Spreading and Overlay Codes for the L1C Signal

摘要

The creation of the new L1C GPS signal presented the opportunity to choose both a family of spreading codes and an associated family of overlay codes. This paper describes the rationale and construction of these families. The families were created from extensive searches with each search requiring its own fine-tuned techniques and search criteria. The L1C spreading codes comprise 210 pilot/data pairs of length-10230 sequences. The data code modulates the data message bits while the pilot code modulates the overlay code symbols, which represents a fixed repeating pattern. The codes are perfectly balanced and exhibit good auto- and cross-correlation (both in the 'odd' case, when there is a bit transition across the code boundary, and the 'even' case when there is no such transition). The length 10230 precluded the immediate adaptation of well-known spreading code families, such as Gold codes. Instead, the relatively new Weil sequence construction was adapted. Weil codes are prime length sequences constructed via shift-and-add from the well-known Legendre sequence and one of its shifts. Weil code correlation sidelobes are bounded by twice the square root of the length, which is no worse than 3 dB from commonly used Gold codes.