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Float Design

Traditionally, float location was based on acoustics. Either the float itself was a transmitter and moored hydrophones were used to locate it (the SOFAR type), or vice-versa - the float was a hydrophone which listened to moored sound sources (the RAFOS type). Both types were vulnerable to mooring or float failure - in such an event, much data could be lost. The advent of global satellite communications and positioning systems made possible the ALACE (Autonomous LAgrangian Circulation Explorer) and its offshoot, the PALACE (Profiling ALACE).

The ALACE float is designed to drift at a selected depth, then every so often (weekly, fortnightly) it will surface and transmit a message back to base. The message includes satellite-derived time and position, and various system diagnostics. It then sinks back to its assigned depth. The cycle repeats for as long as its batteries last, which can be several years.

The basic ALACE has three subsystems: hydraulics, microprocessors and data transmission. The hydraulics control buoyancy adjustment via an inflatable external bladder, so the float can surface and dive; the microprocessors deal with function control and scheduling; and the data transmission system looks after the sending of messages to SystĖme Argos satellites. The float mass is only 23 kg, a substantial reduction over the quarter-tonne SOFAR float. The maximum operating depth for the aluminium pressure case is 2000 m, with a crush depth of over 2600 m. PALACEs typically have temperature and conductivity sensors added so that, when combined with the pressure sensor data, each float ascent is also a temperature-depth or CTD profile. A PALACE is a few kg heavier and 15-20 cm longer than an ALACE. The rise speed is faster than the dive speed because the buoyancy force is intentionally made larger by pumping. A float will take an hour or two to surface at a vertical velocity of 10-20 cm/s, while it will sink at about 5 cm/s.

The ALACE design had some limitations, however. In particular, it could not reduce its buoyancy under pressure, so it could not profile downwards from its drift depth; its internal oil bladder (the reservoir used for filling the external bladder) ruptured at high accelerations, so it could not be deployed from aircraft or from the decks of merchant ships at speed; its energy efficiency was compromised by the high pressure pump, used to move the oil between the reservoir and the external bladder - the pump was inefficient near the surface; and all in all, it was felt that it could be made more reliable.

The result was the present generation of APEX / SOLO floats. The most significant change over the ALACE type is the replacement of the high-pressure pump by a single-stroke pump - it gives the profiling float full ascent / descent control, has no internal oil bladder, and is simpler, more efficient and more reliable.


Simple Mission Operation


Park & Profile Mission Operation



References:

Davis, R. E., D. C. Webb, L. A. Regier and J. Dufour, 1992: The autonomous Lagrangian circulation explorer (ALACE). J. Atmos. Oceanic Tech. 9 264-285.

Davis, R. E., J. T. Sherman and J. Dufour, 2001: Profiling ALACEs and other advances in autonomous subsurface floats. J. Atmos. Oceanic Tech. 18 982-993.

See also the websites for Webb Research, CLS (Systeme Argos), the French float manufacturer Martec.

 

 

Profiling floats at SOC


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Last Modified: 30/04/02
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