Introduction

The radar altimeter transmits short pulses of energy vertically downward toward the ocean surface, then receives the reflected signal. The return yields information on the global distribution and variability of sea surface height, ocean swell amplitude and scalar wind speed. Specifically, the time difference between the transmitted and received signals measures the distance or range between the satellite and the sea surface, the shape of the return yields the significant wave height (SWH) and the magnitude yields the scalar wind speed. If the satellite orbit is precisely determined and the range is corrected for a variety of ionospheric, atmospheric and ocean surface factors, these observations measure, to an accuracy of 2–3 cm, the changes in sea surface height (SSH) associated with tides, geostrophic currents and other oceanic phenomena.

This chapter describes how the altimeter works, discusses its sources of uncertainty and describes some of its oceanographic applications. Wunsch and Stammer (1998) and the collection of papers edited by Fu and Cazenave (2001) contain more detailed and extended discussions of the TOPEX altimeter results, and Chelton et al. (2001b) describe the physics of the TOPEX altimeter and its associated error budget. Morrow and Fu (2010) organized and provided the introduction to a special issue of Marine Geodesy on the JASON-2 mission; see JASON-2 (2010) for a listing of the papers. Fu (2010) describes the current state of the altimeter measurements and their application to the ocean circulation. At scales of 150–200 km, multiple altimeters provide near global coverage of the temporal and spatial scales of ocean variability, the meridional transports of heat and the distribution and properties of ocean eddies.