From 5adbf9b232ce42703d4fdba767ac6152361df298 Mon Sep 17 00:00:00 2001
From: Tom Pittenger <tom.pittenger@intel.com>
Date: Mon, 20 Feb 2017 23:29:34 -0800
Subject: [PATCH] AP_ADSB: populate altitude via pressure for ADSB-Out

fixes https://github.com/ArduPilot/ardupilot/issues/5424
---
 libraries/AP_ADSB/AP_ADSB.cpp | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/libraries/AP_ADSB/AP_ADSB.cpp b/libraries/AP_ADSB/AP_ADSB.cpp
index b467a6dfd79..0b6287fe3a5 100644
--- a/libraries/AP_ADSB/AP_ADSB.cpp
+++ b/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;
+    }