Improving the real‐time ionospheric determination from GPS sites at very long distances over the equator

Abstract

As the authors have shown in previous work, the tomographic approach reduces significantly the mismodeling in the electron content determination using as data the ionosphere‐crossing radio signals of global navigation satellite systems (GNSS) such as the Global Positioning System (GPS). We present in this paper the results of a comprehensive study on the performance of an improved tomographic model of the ionosphere. This model is obtained from the GPS L1 and L2 carrier‐phase data, and it is used to estimate undifferenced and double‐differenced ionospheric corrections in real time and at very long distances between receivers (500–3000 km). The key point of the strategy is the combination of real‐time geodetic and ionospheric techniques to achieve a significant improvement in the reliability of carrier‐phase ambiguity resolution. This new approach includes also the use of smoothed pseudo‐ranges to help in ambiguity resolution. It has been tested under difficult ionospheric conditions, during four consecutive weeks in March–April 2001, at Solar Maximum, and at latitudes ranging from −40 to +40 degrees, so as to include the equatorial region. Dual‐frequency satellite altimetry from TOPEX‐Poseidon has been used to verify the accuracy of the ionospheric model over the oceans in that region.