The performance of GPS, namely vertical guidance accuracy near the runway threshold and the integrity of the system have not been able to match historical ICAO standards and practices. Greater GPS accuracy could be provided by sending out "correcting signals" from ground-based stations, which would improve the accuracy to about 10 m in the worst case, far outperforming MLS. Initially it was planned to send these signals out over short-range FM transmissions on commercial radio frequencies, but this proved to be too difficult to arrange.