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ca_code_gen.py
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ca_code_gen.py
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#!/usr/bin/env python3
# Copyright (c) 2018 Marco Zollinger <[email protected]>
def ca_code_gen(sv):
"""GPS coarse acquisition (C/A) code generator (gold code)
Arguments:
sv -- space vehicle PRN number
Returns:
infinite symmetric C/A code sequence as generator for a single int (-1/1)
"""
# Initialization constants for LFSR-2 (init code implementation)
init_codes = {1: [1, 1, 1, 1, 1, 0, 1, 1, 0, 0], # SV:1
2: [1, 1, 1, 1, 0, 1, 1, 0, 0, 0], # SV:2
3: [1, 1, 1, 0, 1, 1, 0, 0, 0, 0], # SV:3
4: [1, 1, 0, 1, 1, 0, 0, 0, 0, 0], # SV:4
5: [0, 0, 1, 0, 0, 1, 0, 1, 1, 0], # SV:5
6: [0, 1, 0, 0, 1, 0, 1, 1, 0, 0], # SV:6
7: [0, 1, 1, 0, 0, 1, 0, 1, 1, 0], # SV:7
8: [1, 1, 0, 0, 1, 0, 1, 1, 0, 0], # SV:8
9: [1, 0, 0, 1, 0, 1, 1, 0, 0, 0], # SV:9
10: [1, 1, 0, 1, 1, 1, 0, 1, 0, 0], # SV:10
11: [1, 0, 1, 1, 1, 0, 1, 0, 0, 0], # SV:11
12: [1, 1, 1, 0, 1, 0, 0, 0, 0, 0], # SV:12
13: [1, 1, 0, 1, 0, 0, 0, 0, 0, 0], # SV:13
14: [1, 0, 1, 0, 0, 0, 0, 0, 0, 0], # SV:14
15: [0, 1, 0, 0, 0, 0, 0, 0, 0, 0], # SV:15
16: [1, 0, 0, 0, 0, 0, 0, 0, 0, 0], # SV:16
17: [1, 0, 0, 0, 1, 0, 0, 1, 1, 0], # SV:17
18: [0, 0, 0, 1, 0, 0, 1, 1, 0, 0], # SV:18
19: [0, 0, 1, 0, 0, 1, 1, 0, 0, 0], # SV:19
20: [0, 1, 0, 0, 1, 1, 0, 0, 0, 0], # SV:20
21: [1, 0, 0, 1, 1, 0, 0, 0, 0, 0], # SV:21
22: [0, 0, 1, 1, 0, 0, 0, 0, 0, 0], # SV:22
23: [0, 0, 1, 1, 0, 0, 1, 1, 1, 0], # SV:23
24: [1, 0, 0, 1, 1, 1, 0, 0, 0, 0], # SV:24
25: [0, 0, 1, 1, 1, 0, 0, 0, 0, 0], # SV:25
26: [0, 1, 1, 1, 0, 0, 0, 0, 0, 0], # SV:26
27: [1, 1, 1, 0, 0, 0, 0, 0, 0, 0], # SV:27
28: [1, 1, 0, 0, 0, 0, 0, 0, 0, 0], # SV:28
29: [0, 0, 0, 1, 0, 1, 0, 1, 1, 0], # SV:29
30: [0, 0, 1, 0, 1, 0, 1, 1, 0, 0], # SV:30
31: [0, 1, 0, 1, 0, 1, 1, 0, 0, 0], # SV:31
32: [1, 0, 1, 0, 1, 1, 0, 0, 0, 0]} # SV:32
# LFSR-1: init with all 1, feedback taps @3, 10
lfsr1 = lfsr([1] * 10, [0, 0, 1, 0, 0, 0, 0, 0, 0, 1])
# LFSR-2: init with init code, feedback taps @2, 3, 6, 8, 9, 10
lfsr2 = lfsr(init_codes[sv], [0, 1, 1, 0, 0, 1, 0, 1, 1, 1])
while True:
asymmetric = next(lfsr1) ^ next(lfsr2)
yield {0: -1, 1: 1}[asymmetric]
def lfsr(seed, taps):
"""Linear feedback shift register with arbitrary length for PRN sequence
Arguments:
seed -- initialisation seed as list of int (0/1)
taps -- position of feedback taps as list of int (0/1)
Returns:
infinite LFSR output as generator for a single int (0/1)
"""
state = seed
while True:
feedback = 0
for bit, tap in zip(state, taps):
if tap == 1:
feedback ^= bit
state.insert(0, feedback)
yield state.pop()
# Output the first 10 C/A chips for each of the 32 SVs
if __name__ == "__main__":
for sv_no in range(1, 33):
gen = ca_code_gen(sv_no)
chips = []
for i in range(10):
chips.append(next(gen))
print("SV PRN: {:>2}, first 10 chips: {}".format(sv_no, chips))