The WII6 buoy

• 1: Electronics
• 2: Hulls and boxes

A WII6 buoy sits on sea ice or floats at the ice edge and measures how the surface moves. It records motion, computes the wave statistics on board, and sends the selected data over Iridium. The full record is written to the card.

Built to your specification

Each buoy is built for its deployment. The figures on this site are examples; these are the choices you make:

Choice	Examples
Motion sensor	a single-axis Kistler accelerometer, a Sparton AHRS-M2, or the sensor your analysis needs
Sampling rate	8 Hz and 64 Hz are typical
Capture length	fifteen minutes is typical; shorter or longer to suit the site
Reporting interval	from every few minutes to once a month
Heading	from the motion sensor, or dual-antenna GNSS for accurate heading near the poles
Temperature	a probe arrangement to suit the deployment
Enclosure	an ice-surface box, a floating hull, or your own platform

What it measures

Sensor	Measurements and purpose
Motion sensor	acceleration and orientation for the wave maths
GNSS	position and time from a single receiver
Heading	from the motion sensor or a dual-antenna GNSS receiver
Temperature probes	air and water temperature, according to the deployment’s probe arrangement
Power monitoring	power and voltage measured on multiple buses to track the buoy’s health

What it sends

The smallest message is 326 bytes of wave results: significant wave height, peak period, the power spectrum, direction and quality flags. Position and buoy health can be sent alongside them.

You choose which captures send just the power spectrum, which send a 2 Hz record, and which send the full-rate record. For example, for a fifteen-minute capture the 2 Hz record is about 56 kB and fits in a single Iridium Certus message. It lets you rerun the later processing stages on shore with different settings.

Full-rate transfers use more airtime and cost more, so the 2 Hz record is the practical choice for most satellite reprocessing requests. The full record remains on the card for recovery. Every record, the full record included, carries the filtering the motion sensor applies internally.

What changed from WII5

Feature	WII5 v2	WII6
Main processor	ATmega2560 at 11 MHz, 8 kB RAM	STM32U575, Cortex-M33 at 160 MHz, 786 kB SRAM
Wave maths	on a Raspberry Pi woken for the job	on the main processor, using SRAM
Satellite	Iridium SBD, 340 bytes a message	RockBLOCK 9704, Iridium Certus, about 100 kB a message
Firmware	Arduino	Zephyr RTOS
Debug and local access	Raspberry Pi	ESP32-S3 with WiFi, off in deployment
Storage	two SD cards	one SD card, a write-once record layout
Power	per-peripheral switches	switched supplies, with power and voltage measured on multiple buses

Earlier generations used a Kistler single-axis accelerometer with a lower-cost inertial measurement unit (IMU); WII5 used a Sparton motion sensor. Either can be fitted to WII6. The heritage page traces that development.

Where it stands

The WII6 electronics are on the bench on development boards. The Sparton streams into the maths on the STM32, captures go to the card, bench buoys post to the WII6 cloud and take commands back, and the sleep and the firmware update have been exercised. A bench buoy has run its cycle unattended for a week: an hourly capture, its result posted to the cloud, and the main processor asleep in between. Revision A of the main board is drawn and routed. The news has the detail. If you are planning a season, get in touch for the current development schedule.

• Electronics: the board, power monitoring and firmware.
• Hulls and boxes: what the enclosure has to survive and the forms it takes.
• Uses: where a buoy can go, and what it can be fitted to.

1 - Electronics

The board

Satellite

The modem is a RockBLOCK 9704, Ground Control’s Iridium Certus modem: messages of up to 100 kB each way through the Iridium network, which is what lets a 2 Hz record travel, and the path for commands and firmware images from shore. The firmware talks to it through rockblock9704-zephyr, a Zephyr module built on Ground Control’s own C library, pinned and unchanged, with the one thread the library needs, a devicetree binding and a shell command. The module is open source under the MIT licence and is shared with Digital Dimensions’ other remote monitoring products. On the bench the modem’s identity and signal have been read through the module; the first message over the network is next, once the account is provisioned.

Power

Power and voltage are measured on multiple buses, alongside battery monitoring, so a deployment can track energy use and the buoy’s health. Switched supplies let the firmware power down peripherals between jobs, and between captures the main processor sleeps in a low-power stop mode.

Duty cycle

Capture length and reporting interval are set for each deployment, from reports every few minutes to once a month.

The power budget depends on time spent sampling, processing and sending data, as well as consumption between wake-ups. Sleep consumption matters most when the buoy spends long periods waiting for its next capture.

Firmware

Zephyr RTOS, with the wave maths as a library that builds and tests the same way on a workstation and on the processor. The bench image is a hardware test suite: each peripheral exercised on its own, with a shell for all of it and a JSON status frame once a second to the dashboard.

Both processors boot through MCUboot. A new image reaches the main processor through the debug processor, runs on trial until its first transmit confirms it, and is reverted by a reset before then. A tampered or unsigned image is refused. The maths library, the card layout, the link protocol and the update path are documented in the engineering repository. Ask us for the documentation if you are planning an integration.

2 - Hulls and boxes

The enclosure drives almost everything downstream: battery size, sensor mounting, what the antenna can see, and how the buoy behaves when the ice moves. WII6 is designed to fit the hulls that carried WII5, and the same rules apply.

Ice survival

The enclosure lives on or in sea ice for months: freeze-in, freeze-thaw cycles, ridging, and the loads when floes collide.

• Sealing under thermal cycling. Gaskets that work in a warm workshop can fail in a polar winter. Materials are chosen for low-temperature flexibility and the full assembly is cold-soaked before deployment.
• No external moving parts. Anything that hinges, slides or rotates can freeze open or shut.
• Drainage. The outer hull needs to shed water before it collects and freezes. The electronics compartment stays sealed, with drainage paths kept outside it.

Temperature

The enclosure has to cope with Antarctic winter temperatures and direct sun on a dark surface in spring. Wind increases heat loss from a warm enclosure. Battery chemistry and insulation are chosen for the expected temperatures and the current needed while the buoy is awake.

Heat from the electronics contributes to the internal temperature. The battery also needs to work after long periods asleep. Cold testing checks the assembled system. Desiccant and a small internal air volume help manage condensation as it warms up.

Weight

Weight matters three times: at deployment, where buoys go over the side by hand from ships, hovercraft and helicopters; on the ice, where a buoy too light blows around and one too heavy punches through; and in shipping, where every kilogram is multiplied by the fleet.

Staying put

A buoy on the surface has to stay where it was placed. Spikes on the underside lock it to the ice without penetrating so far that it cannot be recovered when the surface refreezes. The spike pattern and material are matched to the ice regime.

Antenna and sky

The Iridium and GNSS antennas need a clear view of the sky. The top is radio-transparent or carries an external antenna, and nothing on the surface shadows it or collects snow that would.

Forms

• An ice-surface box with spikes underneath.
• A floating hull with a weighted keel for the ice edge.
• A tethered variant for specific deployment geometries.

Talk to us about the site and the season and we will recommend the form.