Every GPS satellite broadcasts its own 1023-chip C/A code, a pseudo-random sequence that repeats every millisecond. All satellites share the same frequency, so a receiver tells them apart by their codes.

The code is generated by two 10-stage shift registers, G1 and G2, with stages numbered 1 to 10 and every stage initially set to 1. For each chip:

1. Output G1₁₀ ⊕ G2_a ⊕ G2_b, where a and b depend on the satellite's PRN number (see table).
2. Shift both registers one stage towards stage 10. The new stage 1 of G1 is G1₃ ⊕ G1₁₀, the new stage 1 of G2 is G2₂ ⊕ G2₃ ⊕ G2₆ ⊕ G2₈ ⊕ G2₉ ⊕ G2₁₀, both computed before shifting.

For example, the code of PRN 1 starts 1100100000111001…

Each argument is a recording of 1023 chips from one satellite. The recording may start anywhere within the repeating code, and up to eight chips have been flipped by noise. For each argument, print the PRN number (1 to 32) of the satellite that sent it.

External links: IS-GPS-200, Wikipedia