GPS profiling of the lower troposphere from space: Inversion and demodulation of the open‐loop radio occultation signals

Abstract

Global Positioning System (GPS) radio occultation (RO) is a space‐borne remote sensing technique providing accurate, all‐weather, high vertical resolution atmospheric parameters, including pressure, temperature and humidity in the troposphere and stratosphere. In the moist lower troposphere (LT) RO encounters known problem related to the phase‐locked loop (PLL) tracking technique applied in standard GPS receivers and the complicated structure of LT RO signals. This problem has been overcome by developing an open‐loop (OL) tracking technique. This paper outlines post‐processing of OL RO data. In order to invert OL RO signals, the GPS navigation data modulation (NDM) has to be removed in post‐processing. This paper demonstrates that some tropical occultations are not accurately inverted (associated refractivity inversion errors exceed 5%) without the use of externally supplied NDM bit sequences. This result has important implications for the use of RO data from future RO missions for climate research and weather forecasting.