AP_ADSB: populate altitude via pressure for ADSB-Out

fixes https://github.com/ArduPilot/ardupilot/issues/5424

libraries/AP_ADSB/AP_ADSB.cpp

@@ -518,7 +518,12 @@ void AP_ADSB::send_dynamic_out(const mavlink_channel_t chan)
     // --------------
     // Unknowns
     // TODO: implement http://www.srh.noaa.gov/images/epz/wxcalc/pressureAltitude.pdf
-    int32_t altPres = INT_MAX; //_ahrs.get_baro().get_altitude() relative to home, not MSL
+    AP_Baro baro = _ahrs.get_baro();
+    int32_t altPres = INT_MAX;
+    if (baro.healthy()) {
+        // Altitude difference between 101325 (Pascals) and current pressure. Result in millimeters
+        altPres = baro.get_altitude_difference(101325, baro.get_pressure()) * 1E3; // convert m to mm;
+    }