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electron content to be observed by measuring range and/or phase at two or more frequencies, or through the dispersion of code and carrier. Atmospheric attenuation and delay observed by ground stations can be modeled/observed by estimating parameters associated with empirical models. Measuring the phase delay and amplitude variations of occulting signals from aircraft or satellites enables high-resolution retrieval of atmospheric density, which also exposes temperature and water vapor content. Signals reflected by Earth contain information on surface properties including roughness and reflectivity. Measuring reflected signals from ground, aircraft, or spacecraft enables retrieval of surface conditions including soil moisture, sea ice type, and ocean surface winds.

ATMOSPHERIC SENSING

Ground-based receivers at known locations measuring pseudorange and phase to all GNSS satellites in view provide useful observations for estimating precipitable water vapor (PWV) in the atmosphere. The integrated atmospheric effect along the satellite to receiver ray path can be isolated by removing all other sources of error. The technique was established in the early 1990s (Bevis et al., 1992; Rocken et al., 1993) and is done fairly routinely today (Rocken et al., 2005), although the estimates are not sufficiently widespread to be operationally incorporated in the weather prediction models. Ground-based GNSS PWV sensing relies on a network of geodetic-quality dual-frequency receivers at known locations, precise orbit models, and accurate models relating temperature and moisture content in the atmosphere. High-precision software like Bernese (Dach et al., 2007) must be used to ensure that all other error sources have been eliminated.

A more powerful worldwide approach to exploit GNSS for atmospheric sensing relies on radio occultations measured by orbiting satellites. Researchers have studied the atmospheres of Mars, Venus, and Jupiter since the 1970s through radio occultation. The use of GPS for radio occultation (GPS-RO) measurements from LEO was first initiated in 1995 by the GPS/MET experiment (Schreiner et al., 1998; Yunck et al., 2000). This satellite, placed in a 735 km orbit at 70 degree inclination, flew a modified TurboRogue receiver designed to track signals from above for precise orbit determination and from a separate antenna, signals passing close to the limb of Earth. Now, in 2011, there are about a dozen satellites flying operational or experimental occultation payloads including the six-satellite COSMIC constellation (Anthes et al., 2008), CHAMP, SAC-C, GRAS/Metop, C/NOFS, GRACE-A, and TerrSAR-X. These platforms use forward and backward facing antennas to measure GNSS signals as they rise or set and are occulted by Earth’s atmosphere as shown in Figure 1. A vertical profile of the bending angle through the ionosphere, stratosphere, and troposphere is determined from the excess phase measurements. Refractivity is derived from the bending angles and then further analyzed to determine electron density profiles, temperature, pressure, and water vapor. At its peak performance, COSMIC provided approximately



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