Limitation of VHF tracking caused by ionospheric correction uncertainties

An overview is given of the problems encountered at the European Space Operations Center (ESOC) in determining orbits when working with VHF tracking measurements. Comparisons between VHF ranging measurements and distances derived from S-band orbit determinations reveal larger uncertainties in the ionospheric modeling, especially in the equatorial region, if it is assumed that the differences between the two sets compared can be attributed solely to the ionospheric delay. The errors in modeling the ionospheric delay are up to 20-30% for the mid-latitude station and 30-50% for the equatorial station.