Profiling the atmosphere with the airborne GPS radio occultation technique using open-loop tracking

Muradyan, P, Haase JS, Acikoz U, Garrison JL, Xie F, Lulich T, Ventre BD.  2013.  Profiling the atmosphere with the airborne GPS radio occultation technique using open-loop tracking. Journal of Geophysical Research. in review


The GNSS Instrument System for Multistatic and Occultation Sensing (GISMOS) is designed for dense sampling of meteorological targets using airborne radio occultation (RO). This limb-sounding technique measures the signal Doppler shift due to refraction and retrieves refractivity profiles that are directly related to pressure, temperature and moisture. These first results from the airborne RO system demonstrate the potential to contribute to numerical weather prediction by reliably providing many high vertical resolution profiles in an area of interest compared to similar measurements made from space. GISMOS includes a Global Positioning System radio frequency signal recorder for open-loop (OL) tracking of the signal in the lower troposphere, where conventional closed-loop receivers fail. The first comprehensive performance analysis of the airborne OL profiling method is presented, showing that OL tracking consistently samples as low as 0.3 to 3.4 km altitude for both rising and setting occultations. The only missed occultations during the 5-hour flight are due to missing global tracking network data and aircraft turns. The system on a straight flight path would measure, on average, 3 occultations per hour of flight. The refractivity profiles found using a geometric optics retrieval algorithm closely follow the vertical variations seen in the European Center for Medium Range Weather Forecasting analysis with a standard deviation of 1.5\% at upper and mid-tropospheric levels, well within the range of observation errors typically assigned during assimilation of RO data. However, the data currently have large biases. Potential causes for the retrieval biases are discussed.