Simulation studies of GPS radio occultation measurements

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 receiver 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.