Ice-Tethered Profiler (ITP) Technology

Introduction

A complete description of the ITP technology is provided in WHOI Technical Report 2006-11, down-loadable to the right.

The ITP system consists of three components: a surface instrument package that sits atop an ice floe or floats in open water, a weighted, wire-rope tether of arbitrary length (up to 800 m) suspended from the surface package, and an instrumented profiler that travels up and down the wire tether.  A schematic drawing of the instrument is given to the right.

Surface package

The surface expression of the ITP is a cylindrical/conical buoy that is deployed on an ice floe or floating in the water.  The surface capsule houses a controller, inductive modem electronics, a GPS receiver and an Iridium satellite phone with associated antennae and batteries within a water tight aluminum housing capped by an ultra high molecular weight (UHMW) polyethylene dome.  The electronics case sits within a cylinder of Surlyn ionomer foam designed to provide buoyancy for the plastic-jacketed wire rope tether and end weight should the ice fracture or melt (the latter designed to minimize wire angles when the supporting ice floe moves), and provide modest protection in the event of ice ridging. The Iridium and GPS antennae are mounted internally on a chassis beneath the microwave-transparent UHMW hemispherical radome that forms the top endcap.

Individual battery slices (each nominally 1250 Wh) supply power to the surface electronics. The surface package can operate independently after the profiler exhausts its battery or the tether is damaged by ridging events, with lifetimes of up to 5 years.

Wire tether

The tether is constructed from conventional plastic-jacketed wire rope commonly used in ocean mooring applications.  The upper 5-m of the wire tether is cast within a thick protective urethane jacket that also houses an electrical ground lead for the inductive modem.  A custom termination is used to mechanically join the tether to the surface unit and preserve the electrical isolation of the wire tether from the sea water. Shock-absorbing spring bumpers are clamped onto the wire at the top and bottom of the programmed profiling interval to prevent the profiler from impacting the tether terminations.  A 250 lb ballast weight (made of four 65 lb pieces to facilitate transportation) is fixed to the bottom wire termination to add tension to the wire and minimize its catenary when the supporting ice floe moves.

Profiler

A variation of the WHOI Moored Profiler (in shape and size much like an Argo float) mounts on the tether and cycles vertically along it.   But unlike a float that changes its buoyancy to profile, the ITP uses a small traction drive wheel mounted midway along its body to move up and down.  Typically the drive system draws only 1 W of power as the ITP profiles at 0.25 m/s.  The ITP employs the same CTD sensor package that is currently used on many of the Argo floats (the Sea Bird Electronics, Inc. model 41CP) mounted in the instrument’s top hemispherical end cap. Communication between the Profiler and surface controller is supported by an inductive modem utilizing the wire tether.

The cylindrical pressure case houses the batteries, drive system, instrument controller, CTD and underwater inductive modem (UIM) as well as provides buoyancy for the unit.  The underwater vehicle is ballasted to be neutrally buoyant near the midpoint of its profiling interval.  The instrument controller and software are products of McLane Research Laboratories, Inc. and use the same electronic hardware and software scheme as the full-depth-capable McLane Moored Profiler.  The instrument acquires profiles based on user programmable sampling depths and schedule. The lifetime of the profiler is up to 2 years when collecting two 750 m profiles per day.

Data

The raw CTD and associated engineering data files are relayed from the underwater vehicle to the surface buoy at the completion of each one-way profile, which then transmits them to a logger computer at WHOI via satellite.  Full-resolution CTD and engineering data are transmitted to shore.

At regular interval thoughout the day, the data files that arrived on the WHOI logger computer are accessed by a separate computer that unpacks the binary data files, performs basic edits, averages the profile data into convenient 2 -db pressure bins, produces plots of the CTD and engineering data and saves the data.  These plots and the associated data files are accessible from the ITP Data page.  Additional documentation of the data processing procedures are provided there.

Since all acquired data are transmitted to shore at full resolution and the hardware is relatively low-cost (in comparison to ship time), ITP systems may be considered expendable (thus alleviating the need for expensive recovery operations to collect the data).

Hardware specifications

• Tether length: 10 up to 800 m
• Profiling range: 1,500,000 m on standard battery pack
• Duration: 2.5 to 3 years returning two 750 m profiles (one profile in each direction) per day (2000 total)
• Temperature specification: approximately -25° C (Iridium transmitter is limiting component)
• Data rate: typically 50 Kbytes per profile (totaling 100 Mbytes over 3 years)
• Telemetry: Seabird inductive link from profiler to surface unit; Iridium satellite link to shore
• Data backup: on-board storage in both underwater and surface units; data retained until successfully telemetered
  
• Sensors: Seabird CP-41 CTD (conductivity, temperature, pressure); optionally dissolved oxygen, chlorophyll fluorescence, optical backscatter, ocean color, photosynthetically-available radiation, velocity
• Power: lithium BCX “DD” battery packs; 2600 Wh in surface package (after derating for temperature), 2500 Wh in profiler
• Size: Profiler fits through an 11 inch hole in the ice