fix(bandopen): the moon is not a band opening

A 2 m opening was announced at 9650 km towards Japan. The callsigns gave it
away — 7M4RRM, JA7RPC, JF1AWC — all working EME, which is routine on 2 m and
reported to PSK Reporter like anything else. Real contacts, real grids, and no
evidence whatsoever about the band: an operator turning a beam on that bearing
would hear nothing.

Removing the flat 2400 km ceiling was right; it threw away genuine multi-hop Es
on 6 m. "No limit anywhere" was the overcorrection. The limit is per band now,
and it is physics rather than a threshold: 2 m reaches a few thousand kilometres
by tropospheric duct or a chain of Es clouds and no further, so 3500 km, and
4 m 4000. Six and ten metres keep no ceiling — multi-hop Es and F2 genuinely do
go round the world, which is the case that started all this.

Pinned from both sides: the 9650 km Japanese burst produces nothing, and a
2000 km 2 m burst in one sector still counts.

changelog.json

@@ -3,10 +3,12 @@
     "version": "0.24.7",
     "date": "",
     "en": [
-      "QSOs logged from WSJT-X now carry the space weather and the distance, like hand-logged ones. The UDP path stamped the station profile, the DXCC and the QSL defaults but not SFI, A, K or distance — so an operator running digital had those fields empty across the whole log. Space weather is only stamped on a contact less than a day old: a logger re-broadcasting its backlog would otherwise be handed this morning readings for last month contacts."
+      "QSOs logged from WSJT-X now carry the space weather and the distance, like hand-logged ones. The UDP path stamped the station profile, the DXCC and the QSL defaults but not SFI, A, K or distance — so an operator running digital had those fields empty across the whole log. Space weather is only stamped on a contact less than a day old: a logger re-broadcasting its backlog would otherwise be handed this morning readings for last month contacts.",
+      "Band openings: EME contacts are no longer mistaken for an opening. A 2 m opening was announced at 9650 km towards Japan on stations working moonbounce — real contacts, but the moon says nothing about the band, and an antenna pointed that way finds nothing. Each band now has the longest path the atmosphere can actually carry: 3500 km on 2 m, 4000 on 4 m, and no limit at all on 6 and 10 m where multi-hop really does go round the world."
     ],
     "fr": [
-      "Les QSO enregistrés depuis WSJT-X portent désormais la météo spatiale et la distance, comme ceux saisis à la main. Le chemin UDP posait le profil station, le DXCC et les défauts QSL mais ni SFI, ni A, ni K, ni distance — un opérateur en numérique avait donc ces champs vides sur tout son log. La météo spatiale n est posée que sur un contact de moins d un jour : sinon un logiciel qui rediffuse son historique se verrait attribuer les relevés de ce matin sur des contacts du mois dernier."
+      "Les QSO enregistrés depuis WSJT-X portent désormais la météo spatiale et la distance, comme ceux saisis à la main. Le chemin UDP posait le profil station, le DXCC et les défauts QSL mais ni SFI, ni A, ni K, ni distance — un opérateur en numérique avait donc ces champs vides sur tout son log. La météo spatiale n est posée que sur un contact de moins d un jour : sinon un logiciel qui rediffuse son historique se verrait attribuer les relevés de ce matin sur des contacts du mois dernier.",
+      "Ouvertures de bande : les contacts EME ne sont plus pris pour une ouverture. Une ouverture 2 m était annoncée à 9650 km vers le Japon sur des stations en rebond lunaire — de vrais contacts, mais la Lune ne dit rien de la bande, et une antenne pointée par là ne trouve rien. Chaque bande a désormais la distance maximale que l atmosphère peut réellement porter : 3500 km en 2 m, 4000 en 4 m, et aucune limite en 6 et 10 m où les sauts multiples font vraiment le tour du monde."
     ]
   },
   {

internal/bandopen/bandopen.go

@@ -81,6 +81,28 @@ var watched = map[string]bool{"10m": true, "6m": true, "4m": true, "2m": true}
 // Watched reports whether a band is one the detector looks at.
 func Watched(band string) bool { return watched[strings.ToLower(strings.TrimSpace(band))] }
 
