Dual frequency DGPS service for combating ionospheric interference

The 11-year solar sunspot cycle is approaching a maximum in 2000. The sunsp ot activity causes an increase in the solar flux, charged particles and ele ctromagnetic rays emitted from the Sun. This solar flux affects the ionosph ere, thus influencing the transmission of radio waves through the ionospher e. 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 s cintillations, rapidly varying amplitude and phase of the GPS signal. The e quatorial and high latitude regions are most severely affected by this incr eased ionospheric activity. Experience has shown that, in equatorial region s, errors of 10-20 m may be introduced in Differential GPS services, even o n distances down to a few hundred kilometres from the reference station. Du ring the peak of the solar cycle, for the next 3-4 years, this situation wi ll rather be the rule than the exception in the affected areas. Fugro has i ntroduced an enhanced real-time DGPS service utilising dual frequency GPS c alled Starfix-Plus in selected areas. This service removes ionospheric dela y errors by calculating the delay using dual frequency GPS receivers on the reference stations and the mobiles. The resulting accuracy is down to a fe w metres, even using reference stations up to 2000 km away. This increases the availability and redundancy of usable reference stations in a region, i ncreasing the probability that the required accuracy is available, even if individual stations are not available due to scintillations or for other re asons.