Assessing and mitigating the effects of the ionospheric variability on DGPS

The differential GPS (DGPS) performance can be affected by the ionospheric variability, especially in low-latitude areas. We evaluated the behavior of the ionospheric variability in low-latitude areas in the 11-year cycle of the solar activity. During periods of solar maximum, the percentage of the daily maximum gradient that exceeds 50 mm/km has reached 73 % in 2001. Assuming that the baseline length is 20 km, the gradient larger than 50 mm/km can lead to more than 1.0-m ranging error, which is significant if we want to achieve meter-level accuracy with DGPS. An ionospheric gradient model built from a number of reference stations is proposed for estimating the differential ionospheric delays. The effectiveness of the model is demonstrated under both the quiet and active ionospheric conditions in low-latitude areas. For a short baseline length of 16.9 km in Hong Kong, the horizontal positioning accuracy can be improved by 41 and 61 % during active solar years and geomagnetic storms, respectively. The improvements show that the model is capable of reducing the spatial decorrelation caused by the ionospheric variability, and the model can be used in DGPS especially over the regions, such as low-latitude areas, where the large ionospheric variability happens frequently.