Sea-Bird is developing a new, low-power CTD for autonomous gliders with the high accuracy necessary for research, updating ocean models, assessing sensor stability on moored observatories, and leveraging data collection opportunities from operational vehicle missions. Very recent improvements in our pump efficiency, coupled with new low-power electronics and high-efficiency energy conversion, have resulted in a continuously pumped CTD that consumes only 150 mW while sampling at 1 Hz. To put this into perspective, the energy contained in one D-size Alkaline cell (e.g., Duracell MN1300, 21 Watt-hours or 75.6K Joules, nominal) would power the CTD continuously for nearly 140 hours.
As gliders become commercial platforms used for scientific and operational purposes, the CTDs presently implemented on gliders need to transition from early designs that were made to function on extremely limited power reserves during the gliders' proof-of-concept phase, to commercially supportable payload sensors that are now practical with improvements made in glider power systems and efficiencies. The deeply embedded OEM-style integration of CTD components currently implemented in gliders greatly complicates field support, increases vehicle down-time for routine calibrations or service, and risks incidental parts damage to delicate components exposed to handling. Glider manufacturers are exposed to increased warranty risk because the glider pressure hull must be opened to remove the CTD, and end users experience unnecessarily degraded data accuracy and a lower level of support than Sea-Bird can provide for its standard commercial products.
Sea-Bird Glider Payload CTD.The Glider Payload CTD employs constant pumped flow through ducted temperature and conductivity sensors, and supports an optional modular dissolved oxygen sensor (SBE 43F). The superior dynamic accuracy of Sea-Bird's pumped and ducted measurement technique is well proven to produce more accurate data than non-pumped CTDs. Free-flushed (non-pumped) sensors experience variable flow rates along the glider flight path. Using glide slope and dP/dT (for example) to calculate flow rate is not sufficiently accurate to correct large dynamic errors induced at the top and bottom sections of the CTD profile by glider maneuvers, making substantive salinity error corrections in post-processing very tedious and unique to each profile. The Glider Payload CTD makes measurements at a known constant flow rate that are spatially and temporally well-coordinated. This allows application of proven processing algorithms common to all profiles and defensible on physical principles, to minimize unavoidable dynamic errors that cannot be addressed in-situ.
The field-swappable payload modularity necessitates that the sensors, electronics, and modular pump (and optional DO sensor) be a self-contained integrated package that should reside in a flooded space under the glider fairing. Only a small streamlined sail and trailing conductivity cell need to be positioned on the outside of the glider fairing. The CTD electronics housing is 6.35 cm (2.5 inches) diameter and 17.8 cm (7 inches) long. The CTD and pump will require a modest fixed-buoyancy offset in present gliders of approximately 250 grams (for 350-meter depth) or 490 grams (for 1500-meter depth). The new payload CTD design is inspired by the sail and pumped flow elements pioneered by Scripps on the Spray glider. End users will benefit from high-accuracy salinity data, efficient and affordable calibrations, and hardware and data support services from Sea-Bird. Glider makers will realize greatly reduced service, support, and warranty burden, compared to a CTD that is delivered as a tightly integrated set of OEM components.
The cost will be under $10,000 USD for either 350-meter or 1500-meter CTDs, commensurate with commercial off-the-shelf CTDs having equal performance and capabilities. A faster sampling version (e.g., 16 Hz) of the Glider Payload CTD with higher flow rate (and power consumption) is envisioned for larger, higher-speed, propelled AUVs at approximately the same prices, and a deeper version is possible at a small premium. We encourage feedback from the Glider and AUV community, particularly from those who wish to participate in a vehicle-specific applications dialog. The conceptual design of the Glider Payload CTD permits flexibility in the orientation of the sensors in relation to the main housing by modifying the neck connecting the sail to the housing. Other housing dimensions could also be considered.
Preliminary brochure / specification sheet for the Glider Payload CTD.