These are able to occupy, on a monthly basis, all the 48 hydrographic sections that took the WHP 12 years to do once. We acquire data from this widely dispersed network via satellite telemetry. It is something of a wonder to follow the intricate patrol pattern of the Slocums assigned to the various sections. For example, there are ten assigned to patrol a section along 24N in the Atlantic. Each patrols a six degree segment of longitude.
Pairs meet once a month to do an intercalibration.
Sometimes we schedule an intricate cotillion-like dance of the Sentinel Slocums in which they interchange patrols, thus obtaining, over the course of a year, a complete intercalibration of all Slocums on the section. Each Slocum reports into Mission Control via satellite about six times a day. The satellite also reports on the Slocum's position, which informs us how much the float has been set by the ocean currents it encounters and permits its automatic pilot to be reset according to a course-correction algorithm. Thus we have a basic set of Sentinels routinely patrolling the ocean between 50S and 50N, equivalent to a fleet of 48 full-time hydrographic survey ships, making no port stops, using no fuel, and by international law exempt from the restrictions on passage through territorial waters exacted of manned vessels. The prototype Sentinel was launched in 1994, before the Mission Control Center was built, and the little group of originators was still working in the attic of the Bigelow building at the Woods Hole Oceanographic Institution.