Devel/flarm receiver (#8)

* Add info to main status help page.

Fixes cyoung#728.

* Adds AU tail number decoding from ICAO addr.

Contribution by @armeniki. cyoung#736.

* update readme

Remove line about dangerzone and added kwikEFIS to the list of apps that support Stratux

* Revert 0d93623.

* Add SoftRF udev rules and "ping" code hack (temporary).

* Update gen_gdl90.go

* Update flarm.go

main/flarm.go

@@ -382,7 +382,9 @@ func processAprsData(aprsData string) {
 
 		// set altitude
 		ti.Alt = int32(data.Altitude)
-		ti.AltIsGNSS = true
+		ti.Alt = ti.Alt - int32(mySituation.GPSGeoidSep) // fix geoid sep
+		ti.Alt = ti.Alt - int32(mySituation.GPSAltitudeMSL) + int32(mySituation.BaroPressureAltitude) // calc traffic's baro alt
+		ti.AltIsGNSS = false
 		ti.Last_alt = stratuxClock.Time
 
 		// set vertical speed

main/gen_gdl90.go

@@ -440,10 +440,10 @@ func makeOwnshipGeometricAltitudeReport() bool {
 	}
 	msg := make([]byte, 5)
 	// See p.28.
-	msg[0] = 0x0B                                // Message type "Ownship Geo Alt".
-	alt := int16(mySituation.GPSAltitudeMSL / 5) // GPS Altitude, encoded to 16-bit int using 5-foot resolution
-	msg[1] = byte(alt >> 8)                      // Altitude.
-	msg[2] = byte(alt & 0x00FF)                  // Altitude.
+	msg[0] = 0x0B                                		// Message type "Ownship Geo Alt".
+	alt := int16(mySituation.GPSHeightAboveEllipsoid / 5)	// GPS Altitude, encoded to 16-bit int using 5-foot resolution
+	msg[1] = byte(alt >> 8)                      		// Altitude.
+	msg[2] = byte(alt & 0x00FF)                  		// Altitude.
 
 	//TODO: "Figure of Merit". 0x7FFF "Not available".
 	msg[3] = 0x00
@@ -671,7 +671,7 @@ func makeFFIDMessage() []byte {
 	}
 	copy(msg[19:], devLongName)
 
-	msg[38] = 0x01 // Capabilities mask. MSL altitude for Ownship Geometric report.
+	msg[38] = 0x00 // Capabilities mask. Altitude above the WGS84 ellipsoid for Ownship Geometric report.
 
 	return prepareMessage(msg)
 }