What the number means
The drifter reports significant wave height, the standard sea-state figure that buoy networks and marine forecasts use. It is close to what an experienced observer would call the wave height by eye: the average height, trough to crest, of the largest one-third of the waves passing through. A reading of 0.5 m means the bigger waves in the current sea are running about half a meter. Smaller ripples are still there; they just don't drive this number.
It appears on the map and in the observation feed next to the drifter, in meters, alongside its position and the other readings the beacon sends.
How the drifter measures it
The drifter floats freely and carries a motion sensor, the same kind of accelerometer-and-gyroscope unit that steadies a phone or a drone. As waves lift and drop the hull, that sensor feels the motion. Turning that raw motion into an honest wave height takes three steps.
1. Find which way is up
A free-floating drifter tilts and spins; its sensor's idea of "up" is wherever the hull happens to be pointing. Before it can measure vertical motion, the drifter works out its true orientation by combining two signals: the gyroscope, which tracks turning accurately over short spans, and gravity, which gives a steady long-term reference for down. Trusting the gyroscope moment to moment keeps the orientation steady while the drifter bobs, so wave motion isn't mistaken for tilt.
2. Isolate the up-and-down motion
With orientation known, the drifter projects its motion onto the true vertical and subtracts gravity. What is left is heave: the pure rise and fall of the surface, with rocking and sideways drift removed. On flat water this settles to about zero.
3. Turn motion into a wave height
The drifter collects about three and a half minutes of heave, then sorts that motion by frequency to see how much wave energy sits at each wave speed. The total energy across the wave band is the variance of the sea surface, and significant wave height follows from it by the standard oceanographic relationship.
The formula. Significant wave height is four times the square root of the surface-height variance (Hs = 4 × √variance). It is the same definition national buoy networks publish, so a Hover drifter's reading lines up with what marine forecasters expect.
What to expect from a reading
| Property | Behavior |
|---|---|
| Units | Meters, on the map and in the observation feed. |
| First reading | About three and a half minutes after the drifter starts, once it has gathered a full window of motion. Before that it reports nothing. |
| Calm water | Reads close to zero. A built-in floor keeps slow sensor drift from registering as a false swell on calm water. |
| Wave range | Tuned for waves spaced roughly 2 to 12.5 seconds apart, which covers typical coastal and bay conditions. Very long-period ocean swell beyond that range is not captured. |
| Update | Each reading reflects the last few minutes of sea state, so it tracks changing conditions smoothly rather than reacting to a single big wave. |
Because the measurement averages minutes of motion across the wave spectrum, it stays steady and meaningful instead of spiking on a single large wave. A growing sea shows up as a rising height across successive readings.
Next: the companion reading from the same motion analysis.
Wave period →