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

Abstract

[1] 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.

Forecast impact experiment with a constellation of GPS radio occultation receivers

“…The increase in the O − B standard deviation near 11 km is a robust feature in the statistics, and it is evident in the Tropics, Southern and Northern Hemispheres for all six COSMIC satellites (it is also observed in experiments covering other time periods). In the processing of the COSMIC GPSRO measurements at UCAR, there is a transition for ray tangent heights near 10 km, where PLL signal tracking is used above 10 km and OL signal tracking is used below 10 km (Sokolovskiy et al, 2006). It is possible that this transition produces different error characteristics for setting and rising occultations (Sergey Sokolovskiy pers.…”

GPS Radio Occultation: Results from CHAMP, GRACE and FORMOSAT-3/COSMIC

“…The mea surements were an a lyzed us ing the at mo spheric ex cess phase data pro vided by UCAR. The re con struc tion of the fi nal phase from the model and the dif fer en tial phase was performed us ing the nav i ga tion bit in for ma tion from a sup porting GPS ground net work, also pro vided by UCAR (us ing the al go rithm ac cord ing to Sokolovskiy et al 2006a). The GFZ anal y sis re sults (refractivity and dry tem per a ture pro files) agree very well with UCAR.…”

Open-loop GPS signal tracking at low elevation angles from a ground-based observation site

“…To address premature signal loss GNSS-RO receiver tracking algorithms based on closed feedback loops were replaced by "open-loop" techniques (see e.g. Sokolovskiy, 2001b;Sokolovskiy et al, 2006;Ao et al, 2009;Bonnedal et al, 2010;Wang et al, 2016). If a receiver operates in open-loop mode the feedback loop is opened and the NCO producing the replica signal is steered (in part or fully) from model parameters.…”

“…Whereas the "fly wheeling" tracking method of JPL's "Blackjack" GPS receivers mounted on CHAMP and GRACE Hajj et al, 2004) showed some progress, significant improvements with respect to probing of the planetary boundary layer (PBL), in particular at low latitudes, were obtained with the introduction and implementation of open-loop (O/L) tracking (or raw-sampling) techniques (see e.g. Sokolovskiy, 2001b;Sokolovskiy et al, 2006;Ao et al, 2009;Bonnedal et al, 2010).…”

“…The open-loop signal tracking mode successfully resolves the problem of premature loss of signal in the lower troposphere at low latitudes (Sokolovskiy, 2001b;Sokolovskiy et al, 2006). In contrast to closed-loop (C/L) tracking, a receiver operating in open-loop mode partially or completely disregards the tracking loop feedback values from the carrier and code discriminators, but instead steers the corresponding numerically controlled oscillators (NCOs) using a priori parameters.…”