The 3-meter discus buoy

An aluminum disk 3.0 m across, or about 3.35 m including its black rubber fender. NDBC adapted it in 1982–83 from a Woods Hole design. The flat yellow deck is about 0.41 m above the water, and the hull has a draft of 0.46 m: below a chine just under the waterline, it narrows in a shallow cone. It weighs about 1,720 kg. Aluminum is less likely to corrode, and it doesn't disturb the compass.

Inside, six watertight buoyancy compartments surround a central well 0.91 m across and 1.02 m deep, under a hatch in the middle of the deck. The well once held the electronics and batteries on a rack. The current payload is mounted above the deck, so about 450 kg of ballast goes into the hull instead. The figure shows the six bulkheads evenly spaced, because their exact layout isn't published.

Three lifting eyes stand at the edge of the deck, 120° apart, where the legs of the frame meet the deck. Under the hull, a rigid three-legged steel bridle meets at a point 2.5 to 2.9 m down, where the mooring hangs from a swivel. The mooring is all chain in shallow water, and nylon and polypropylene in deep water, down to an anchor of a few metric tons. The bridle steadies the hull, at a small cost to how closely the hull follows the slope of the waves. Buoys in the Pacific also carry a seal cage, a ring of bars around the frame (not drawn).

NDBC's current payload, SCOOP, puts the electronics in sealed tubes on the frame instead of in the hull. The weather tube carries two masts. The taller one holds an R.M. Young propeller-vane anemometer: an 18 cm four-bladed propeller, kept facing the wind by its vane. The shorter one holds an all-in-one sensor that has no moving parts and measures wind with sound, along with air temperature, humidity and pressure. NDBC's documents disagree about which of the two is the primary wind sensor.

Below the weather tube, four solar panels lean around a lithium battery. The wave tube stands near the center of the hull, close to the waterline. The tubes communicate with each other by short-range radio, and with shore by Iridium satellite. A yellow LED light marks the buoy at night.

Nothing on the buoy measures the water's surface. The wave tube holds a motion sensor (accelerometers, angular-rate sensors and a magnetometer on three axes). Twice an hour, it records the hull's heave, pitch, roll and heading 1.7066 times a second for 20 minutes. A disk this size follows the surface, rising and falling with the water at its center and tilting with its slope, so NDBC treats the hull's motion as the height of the water and its slope in two directions.

On board, the angular rates are integrated into pitch and roll, and the compass converts them into the slope of the water toward the east and toward the north. The record is turned into spectra in 46 frequency bands, from 0.0325 Hz to 0.485 Hz (periods of about 31 s down to about 2 s), and only the spectra are sent from the buoy. On shore, the heave spectrum is cleaned of low-frequency noise, corrected for the response of the hull and mooring, and divided by (2πf)⁴ to convert acceleration into height. Significant wave height is four times the square root of the area under the spectrum.

Direction comes from how the hull tilts as it rises and falls. At each frequency, the cross-spectra of heave and slope give a mean direction and a measure of spread, which NDBC publishes as α1, α2, r1 and r2. They describe only a broad distribution (its half-power width is about 88°). Waves shorter than about 6.6 m (above 0.485 Hz), about twice the hull's width, aren't reported, because the hull rides over them. NDBC states that its wave measurements are accurate to 0.2 m in height, 1 s in period and 10° in direction. Wave Intelligence separates the wave systems in these spectra.

Wind is averaged over 10 minutes, six times an hour: the speed as a simple mean, and the direction as a mean of unit vectors. The gust is the highest 5-second running mean in the period. The vane reads the direction relative to the hull, which turns freely on its mooring, so the buoy's compass converts the reading into a direction from magnetic north on board, and from true north on shore.

The anemometer is 3.8 m above the water. NDBC publishes the wind as measured at that height. It also estimates the wind at 10 m from the air and sea temperatures; in neutral air, that estimate reads roughly a tenth higher.

Air temperature and humidity come from the all-in-one sensor 3.4 m up, averaged over 10 minutes; the dew point is calculated from them on shore. Pressure is measured 2.4 m above the water as a 1-minute mean, six times an hour, and corrected on shore for that height. The 3-hour change in pressure is reported on the hour.

Sea temperature comes from a digital probe clamped to the bridle 2 m below the waterline. It is read six times an hour and reported as measured. Earlier payloads used a thermistor inside the hull, 0.7 m down.

Weather readings go to shore every 10 minutes and wave readings every 30 minutes, by Iridium satellite to a ground station in Hawaii. NDBC combines each buoy's messages into one report in about five minutes, checks it automatically and publishes it as provisional data. Analysts review it within about a day.