fix(rotor): wind is not rotation

"I thought we had fixed this" — and we had not, we had only raised the number.
The antenna moves a few degrees in the wind and the Stop button lights up and
goes out, all evening, on a mast that has not turned.

A threshold cannot tell the two apart at any setting. A gust pushes a beam past
four degrees and brings it back — 100°, 105°, 100°, 106° — and every one of those
excursions cleared the threshold. Raising it only raises the wind speed it takes,
and blunts the detection of a rotor genuinely creeping.

What separates a rotation from the weather is not amplitude, it is SIGN. A rotor
under power advances; a gust reverses. So a step is only movement when the
previous step went the same way: wind gives +5, −5, +5 and never two in a row,
while a rotor gives +4, +4, +4 and is announced on the second — one poll, about
a second, on a mast that takes half a minute to cross a pass.

The threshold stays, doing the job it is actually good at: ignoring the degree
or two of potentiometer noise that is not a step at all.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

frontend/src/components/RotorCompass.tsx

@@ -45,15 +45,17 @@ type RotorPreset = { label: string; azimuth: number };
 // rather than reported: a heading that changes by more than a degree means it
 // is, and it is considered stopped once the readout has been still for a while.
 const MOVEMENT_SETTLE_MS = 1600;
-// Four degrees, not one.
+// Four degrees: the band inside which a reading is not a step at all.
 //
-// A rotor at rest does not report a constant heading: the potentiometer and the
-// controller's rounding walk the reading a degree or two either side, and at one
-// degree that jitter WAS movement — Stop lit for a second and a half, went out,
-// and lit again, for an antenna that had not turned all evening. The reference
-// is only moved when the threshold is crossed, so a rotor genuinely turning
-// accumulates towards it however slowly it goes; noise around a value never
-// gets there.
+// A rotor at rest does not report a constant heading — the potentiometer and
+// the controller's rounding walk the reading a degree or two either side. But
+// the threshold is only half the answer, and on its own it was not enough: WIND
+// moves a beam further than four degrees and back again, and every one of those
+// excursions counted, so Stop lit and went out all evening on an antenna that
+// had not turned. Raising the number only raises the wind speed it takes.
+//
+// What actually separates a rotation from the weather is the DIRECTION — see the
+// movement effect below.
 const MOVEMENT_TRIGGER_DEG = 4;
 // How long an order is given to produce movement before the widget stops
 // claiming the antenna is turning — the rotor may already have been there.
@@ -517,6 +519,10 @@ export function RotorCompass({
   const latestAzimuthRef = useRef<number | null>(displayAzimuth);
   const movementReferenceRef = useRef<number | null>(displayAzimuth);
   const movementSeenRef = useRef(false);
+  // Which way the last accepted step went (+1 CW, −1 CCW, 0 none), and how many
+  // in a row have gone that way. Two make it a rotation; one is weather.
+  const movementDirRef = useRef(0);
+  const movementRunRef = useRef(0);
 
   const rememberTarget = (value: number | null) => {
     if (value == null) rememberedTargets.delete(rotorKey);
@@ -547,6 +553,8 @@ export function RotorCompass({
     setTargetFading(false);
     setIsMoving(rememberedTarget != null);
     movementSeenRef.current = false;
+    movementDirRef.current = 0;
+    movementRunRef.current = 0;
     window.clearTimeout(movementTimerRef.current);
     window.clearTimeout(commandTimerRef.current);
     window.clearTimeout(targetArrivalTimerRef.current);
@@ -564,21 +572,49 @@ export function RotorCompass({
 
   // Movement is inferred from the readout itself, so a rotor turned by its own
   // controller — or by another program — reads as moving here too.
+  //
+  // The test is the DIRECTION, not the size of the step.
+  //
+  // A threshold alone does not work, whatever it is set to. Wind pushes a beam
+  // off its bearing and back — 100°, 105°, 100°, 106° — and every one of those
+  // excursions clears a four-degree threshold, so Stop lit and went out all
+  // evening on an antenna that had not turned. Raising the number only raises
+  // the wind speed it takes.
+  //
+  // What separates the two is not amplitude but sign: a rotor under power
+  // advances, gust after gust reverses. So a step is only movement when the
+  // PREVIOUS step went the same way. Wind gives +5, −5, +5 and never two in a
+  // row; a rotor gives +4, +4, +4 and is announced on the second — one poll,
+  // about a second, on a mast that takes half a minute to cross a pass.
   useEffect(() => {
     if (rawAzimuth == null) return;
     const reference = movementReferenceRef.current;
     if (reference == null) { movementReferenceRef.current = rawAzimuth; return; }
-    if (angularDistance(rawAzimuth, reference) >= MOVEMENT_TRIGGER_DEG) {
-      movementSeenRef.current = true;
-      setIsMoving(true);
-      window.clearTimeout(commandTimerRef.current);
-      window.clearTimeout(movementTimerRef.current);
-      movementTimerRef.current = window.setTimeout(() => {
-        setIsMoving(false);
-        movementSeenRef.current = false;
-      }, MOVEMENT_SETTLE_MS);
-      movementReferenceRef.current = rawAzimuth;
+    // Signed, and the short way round: crossing north is a small step, not 350°.
+    const delta = ((rawAzimuth - reference + 540) % 360) - 180;
+    if (Math.abs(delta) < MOVEMENT_TRIGGER_DEG) return; // inside the noise band
+    const sign = delta > 0 ? 1 : -1;
+    if (movementDirRef.current === sign) {
+      movementRunRef.current += 1;
+    } else {
+      movementDirRef.current = sign;
+      movementRunRef.current = 1;
     }
+    movementReferenceRef.current = rawAzimuth;
+    if (movementRunRef.current < 2) return; // one step either way is weather
+
+    movementSeenRef.current = true;
+    setIsMoving(true);
+    window.clearTimeout(commandTimerRef.current);
+    window.clearTimeout(movementTimerRef.current);
+    movementTimerRef.current = window.setTimeout(() => {
+      setIsMoving(false);
+      movementSeenRef.current = false;
+      // Forget the direction too: the next real move starts its own run rather
+      // than inheriting one from a rotation that finished minutes ago.
+      movementDirRef.current = 0;
+      movementRunRef.current = 0;
+    }, MOVEMENT_SETTLE_MS);
   }, [rawAzimuth, rotorKey]);
 
   // Arrival: confirmed over time, then faded. Every check re-reads the