Robust Detection of Ionospheric Irregularities

The Wide Area Augmentation System (WAAS) will provide real-time differential GPS corrections and integrity information for aircraft navigation use. The most stringent application of this system will be precision approach, where the system guides the aircraft to within a few hundred feet of the ground. Precision approach operations require the use of differential ionospheric corrections. WAAS must incorporate information from reference stations to create a correction map of the ionosphere. More importantly, this map must contain confidence bounds describing the integrity of the corrections. The confidence bounds must be large enough to describe the error in the correction, but tight enough to allow the operation to proceed. The difficulty in generating these corrections is that the reference station measurements are not co-located with the aviation user measurements. For an undisturbed ionosphere over the Conterminous United States (CONUS), this is not a problem as the ionosphere is nominally well behaved. However, a concern is that irregularities in the ionosphere will decrease the correlation between the ionosphere observed by the reference stations and that seen by the user. Therefore, it is essential to detect when such irregularities may be present and adjust the confidence bounds accordingly. The approach outlined in this paper conservatively bounds the ionospheric errors even for the worst observed ionospheric conditions to date, using data sets taken from the operational receivers in the WAAS reference station network. As we progress through the current solar cycle and gather more data on the behavior of the ionosphere, many of our pessimistic assumptions will be relaxed. This will result in higher availability while maintaining full integrity. INTRODUCTION The nominal or quiet ionosphere above the Conterminous United States (CONUS) is smooth and easily estimated. However, there are times when the ionosphere is more difficult to describe, particularly during geomagnetic and ionospheric storms. Under disturbed conditions smaller scale features may be difficult to observe or estimate. The Wide Area Augmentation System (WAAS) must correct for the users’ ionospheric delay errors and place strict confidences on those corrections under all conditions, but its measurements are not co-located with the users’. Therefore, we have to translate the knowledge we gain through our measurements, from their locations, to any possible user location. The mechanism specified in the WAAS Minimum Operational Performance Standards (MOPS) [1] is the vertical ionospheric delay grid. The MOPS specifies transmission of vertical delay values and confidences at discrete grid locations. These confidences must bound the errors not only at the grid locations, but for all interpolated regions between the grid points. In addition we must bound the errors both for the nominal and the disturbed ionosphere. If we cannot distinguish between nominal and disturbed conditions, then we must always assume disturbed conditions are present. Instead, we would prefer to detect ionospheric irregularities so that we can provide a high level of service during nominal periods. A reduced level of service would only be necessary during periods of detected disturbances. The detection scheme must be extremely robust in order to provide the necessary level of protection. The integrity requirements for precision approach guidance set the probability of hazardously misleading information below 10-7 per approach. Therefore, the chance of an undetected ionospheric irregularity must be at a similarly small level. N um be r of P oi nt s pe r P ix el 10 0 10 1 10 2 10 3 10 4 0 50