Galileo Signal Design: State of Art

The Global Positioning System (G.P.S.) has been a revolutionary system that opened new opportunities and enabled innovative services for both governmental and civilian applications. Rapidly, an efficient and reliable positioning system has become mandatory for public protection and security applications. As consequence Europe has realized the need to develop an independent positioning system (Galileo) with enhanced capabilities, performance and an unconditioned reliability. Galileo has pursued from the very first moment the goal of having wide band signals in all its assigned frequency bands but it was a particularly difficult task because the band E1 and L1 had already congested. It will be at the same time compatible and inter operable with American G.P.S. Compatibility refers to the ability of space based positioning, navigation and timing (P.N.T.) services to be used separately or together without interfering with each individual service or signal, and without adversely affecting navigation warfare. Interoperability refers to the ability of civil space base P.N.T. services to be used together to provide better capabilities at the user level than would be achieved by relying solely on one service or signal. To achieve this it was necessary planning special signal with particular waveform. An important aspect in designing the modulation scheme is obtain good spectral properties and suitable spectral shaping, low interference with existing G.P.S. signals, good root mean square (R.M.S.) bandwidth, good time resolution (in order to allow the separation between channel paths and to decrease the synchronization errors). The family of modulations that allowed this was the B.O.C. (Binary Offset Carrier). This paper is concerned with this modulation: it will be described their performance both the transmitter that the receiver point of view. Moreover after a brief description of Galileo signal in which will highlight the differences between the various services to be provided by Galileo itself, will be introduced the AltBOC modulation used on E5. Attention will focused on the signal processing techniques required to process the AltBOC modulation because they are much more challenging than those for traditional BPSK or even for the usual B.O.C. modulation. This stems from the extremely large bandwidth and from the complex interaction of 4 components of spreading code.