Combining Pseudoranges from GPS and Loran-C for Air Navigation

User equipment that combines pseudoranges from GPS and Loran would be more reliable than an unaided GPS receiver or a Loran-only receiver. It could remove coverage outages caused by satellite shutdowns and/or bad satellite geometry. Similarly, it could remove most of the coverage outages caused by Loran station shutdowns, high atmospheric noise levels, or precipitation static. A hybrid GPS/Loran receiver could also reliably provide self-contained fault detection and isolation (otherwise known as receiver autonomous integrity monitoring, or RAIM). This paper treats position fixing, receiver autonomous fault detection, and receiver autonomous fault isolation as system services, and computes the probability of outage for these three services. Our analysis computes the outage probability for the level of service required for nonprecision approach and en route navigation in the National Airspace System. It assumes that GPS is configured in the Optimal 21 Satellite Constellation, and includes the effects of satellite as well as Loran station shutdowns. Finally, our analysis examines whether it is important to synchronize Loran transmissions to GPS time.