GPS World, June 2009

SYSTEM DESIGN TEST Galileo L1 E5 Receiver Pulling in Wideband A direct conversion receiver architecture meets the challenges of wideband GNSS signals such as Galileos E5 signal with bandwidth greater than 50 MHz Ruediger M Weiler Paul Blunt Philip Jales Martin Unwin and Stephen Hodgart N ew generation GNSS signals can be combined to offer the use of dual frequency techniques for the general public The Galileo signal in the E5 band exemplifi es such a wideband widely available signal Modulated by the alternate binary offset carrier AltBOC 15 10 with a bandwidth greater than 50 MHz it makes receiver design challenging The direct conversion technique explained here enables a prototype dual frequency receiver We analyze its performance with live Galileo in orbit validation element GIOVE signals on the L1 and E5 band Ongoing GNSS evolution introduces more wideband signals greater than 10 MHz intended to deliver better tracking accuracy and greater robustness against code multipath These wideband signals are combined together with narrowband signals onto different carriers FIGURES 1 and 2 overview the spectral use of modernized GPS and Galileo The L1 and the L5 E5a frequency bands are common to both GPS and Galileo Not considered here are GLONASS and Compass signals but the principles and methods described apply equally The binary offset carrier BOC is used for some signals to ensure spectral separation In the accepted notation BOC f subcarrier f code both frequencies are written as a multiple of the fundamental GNSS frequency 1023 MHz The subcarrier is always selected as equal or higher than the code frequency as it specifies how far the main lobes of the signal spread out For BOC 10 5 modulation each chip of the code is modulated by this pattern FIGURE 3 shows the autocorrelation function Power RF input Gain Heritage front end p HARDWARE SET UP for the prototype direct conversion based dual frequency GNSS receiver The subcarrier spreads the spectral energy of the signal away from the center of the band leaving a null at the center frequency FIGURE 4 an important feature for direct conversion receivers The autocorrelation function shows a multi peak characteristic In addition to wideband signals using BOC modulation other signals are modulated by phase shift keying PSK with a higher bandwidth than the common C A code signal from GPS TABLE 1 summarizes the Galileo signals used in this article The E5a b signal has a bandwidth of more than 40 MHz p FIGURE 1 Spectral use of GPS open service in blue restricted services in green and red p FIGURE 2 Spectral use of Galileo open services shown in blue commercial services in green and restricted in red GPS World June 2009 www gpsworld com 12