Assimilative modeling of ionospheric dynamics for nowcasting of HF propagation channels in the presence of TIDs

The ionospheric data assimilation algorithm called GPS Ionospheric Inversion (GPSII; pronounced gypsy) has been extended and employed to model the dynamic ionosphere, including medium-scale traveling ionospheric disturbances (MS-TIDs). MS-TIDs are characterized by periods of 10-30min. GPSII can assimilate many forms of ionospheric-related data, including ionogram data and GPS L1/L2 beacon data. For this present effort, GPSII was extended to assimilate propagation time delay, integrated Doppler shift, and angle-of-arrival (AoA) measurements of HF transmissions from known reference points (KRPs). GPSII applies a regularization technique that constrains the solver to find the smoothest 3-D ionosphere model that still reproduces the input data to within their respective errors of measurement. A companion paper documents the development of the assimilation capability for KRPs. In this paper we show test results of the model's performance in reproducing measured AoA variations in the presence of medium-scale traveling ionospheric disturbances (MS-TIDs) using near vertical incidence skywave data collected at White Sands Missile Range by the Intelligence Advanced Research Projects Activity HFGeo Program Government team. We find that using three KRPs within approximately 50km of reference/check/nonassimilated transmitters, we can reproduce the measured AoAs of the nonassimilated transmitters to within 1.9 degrees with 90% confidence even in the presence of highly dynamic MS-TIDs.