A GPS anchor drag alarm turns a string of satellite fixes into a single yes-or-no question: is my boat still where I anchored it? Here's exactly how it does that, and why the quality of the GPS makes or breaks the answer.
Step 1: Set the reference point
When you drop the hook and settle back, you tell the alarm "this is home." It records your current GPS position as the reference point. Ideally you set it once the anchor has bitten and you've backed down on it, so the reference reflects where the boat actually rests — not where you first dropped.
Step 2: Define the watch radius
You then set a watch radius — a circle around the reference point that represents your safe swinging room. Choose it from your boat length, the scope you've let out, and the depth. A boat in 5 m of water on a 5:1 scope can legitimately swing across a wide circle, so the radius has to be generous enough to cover normal movement but tight enough to catch real drag early.
Step 3: Watch the swing circle
Here's the subtlety. A boat at anchor never sits still — it sails back and forth across the full width of its swing circle as the wind shifts. A good drag alarm understands that this arc is normal. It only raises the alarm when your position crosses the radius boundary in a way that indicates the whole circle has moved, i.e. the anchor has let go and you're being carried downwind.
Why GPS accuracy decides everything
This is where hardware matters. If your GPS drifts 5–15 m (typical for a phone), the alarm can't tell a wandering fix from real movement — so it either false-alarms constantly or has to use a radius so wide it's useless. A dedicated multi-GNSS receiver listens to GPS, GLONASS, Galileo and BeiDou at once, holding 1–2 m accuracy with a one-second update rate. Tighter, steadier fixes mean the alarm can use a smaller, safer radius and still avoid waking you for nothing.
That accuracy gap is the single biggest reason a free app cries wolf — more in why anchor alarm apps fail. To put the right radius in practice, see how to set an anchor alarm, or step back to the complete guide to anchor alarms.
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