AP_Terrain: added status() interface for reporting

libraries/AP_Terrain/AP_Terrain.cpp

@@ -262,4 +262,25 @@ void AP_Terrain::update(void)
     height_amsl(ahrs.get_home(), height);
 }
 
+/*
+  return status enum for health reporting
+*/
+enum AP_Terrain::TerrainStatus AP_Terrain::status(void)
+{
+    if (!enable) {
+        return TerrainStatusDisabled;
+    }
+    Location loc;
+    if (!ahrs.get_position(loc)) {
+        // we don't know where we are
+        return TerrainStatusUnhealthy;
+    }
+    float height;
+    if (!height_amsl(loc, height)) {
+        // we don't have terrain data at current location
+        return TerrainStatusUnhealthy;        
+    }
+    return TerrainStatusOK; 
+}
+
 #endif // HAVE_AP_TERRAIN

libraries/AP_Terrain/AP_Terrain.h

@@ -83,11 +83,20 @@ public:
     AP_Int8  enable;
     AP_Int16 grid_spacing; // meters between grid points
 
+    enum TerrainStatus {
+        TerrainStatusDisabled  = 0, // not enabled
+        TerrainStatusUnhealthy = 1, // no terrain data for current location
+        TerrainStatusOK        = 2  // terrain data available
+    };
+
     static const struct AP_Param::GroupInfo var_info[];
     
     // update terrain state. Should be called at 1Hz or more
     void update(void);
 
+    // return status enum for health reporting
+    enum TerrainStatus status(void);
+
     // send any pending terrain request message
     void send_request(mavlink_channel_t chan);