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.go

@@ -69,6 +69,7 @@ type satTracker struct {
 	rotStep   float64
 	rotMinE   float64
 	rotPark   bool
+	rotAzOnly bool
 	rotAz     float64 // last commanded, so a step smaller than the beamwidth costs nothing
 	rotEl     float64
 	rotSent   bool
@@ -101,6 +102,10 @@ type SatTrackStatus struct {
 	RotAz   float64 `json:"rot_az"`
 	RotEl   float64 `json:"rot_el"`
 	RotLive bool    `json:"rot_live"`
+	// RotAzOnly: the elevation is not being driven and RotEl means nothing.
+	// Sent so the panel can leave it out rather than draw an antenna lying on
+	// the horizon, which is what an undriven zero looks like.
+	RotAzOnly bool `json:"rot_az_only"`
 }
 
 // StartSatelliteTracking arms the radio and starts following the satellite.
@@ -138,6 +143,8 @@ func (a *App) StartSatelliteTracking(name string, transponder int) error {
 		} else {
 			t.rot = r
 			t.rotStep, t.rotMinE, t.rotPark = float64(set.RotStep), float64(set.RotMinEl), set.RotPark
+			t.rotAzOnly = set.RotAzOnly
+			t.status.RotAzOnly = set.RotAzOnly
 		}
 	}
 
@@ -415,7 +422,14 @@ func (t *satTracker) pointRotator(pos sat.Position, geostationary bool) {
 			return
 		}
 	}
-	if t.rotSent && math.Abs(az-t.rotAz) < t.rotStep && math.Abs(el-t.rotEl) < t.rotStep {
+	// In azimuth-only mode the elevation is never commanded, so comparing it
+	// would find a difference on every tick and send a command for nothing —
+	// the antenna ordered to the same bearing once a second for the whole pass.
+	moved := math.Abs(az-t.rotAz) >= t.rotStep
+	if !t.rotAzOnly {
+		moved = moved || math.Abs(el-t.rotEl) >= t.rotStep
+	}
+	if t.rotSent && !moved {
 		return
 	}
 	if err := t.rot.Point(az, el); err != nil {