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Notice for CDAAC data users

Current radio occultation missions (CHAMP and SACC) are experimental missions and their results have only preliminary error characterization. More complete error characterization will be available in future missions.

At present, for these missions, the quality of the retrieved refractivity profiles in statistical sense can be characterized through comparisons to other observational data, such as radiosondes and atmospheric models. Such comparisons show the promise of these data. Figure 1 shows a global refractivity comparison between the ECMWF model and CDAAC-processed CHAMP from 2002. Comparison of the GPS/MET data to atmospheric models and other observational data are available in

Rocken et al., Analysis and validation of GPS/MET data
in the neutral atmosphere, JGR, 102, D25, 29,849-29,866, 1997
For CHAMP, see
Wickert, J., C. Reigber, G. Beyerle, R. Konig,
C. Marquardt, T. Schmidt, L. Grunwaldt, R. Galas, T.K. Meehan,
W.G. Melbourne and K. Hocke, Atmosphere Sounding by GPS Radio
Occultation: First Results from CHAMP (2001), GRL, 28, 3263
More publications on CHAMP and SACC data are expected to appear in the near future.

Figure 1 -- 2002 refractivity comparison. CHAMP vs. ECMWF model

CDAAC data processing employs extensive quality control, which is briefly outlined below.

Prior to inversion, an L1 Doppler model based on known GPS and LEO orbits and refractivity climatology is computed and compared to the observed Doppler. Large deviations, which commonly occur in the moist troposphere, indicate tracking errors and are used for truncating occultation data below that altitude. However, smaller tracking errors, which are less common but may happen at any altitude, can still affect the inversions.

In the lower troposphere, most occultations have low quality L2 signal that is not useable for ionospheric calibration. So, below a certain altitude the L2 signal is no longer used. The quality of the L2 signal and the altitude below which it is discarded are assessed individually for each occultation. This assesment is based on a sharp increase of L2 Doppler fluctuation and its mean deviation from L1 Doppler.

Along with each retrieved bending angle and refractivity profile, CDAAC inversion software provides a set of scalar parameters which can be used to assess the quality of the occultation in different respects. Below is a description of the most important scalar parameters. In the future, additional scalar parameters can be added.

stdv (rad)
Standard deviation of the observed ionosphere free bending angle from climatology between 60 and 80 km (where the noise overshadows the signal from the neutral atmosphere). The dominant source of noise is the ionosphere and the magnitude of stdv spans a rather large range. This parameter is used for optimal estimation of the bending angle profile prior to the Abel inversion. [Sokolovskiy, S. and D. Hunt, Statistical Optimization Approach for GPS/MET data inversions. URSI GPS/MET Workshop, Tucson, AZ, 1996]:
alpha(h)=w(h)*alpha_obs(h)+[1-w(h)]*alpha_clim(h)
where alpha is the bending angle, h is the height of ray asymptote.

Figure 2 -- Weighting functions w(h) for different magnitudes of stdv:
1) 1E-6; 2) 2E-6; 3) 5E-6; 4) 1E-5; 5) 2E-5; 6) 5E-5.


NOTE: The estimation of the weighting profile which was formerly based on stdv alone is now more complex in the newest version of ROAM. [Lohmann, S. Statistical Optimization of Radio Occultation Data with Dynamical Estimation of Error Covariances. COSMIC science workshop, Taiwan 2004]

smean (rad)
Mean deviation of the observed ionosphere free bending angle from climatology between 60 and 80 km. Normally (for most occultations) smean is much smaller than stdv. If smean is large (>~5E-5) and comparable to stdv, we don't recommend using the occultation at high altitudes (>~20km).
znid (km)
Altitude below which the L2 signal has low quality and is not used for the ionospheric calibration.
S4
Normalized standard deviation of the L1 signal amplitude at high altitudes. If large, this indicates strong ionospheric scintillation and may cause tracking errors on both L2 and L1 at all altitudes.
difmaxion (rad)
Maximal difference between L1 and L2 bending angles above znid. If large, can indicate extreme ionospheric conditions or L2 tracking errors.
difmaxref
Maximal fractional deviation of retrieved refractivity from climatology (CIRA 86).

To determine which occultations to use, one can either rely on the bad flag, which is set for each occultion based on the values of the above scalar parameters (bad = 1 means don't use this occultion!) or examine these parameters and make the right choice for their particular situation. In general, stricter checking results in fewer, but higher quality occultations. We recommend making the decision based on histograms of the scalar parameters plotted for the period of interest.

For those using the bad flag, here are the parameters used in determining it:

  • s4 > 0.1
  • difmaxref > 0.3
  • difmaxion > 0.001 rad
  • stdv > 1.5e-4 rad
  • abs(smean) > 1.0e-4 rad
If any of the above tests fail, then bad is set to 1 (TRUE)

We note that during some observational periods CHAMP and SACC signals contain periodic phase modulation (1 sec and 5 sec spikes in Doppler) which is not completely removed by filtering (tuned up to be consistent, on average, with the Fresnel zone), and manifest itself as periodic structures in the retrieved temperature profiles with period ~1-3 km, above the tropopause. If the data are intended for study of the gravity waves, we recommend to contact the CDAAC team first.