The impact of the second civil frequency will completely remove the requirement for ionospheric corrections for users equipped to take advantage of this feature, and it will improve the corrections provided by WAAS. If, at some future time, the full community were to shift to dual-frequency user equipment, the WAAS ground station requirements could be reduced significantly. The density of WAAS reference stations required for ionospheric correction is greater than that required for orbit determination or for integrity monitoring. Furthermore, the second civil frequency, and the proposed higher signal power, will mitigate interference concerns.
ES.2 RISKS
The only risks that proved significant are interference (unintentional and intentional) and ionospheric propagation effects (high sunspot cycle and scintillation); these risks are discussed in the following subsections.
ES.2.1 UNINTENTIONAL INTERFERENCE
Although there have been few reports of GPS receiver interference from the many Government and commercial transmitters currently operating in the NAS, a review of interference sources identified in RTCA DO-235 indicates that several have the potential for GPS signal disruption. Three potential interference sources were singled out for further analysis. The first and potentially most serious one is television broadcast. The current Federal Communications Commission (FCC) specifications allow out-of-band emissions of sufficiently high levels to interfere with GPS signal reception. A simulation effort, undertaken to evaluate television emissions, indicated that stations transmitting on channel 23 within line of sight of aircraft approach paths could readily deny GPS signal reception. However, this threat is easily managed by modifying television broadcast regulations to exclude unacceptable power levels in satellite radio-navigation bands, by testing for interference when FAA instrument approaches are first established, and by adding filtering to the television transmitter output that are found to interfere with GPS reception.
The second area of concern is commercial very high frequency (VHF) broadcast (e.g.,taxi dispatch). The levels of power and typical antenna configurations restrict this threat to small regions near runways. VHF broadcast interference would also be managed by the same measures indicated for television broadcast.
The third possible threat is from over-the-horizon (OTH) military radar. OTH radar interference was not analyzed because insufficient information was available during this study. This threat is very restricted with regard to number and geography; therefore, it is not expected to be a significant risk. However, it is recommended that this emission source be further reviewed to ensure the risk is truly insignificant.
In summary, unintentional interference is not a major risk factor. Most interference difficulties reported by the aircraft community thus far have been the result of onboard interference, which is necessarily resolved during certification. While it is not possible to rule out future interference from offboard emitters, remedying such problems should not be difficult. The introduction of a second civil frequency will further reduce concerns about unintentional interference. Furthermore, the actions required to counter intentional interference will readily address this risk.
