A model for the tropospheric excess path length of radio waves from surface meteorological measurements

Two models are proposed here for the prediction of the excess path length of radio waves from ground‐based measurements of pressure, temperature, and humidity, at zenith and at a given apparent elevation angle. They can be applied to the correction of high‐accuracy range measurements between ground beacons and orbiting satellites. Both of these models use the same formulation for the hydrostatic component of the excess path, Δ L h , with an accuracy better than 5 mm, as compared to measurements of Δ L h . However, two distinct methods have been developed for the wet component Δ L w of the excess path length. The first is based on a theoretical assumption regarding the relative humidity profile and leads to an appropriate accuracy for locations corresponding to standard conditions (≈ 2 cm) with a very small mean deviation (< 5 mm). However, this purely theoretical method must be replaced by a semiempirical one when predictions of Δ L w are made from ground measurements in other types of climates, such as equatorial, tropical, or oceanic climates, because of the much poorer accuracy of ground measurements and greater variability of the humidity profile in such locations (rms deviation > 4 cm, offsets as large as 4 cm). The second model is implemented by the use of an extended radiosonde data base covering the whole world and a 1‐year period. It leads to an rms deviation from measured data comparable to the first method, but to reduced values of the offset, and thus to an overall improvement of the prediction. One advantage of such a method is that it provides a simple global model of the excess path length, with an accuracy comparable to other sophisticated ones.