Ionospheric and geomagnetic conditions during periods of degraded GPS position accuracy: 1. Monitoring variability in TEC which degrades the accuracy of Real-Time Kinematic GPS applications

The Real-Time Kinematic (RTK) positioning technique, providing centimetre-level accuracy, is most vulnerable to the ionospheric irregularities having a size comparable with the distance between the reference station and the users. In practice this distance is of order of 10–20 km. These irregularities can severely degrade the position accuracy. To monitor and study these smaller-scale ionospheric disturbances, a new method is developed, using the GPS derived TEC. The method calculates time derivatives (rate of change) from successive TEC values taken from individual satellites, de-trend their variations in 15 min intervals with low order polynomial and calculates the standard deviation from residuals. This standard deviation is a measure of amplitudes of ionospheric irregularities with characteristic period of 5–10 min and wavelength of 30–60 km. By changing the time of accumulation, the method becomes sensitive to smaller or higher scale irregularities. The standard deviation is quantified in nine grades and characterizes the level of disturbance, named RTK ionospheric intensity. The RTK ionospheric intensity above a define level is called RTK ionospheric event. It is found that the RTK ionospheric intensity has well expressed diurnal, seasonal and solar cycle occurrence. The probability of degraded positioning accuracy increases in morning hours in winter at high solar activity. It is shown that the RTK ionospheric intensity can be used as an effective tool in studying the smaller-scale ionospheric disturbances.