An improvement to ionospheric delay correction for single-frequency GPS users - the APR-I scheme

For the commonly used GPS wide-area augmentation systems (WAAS) with a grid ionospheric model, the efficient modelling of ionospheric delays in real t ime, for single-frequency GPS users, is still a crucial issue which needs f urther research. This is particularly necessary when differential ionospher ic delay corrections cannot be broadcast, when users cannot receive them, o r when there are ionospheric anomalies. Ionospheric delays have a severe ef fect on navigation performance of single-frequency receivers. A new scheme is proposed which can efficiently address the above problems. The robust re currence technique is based on the efficient combination of single-frequenc y GPS observations by users and the high-precision differential ionospheric delay corrections from WAAS. Its effectiveness is verified with examples.