Gregory Bishop

The contribution of the protonosphere to GPS total electron content: Experimental measurements

Global Positioning System (GPS) satellites have orbital altitudes of about 20,200 km, while satellites in the Navy Ionospheric Monitoring System (NIMS) constellation are in circular orbits at heights of about 1100 km. Independent measurements of the electron content in the ionized atmosphere can be made using the radio signals from both satellite constellations. Differences between the two estimates can be related to the electron content on the GPS ray paths above 1100 km, through the tenuous plasma of the protonosphere. Results are reported from some 21 months of simultaneous observations of both GPS and NIMS transmissions at a European midlatitude station at solar minimum. It is shown that the average differences between the electron contents measured by the two systems are in broad agreement with the predictions from an earlier modeling study of the effects of the protonosphere on GPS total electron content. The expected influence of ray path / flux tube geometry and the rapid depletion and slow refilling of the protonosphere in response to geomagnetic storm activity can be seen in the averaged measurements.

The effect of the protonosphere on the estimation of GPS total electron content: Validation using model simulations

Simulated observations of total electron content (TEC) along ray paths from Global Positioning System (GPS) satellites have been used to validate the estimation of TEC using GPS measurements. The Sheffield University plasmasphere ionosphere model (SUPIM) has been used to create electron densities that were integrated along ray paths from actual configurations of the GPS constellation. The resultant slant electron contents were then used as inputs to validate the self-calibration of pseudo-range errors (SCORE) process for the determination of TEC from GPS observations. It is shown that if the plasma resides only in the ionosphere below 1100 km, then the SCORE procedure determines the TEC to a high degree of accuracy. When the contribution of the electrons in the protonosphere above 1100 km is included, the analysis results in TEC estimates that are high by some 2 TEC units (TECU) for conditions appropriate to European midlatitudes at solar minimum. However, if a restriction is placed in the analysis on use of observations equatorward of the station, then allowance can be made for the effect of the protonosphere. It is shown that with appropriate selection of the boundary for the observations, TEC can be estimated by SCORE to better than 1 TECU for the conditions of the simulation. Sample results are included from actual experimental observations using GPS to demonstrate the effect of compensation for the protonospheric plasma.

The protonospheric contribution to GPS total electron content: Two-station measurements

Results are presented from an experiment to estimate the contribution of plasma on ray paths through the protonosphere to measurements of total electron content (TEC) using Global Positioning System (GPS) signals. Simulations using the Sheffield University plasmasphere ionosphere model show that observations of GPS satellites made at two stations separated by a few degrees of latitude could involve a common ionospheric volume but very different intersection geometries of the ray paths with protonospheric flux tubes. Experimental results demonstrate that, on average, higher equivalent vertical TECs are measured on ray paths to the south than those to the north of the European midlatitude stations considered here. The observations are discussed in terms of the known asymmetries of the protonospheric flux tubes, and caution is advised in the use of thin-shell ionospheric models for precise determination of TEC or correction for its effects on GPS systems.

An application of Parameterized Real-Time Ionospheric Specification Model to regional ionospheric specification

We are currently utilizing the Parameterized Real-Time Ionospheric Specification Model (PRISM) as a platform for deriving real-time regional ionospheric specification. This application involves supporting PRISM with a limited set of real-time sensor input and obtaining from PRISM a specification of ionospheric densities over a region extending to a radius of 2000–3000 km. In the initial work on this effort we have examined both data assimilation and model validation techniques, on a regional basis. We seek improved techniques for regional ionospheric specification using primarily GPS and ionosonde data as model input and mechanisms for both after-the-fact and real-time assessment of the quality of the resulting specification. We will examine, for one region, data selection and application, data quality and accuracy, and approaches for obtaining a figure of merit on the specification product. We will present some results from a short regional test of these methods and discuss issues involved in generalizing to larger studies and other regions.