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Wang, KN, Garrison JL, Acikoz U, Haase JS, Murphy BJ, Muradyan P, Lulich T.  2016.  Open-loop tracking of rising and setting GPS radio-occultation signals from an airborne platform: Signal model and error analysis. Ieee Transactions on Geoscience and Remote Sensing. 54:3967-3984.   10.1109/tgrs.2016.2532346   AbstractWebsite

Global Positioning System (GPS) radio-occultation (RO) is an atmospheric sounding technique utilizing the received GPS signal through the stratified atmosphere to measure refractivity, which provides information on temperature and humidity. The GPS-RO technique is now operational on several Low Earth Orbiting (LEO) satellites, which cannot provide high temporal and spatial resolution soundings necessary to observe localized transient events, such as tropical storms. An airborne RO (ARO) system has thus been developed for localized GPS-RO campaigns. RO signals in the lower troposphere are adversely affected by rapid phase accelerations and severe signal power fading. These signal dynamics often cause the phase-locked loop in conventional GPS survey receivers to lose lock in the lower troposphere, and the open-loop (OL) tracking in postprocessing is used to overcome this problem. OL tracking also allows robust processing of rising GPS signals, approximately doubling the number of observed occultations. An approach for "backward" OL tracking was developed, in which the correlations are computed sequentially in reverse time so that the signal can be acquired and tracked at high elevations for rising occultations. Ultimately, the signal-to-noise ratio (SNR) limits the depth of tracking in the atmosphere. We have developed a model relating the SNR to the variance in the residual phase of the observed signal produced from OL tracking. In this paper, we demonstrate the applicability of the phase variance model to airborne data. We then apply this model to set a threshold on refractivity retrieval based upon the cumulative unwrapping error bias to determine the altitude limit for reliable signal tracking. We also show consistency between the ARO SNR and collocated COSMIC satellite observations and use these results to evaluate the antenna requirements for an improved ARO system.

Murphy, BJ, Haase JS, Muradyan P, Garrison JL, Wang KN.  2015.  Airborne GPS radio occultation refractivity profiles observed in tropical storm environments. Journal of Geophysical Research-Atmospheres. 120:1690-1709.   10.1002/2014jd022931   AbstractWebsite

Airborne GPS radio occultation (ARO) data have been collected during the 2010 PRE-Depression Investigation of Cloud systems in the Tropics (PREDICT) experiment. GPS signals received by the airborne Global Navigation Satellite System Instrument System for Multistatic and Occultation Sensing (GISMOS) are used to retrieve vertical profiles of refractivity in the neutral atmosphere. The system includes a conventional geodetic GPS receiver component for straightforward validation of the analysis method in the middle to upper troposphere, and a high-sample rate (10 MHz) GPS recorder for postprocessing complex signals that probe the lower troposphere. The results from the geodetic receivers are presented here. The retrieved ARO profiles consistently agree within similar to 2% of refractivity profiles calculated from the European Center for Medium-Range Weather Forecasting model Interim reanalyses as well as from nearby dropsondes and radiosondes. Changes in refractivity obtained from ARO data over the 5days leading to the genesis of tropical storm Karl are consistent with moistening in the vicinity of the storm center. An open-loop tracking method was implemented in a test case to analyze GPS signals from the GISMOS 10 MHz recording system for comparison with geodetic receiver data. The open-loop mode successfully tracked similar to 2 km deeper into the troposphere than the conventional receiver and can also track rising occultations, illustrating the benefit from the high-rate recording system. Accurate refractivity retrievals are an important first step toward the future goal of assimilating moisture profiles to improve forecasting of developing storms using this new GPS occultation technique.

Lesne, O, Haase J, Kirchengast G, Ramsauer J, Poetzi W.  2002.  Sensitivity Analysis of GNSS radio occultation for airborne sounding of the troposphere. Phys. and Chem. of the Earth. 27:291-299. AbstractWebsite

The usual geometry for radio occultation sounding using GNSS (Global Navigation Satellite System) signals has the receiver placed on a LEO (Low Earth Orbit) satellite. We investigate a new geometric approach, assuming an airborne rather than a spaceborne receiver. Information on the refractivity structure and hence the pressure, temperature, humidity can be retrieved from accurate airborne measurements of amplitude and phase delay of the signals occulted by the troposphere. We present some advantages and disadvantages for the concept of making measurements from commercial aircraft equipped with proper GNSS receivers and antennae compared to the spaceborne case. We simulated realistic airborne occultation observations and assessed the characteristics of their geometry and sampling. We also compared the dynamic range of the signal with the magnitude of error sources that affect the measurements. Findings include that an airborne system has the potential to provide many more profiles below 10 km height than a single LEO or constellations of up to 25 satellites over the North Atlantic (though with inferior global coverage), and that the SNR (signal to noise ratio) should be better below 5 km than in the LEO case. Though the receiver velocity error is larger than for the LEO system, it is still small enough relative to the signal level to retrieve useful information. Because of the large horizontal drift of the tangent point of up to 450 km, the assumption of spherical symmetry in the existence of significant 3D variations in structure is expected to be a major error source, in addition to the airplane velocity uncertainty.