WAAS utilization of the new civil signal at L2

GPS modernization not only adds an additional new GPS frequency, L5, in an Aeronautical Radio Navigation Service (ARNS) band but also adds the civil code to L2. Adding the civil code to L2 will improve the use of GPS for civilian applications because it eliminates the need for the fragile semi-codeless tracking technique now used to acquire L2 measurements. The ionospheric delay is inversely proportional to the square of signal frequency. Therefore, the ionospheric delay at L2 is 65% larger than at L1, and at L5 it is 80% larger than L1. While L2 outperforms L5 in ionospheric delay, L2 is not in an ARNS band and FAA currently does not plan to use its civil signal for aviation applications. However, in order to maximize the civil signal benefits to WAAS, there is ongoing discussion to move L2 to 1278.75 MHz (on top of the Galileo E6). This paper investigates the possible use of L2 for civil aviation applications. The design evaluation criterion compares the coverage of availabilities versus the Vertical Alert Limit (VAL) under two cases: L2-only single frequency GPS/WAAS user and L2-L5 dual-frequency GPS/WAAS user. As discussed in[1], a dual-frequency user can directly estimate the ionospheric delay in the airplane, and then subtract this estimation from the pseudorange measurement. In general, this direct use of dualfrequencies should be more accurate and offer higher availability. However, when a three-frequency user loses the L1 frequency to Radio Frequency Interference (RFI), the most worrisome dual-frequency user case, L2-L5, is introduced. This paper shows why the direct use of the L2-L5 dual-frequency to estimate the ionospheric delay does not provide higher availability. This paper also discusses the changes in the protection level calculation for an L2-L5 dual-frequency user, and then this paper shows the availability simulation results for an L2-L5 dual-frequency user in the Conterminous United States (CONUS). This paper then investigates the effect of moving L2 to the Galileo E6 frequency (1278.75 MHz). This move will increase the separation between the L2 and L5 frequencies, and the L2-L5 dual-frequency ionospheric delay estimation will therefore be improved. The evidence suggests that the new L2 at Galileo E6 frequency (1278.75 MHz) should be used for civil aviation application. However, it will be very difficult to form an international agreement to assign L2 into the international ARNS band.