (SBAS) ionospheric estimation algorithms in equatorial regions was analyzed using recorded Brazilian GPS data. Algorithms similar in design to those employed by the U.S. Wide Area Augmentation System (WAAS) were used. The results of the analysis confirm those of a previous analysis that was based on data generated with an ionospheric model. The structure of the equatorial ionosphere, which causes GPS ionospheric range delays to vary from one point in space and time to another, is much more complex than the mid-latitude ionosphere and presents a difficult challenge to the development of a single-frequency SBAS capable of supporting a high availability of vertically guided approaches. The combined effects of the equatorial slant-to-vertical conversion errors, spatial and temporal decorrelation errors, modeling and estimation errors, and the demanding integrity requirements imposed on the error bounds broadcast by an SBAS, result in large error bounds that will severely limit the availability of vertically guided approach procedures during the local evening hours near the peak of the solar cycle.

R. Lejeune

Papers

Performance of SBAS Ionospheric Estimation in the Equatorial Region