APM: added ALT_OFFSET parameter

useful to adjust for barometric pressure changes during a long flight

ArduPlane/ArduPlane.pde

@@ -1175,10 +1175,10 @@ static void update_current_flight_mode(void)
             if (g.flybywire_elev_reverse) {
                 elevator_input = -elevator_input;
             }
-            if ((current_loc.alt >= home.alt+g.FBWB_min_altitude_cm) || (g.FBWB_min_altitude_cm == 0)) {
+            if ((adjusted_altitude_cm() >= home.alt+g.FBWB_min_altitude_cm) || (g.FBWB_min_altitude_cm == 0)) {
                 altitude_error_cm = elevator_input * g.pitch_limit_min_cd;
             } else {
-                altitude_error_cm = (home.alt + g.FBWB_min_altitude_cm) - current_loc.alt;
+                altitude_error_cm = (home.alt + g.FBWB_min_altitude_cm) - adjusted_altitude_cm();
                 if (elevator_input < 0) {
                     altitude_error_cm += elevator_input * g.pitch_limit_min_cd;
                 }

ArduPlane/Parameters.h

@@ -76,6 +76,7 @@ public:
         k_param_crosstrack_min_distance,
         k_param_rudder_steer,
         k_param_throttle_nudge,
+        k_param_alt_offset,
 
         // 110: Telemetry control
         //
@@ -299,6 +300,7 @@ public:
     AP_Int16 roll_limit_cd;
     AP_Int16 pitch_limit_max_cd;
     AP_Int16 pitch_limit_min_cd;
+    AP_Int16 alt_offset;
 
     // Misc
     //

ArduPlane/Parameters.pde

@@ -160,6 +160,15 @@ const AP_Param::Info var_info[] PROGMEM = {
     // @User: Advanced
     GSCALAR(alt_control_algorithm, "ALT_CTRL_ALG",    ALT_CONTROL_DEFAULT),
 
+    // @Param: ALT_OFFSET
+    // @DisplayName: Altitude offset
+    // @Description: This is added to the target altitude in automatic flight. It can be used to add a global altitude offset to a mission, or to adjust for barometric pressure changes
+    // @Units: Meters
+    // @Range: -32767 32767
+    // @Increment: 1
+    // @User: Advanced
+    GSCALAR(alt_offset, "ALT_OFFSET",                 0),
+
     GSCALAR(command_total,          "CMD_TOTAL",      0),
     GSCALAR(command_index,          "CMD_INDEX",      0),
 

ArduPlane/commands_logic.pde

@@ -302,7 +302,7 @@ static bool verify_takeoff()
         calc_bearing_error();
     }
 
-    if (current_loc.alt > takeoff_altitude)  {
+    if (adjusted_altitude_cm() > takeoff_altitude)  {
         hold_course = -1;
         takeoff_complete = true;
         next_WP = current_loc;
@@ -322,7 +322,7 @@ static bool verify_land()
     // Set land_complete if we are within 2 seconds distance or within
     // 3 meters altitude of the landing point
     if (((wp_distance > 0) && (wp_distance <= (g.land_flare_sec*g_gps->ground_speed*0.01)))
-        || (current_loc.alt <= next_WP.alt + g.land_flare_alt*100)) {
+        || (adjusted_altitude_cm() <= next_WP.alt + g.land_flare_alt*100)) {
 
         land_complete = true;
 
@@ -441,8 +441,10 @@ static void do_change_alt()
 {
     condition_rate          = labs((int)next_nonnav_command.lat);
     condition_value         = next_nonnav_command.alt;
-    if(condition_value < current_loc.alt) condition_rate = -condition_rate;
-    target_altitude_cm      = current_loc.alt + (condition_rate / 10);                  // Divide by ten for 10Hz update
+    if (condition_value < adjusted_altitude_cm()) {
+        condition_rate = -condition_rate;
+    }
+    target_altitude_cm      = adjusted_altitude_cm() + (condition_rate / 10);                  // Divide by ten for 10Hz update
     next_WP.alt             = condition_value;                                                                  // For future nav calculations
     offset_altitude_cm      = 0;                                                                                        // For future nav calculations
 }
@@ -467,7 +469,8 @@ static bool verify_wait_delay()
 
 static bool verify_change_alt()
 {
-    if( (condition_rate>=0 && current_loc.alt >= condition_value) || (condition_rate<=0 && current_loc.alt <= condition_value)) {
+    if( (condition_rate>=0 && adjusted_altitude_cm() >= condition_value) || 
+        (condition_rate<=0 && adjusted_altitude_cm() <= condition_value)) {
         condition_value = 0;
         return true;
     }

ArduPlane/geofence.pde

@@ -153,7 +153,7 @@ static bool geofence_check_minalt(void)
     if (g.fence_minalt == 0) {
         return false;
     }
-    return (current_loc.alt < (g.fence_minalt*100.0) + home.alt);
+    return (adjusted_altitude_cm() < (g.fence_minalt*100.0) + home.alt);
 }
 
 /*
@@ -167,7 +167,7 @@ static bool geofence_check_maxalt(void)
     if (g.fence_maxalt == 0) {
         return false;
     }
-    return (current_loc.alt > (g.fence_maxalt*100.0) + home.alt);
+    return (adjusted_altitude_cm() > (g.fence_maxalt*100.0) + home.alt);
 }
 
 

ArduPlane/navigation.pde

@@ -128,7 +128,7 @@ static void calc_altitude_error()
         target_altitude_cm = next_WP.alt;
     }
 
-    altitude_error_cm       = target_altitude_cm - current_loc.alt;
+    altitude_error_cm       = target_altitude_cm - adjusted_altitude_cm();
 }
 
 static int32_t wrap_360_cd(int32_t error)

ArduPlane/sensors.pde

@@ -65,3 +65,14 @@ void read_receiver_rssi(void)
     receiver_rssi = constrain(ret, 0, 255);
 #endif
 }
+
+
+/*
+  return current_loc.alt adjusted for ALT_OFFSET
+  This is useful during long flights to account for barometer changes
+  from the GCS, or to adjust the flying height of a long mission
+ */
+static int32_t adjusted_altitude_cm(void)
+{
+    return current_loc.alt - (g.alt_offset*100);
+}