IS-GPS-800A - US Coast Guard Navigation Center

The above Legendre sequence is used to construct the unique length-10223 sequence used for each ranging code.This sequence, called a Weil-code, is the exclusive-or of L(t) and a shift of L(t). A Weil-code W i (t ; w) is specifiedby Weil Index w, ranging from 1 to 5111, which represents the shift of L(t) and is defined as,W i (t ; w) = L(t) L( (t + w) modulo 10223 ) for t = 0 to 10222.Assignment of Weil Index, w, by PRN signal number, i, is given in Table 3.2-2.Finally, the ranging code is constructed by inserting a fixed expansion sequence into the Weil-code. The expansionsequence is composed of seven bit values 0 1 1 0 1 0 0. The insert point is specified by Insertion Index p, where p =1 to 10223. The expansion sequence is inserted before the p th value of the Weil-code.Thus the ranging code L1C Pi (t)/L1C Di (t) is defined as,L1C Pi (t)/L1C Di (t) = W i (t ; w), for t = 0, 1, . . . , p – 2;L1C Pi (t)/L1C Di (t) = 0, for t = p – 1;L1C Pi (t)/L1C Di (t) = 1, for t = p;L1C Pi (t)/L1C Di (t) = 1, for t = p + 1;L1C Pi (t)/L1C Di (t) = 0, for t = p + 2;L1C Pi (t)/L1C Di (t) = 1, for t = p + 3;L1C Pi (t)/L1C Di (t) = 0, for t = p + 4;L1C Pi (t)/L1C Di (t) = 0, for t = p + 5;L1C Pi (t)/L1C Di (t) = W i (t – 7 ; w), for t = p + 6, p + 7, . . ., 10229.Assignment of Insertion Index, p, by PRN signal number, i, is given in Table 3.2-2. The generation of L1C P -codeand L1C D -code is conceptually described in Figure 3.2-1.3.2.2.1.2 Overlay Code StructureThe overlay codes L1C Oi (t) are constructed using Linear Feedback Shift Register (LFSR) methods. These codes are2047 bits long truncated to 1800-bit long sequences and are constructed using 11-stage LFSR generators asdescribed below. This section provides the code generation method for the first 63 PRN numbers and Section6.3.1.2 provides the generation method for the additional PRN signal numbers 64 through 210.8 IS-GPS-800A8 June 2010