Observing Earth's atmosphere with radio occultation measurements using the Global Positioning System

Abstract

Abstract. The implementation of the Global Positioning System (GPS) network of satellites and the development of small, high-performance instrumentation to receive GPS signals have created an opportunity for active remote sounding of the Earth's atmosphere by radio occultation at comparatively low cost. A prototype demonstration of this capability has now been provided by the GPS/MET investigation. Despite using relatively immature technology, GPS/MET has been extremely successful [Ware et al., 1996; Kursinski et al., 1996], although there is still room for improvement. The aim of this paper is to develop a theoretical estimate of the spatial coverage, resolution, and accuracy that can be expected for atmospheric profiles derived from GPS occultations. We consider observational geometry, attenuation, and diffraction in defining the vertical range of the observations and their resolution. We present the first systematic, extensive error analysis of the spacecraft radio occultation technique using a combination of analytical and simulation methods to establish a baseline accuracy for retrieved profiles of refractivity, geopotential, and temperature. Typically, the vertical resolution of the observations ranges from 0.5 km in the lower troposphere to 1.4 km in the middle atmosphere. Results indicate that useful profiles of refractivity can be derived from -60 km altitude to the surface with the exception of regions less than 250 m in vertical extent associated with high vertical humidity gradients. Above the 250 K altitude level in the troposphere, where the effects of water are negligible, sub-Kelvin temperature accuracy is predicted up to -40 km depending on the phase of the solar cycle. Geopotential heights of constant pressure levels are expected to be accurate to ~ 10 m or better between 10 and 20 km altitudes. Below the 250 K level, the ambiguity between water and dry atmosphere refractivity becomes significant, and temperature accuracy is degraded. Deep in the warm troposphere the contribution of water to refractivity becomes sufficiently large for the accurate retrieval of water vapor given independent temperatures from weather analyses . The radio occultation technique possesses a unique combination of global coverage, high precision, high vertical resolution, insensitivity to atmospheric particulates, and long-term stability. We show here how these properties are well suited for several applications including numerical weather prediction and long-term monitoring of the Earth's climate.

Greenhouse gas profiling by infrared-laser and microwave occultation: retrieval algorithm and demonstration results from end-to-end simulations

“…It follows very closely the Bönsch and Potulski (1998) formulation (at λ > 0.5 µm) and can be considered an improved version of the classical very similar optical refractivity formula developed by Edlén (1966). Different from the SmithWeintraub formula of microwave refractivity used in LMO (Schweitzer et al, 2011b), the water vapor term in this optical refractivity formula is essentially negligible because the frequencies are much too high for the permanent dipole moments of the water vapor molecules to contribute an orientation polarization term (the "wet term" in the microwave formula; e.g., Kursinski et al, 1997). The difference of the MW refractivity and the IR refractivity is illustrated over the range 2 µm to 3 µm by Schweitzer et al (2011a).…”

“…As described by Kirchengast and Schweitzer (2011), the LMIO method can be considered as a next generation of the well established and successful GNSS-LEO radio occultation (GRO) method (Ware et al, 1996;Kursinski et al, 1997;Steiner et al, 2001;Anthes et al, 2008;Luntama et al, 2008;Steiner et al, 2009;Ho et al, 2009). LMIO and GRO share the occultation measurement principle (Phinney and Anderson, 1968; and the use of highly coherent and stable inter-satellite signals, and therefore the potential of providing accurate, long-term, consistent benchmark data with high vertical resolution and global coverage.…”

Quantifying residual ionospheric errors in GNSS radio occultation bending angles based on ensembles of profiles from end-to-end simulations

“…Detailed analyses of GNSS RO errors have been conducted by many scientists (Kursinski et al, 1997;Rieder and Kirchengast, 2001;Steiner and Kirchengast, 2005;ScherllinPirscher et al, 2011a, b). These errors mainly include the satellites' orbital error, clock biases, systematic hardware delay, antenna phase center variation, cycle slips, ionospheric refraction, atmospheric multipath and scintillations.…”

“…Global Navigation Satellite System (GNSS) radio occultation (RO) (Melbourne et al, 1994;Kursinski et al, 1997;Hajj et al, 2002) is a relatively new atmospheric remote sensing technique. It can deliver data traceable to the international standard of time (the SI second) and has the potential for monitoring decadal-scale climate change (Steiner et al, 2009;Scherllin-Pirscher et al, 2011b;Lackner et al, 2011) due to its unique characteristics such as high vertical resolution, high accuracy and long-term stability of its observations, as well as self-calibration capability and global coverage (Gobiet and Kirchengast, 2004;Steiner et al, 2011).…”

Statistical estimations of atmospheric duct over the South China Sea and the tropical eastern Indian Ocean

“…(1), we could see that a duct occurrence is usually accompanied by temperature inversion and/ or rapid decrease of moisture. Previous radiosonde refractivity studies pointed out that the most ducting events were found below 2 km [9,10]. Thus, the maximum computation height in this paper was limited to 2 km.…”