A variety of methods exists to obtain vertical profiles of currents in the ocean: Current meter moorings, while yielding very useful time series of such profiles, are very expensive to maintain and thus can only be realized at a few locations. Geostrophic velocity profiles can be deduced from the density field which is derived from Hydrographic data. However, assumptions have to be made about reference levels and the underlying dynamics which are only useful for the interior of the ocean. Moreover, the traditional geostrophic assumption breaks down near the equator, in flows with strong curvature and close to boundaries. Direct velocity measurements from free-fall probes such as the acoustically tracked `Pegasus' profiler (Spain et al. 1981) and electro magnetic current meter (Sanford et al., 1978) are flexible in that measurements can be made during an oceanographic cruise. Both systems allow precise measurements of the velocity profiles with expected errors of less than 1~cm/s. However, they require significant extra ship time for deployment and recovery of the probes. Moreover, they are delicate instruments and skilled technicians are needed to operate them. Finally, the `Pegasus' profiler is restricted to locations where usually non-recoverable bottom mounted transponders are deployed. Velocity profiles of the upper ocean are frequently obtained with ship mounted acoustic Doppler current profilers (ADCP) (e.g.~Joyce et al., 1982) combined with accurate navigation systems such as the Global Positioning System (GPS). They have become a standard tool in physical oceanography and allow researchers to probe the upper 200--400~m of the water column while the ship is underway or on station.
A few years ago the vertical range of ADCP's was extended by attaching a self contained unit to a CTD package (Firing and Gordon, 1990). After careful data analysis it is possible to obtain full ocean depth velocity profiles (e.g.~Firing and Hacker, 1992; Fischer and Visbeck, 1993a). The flexibility and low costs of the lowered ADCP system (LADCP) makes it a very attractive add-on (e.g. Wilson, 1994; Hinrichsen and Lehmann, 1994) for Hydrographic cruises, despite the somewhat lowered expectations of accuracy ($\sim$ 1-3 cm/s) when compared to the recoverable free fall probes.
Currently on almost every NSF funded WOCE hydrographic cruise LADCP measurements are taken. And the groups who do not own or have access to such a system are thinking about purchasing one.