Improved Mapping of Tropospheric Delays

The authors compare several methods to map the a priori tropospheric delay of global positioning system (GPS) signals from the zenith direction to lower elevations. This is commonly achieved with so-called mapping functions. Dry mapping functions are applied to the hydrostatic delay; wet mapping functions are used to map the zenith wet delay to lower elevation angles. The authors compared the following mapping techniques against raytraced delays computed for radiosonde profiles under the assumption of spherical symmetry: (a) the Niell mapping function; (b) mapping through the COSPAR International Reference Atmosphere with added water vapor climatology; (c) the same as b with added use of surface meteorological temperature, pressure, and humidity; and (d) use of the numerical reanalysis model of the National Centers for Environmental Prediction-National Center for Atmospheric Research. Based on comparisons with all available global radiosondes (1000 per day), for every fifth day of 1997 (73 days), the authors found that dry mapping based on method d performs 2 - 3 times better than a for elevations 15° and below. The authors further report that b and c perform better dry mapping than a, with an improvement of 50%. Smaller improvements are also shown for wet delay mapping by b, c, and d as compared to a. At 5° and below, the Niell dry mapping function has biases that vary with season by 1%, and it displays significant systematic errors (2% - 4% at 5° elevation) between 30° and 90° southern latitude during the northern winter months. It is concluded that the most demanding meteorological and geodetic GPS applications should use location- and time-specific "direct" mapping functions such as b, c, or d rather than parameterized functions, especially if low elevation observations are used. The authors describe how this improved mapping can be implemented in GPS analysis software.