Читать книгу Global Navigation Satellite Systems, Inertial Navigation, and Integration - Mohinder S. Grewal - Страница 30

This band, which spans the frequency range from 1164 to 1214 MHz, contains two signals, denoted E5a and E5b, which are centered at 1176.45 and 1207.140 MHz, respectively. Each signal has an in‐phase component and a quadrature component. Both components use spreading codes with a chipping rate of 10.23 Mcps (million chips per second). The in‐phase components are modulated by navigation data, while the quadrature components, called pilot signals, are data‐free. The data‐free pilot signals permit arbitrarily long coherent processing, thereby greatly improving detection and tracking sensitivity. A major feature of the E5a and E5b signals is that they can be treated as either separate signals or a single wide‐band signal. Low‐cost receivers can use either signal, but the E5a signal might be preferred, since it is centered at the same frequency as the modernized GPS L5 signal and would enable the simultaneous reception of E5a and L5 signals by a relatively simple receiver without the need for reception on two separate frequencies. Receivers with sufficient bandwidth to receive the combined E5a and E5b signals would have the advantage of greater ranging accuracy and better multipath performance.

Even though the E5a and E5b signals can be received separately, they actually are two spectral components produced by a single modulation called alternate binary offset carrier (AltBOC) modulation. This form of modulation retains the simplicity of standard binary offset carrier (BOC) modulation (used in the modernized GPS M‐code military signals) and has a constant envelope while permitting receivers to differentiate the two spectral lobes.

The in‐phase component of the E5a signal is modulated with 50 sps (symbols per second) navigation data without integrity information, and the in‐phase component of the E5b signal is modulated with 250 sps data with integrity information. Both the E5a and E5b signals are available to the OS and CS services.