def calibrate_on_noaa(device_index=0,
calibrator_freq=noaa_freq,
bandwidth=1.0 * u.MHz,
passes=20,
max_offset=1000,
default_offset=325):
"""
Calibrate the RTL-SDR using a known NOAA weather station.
Parameters
----------
device_index : int
The USB ID of the device. Usually 0, sometimes 1.
calibrator_freq : Quantity, MHz
The frequency of the weather station. The active one in Gainesville is
preselected.
bandwidth : Quantity, MHz
The bandwidth of the observation to obtain. Generally limited to <2.4
MHz. Will be used to set the sample rate of the SDR.
passes : int
Number of passes (single-integrations) to average prior to measuring
the offset
max_offset : int
The maximum offset to search for, in parts-per-million. This is set
to avoid possibly detecting other (RFI) signals in-band
default_offset : int
The default offset to use when measuring the frequency offset. A
nonzero offset is needed to avoid having the signal channel landing on
the central DC channel, which generally does not have a good
measurement
"""
sdr = RtlSdr(device_index=device_index)
try:
sdr.sample_rate = bandwidth.to(u.Hz).value
sdr.gain = 15
numsamples = 2048 * 4
sdr.center_freq = calibrator_freq.to(u.Hz).value
assert sdr.get_freq_correction() == 0
if default_offset != 0:
sdr.set_freq_correction(default_offset)
foff = sdr.get_freq_correction()
pses = []
frq = np.fft.fftfreq(numsamples)
idx = np.argsort(frq)
for ii in range(passes):
sdr.center_freq = calibrator_freq.to(u.Hz).value
samples = sdr.read_samples(numsamples)
ps = np.abs(np.fft.fft(samples))**2
# this seems to be an unneeded hack
# (but it might help avoid a spike at 0-offset?)
ps[0] = np.mean(ps)
pses.append(ps[idx])
print(".", end='')
mean_ps = np.mean(pses, axis=0)
frequency = u.Quantity(sdr.fc * (1 - foff / 1e6) + sdr.rs * frq[idx],
u.Hz)
print()
print(f"sdr.fc={sdr.fc}, sdr.center_freq={sdr.center_freq}")
cutout = ((frequency > calibrator_freq * (1 - max_offset)) &
(frequency < calibrator_freq * (1 + max_offset)))
max_ind = np.argmax(mean_ps[cutout])
meas_freq = frequency[cutout][max_ind]
meas_offset = (meas_freq - calibrator_freq) / calibrator_freq
finally:
sdr.close()
print(
f"Measured frequency offset is {meas_offset.decompose().value*1e6} parts per million (ppm)"
)
return frequency, mean_ps, meas_freq, meas_offset.decompose()
