A Method in Determining Ionospheric Total Electron Content Using GNSS Data for non-IGS Receiver Stations

Ionospheric total electron content (TEC) is an essential parameter in the study of space weather. With the advent Global Navigation Satellite System (GNSS) technology, ionospheric TEC can be measured using dual-frequency GNSS receivers. TEC can be estimated along the signal path connecting the satellite and receiver. However, an unknown instrumental bias called the differential code bias (DCB), is inherent upon measurements which causes errors on TEC calculations. These are usually in the form of biases in TEC values. The International GNSS Service (IGS) provides these biases but only for its own network. In this study, a method for determining the unknown TEC bias for stations outside the IGS network is presented. This is done by first determining a stable absolute minimum TEC on a given day provided by the Global Ionosphere Maps (GIM). The TEC derived from GNSS is then matched with the absolute TEC from GIM at a given location. The difference between the GNSS TEC and GIM TEC will be used to estimate the TEC bias. This study used GNSS raw data from selected six IGS stations across different latitudes in 120°E longitude sector during five most quiet and five most disturbed days for every month of 2015. This is to evaluate the performance of the proposed method that will be used for non-IGS stations across different latitudes, month, and geomagnetic activity. Correlation results show that the proposed method is strongly correlated (R > 0.990) with the standard method of calculating absolute TEC using the DCBs from IGS. The monthly absolute mean differences and root mean square differences (RMSD) are both < 7 TECU, respectively. It is also observed that the absolute mean differences and RMSDs are higher for stations in the low-latitude and equatorial regions. This is due to the high TEC gradients in these regions. For example, PIMO has the highest yearly mean difference (~2.7 TECU) and RMSD (~3.1) for both quiet and disturbed days while (insert stations statistics). This shows that the proposed method can provide a reliable result in calculating TEC regardless of the station's latitude, month, and geomagnetic activity.