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TECHNICAL ENHANCEMENTS FOR FUTURE CONSIDERATION 127 (1) The same antenna can be used. In this case the signals can be tracked to lower elevation angles. Given a typical gain fall-off from a survey-type antenna, the effective tracking limit would move from 15 degrees elevation to 5 degrees. Tracking to a 5-degree elevation angle is required for WAAS reference station sites and is beneficial for all differential GPS networks. Tracking to low elevation angles is also important when dual-frequency Y-codeless GPS is used on kinematic platforms such as aircraft, where bank angles can reduce antenna gain toward satellites at relatively high elevation angles. In addition, reducing the minimum tracking angle from 15 degrees to 5 degrees will increase the maximum tropospheric signature by about a factor of three. For high-accuracy GPS users who solve for tropospheric delays either to remove it as an error source from baseline measurements or to monitor tropospheric parameters such as water vapor content, the lower elevation tracks give about a threefold increase in accuracy. (2) In applications where the limiting error is signal multipath originating from reflectors at low elevation, the system designer may decide to exploit improved signal-to-noise ratio by specifying an antenna with more rejection at low-elevation angles. (3) Under some conditions, ionospheric variations cause a Y-codeless receiver's L2 tracking loop to slip cycles.8 Given an L2 signal with 6 dB more power, the receiver's L2 tracking loop bandwidth could be increased by a factor of two. L2 Squaring Y-Codeless Receivers This receiver recovers the L2 observables by multiplying the L2 Y-code by itself. A 6-dB increase in the GPS L2 signal causes a 12-dB signal-to-noise ratio increase in the reconstructed L2 carrier phase and pseudorange. These same benefits apply to the squaring receiver, with increased effects. Y-Code Tracking PPS Receivers For the military, a 6-dB increase in L2 signal strength would assist in direct Y-code acquisition and would improve the anti-jam margin, especially if L1 was jammed during a conflict. For example, if the power of the L2 signal is increased by 6 dB, then 6 dB in anti-jam capability could be provided to military users. The important parameter is the ratio between the power of the desired signal and the jammer power. Since the latter decreases 9 Additional information on wide-band signals is given in Appendix L.
