Availability benefit of future dual frequency GPS avionics under strong ionospheric scintillation

A Global Positioning System (GPS) receiver may lose carrier tracking lock to the GPS signal under deep and frequent fades due to ionospheric scintillation. The frequent loss of lock observed during a strong scintillation period from the past solar maximum can significantly reduce GPS aviation availability. However, the frequency diversity (L1 and L5 frequencies) from the future GPS is expected to mitigate scintillation impact on GPS aviation. In order to assess its mitigation effectiveness, we propose a way to generate correlated fading processes based on our definition of a correlation coefficient between fading channels. Using the correlated fading process model that we propose, navigation availability of Localizer Performance with Vertical guidance (LPV)-200 during severe scintillation is parametrically studied. We then present results showing that high navigation availability is attainable if a receiver reacquires the lost channel within 1 or 2 seconds. Based on this result, we propose a new performance requirement for the future dual frequency GPS aviation receiver performance standards to guarantee high navigation availability during strong scintillation. strong scintillation are much shorter than 30 s and are actually shorter than 1 s. In order to provide high navigation availability under strong scintillation, we suggest an additional requirement for a future dual frequency MOPS. The new requirement that we propose is, "For satellite signal outages of 1 second or less ... the equipment shall reacquire the satellite within 1 second from the time the signal is reintroduced [17]." If a receiver is propagating its tracking loop for brief outages instead of going through its full reacquisition procedures, it can immediately retrack the lost channel when the signal comes back. Hence, fast reacquisition after a brief outage is technically possible. However, if a receiver performs extensive safety checks before reintroducing the lost channel into position solutions, the reacquisition time can be much longer. Therefore, this new proposal needs to be evaluated against RFI threats as well. This new requirement was proposed at the RTCA Special Committee-159 (GPS) Working Group-2 (GPS/WAAS) Meeting on 24 June 2009.