Profiling the atmosphere with the airborne radio occultation technique

Paytsar Muradyan, Purdue University

Abstract

The GNSS Instrument System for Multistatic and Occultation Sensing (GISMOS) was designed for dense sampling of meteorological targets using the airborne radio occultation (RO) technique. Airborne RO refers to an atmospheric limb sounding technique in which Global Positioning System (GPS) signals are recorded at a receiver onboard an aircraft as the satellites descend beyond the limb of the Earth. The GPS signals, that are unaffected by clouds and precipitation, experience refractive bending as well as a delay in the travel time through the atmosphere. Bending can be used to retrieve information about atmospheric refractivity, which depends on atmospheric moisture and temperature. The new system has the potential for improving numerical weather prediction (NWP) forecasts through assimilation of many high-resolution atmospheric profiles in an area of interest, compared to spaceborne RO, which samples sparsely around the globe. In February 2008, GISMOS was deployed on the National Science Foundation Gulfstream-V aircraft to make atmospheric observations in the Gulf of Mexico coastal region with an objective to test the performance of the profiling system. Recordings from this flight campaign made with the conventional phase lock loop GPS receivers descend from flight level to 5 km altitude. However, below that level strong refractivity gradients, especially those associated with the boundary layer, cause rapid phase accelerations resulting in loss of lock in the receiver. To extend the RO profiles deeper in the atmosphere, the GISMOS system was also equipped with a GPS Recording System (GRS) that records the raw RF signals. Post-processing this dataset in open-loop (OL) tracking mode enables reliable atmospheric profiling at lower altitudes. We present a comprehensive analysis of the performance of the airborne system OL tracking algorithm during a 5 hour flight on 15 February 2008. Excess phase and amplitude profiles for 5 setting and 5 rising occultations were successfully retrieved out of the 19 possible cases. Profiles from rising occultations were retrieved with comparable quality to setting occultations. The only missed occultations were due to missing or poor quality ancillary navigation data from the global tracking network and the aircraft turns. We demonstrate that the OL tracking receiver performs much better than the conventional receivers, consistently tracking as low as 0.5 to 3.4 km. Based on this success rate and the improved global network coverage since 2008 providing navigation data bits, the airborne RO system on a straight flight path today would achieve 3 occultations per hour of flight time. The refractivity profiles retrieved with a geometric optics method show a bias with respect to the European Center for Medium Range Weather Forecasting (ECMWF) analysis profiles. The data were compared with a co-located spaceborne RO profile, and although the airborne data shows a larger bias with respect to ECMWF profiles, there is a correlation of the vertical variations observed with both datasets. The standard deviation of the difference with the ECMWF profile refractivity is less than 1 % in terms of refractivity. The comparison of the retrieved refractivity and a co-located radiosonde station profile shows a bias as well, with a standard deviation of 2.3 % from 5-12 km altitude. Future efforts should be directed at resolving the source of the bias, in which case the data will be quite useful for assimilation. The differences are within the range of the observation errors typically assigned to RO data below 10 km during assimilation. Signal tracking and retrieval in the lower troposphere continues to be a major challenge for spaceborne RO, and has limited the impact of all RO data in NWP in the lower troposphere. Full bandwidth signals from airborne measurements could provide a testbed for improving the quality of future spaceborne RO measurements. The airborne RO technique could potentially be implemented on commercial aircraft to provide dense measurements to improve weather forecasting in busy flight corridors.

Degree

Ph.D.

Advisors

Garrison, Purdue University.

Subject Area

Atmospheric sciences|Remote sensing

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