feat(sat): follow the azimuth only

A satellite tracker that insists on an elevation motor is a tracker switched off
for nearly everybody. A pass at the edge of the footprint — which is most of
them — never climbs above ten or fifteen degrees for its whole length, and a
yagi's beamwidth swallows that: the bearing alone is enough, and it is how most
stations that work satellites are actually built. The same switch rescues an
az/el station whose elevation motor has failed.

So it is an option, not a silent fallback, because it does cost something: a
bird straight overhead is a moving azimuth and a bearing that means nothing, and
whether to accept that is the operator's call. With it on, any rotor in the list
can be chosen — the PstRotator, the Rotator Genius, the ARCO, the tower already
turned for HF.

That works because the per-backend command dispatch moved out of the three
RotatorGoTo/Stop/Heading methods into linkGoTo/linkStop/linkHeading, so the
satellite tracker drives any of the seven backends through the same code the
compass uses instead of a second implementation of each. GetRotatorHeading loses
sixty lines of near-duplicate switch in the process, and a rotor with no
elevation axis now says so (HasElevation) rather than reporting a zero that
looks like a real bearing.

One trap, with a test on it: the step check compared both axes, so with the
elevation never commanded its difference stayed above the step for the whole
pass and every tick ordered the antenna to the bearing it was already on. A mast
has a finite number of turns in it.

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

app_sat_track_test.go

@@ -45,3 +45,64 @@ func TestSatModeLetters(t *testing.T) {
 		}
 	}
 }
+
+// Azimuth-only tracking must not command the rotor once a second.
+//
+// The step check used to compare BOTH axes, so with the elevation never
+// commanded its difference stayed above the step for the whole pass and every
+// tick sent the antenna to the bearing it was already on. A rotator is a
+// mechanical thing with a finite number of turns in it.
+func TestPointRotatorAzOnlyIgnoresElevation(t *testing.T) {
+	rec := &countingRotator{}
+	tr := &satTracker{rot: rec, rotStep: 5, rotAzOnly: true}
+
+	// The satellite climbs while the bearing barely moves — a pass going
+	// overhead from the side, which is the shape that provoked this.
+	for _, p := range []sat.Position{
+		{Az: 100, El: 5},
+		{Az: 101, El: 20},
+		{Az: 102, El: 45},
+		{Az: 103, El: 70},
+	} {
+		tr.pointRotator(p, false)
+	}
+	if rec.n != 1 {
+		t.Errorf("azimuth-only sent %d commands for 3° of bearing, want 1", rec.n)
+	}
+	if rec.lastAz != 100 {
+		t.Errorf("commanded azimuth %v, want the first one", rec.lastAz)
+	}
+
+	// And it still follows the azimuth when the azimuth actually moves.
+	tr.pointRotator(sat.Position{Az: 130, El: 70}, false)
+	if rec.n != 2 {
+		t.Errorf("a 30° swing was not followed: %d commands", rec.n)
+	}
+}
+
+// With an elevation axis, a climb is still followed.
+func TestPointRotatorFollowsElevationWhenItCan(t *testing.T) {
+	rec := &countingRotator{}
+	tr := &satTracker{rot: rec, rotStep: 5}
+	tr.pointRotator(sat.Position{Az: 100, El: 5}, false)
+	tr.pointRotator(sat.Position{Az: 101, El: 40}, false)
+	if rec.n != 2 {
+		t.Errorf("a 35° climb was not followed: %d commands", rec.n)
+	}
+	if rec.lastEl != 40 {
+		t.Errorf("commanded elevation %v, want 40", rec.lastEl)
+	}
+}
+
+type countingRotator struct {
+	n              int
+	lastAz, lastEl float64
+}
+
+func (c *countingRotator) Point(az, el float64) error {
+	c.n++
+	c.lastAz, c.lastEl = az, el
+	return nil
+}
+func (c *countingRotator) Heading() (float64, float64, bool, error) { return 0, 0, false, nil }
+func (c *countingRotator) Close()                                   {}