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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 88 (4) The signal should optimally be spaced for ionospheric correction and wide lane ambiguity resolution. The NRC committee determined that ideally, the new GPS signal should be on an L-band frequency sufficiently offset from L1 to permit user correction of ionospheric delay, which would improve user accuracy yet be close enough to L1 to allow fast, wide-lane cycle ambiguity resolution, also termed wide-laning.19 For adequate ionospheric correction, the separation between L1 and a new frequency should be at least 200 MHz.20 For optimal wide-lane ambiguity resolution, the frequency difference between L1 should be no greater than 350 MHz. (5) The signal should occupy a wide frequency band The signal should occupy a wide frequency band, that is, around 10 MHz, to reduce the effects of multipath and improve resistance to unintentional RF interference. A wide-band signal has two main advantages over a narrow-band signal.21 First, use of a wide-band signal allows about a 10-dB improvement in interference rejection over a narrow-band signal. This is significant for both stand- alone and differential users needing improved availability in the presence of wide-band or continuous wave interference. The second advantage is that upon signal reacquisition, a wide-band signal can recover submeter pseudorange accuracy faster than a narrow-band signal in both low- and high-multipath environments. For example, as discussed in Appendix G, in a high multipath environment, such as around buildings, a narrow-band signal will have an error larger than a wide- band signal after signal reacquisition. Many important real-time vehicular applications, such as aircraft precision approach and land vehicle guidance, would benefit from the faster accuracy recovery obtained with a wide-band, faster chipping-rate signal. New Signal Structure Options Ten signal structure enhancements to the current GPS signal structure were considered and are described in Appendix H. Each option involved possible changes to L1 or L2, as well as possible signal transmissions on a new frequency. Using the previously 19 Wide-lane ambiguity resolution (wide-laning) is a processing technique developed by civilian DGPS users to process carrier phase data. With wide-laning, the two carrier frequencies are mixed to provide a difference frequency of about 45 times longer wavelength, improving the speed and reliability of cycle ambiguity resolution. The wide-laning technique is available to cross-correlation types of receivers today, but at a serious loss in effective carrier-to-noise ratio as compared with a dual-frequency code-tracking receiver. 20 Letter from J. A. Klobuchar, U.S. Air Force Geophysics Laboratory, 22 December 1994. 21 A wide-band signal is generally defined to be around 20 MHz wide; a narrow-band signal around 2 MHz wide.
