Bandwidth criteria for GNSS signals

In a digital communication system, in the context of bandwidth efficiency, modulation schemes are typically compared in terms of power containment within a bandwidth such as 95% or 99% bandwidths. These bandwidths are in turn related to the average bit error rate (BER) attainable by the communication systems and also are a measure of the out-of-band interference to other systems in the neighborhood of the frequency band. Global Positioning System (GPS) is a satellite-based navigation system in which navigation solution accuracy is more important than the average BER. Currently GPS has a protected bandwidth of 24 MHz although the GPS signal itself occupies a much wider bandwidth. Just like typical digital communication systems, current GPS also is based on a non-return to zero (NRZ) formatting of the data and the spreading codes in which the signal power spectral density (psd) has a peak at the carrier frequency and follows a sine2 function away from the center. The modernized GPS signal would be employing a formatting scheme called binary offset carrier (BOC). For BOC signals the spectral peaks (there are two main peaks) occur away from the center and any bandwidth definition based on the NRZ format may not fully satisfy the need of the BOC signals. Given that GPS today has a protected bandwidth of 24 MHz. So which bandwidth should be the protected bandwidth for the modernized signals and which criterion should be used in the definition of such a bandwidth are the questions that need a careful study. Since Galileo, a European version of the GPS, which is a part of the GNSS (global navigation satellite system), is adopting some form of BOC signals for its use in the same frequency band as the GPS, a proper definition of bandwidth for the navigation signals to be protected is sorely needed. A proper definition for the bandwidth would also help setting power flux densities to and from other systems in the regulatory context. In this paper a description of some of the current and the future GNSS signals adopted by the GPS and Galileo is given, followed by a discussion on criteria that may be appropriate for use in calculating the bandwidth to be protected for GNSS signals.