Ionospheric Variations Affecting Altimeter Measurements: A Brief Synopsis

Abstract

Ionospheric variations occur on all scales, from meters to thousands of kilometers. The fluctuations affect space‐borne, high‐precision (2 to 10 cm) altimeter measurements at gigahertz frequencies directly and also indirectly, through the accuracy of orbit determination from radio metric data. Data on quiet‐time ionospheric fluctuations on scales from 1 km to thousands of kilometers are reviewed. Large‐scale changes are considered to be those on scales greater than 500 km or 1 hr. Temporal changes occur on solar cycle (eleven year), seasonal, and diurnal scales. Spatial variations include the equatorial anomaly (greater density near ± 15° latitude than at the equator) and the midlatitude trough (low density at 50 to 65° latitude at night). The correlation distance of the ionosphere (70% correlation) is approximately 2500 to 3000 km east‐west and 1600 km north‐south.

Small‐scale fluctuations (<500 km) are well described by a power law spectrum with a one‐dimensional index of ‐1.5 to ‐2.0. The irregularities are aligned along the earth's magnetic field and are generally believed to be rod‐like, with lengths five to ten times their thickness. The observed spectrum of the density fluctuations gives vertical line‐of‐sight path length fluctuations of 1 to 40 mm on 500‐km horizontal scales for a radio signal at 13.7 GHz for a satellite well above the ionospheric density maximum at 350 to 400 km.