Analysis of Mid-Latitude Space Weather Events and their User Impacts

The ionosphere is one of the last remaining error sources in GPS, and its effects are largest during space weather events. In particular, space weather events in the mid-latitudes, which is where the majority of GPS users reside, can impact many practical applications of GPS. For example, gradients greater than 50 TEC units per degree have been observed near many large airports in the Northeast and Northwest continental US associated with storm enhanced density (SED). These local gradients are known to have limited the availability of the FAA's Wide Area Augmentation System (WAAS) for commercial air travel during those time periods. TEC gradients observed in 2003 resulted in positioning errors larger than 20 meters on differential GPS baselines as short as 150 km. These short-scale density TEC gradients will have consequences for both the FAA's developing Local Area Augmentation System (LAAS) and for maritime navigation. For example, marine users have horizontal positioning requirements of 2-5 meters at a 95 percent confidence level for safety of navigation in inland waterways. DGPS services are generally used to meet such accuracy requirements. In the future, accuracy demands on GPS and DGPS are predicted to increase. Applications include in-vehicle navigation systems, railway control, highway traffic management, emergency response, and commercial aviation. Service providers will be challenged to achieve improved positioning accuracies with adequate reliability.