Use of GPS for estimation of bending angles of radio waves at low elevations

Abstract

The paper compares three methods of calculating the bending angles of radio waves propagated from space to a ground-based receiver: (1) from refractivity climatology corrected for refractivity at the receiving antenna, (2) from radiosonde refractivity profiles, and (3) from the refractivity at the antenna and the measured Doppler frequency shift of the GPS signals. The methods are tested with the use of radiosonde and GPS observations collocated in space and in time. We analyzed seven cases during October-November 1999 where GPS satellites were observed to below 0.5° elevation from Point Loma, California, and which coincided closely in time with radiosonde launches from the nearby Miramar station. In all cases the bending angles calculated from Doppler and from radiosondes agree fairly well at all elevations, but in a number of cases both differ significantly at low elevations from the bending angles calculated from climatology corrected for the refractivity at the antenna. Thus GPS has the potential of being used for the correction of radar observations at low elevations instead of (or complementary to) radiosondes. The differences between the bending angles calculated from climatology corrected for the refractivity at the antenna and those calculated from the Doppler frequency shift indicate anomalies in the refractivity profile in the lower troposphere and can thus be used as an indicator of ducting conditions.