RETRIEVAL OF REFRACTIVITY PROFILE WITH GROUND-BASED RADIO OCCULTATION BY USING AN IMPROVED HARMONY SEARCH ALGORITHM
Mu-Min Chiou, Jean‐Fu Kiang · Progress In Electromagnetics Research M · 2016
A ground-based radio occultation (RO) technique is proposed to retrieve the atmospheric refractivity profile around a specific region at a higher sampling rate than conventional space-based RO techniques, making it more suitable for regional weather studies.A harmony search (HS) algorithm with ensemble consideration (HS-EC) based on atmospheric physics is proposed to retrieve the refractivity profile more efficiently without being trapped in suboptimal solutions.The highest altitude of profile is extended to 95 km from 40 km adopted in conventional ground-based RO techniques, leading to more accurate results. INTRODUCTIONThe height profile of refractivity used to be retrieved by using space-based radio occultation (RO) technique [1,2], where excess phase-paths of GPS signals received at low-earth orbit (LEO) satellites and the bending angles of ray paths were used to reconstruct a height profile of refractivity via Abel transform [3,4].Global coverage of refractivity profiles is useful to predict weather conditions in the troposphere and the lower stratosphere [5,6], to study heavy precipitation [7], Arctic atmosphere [8], severe weather events [9, 10], and over-the-horizon communication channels [11].The space-based RO technique becomes less accurate in the lower troposphere, especially when a super-refraction (SR) layer is present [12], which happens frequently above sea surfaces [13].Currently available space-based RO missions can provide only relatively sparse data for any specific region of interest [14], which is not sufficient for regional weather studies.A tomography technique was applied to retrieve meteorological profiles in the atmosphere by deploying a dense network of GPS receivers [15].However, to monitor the temporal and spatial variations around a specific region demands even more receivers [16].Ground-based RO technique was proposed for regional weather studies, which demanded a high sampling rate [14,17].A practical ground-based RO technique can be implemented at a specific site, by installing a receiver on building roof, vehicle or ship [18].A ray-tracing model was applied to retrieve the atmospheric refractivity profile from the measurement data of GPS tropospheric delays [19].In [20], a three-level model was proposed to characterize the ducting conditions near coast.Temporal and spatial variations in temperature, pressure and water vapor create a volatile refractivity profile, making its retrieval a challenging task.In [21], GPS signals at a specific ground-based receiver were used to retrieve height profile of maritime refractivity by using an artificial neural network technique, which was validated by measurement data on the coast of the Yellow sea.In [18], an exhaustive search (ES) method was applied to retrieve the refractivity profile from the boundary layer up to 10 km above ground, with data from a single ground-based GPS receiver.The computational time of the exhaustive search method grows exponentially with the number of