Description: Within the first two years following the activation of the GPS radio occultation experiment aboard the geoscience satellite CHAMP more than 100,000 occultation events have been observed. Between 70 and 80% of these observations were successfully processed to yield vertical profiles of atmospheric refractivity, temperature and humidity. In the upper troposphere and stratosphere the derived atmospheric refractivities agree with ECMWF meteorological analyses to better than 0.5%; in the lower troposphere, however, a negative bias exceeding several percent is observed. End-to-end simulation studies were performed to investigate possible causes for the observed refractivity bias. Using the multiple phase screen method C/A-code modulated L1 signals are propagated through a spherically symmetric refractivity field derived from a high-resolution radio sonde observation. The propagated signals are tracked by a software GPS receiver and converted to refractivity profiles using the canonical transform technique and the Abel inversion. Ignoring noise and assuming an ideal receiver tracking behaviour the true refractivity profiles are reproduced to better than 0.1% at altitude above 2 km. The non-ideal case is simulated by adding between 14 and 24 dB of Gaussian white noise to the signal and tracking the signal with receivers operating at 50 and 200 Hz sampling frequency using two different carrier phase detectors. In the upper troposphere and stratosphere the receiver models reproduce the true refractivity profile to better than 0.1%. However, in the mid-troposphere down to altitudes of about 2 km a Costas-type phase-locked loop tracking induces negative refractivity biases on the order of −1 to −2% at 50 Hz sampling frequency. Modifications to the receiver tracking algorithm improve the retrieved signal signifi- cantly. Based on these simulation results a heuristic procedure based on the canonical transform method and the sliding spectral technique is proposed. The procedure is applied to simulated profiles as well as observations within the existing CHAMP data set.

Description: [1] Analysis of atmospheric occultation data from the GPS Meteorology experiment has revealed that the refractivity retrievals in the lower troposphere were systematically smaller than those obtained with numerical weather prediction models. It has been suggested that the bias was due to a combination of atmospheric multipath, critical refraction, and receiver tracking errors. In this paper, we show that a similar bias exists in the CHAMP and SAC-C data and describe the characteristics of the bias based on over 6700 soundings from October 2001. Retrievals obtained using the recently introduced canonical transform method are shown to markedly reduce the refractivity bias; however, a significant bias still remains below 2 km altitude. To better understand the underlying causes of the bias, we perform an end-to-end simulation study that incorporates full-wave signal propagation and realistic receiver tracking effects using an ensemble of atmospheric profiles. We find that atmospheric ducting effects associated with the top of the planetary boundary layer (PBL) at 1�2 km altitude would cause retrieval errors at and below the PBL even in the absence of the receiver errors. Furthermore, current implementation of the receiver tracking algorithm based on an enhanced version of the phase-locked loop could introduce additional errors under the low signal-to-noise ratio conditions that are often encountered in the lower troposphere. The latter problem is expected to be resolved in the near future through the adoption of open-loop tracking and the removal of the navigation modulation from the GPS signal.

Description: The atmospheric propagation of GPS signals under multipath conditions and their detection are simulated. Using the multiple phase screen method C/A-code modulated L1 signals are propagated through a spherically symmetric refractivity field derived from a high-resolution radio sonde observation. The propagated signals are tracked by GPS receivers implemented in software and converted to refractivity profiles by the canonical transform technique and the Abel inversion. Ignoring noise and assuming an ideal receiver tracking behaviour the true refractivity profile is reproduced to better than 0.1% at altitude above 2 km. The non-ideal case is simulated by adding between 14 and 24 dB of Gaussian white noise to the signal and tracking the signal with receivers operating at 50 and 200 Hz sampling frequency using two different carrier phase detectors. In the upper troposphere and stratosphere the reciever models used in this study yield comparable refractivity profiles. However, in the mid-troposphere down to altitudes of about 2 km a Costas-type phase-locked loop tracking induces negative refractivity biases on the order of -1 to -2% at 50 Hz sampling frequency. Modifications to the receiver tracking algorithm significantly improves the retrieval results. In particular, replacing the Costas-loop's two-quadrant phase extractor with a four-quadrant discriminator reduces the refractivity biases by a factor of 5; increasing the sampling frequency from 50 to 200 Hz gains another factor of 2.