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.

No abstract

No abstract

Data from the Palomar Testbed Interferometer, with a baseline length of 110 m and an observing wavelength of 2.2 km, were used to derive information on atmospheric turbulence on 64 nights in 1999. The measured two-aperture variance coherence times at 2.2 km ranged from 25 to 415 ms (the lower value was set by instrumental limitationsÈthe interferometer could not operate when the coherence time was lower than this). On all nights, the spectrum of the short-timescale (less than 600 ms) delay Ñuctua-tions had a shallower spectrum than the theoretical Kolmogorov value of 5/3. On most nights, the mean value of the power-law slope was between 1.40 and 1.50. Such a sub-Kolmogorov slope will result in the seeing improving as the B0.4 power of wavelength, rather than the slower 0.2 power predicted by Kolmogorov theory. On four nights, the combination of delay and angle-tracking measurements allowed a derivation of the (multiple) wind velocities of the turbulent layers, for a frozen-Ñow model. The derived wind velocities were all ¹4 m s~1, except for a small 10 m s~1 component on one night. The combination of measured coherence time, turbulence spectral slope, and wind velocity for the turbulent layer(s) allowed a robust solution for the outer scale size (beyond which the Ñuctuations do not increase). On the four nights with angle-tracking data, the outer scale varied from 6 to 54 m, with most values in the 10È25 m range. Such small outer scale values cause some components of visibility and astrometric errors to average down rapidly.

No abstract

No abstract

No abstract

Tracking Systems and Applications Sections The Deep Space Network is establishing a high-accuracy VLBI celestial reference frame. This article presents VLBI results of observations of 416 radio sources with declinations north of -45 deg which have been conducted at frequencies of 2.3 GHz and 8.4 GHz. At 2.3 GHz, 323 of 391 radio sources observed were detected with a fringe spacing of 3 milliarcsec and a detection limit of _0.1 Jy. At 8.4 GHz, 278 of 416 radio sources were detected with a fringe spacing of 1 milliarcsec and a detection limit of _0.1 Jy. This survey was conducted primarily to determine the strength of compact components at 8.4 GHz for radio sources previously detected with VLBI at 2.3 GHz. Compact extragalactic radio sources with strong correlated flux densities at both frequencies are used to form a high-accuracy reference frame.