The Sofar Spotter

Yellow and five-sided, 42 cm across and 31 cm tall, with a rounded bowl below. It weighs 7.45 kg; in Sofar's photographs it floats with its flange at the waterline. Each of the five sloping faces carries one of five 2-watt panels, which charge a 13,400 mAh lithium-ion battery. In the photographs, each of the five corners of the flange has a hole through it, which serves as a handhold. Under the bowl, a stainless steel fitting is the point where the mooring line is shackled.

On top, inside a black plastic flange, a clear lid held by captive screws covers the electronics; the switch and status lights show through it. An amber light flashes for half a second every 2.5 seconds, visible at least a mile away in normal conditions. The GPS antenna needs an open view of the sky, so the figure places it under the lid, with the battery below it. Both positions are estimates, since the layout inside isn't published.

A Spotter can drift free or be moored. Sofar calls mooring one a compromise between holding it in place and leaving it free to move, since its motion is the measurement. Its guidelines (a 2023 draft) recommend by default a sinking line in a 10 m loop to a small float, then down to chain and an anchor, so that the line restrains the buoy as little as possible: polypropylene line 8 to 10 mm thick, a 300 mm trawl float with about 10 kg of buoyancy for each loop, a vertical section 1.2 to 1.5 times the depth, 5 m of 6 mm chain, an anchor holding about 70 kg, and fittings of 316 stainless steel. Sofar advises mooring at depths of 5 to 300 m. How the four on Bluegraph are moored isn't published; 42358 is in 62 m of water.

As a wave passes, the water under it moves in circles: up and forward on the crest, down and back in the trough. The circles shrink with depth. The buoy is much smaller than the waves, so it circles with the water at the surface. GPS measures that motion (east, north and up) 2.5 times a second, in records of 30 minutes.

The 3-meter discus buoy measures accelerations and tilts and integrates them; the Spotter instead takes its displacements from GPS. It calculates its motion from the satellites' signals, to about 2 cm; Sofar says that depends on the antenna's view of the sky, the conditions and the state of the GPS system. Sofar doesn't publish how its receiver does this. GPS wave buoys generally use the Doppler shift of the signals, which gives the buoy's velocity far more precisely than its position, and integrate the velocity, filtering out the slow drift.

Deployed beside Datawell buoys, which measure with accelerometers, the Spotter's wave heights agreed within about 10% in one test. In a year-long test, the significant wave height differed by 0.06 m root mean square, the mean period by 0.27 s and the peak direction by about 22°.

On board, each 30-minute record is turned into spectra. The record is cut into windows of 256 readings (102.4 s), each overlapping the next by half, 33 in all, and their Fourier transforms are averaged. That gives the spectra of the up, east and north motion and the cross-spectra between them, at steps of about 0.0098 Hz. The statistics cover 0.03 to 0.8 Hz, periods of 33 s down to 1.25 s. The significant wave height is four times the square root of the area under the spectrum of the up motion.

Direction comes from how the horizontal motion lines up with the vertical motion. The water moves forward on the crest and back in the trough, so at each frequency the cross-spectra of up with east and with north point along the direction the waves travel. From the spectra, Sofar computes four numbers at each frequency, a1, b1, a2 and b2, which give the mean direction and how widely it spreads, and states directions as where the waves come from, clockwise from north.

In its spectrum mode a Spotter sends the spectrum itself every 1 to 6 hours. For the four on Bluegraph, NDBC publishes only the summaries (the wave height, the dominant period and the mean direction), so their station pages show those and no spectrum.

The sea-temperature sensor is in an insulated stainless steel penetrator in the hull. Sofar gives it an accuracy of 0.1 °C and a resolution of 0.02 °C, from −5 to 50 °C. NDBC gives its depth as 0.2 m at the three Great Lakes buoys and 1 m at 42358. A depth of 1 m suggests a sensor on the mooring line rather than in the hull (Sofar makes moorings that carry them), but the station page doesn't say.

A barometer inside measures air pressure, to 0.5 hPa, from 700 to 1,100 hPa. Of the four, only 42358 reports pressure to NDBC.

A Spotter has no anemometer. Sofar estimates the wind from the short waves, which grow as the wind strengthens; it reports errors of about 2 m/s for winds of 3 to 12 m/s. 42358 reports a wind speed in whole meters per second, and no direction. That is probably this estimate, derived from the waves rather than measured, though NDBC's page doesn't say.

The electronics include the GPS receiver, the processor that calculates the spectra, the barometer, and the satellite and cellular radios. Their layout inside isn't published. By default, a Spotter sends the wave statistics of two 30-minute records each hour, by Iridium satellite, or by a cellular network where one is available, falling back to satellite when that signal fails. On a cellular network it can be set to send as often as every 15 minutes. A memory card on board keeps the full record of its motion, spectra and positions, which is read when the buoy is recovered.

NDBC publishes the four buoys' reports along with its own. On Bluegraph they give the wave height, the dominant period, the mean wave direction and the sea temperature, and at 42358 the pressure and the wind speed too. In September and October 2026 the Great Lakes buoys reported about once an hour, and 42358 its pressure every half hour.