The Need for Space Weather Monitors: Ionospheric Scintillation
As the national and global economy becomes more advanced, it exhibits a greater dependence on satellite based navigation systems such as GPS. This dependency was discussed in a recent AMS Policy Workshop Report on “Satellite Navigation & Space Weather: Understanding the Vulnerability & Building Resiliency”. The Figure above (taken from that report) indicates that precision GPS systems are now integral to many commercial enterprises including air transportation, oil exploration, road building, agriculture, surveying, shipping and transportation. GPS timing is integral to the banking and investment system. The FAA has introduced a GPS augmentation system for aviation called the Wide Area Augmentation System (WAAS) that is susceptible to GPS degradation and outage.
The ionosphere is known to have two types of detrimental effects on the radio signals used for satellite-based navigation systems such as the Global Positioning System (GPS). The first effect is the ionospheric delay experienced by the signals, which affects the estimation of range (Klobuchar 1987). The ionospheric delay depends on the integrated electron concentration along the propagation path from the GPS satellite to the GPS receiver. This integrated quantity is called the Total Electron Content (TEC). The TEC can be measured and the corresponding effects of the ionospheric delay can be mitigated, using an estimate of the phase difference between two GPS signals with closely spaced frequencies (Klobuchar 1996). This is the approach used by dual-frequency GPS receivers. Single-frequency receivers, however, must rely on independent estimates of the TEC and corresponding ionospheric delay for corrections in their position estimates. For GPS users, various agencies provide global maps of the ionospheric TEC (vertically integrated) in real-time that can be used to provide corrections to GPS positioning and navigation services. The remote configurability and low cost of CASES will allow deployment of a large number of stations, which will greatly improve the resolution of global TEC maps.
The second effect on GPS systems is characterized by fluctuations in the measured amplitude or phase of the received signals. These fluctuations are referred to as ionospheric scintillation (e.g., Fremouw et al. 1978; Yeh and Liu 1982). Scintillation is caused by electron density fluctuations in the ionosphere across the path traveled by the radio signal from the satellite to the receiver. The fluctuations in electron density cause perturbations in the index of refraction (n) of the medium (the ionosphere), which in turn cause diffraction of the radio waves. Multiple diffracted wave-fronts, with different phases, reach the antenna receiver at the same time. Because both the ionospheric irregularities causing the diffraction and the GPS satellites are moving, the receiver experiences alternating periods of constructive/destructive signal interference, and fluctuations in the phase and amplitude of the received signal develop. Ionospheric scintillation is known to affect the performance of GPS receivers at various levels ranging from degradation to denial of service (Aquino et al. 2005; Beniguel et al. 2004; Conker et al. 2003). The strongest scintillation occurs at low latitudes during solar maximum in the post-sunset hours before midnight. At high latitudes, both the auroral regions and polar cap can exhibit significant scintillation activity at all local times and during all phases of the solar cycle. Severe scintillation can result in degradation or disruption of the GPS signal, which can prevent GPS receivers from locking onto the GPS satellite signal and from determining position at all (denial of service).
The effects of scintillation are difficult to mitigate. However, the starting point is to identify for users that scintillation conditions are or will be occurring, so that users know the information from their GPS systems may not be trustworthy. Ideally, the scintillating GPS signal can be measured, but this requires the application of specialized signal processing techniques within the GPS receiver so that it does not lose track of the signal carrier and code.
The CASES receiver differs from typical GNSS receivers in two key ways: it has been specially designed to measure TEC and scintillation parameters, and special features have been implemented that allow it to operate robustly in the presence of vigorous ionospheric scintillation. Several techniques have been implemented to make the receiver particularly well suited for scintillation monitoring. These include the careful choice of parameters in the tracking loops, and a technique known as data bit prediction. The receiver implements a data bit prediction algorithm that makes operation of the receiver phase-locked loop more robust under ionospheric scintillation or similarly adverse conditions. By recording previously observed data bits, the receiver can guess with a high degree of certainty what future data bits will be (or it can calculate them, for example in the case of time-of-week data, where it is known how the data changes). This processing technique also improves performance under nominal operating conditions.
The custom hardware platform on which the software runs is very compact while still remaining quite flexible and extensible. The CASES receiver software includes several novel additions that make this instrument ideal as a space weather monitoring tool: advanced triggering techniques to accurately determine the onset of ionospheric scintillation, buffering of data to allow for observation of the onset of scintillation, data-bit prediction and wipe-off for robust tracking through scintillation, and eliminating local clock effects by differencing. These techniques have been successfully employed in scintillating environments in Peru and in the Antarctic.