Session F2: Atmospheric Effects on GNSS

Ionospheric TEC Estimations Using Single-Frequency Wideband Low Elevation GNSS Signals
Madeline C. Evans, Brian Breitsch, and Jade Morton, University of Colorado Boulder
Date/Time: Wednesday, Sep. 18, 2:35 p.m.

Peer Reviewed

This study presents the use of wideband, low elevations satellite signals to retrieve ionospheric Total Electron Content (TEC) from GNSS receiver ground station data. TEC is a valuable measurement for space weather analysis and GNSS precision and accuracy purposes. Ionospheric conditions are one of the most unpredictable factors affecting Global Navigation Satellite System (GNSS) positioning, thus ionospheric monitoring is imperative for measuring and predicting the impacts on both space-based and ground-based systems during space weather events. Ground-based monitoring using GNSS signals is advantageous for high spatiotemporal resolution monitoring due to the ubiquitousness of GNSS signals [1]. However, due to lack of GNSS ground-station receivers in high latitudes, insufficient measurements are available for highly accurate ionospheric monitoring. Consequently, global ionospheric models (GIMs), which primarily use GNSS-derived TEC estimates, have degraded performance in these vital regions in comparison to high-precision TOPEX/Jason measurements [2]. Utilizing low elevation signals can extend measurement in high-latitude regions. In polar regions, the scarcity of ground stations is compounded by the lack of high-elevation angle GNSS signals, consequent from satellite orbit inclination angles. This is exemplified in Fig. 1, which shows a heat map of received GPS signals as a function of latitude and satellite elevation angle. High-latitude stations do not have access to high elevation satellites due to the limitation of GNSS satellite orbit inclination angles. Elevation masks are commonly used to limit the use of data at elevation angles below approximately 20 degrees where multipath and carrier phase cycle slips frequently occur. As a result, TEC coverage is even more sparse in the polar regions.
This study presents a method to utilize wideband low elevation GNSS signals to obtain precise TEC estimations and circumvent this large limiting factor to GNSS ground station coverage. By utilizing measurements in the range of elevation of 3 to 20 degrees, we could increase the amount of ionospheric data in certain latitude bands by approximately 2-times the current amount, compared to a 20-degree elevation mask.
This study uses data collected from GNSS ground stations deployed by the University of Colorado in Haleakala, HI (20.71°N latitude) and Toolik Lake, AK (68.63°N latitude). GPS L1 C/A, L2 C, and L5 pseudorange data from these stations are utilized as a case study for single-frequency wideband TEC estimates. In order to calculate TEC using single-frequency observables, several factors need to be assessed, such as receiver clock error, GNSS satellite orbit error, tropospheric propagation error, and receiver position. The receiver clock error can be accurately determined by using higher-elevation signals, while satellite orbit information can be derived from ephemeris data products provided by the International GNSS Service (IGS). Tropospheric error can be measured using established models, and the position of the GNSS ground monitoring station is surveyed beforehand. Single-track TEC estimates found using both a dual-frequency narrowband and our single-frequency wideband method are exhibited in Fig. 2. Notably, the noise level improvement for single-frequency wideband TEC estimates compared to dual-frequency estimates are very distinct at low elevation angles. A comparison of average TEC estimate noise using various TEC estimate methods over one week of satellite passes over the Hawaii station is displayed in Fig. 3. This figure shows that the noise levels of single-frequency wideband-derived TEC are reduced by a factor of ~7 at low elevation angles and ~3 at high elevation angles, compared to dual-frequency-derived TEC. Results from the high-latitude station in Alaska will be included in the final paper. Overall, this technique could enhance the impact of GNSS ground stations to further extend their monitoring capabilities over previously unmonitored regions. This is in effort toward improving the detection of ionospheric irregularities at high latitudes, contributing to a heightened understanding of ionospheric dynamics.
The paper will present the detailed methodology we developed to utilize single-frequency wideband signals to estimate TEC and comparison with dual-frequency TEC estimation results. The method performance using data from our Hawaii and Alaska stations collected over a range of solar/geomagnetic conditions will also be assessed. Approaches to remove TEC bias will also be discussed.