Dual Frequency DGPS Service for Combating Ionospheric Interference

Abstract

The 11-year solar sunspot cycle is approaching a maximum in 2000. The sunspot activity causes an increase in the solar flux, charged particles and electromagnetic rays emitted from the Sun. This solar flux affects the ionosphere, thus influencing the transmission of radio waves through the ionosphere. The GPS satellite navigation system is affected in that the transit time of the signals varies, introducing position errors. Also GPS equipment may experience degraded performance in tracking of the GPS satellites due to scintillations, rapidly varying amplitude and phase of the GPS signal. The equatorial and high latitude regions are most severely affected by this increased ionospheric activity. Experience has shown that, in equatorial regions, errors of 10–20 m may be introduced in Differential GPS services, even on distances down to a few hundred kilometres from the reference station. During the peak of the solar cycle, for the next 3–4 years, this situation will rather be the rule than the exception in the affected areas. Fugro has introduced an enhanced real-time DGPS service utilising dual frequency GPS called Starfix-Plus in selected areas. This service removes ionospheric delay errors by calculating the delay using dual frequency GPS receivers on the reference stations and the mobiles. The resulting accuracy is down to a few metres, even using reference stations up to 2000 km away. This increases the availability and redundancy of usable reference stations in a region, increasing the probability that the required accuracy is available, even if individual stations are not available due to scintillations or for other reasons.