ES.2.2 INTENTIONAL INTERFERENCE
Interntional interference is by far the largest risk area; however, the planned avionics are designed to quickly recognize the onset of this threat. Assuming that sufficient resources are available to vector aircraft away from jammed regions, this threat will pose no safety risk. It can, however, create considerable disruption in traffic control and flight schedules. Methods to detect, locate, and prosecute those who intentionally jam GPS signals must be put in place to discourage such activities. Air traffic control procedures must also be established to manage affected aircraft. The study concludes that there is no credible spoofing threat and that, although real, jamming threats can be managed.
Further refinements of this analysis need to be based on specific threat definitions. The study was based on a threat the study team judged to be plausible with regard to economic and motivational characteristics. It is strongly recommended that the Department of Transportation(DOT), in cooperation with the intelligence community, establish specific threat definitions as a basis for further analysis.
Technologies are emerging that can greatly reduce vulnerability to GPS signal jamming.
Techniques that can add 40 to 50 dB of additional rejection are possible; inclusion of such capabilities would virtually defeat the jamming threat considered in this study.
ES.2.3 LARGE IONOSPHERIC REFRACTION ERRORS
Considerable concern has been expressed about the impact of increased ionospheric refraction errors caused by spatial gradients during peaks of the sunspot cycle. A reasonable model of the ionosphere was created to evaluate this effect. It was found that errors produced did not significantly alter system performance for GPS only or LAAS, but did significantly degrade WAAS. It is important to note that the WAAS results regarding the larger ionospheric errors are sensitive to the ionospheric correction methodology. According to the definitions of the hazard risk index, its risk frequency is classified as "reasonably probable" and its consequence was considered "major" because of possible safety implications. With these classifications, the risk was determined to be "undesirable." This risk can be mitigated by increasing the density of the wide-area reference sites(WRSs) and/or grid points, as well as improving the ionospheric correction algorithm. This area of WAAS ionospheric correction methodology should receive further analysis, but it is JHU/APL's judgement that the WAAS configuration can be designed to meet the needed performance so that risk becomes "acceptable." However, note that when the second civil frequency becomes available, the risk is eliminated.
ES.2.4 IONOSPHERIC SCINTILLATION
Ionospheric scintillation is most severe in equatorial regions and in the auroral region. The most likely means by which ionospheric scintillation affects GPS uses in the Continental United Sates (CONUS)is in viewing GPS satellites through these regions. The auroral region covers the northern part of Canada between 65o and 72 oN geomagnetic latitude, and the equatorial region covers zones at 15°±10°N and at 15°±10°S geomagnetic latitude. Only the northern equatorial zone is seen from the United States and only by two of the locations included in the study.
A conservative model was used to test the overall impact of including this effect in the normal system availability analysis. Its impact was to drop the availability below requirements at a few locations. Therefore, ionospheric scintillation must be considered as a risk factor. According to the definitions of the hazard risk index, its risk frequency is classified as "reasonably probable" and its effect was judged to be "minor." With these classifications, the risk is determined to be "acceptable" with FAA approval.
