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Severe Space Weather Events—Understanding Societal and Economic Impacts: A Workshop Report
GLOBAL POSITIONING SYSTEMS AND AVIATION
The FAA is in the process of transitioning the National Airspace System to utilize space-based navigation as the primary means of navigation. This transition is part of an overall modernization of the National Airspace System to implement integrated Communications Navigation and Surveillance (CNS). CNS services required by the FAA for aviation are provided partially by the FAA and partially by private sector operators. One way of achieving navigation is with GPS and augmentation systems. In his presentation, Thomas McHugh noted that the use of GPS for CNS is an evolving process with several different approaches, each offering advantages and challenges. Surveillance services are planned as part of the Automatic Detection and Surveillance-Broadcast system (ADS-B), and an integrated CNS service is planned through the Next Generation Air Transportation system (NextGen).
CNS is vulnerable to space weather: accuracy and integrity can be lost for non-augmented single-frequency GPS users, and availability can be lost for augmented single-frequency GPS users. All GPS users are vulnerable to loss of availability during extreme events such as radio-frequency interference from solar radio bursts and loss of reception of many or all GPS signals due to scintillation. Additional threats to robust CNS include loss of high frequency for oceanic reporting and disruption of the national power and telecommunications infrastructure during an extreme event. As McHugh noted, “The vulnerabilities to CNS are down in the ionosphere.” These vulnerabilities are mitigated by new signals and codes for the modernized GPS system, backup navigation systems, and autonomous navigation systems.
Space weather vulnerabilities depend critically on the type of navigation employed, which can be divided into two broad categories, non-precision and precision. Non-precision navigation requirements are looser and apply in operations less vulnerable to space weather, whereas precision navigation requirements, used in landing and approach procedures, are strict, and the availability of navigation services is exchanged for safety.
The ionosphere is the primary source of error for users of single-frequency non-augmented GPS, which uses the Klobuchar model7,8 to correct ionospheric ranging errors; frequently these corrections are in error. Since accuracy is degraded during even minor ionospheric events, this technology can be used only for non-precision applications. This technology is also vulnerable to scintillation, which causes temporary loss of GPS reception and affects the availability of Receiver Autonomous Integrity Monitoring (RAIM), which can be interrupted by the loss of even a small number of satellite signals. As discussed by McHugh, certification of aviation technology requires 10−7 probability of not providing misleading information, and “it is extremely difficult to certify the ionosphere.”
Augmented users are less vulnerable to minor and moderate ionospheric disturbances but still can be affected by scintillation, solar radio bursts, and major ionospheric disturbances. The primary source of augmentation over the continental United States, Alaska, and Hawaii is the Wide Area Augmentation System (WAAS). WAAS disables the use of precision navigation in areas affected by ionospheric disturbances and does so using internal detection of the disturbances so that safety is never compromised. When large areas of disturbance are detected, precision navigation is disabled for all areas until 8 hours after disturbances cease. During the October and November 2003 magnetic storms, WAAS was disabled throughout the service area for 30 hours, and similar impacts are expected during the next solar maximum. McHugh expects that for the next solar maximum there will be four or five storms that will lead to widespread outages and that there will be shorter, regional outages “for probably the top 20 storms of the cycle’”
Solutions for the Future
The FAA approach to mitigating space weather impacts is in part to implement new GPS signals and codes and in part to maintain backup systems. Starting with the GPS Block IIF satellites, a new L5 civil GPS signal will be transmitted in an aviation-protected frequency band. The L5 signal along with the L1 civil signal allows GPS receivers to estimate and remove ionospheric errors, a capability that will mitigate the problems with the Klobuchar and SBAS (Satellite-based Augmentation Systems) thin shell models such as WAAS. In addition, the L5 signal design is more robust than the L1C/A signal and will help mitigate unintentional interference. Hegarty stated that