Principles of the open loop tracking of radio occultation

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Principles of the open loop tracking of radio
occultation signals S. Sokolovskiy University
Corporation for Atmospheric Research
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Sliding spectrogram of the simulated RO signal
sampled at 50 Hz (exponential refractivity
profile)
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Sliding spectrogram of the simulated RO signal
sampled at 50 Hz (HiRes radiosonde refractivity
profile)
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Layout of RO signal in frequency domain
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Phase acceleration of RO signal sampled at 50
Hz. Magnitude of random component is much larger
than the mean. In closed loop tracking the
results may depend on the method of projecting
the phase model ahead and the amount of
the previously extracted data used.
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Penetration of GPS/MET occultations during two
observational periods with different tuning of
the tracking firmware.
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Layout of RO signal in frequency domain
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Estimation of the weather-related spread of mean
RO signal frequency shift based on GO simulations
with NCEP T62 NWP model
(ensembles for different latitudes are shifted in
frequency for visualization)
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Sampling RO signal at different rates
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Estimation of the center of spectrum
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Estimation of the mean residual frequency shift
of RO signal by estimating of the center of the
sliding spectrorgram
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Extraction of the accumulated phase from raw
phase (resolving cycle ambiguities)
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In receiver Downconversion of RO signal by use
of the phase model without a feedback from the
signal. The frequency model is based on the
predicted orbits and a refractivity (bending
angle) climatology. Low-pass filtering. Sampling
aliasing must not result in overlapping of
harmonics in the spectra. Sampling rate must not
be smaller than the spread of spectrum. In post
processing The additional downconversion of the
sampled signal (if necessary) to
eliminate (reduce) aliasing. The phase model is
based on the sliding-window spectral analysis of
the sampled signal. Resampling at higher rate
and extraction of the accumulated
phase. Upconversion (with account for clock
solutions). Calculation of the excess phase (with
account for orbit solutions).
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