CTD Module for ALACE

Introduction

The Autonomous Lagrangian Circulation Explorer (ALACE) (Davis et. al., 1992) is a sub-surface oceanographic float manufactured by Teledyne Webb Research Corporation (WRC). Deployed to depths up to 2000 meters, it surfaces at regular intervals for the ARGOS satellite system to determine its new drift position and to transmit in-situ measurements. The Sea-Bird CTD module takes advantage of each ascent to obtain the latest profile of temperature and salinity for transmission via ARGOS.

The value of near-real-time oceanographic data transmission from multiple floats is evidenced by the ability of oceanographers to monitor and predict the influence of El Niño weather patterns on the North Atlantic ocean surface (Riser, 1998).

Sea-Bird CTD Module photos

The CTD sensors and electronics (shown below) are mounted to an ALACE top end cap. (Satellite whip antenna not shown.)

Photo A: Sea-Bird CTD module with guard installed.	Photo B: Module with guard removed to show conductivity cell.
Photo C: Module showing mounted PCB.	Photo D: The opposite side of Photo C, showing the pressure sensor.

Features of the Sea-Bird CTD Module Design

1. Sea-Bird's CTD module uses the proven MicroCAT Temperature, Conductivity, and Pressure sensors. The CTD is shipped fully calibrated.

2.  In addition to its accurate initial calibration, the Sea-Bird CTD has demonstrated excellent long term stability; eliminating the need for post-deployment tampering of the calibration to force agreement with the local TS.

3.  The CTD module has carefully engineered anti-foul protection.

4. The CTD Module has proven TC-Ducted flow over the temperature sensor and into the conductivity sensor. Salinity spiking is minimized because the TC-Duct and pump precisely coordinate the T and C responses.

5. The CTD Module's premium strain-gauge pressure sensor has thermistor correction for ambient temperature effects.

6. The module is ruggedly constructed, with all exposed metal parts being titanium.

7. The pump delivers 40 ml/sec flow for 2.5 seconds per measurement of T, C, & P, and exhausts through a zero-thrust port.

Additional Information
(At right, Module mounted on a float at WRC)

The first Sea-Bird CTD modules were delivered to WRC in May 1997 and were deployed in the North Atlantic, east of Bermuda, by Dr. Steven Riser (School of Oceanography, University of Washington, Seattle) in August 1997.

For summary results of more than a year of profiling for float #63, visit:
- http://flux.ocean.washington.edu/atlantic/info/sbe41.html#063

Drift position and CTD profiles from these floats are provided by Dr. Riser on the web. 
For the latest profile from the module on float #063, visit:
- http://flux.ocean.washington.edu/atlantic/homographs/TP/063.html

Sea-Bird CTD ALACE floats have been launched from underway vessels (Voluntary Observing Ships-VOS). These floats (University of Washington serial numbers 125, 126) have been reporting successfully and their profiles can be found at:
- #125 http://flux.ocean.washington.edu/atlantic/homographs/TP/125.html
- #126 http://flux.ocean.washington.edu/atlantic/homographs/TP/126.html

Information on performance of SBE 41 / 41CP:
- Accuracy and Stability of Argo SBE 41 and SBE 41CP CTD Conductivity and Temperature Sensors (2008)

Information on development of a Surface Temperature Salinity sensor:
- Measurement of Salinity and Temperature Profiles through the Sea Surface on Argo Floats (December 2008)
- STS: An Instrument for Extending Argo Temeprature and Salinity Measurements through the Sea Surface (March 2008)

General information about ALACE floats, go to:
- Teledyne Webb Research Corp.
- WOCE Subsurface Float Data Assembly Center (WFDAC).
- Robot buoys are taking the ocean's pulse, Seattle Times, April 2, 2008.

Information on Oxygen sensors integrated with the SBE 41 / 41CP:
- SBE 43I brochure / specification sheet
- Assessing the Calibration Stability of Oxygen Sensor Data on Argo profiling floats using routine WOCE monitoring data from HOT (2008)
- A Year of Oxygen Measurements from Argo Floats (2003)

References

Davis, R.E., D.C. Webb, L.A. Regier, and J. Dufour (1992) "The Autonomous Lagrangian Circulation Explorer (ALACE)", Journal of Atmospheric and Oceanic Technology, V9, 264-285.

Riser, S.C. (1998). "The Distribution of 18-Degree Water in the North Atlantic During the Autumn of 1997 and the Winter of 1988", AGU 1998 Ocean Sciences Meeting Presentation OS11J-04. 

Last modified: 22-Jun-2009

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