+// maxTerrestrialKm is the longest path a band can carry through the atmosphere.
+// Zero means no limit.
+//
+// The flat 2400 km ceiling was removed because it threw away real multi-hop Es
+// on 6 m, and that was right — but "no limit anywhere" then let something else
+// through. A 2 m opening was announced at 9650 km towards Japan, on stations
+// that were unmistakably working EME: the moon is not an opening, and pointing
+// an antenna at that bearing would find nothing.
+//
+// So the limit is per band, and it is physics rather than a threshold. Two
+// metres reaches a few thousand kilometres by tropospheric duct or a chain of Es
+// clouds and no further; beyond that the path went via the moon or a satellite,
+// neither of which says anything about the band. Six and ten metres have no
+// ceiling at all — multi-hop Es and F2 genuinely go round the world.
+var maxTerrestrialKm = map[string]int{
+	"2m": 3500,
+	"4m": 4000,
+}
+
+// MaxKmFor returns the plausibility ceiling for a band, 0 for none.
+func MaxKmFor(band string) int { return maxTerrestrialKm[strings.ToLower(strings.TrimSpace(band))] }
+
 // Opening is a detected opening, ready to be announced.
 type Opening struct {
 	Band       string `json:"band"`
@@ -130,6 +152,12 @@ func (d *Detector) Add(s Spot, lat float64) *Opening {
 	if s.DistKm < d.cfg.MinKm || (d.cfg.MaxKm > 0 && s.DistKm > d.cfg.MaxKm) {
 		return nil
 	}
+	// Past what the atmosphere can carry on this band, the path went via the moon
+	// or a satellite. Those are real contacts and real reports; they are simply
+	// not evidence about the band.
+	if m := MaxKmFor(band); m > 0 && s.DistKm > m {
+		return nil
+	}
 	d.recent = append(d.recent, s)
 	d.prune(s.At)
 

internal/bandopen/eme_test.go

@@ -0,0 +1,51 @@
+package bandopen
+
+import (
+	"testing"
+	"time"
+)
+
+// A 2 m "opening" was announced at 9650 km towards Japan on stations that were
+// plainly working EME. The moon is not an opening: an operator pointing an
+// antenna at that bearing finds nothing.
+func TestEMEIsNotAnOpeningOnTwoMetres(t *testing.T) {
+	d := New(DefaultConfig())
+	base := time.Date(2026, 8, 12, 6, 0, 0, 0, time.UTC)
+	for i, call := range []string{"7M4RRM", "JA7RPC", "JF1AWC", "JK1TPA", "JH1JCQ"} {
+		if op := d.Add(Spot{
+			Call: call, Band: "2m", DistKm: 9650, Bearing: 40 + i*3,
+			At: base.Add(time.Duration(i) * time.Minute),
+		}, 47.0); op != nil {
+			t.Fatalf("a 9650 km 2 m path was announced as an opening: %+v", op)
+		}
+	}
+}
+
+// But a real 2 m opening — an Es chain at a plausible distance — must survive.
+func TestLongButPlausibleTwoMetresStillCounts(t *testing.T) {
+	d := New(DefaultConfig())
+	base := time.Date(2026, 6, 20, 18, 0, 0, 0, time.UTC)
+	var got *Opening
+	for i, call := range []string{"EA1AA", "CT1BB", "EA7CC", "CT7DD"} {
+		if op := d.Add(Spot{
+			Call: call, Band: "2m", DistKm: 2000 + i*30, Bearing: 200 + i*4,
+			At: base.Add(time.Duration(i) * time.Minute),
+		}, 47.0); op != nil {
+			got = op
+		}
+	}
+	if got == nil {
+		t.Fatal("a 2000 km 2 m burst in one sector was not reported")
+	}
+}
+
+// Six metres keeps no ceiling: multi-hop Es genuinely goes that far, which is
+// why the flat limit was removed in the first place.
+func TestSixMetresHasNoCeiling(t *testing.T) {
+	if MaxKmFor("6m") != 0 || MaxKmFor("10m") != 0 {
+		t.Error("6 m and 10 m must have no distance ceiling")
+	}
+	if MaxKmFor("2m") == 0 {
+		t.Error("2 m must have one")
+	}
+}