Spread Spectrum Codes for GPS L5

Abstract

The Global Positioning System (GPS) is a spread-spectrum system that employs direct-sequence spreading of the spectrum to achieve better ranging accuracy. Although GPS was originally conceived for use by the military in their missions, it has become an important utility for a number of civil and commercial applications. To facilitate improvement in the use of satellite navigation by the civilian users, for example aviation and terrestrial transportation systems, an additional signal called L5 is being added to the current GPS L1 signals. L5 is also a spread spectrum signal in which the spreading code rate is ten times the coding rate of the current coarse acquisition (C/A) codes on L1. Also, the code period is ten times longer than the L1 C/A codes. Higher coding rates and longer code periods are expected to help overcome some of the shortfalls of the C/A codes. An important consideration in the design of a spread-spectrum signal for code division multiple access (CDMA) applications such as in GPS is to select codes to minimize what is known as CDMA noise. The code selection process can be very tedious, depending on the number of codes needed and the number of codes available in a given code set. Code balance, autocorrelation sidelobe peak, cross-correlation peak, and spectral line distributions are some of the measures available in the code selection process, and these measures are not necessarily independent. In this paper we give the details of the code properties and the selection process used in our study. The suggested process can be applied to code selection in any other spread spectrum system.