Overcoming ionospheric scintillation for worldwide GPS aviation [electronic resource]
Keywords: Aeronautics and Astronautics

Together, the Global Positioning System (GPS) and the Wide Area Augmentation System (WAAS) can guide aircraft down to 200 ft above the runway. This approach procedure is referred to as Localizer Performance with Vertical guidance (LPV)-200. Approach guidance using GPS and WAAS has many advantages over traditional precision approach guidance using the Instrument Landing System (ILS) and the number of published LPV approaches in the U. S. is now greater than the number of published Category I ILS approaches. Given the success of LPV service in the U. S., worldwide expansion of the LPV service is being actively pursued. One of the main challenges for the LPV service in the equatorial area, including Brazil and India, is ionospheric scintillation. Due to electron density irregularities inside the ionosphere, transionospheric radio waves interfere constructively and destructively; consequently, a GPS receiver can experience deep and frequent signal fading. This phenomenon is known as ionospheric scintillation. Although the physics of the ionospheric scintillation has been studied for several decades, its impact on GPS aviation has not been well understood. As a result, the current GPS aviation receiver performance standards (RTCA/DO-229D) do not have any specific performance requirements to mitigate the impact of scintillation. Although current aviation receivers do not protect against scintillation, currently it is not a problem because they are only used in the mid-latitude area where scintillation is not usually observed. The advent of a new GPS civilian signal at the L5 frequency and dual frequency airborne receivers will eliminate ionospheric delay errors in the equatorial and arctic regions, but it will not solve the scintillation problems that will arise in those areas. This dissertation analyzes and proposes a solution to scintillation for the dual frequency era so that the LPV service will become practical worldwide. Specifically, this research investigates the characteristics of GPS signal fades due to scintillation from the perspective of GPS aviation. Using high rate GPS data collected during the past solar maximum, scintillation impact on the operational availability of LPV-200 is analyzed. In order to mitigate scintillation impact, satellite-to-satellite correlation and frequency-to-frequency correlation of GPS signal fades under strong scintillation are investigated. Based on the observed data and availability analyses, a new requirement for the future dual frequency aviation receiver performance standards to mitigate the impact of scintillation is proposed. This new requirement mandating fast reacquisition after a very brief outage is under discussion at the standards committee working group (RTCA Special Committee-159, Working Group-2). Without this requirement, a certified aviation receiver may provide less than 50% availability of LPV-200 during severe scintillation. It is demonstrated that with this new requirement in place, it will be possible to achieve high level of availability (more than 99% availability) even during severe scintillation.