The atmospheric limb sounding technique making use of radio signals transmitted by the Global Position System (GPS) has emerged as a promising approach for global atmospheric measurements. As demonstrated by the proof-of-concept GPS Meteorology (GPS/MET) experiment and more recently by the CHAMP and SAC-C missions, the GPS radio occultation (RO) sounding data are of high accuracy and high vertical resolution. Past studies have also shown that the GPS RO measurements are most accurate in the upper troposphere and lower stratosphere.

In this study, we assess the usefulness of GPS RO soundings in detecting diurnal and semidiurnal tides. Retrieved temperatures between 10 and 30 km in the tropics from the CHAMP occultations over more than 4 years are analyzed using space-time spectrum analysis to isolate diurnal and semidiurnal waves. Due to the limited local solar time (LST) coverage for monthly occultations, all available data from several years are superimposed to provide 24-hour LST coverage. The Fourier components are determined from temperatures interpolated onto 5 deg * 1 hour grid in the two-dimensional coordinate of longitude and universal time. The effects of aliasing associated with RO data sampling and of the measurement noise are estimated using synthetic data. Our initial results focus on the seasonal-latitudinal and vertical structures of the diurnal and semidiurnal tides, and we include comparisons between observations and model results from the Canadian Middle Atmosphere Model (CMAM) and Global-Scale Wave Model (GSWM02). The observed diurnal tides are in reasonable agreement with the models, for amplitude, phase and seasonal variations. In contrast, there are several differences between the observed semidiurnal tides and those from models: the observed amplitudes are larger than modeled, and their phases show large monthly variations that are not evident in the models.

In this study we have assumed that the inter-annual variability is negligible, due to the data availability from CHAMP. It would be desirable to remove such assumption in the future. On 15 April 2006, the six-satellite constellation COSMIC mission was launched. COSMIC will provide much more uniform data coverage in terms of LST, and will be valuable for the study of tides in the tropical middle atmosphere.