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.