better traffic altitude computation for flarm targets, fixes flarm altitude in web radar

main/flarm-nmea.go

@@ -106,10 +106,10 @@ func makeFlarmPFLAAString(ti TrafficInfo) (msg string, valid bool) {
 	//}
 
 	altf := mySituation.BaroPressureAltitude
-	if !isTempPressValid() { // if no pressure altitude available, use GPS altitude
+	if !isTempPressValid() && isGPSValid() { // if no pressure altitude available, use GPS altitude
 		altf = mySituation.GPSAltitudeMSL
 	}
-	if ti.AltIsGNSS {
+	if ti.AltIsGNSS && isGPSValid() {
 		// Altitude coming from OGN. We set the geoid separation to 0 in the OGN config, so OGN reports ellipsoid alt - we need to compare to that
 		altf = mySituation.GPSHeightAboveEllipsoid
 	}

main/flarm.go

@@ -389,8 +389,16 @@ func processAprsData(aprsData string) {
 		ti.Last_source = TRAFFIC_SOURCE_FLARM
 
 		// set altitude
-		ti.Alt = int32(data.Altitude)
+		// To keep the rest of the system as simple as possible, we want to work with barometric altitude everywhere.
-		ti.AltIsGNSS = true
+		// To do so, we use our own known geoid separation and pressure difference to compute the expected barometric altitude of the traffic.
+		if isGPSValid() && isTempPressValid() {
+			ti.Alt = int32(float32(data.Altitude) - mySituation.GPSHeightAboveEllipsoid + mySituation.BaroPressureAltitude)
+		} else {
+			ti.Alt = int32(data.Altitude)
+			ti.AltIsGNSS = true
+		}
+
+
 		ti.Last_alt = stratuxClock.Time
 
 		// set vertical speed