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track-gps-l1.py
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#!/usr/bin/env python
import optparse
import numpy as np
import gnsstools.gps.ca as ca
import gnsstools.nco as nco
import gnsstools.io as io
import gnsstools.discriminator as discriminator
import gnsstools.util as util
class tracking_state:
def __init__(self,fs,prn,code_p,code_f,code_i,carrier_p,carrier_f,carrier_i,mode):
self.fs = fs
self.prn = prn
self.code_p = code_p
self.code_f = code_f
self.code_i = code_i
self.carrier_p = carrier_p
self.carrier_f = carrier_f
self.carrier_i = carrier_i
self.mode = mode
self.prompt1 = 0 + 0*(1j)
self.carrier_e1 = 0
self.code_e1 = 0
self.eml = 0
self.carrier_cyc = 0
self.code_cyc = 0
# tracking loops
def track(x,s):
n = len(x)
fs = s.fs
nco.mix(x,-s.carrier_f/fs, s.carrier_p)
s.carrier_p = s.carrier_p - n*s.carrier_f/fs
t = np.mod(s.carrier_p,1)
dcyc = int(round(s.carrier_p-t))
s.carrier_cyc += dcyc
s.carrier_p = t
cf = (s.code_f+s.carrier_f/1540.0)/fs
p_early = ca.correlate(x, s.prn, 0, s.code_p-0.05, cf, ca.ca_code(prn))
p_prompt = ca.correlate(x, s.prn, 0, s.code_p, cf, ca.ca_code(prn))
p_late = ca.correlate(x, s.prn, 0, s.code_p+0.05, cf, ca.ca_code(prn))
if s.mode=='FLL_WIDE':
fll_k = 3.0
a = p_prompt
b = s.prompt1
e = discriminator.fll_atan(a,b)
s.carrier_f = s.carrier_f + fll_k*e
s.prompt1 = p_prompt
elif s.mode=='FLL_NARROW':
fll_k = 0.8
a = p_prompt
b = s.prompt1
e = discriminator.fll_atan(a,b)
s.carrier_f = s.carrier_f + fll_k*e
s.prompt1 = p_prompt
elif s.mode=='PLL':
pll_k1 = 0.1
pll_k2 = 3.5
e = discriminator.pll_costas(p_prompt)
e1 = s.carrier_e1
s.carrier_f = s.carrier_f + pll_k1*e + pll_k2*(e-e1)
s.carrier_e1 = e
# code loop
dll_k1 = 0.00002
dll_k2 = 0.2
s.early = np.absolute(p_early)
s.prompt = np.absolute(p_prompt)
s.late = np.absolute(p_late)
if (s.late+s.early)==0:
e = 0
else:
e = (s.late-s.early)/(s.late+s.early)
s.eml = e
e1 = s.code_e1
s.code_f = s.code_f + dll_k1*e + dll_k2*(e-e1)
s.code_e1 = e
s.code_p = s.code_p + n*cf
t = np.mod(s.code_p,ca.code_length)
dcyc = int(round(s.code_p-t))
s.code_cyc += dcyc
s.code_p = t
return p_prompt,s
#
# main program
#
parser = optparse.OptionParser(usage="""track-gps-l1.py [options] input_filename sample_rate carrier_offset PRN doppler code_offset
Track GPS L1 signal
Examples:
Track with default options:
track-gps-l1.py /dev/stdin 69984000 -9334875 31 1200.0 831.15
Track with pure PLL (no FLL intervals at the start) and with a specified carrier phase:
track-gps-l1.py --carrier-phase 0.214 /dev/stdin 69984000 -9334875 31 1200.0 831.15
Arguments:
input_filename input data file, i/q interleaved, 8 bit signed
sample_rate sampling rate in Hz
carrier_offset offset to L1 carrier in Hz (positive or negative)
PRN PRN
doppler Doppler estimate from acquisition
code_offset Code-offset estimate from acquisition""")
parser.disable_interspersed_args()
parser.add_option("--loop-dwells", default="500,500", help="initial time intervals for wide FLL, then narrow FLL, in milliseconds (default %default)")
parser.add_option("--carrier-phase", help="initial carrier phase in cycles (disables FLL: uses PLL from the start)")
(options, args) = parser.parse_args()
filename = args[0]
fs = float(args[1])
coffset = float(args[2])
prn = int(args[3])
doppler = float(args[4])
code_offset = float(args[5])
loop_dwells = util.parse_list_floats(options.loop_dwells)
carrier_p = 0.0
if options.carrier_phase is not None:
carrier_p = float(options.carrier_phase)
loop_dwells = 0,0
fll_wide_time,fll_narrow_time = loop_dwells
fp = open(filename,"rb")
n = int(fs*0.001*((ca.code_length-code_offset)/ca.code_length)) # align with 1 ms code boundary
x = io.get_samples_complex(fp,n)
code_offset += n*1000.0*ca.code_length/fs
s = tracking_state(fs=fs, prn=prn, # initialize tracking state
code_p=code_offset, code_f=ca.chip_rate, code_i=0,
carrier_p=carrier_p, carrier_f=doppler, carrier_i=0,
mode='FLL_WIDE')
block = 0
coffset_phase = 0.0
samp = 0
while True:
if block>=fll_wide_time:
s.mode = 'FLL_NARROW'
if block>=fll_wide_time+fll_narrow_time:
s.mode = 'PLL'
if s.code_p<ca.code_length/2:
n = int(fs*0.001*(ca.code_length-s.code_p)/ca.code_length)
else:
n = int(fs*0.001*(2*ca.code_length-s.code_p)/ca.code_length)
x = io.get_samples_complex(fp,n)
if x is None:
break
samp += n
nco.mix(x,-coffset/fs,coffset_phase)
coffset_phase = coffset_phase - n*coffset/fs
coffset_phase = np.mod(coffset_phase,1)
p_prompt,s = track(x,s)
vars = block, np.real(p_prompt), np.imag(p_prompt), s.carrier_f, s.code_f-ca.chip_rate, (180/np.pi)*np.angle(p_prompt), s.early, s.prompt, s.late, s.code_cyc, s.code_p, s.carrier_cyc, s.carrier_p, samp
print('%d %f %f %f %f %f %f %f %f %d %f %d %f %d' % vars)
block = block + 1