Plane: allow continued use of rangefinder data for 5s after loss of contact

this allows short outages to be ridden out

ArduPlane/ArduPlane.pde

@@ -307,6 +307,8 @@ static RangeFinder rangefinder;
 static struct {
     bool in_range;
     float height_estimate;
+    float correction;
+    uint32_t last_correction_time_ms;
     uint8_t in_range_count;
 } rangefinder_state;
 

ArduPlane/altitude.pde

@@ -513,7 +513,8 @@ static float lookahead_adjustment(void)
  */
 static float rangefinder_correction(void)
 {
-    if (!rangefinder_state.in_range) {
+    if (hal.scheduler->millis() - rangefinder_state.last_correction_time_ms > 5000) {
+        // we haven't had any rangefinder data for 5s - don't use it
         return 0;
     }
 
@@ -523,9 +524,46 @@ static float rangefinder_correction(void)
                               (flight_stage == AP_SpdHgtControl::FLIGHT_LAND_APPROACH ||
                                flight_stage == AP_SpdHgtControl::FLIGHT_LAND_FINAL));
     if (!using_rangefinder) {
-        return false;
+        return 0;
     }
 
+    return rangefinder_state.correction;
+}
+
+
+/*
+  update the offset between rangefinder height and terrain height
+ */
+static void rangefinder_height_update(void)
+{
+    uint16_t distance_cm = rangefinder.distance_cm();
+    int16_t max_distance_cm = rangefinder.max_distance_cm();
+    if (rangefinder.healthy() && distance_cm < max_distance_cm) {
+        // correct the range for attitude (multiply by DCM.c.z, which
+        // is cos(roll)*cos(pitch))
+        float corrected_range = distance_cm * 0.01f * ahrs.get_dcm_matrix().c.z;
+        if (rangefinder_state.in_range_count == 0) {
+            // we've just come back into range, start with this value
+            rangefinder_state.height_estimate = corrected_range;
+        } else {
+            // low pass filter to reduce noise. This runs at 50Hz, so
+            // converges fast. We don't want too much filtering
+            // though, as it would introduce lag which would delay flaring
+            rangefinder_state.height_estimate = 0.75f * rangefinder_state.height_estimate + 0.25f * corrected_range;
+        }
+        // we consider ourselves to be fully in range when we have 10
+        // good samples (0.2s)
+        if (rangefinder_state.in_range_count < 10) {
+            rangefinder_state.in_range_count++;
+        } else {
+            rangefinder_state.in_range = true;
+        }
+    } else {
+        rangefinder_state.in_range_count = 0;
+        rangefinder_state.in_range = false;
+    }
+
+    if (rangefinder_state.in_range) {
         // base correction is the difference between baro altitude and
         // rangefinder estimate
         float correction = relative_altitude() - rangefinder_state.height_estimate;
@@ -533,10 +571,14 @@ static float rangefinder_correction(void)
 #if AP_TERRAIN_AVAILABLE
         // if we are terrain following then correction is based on terrain data
         float terrain_altitude;
-    if (g.terrain_follow && terrain.height_above_terrain(terrain_altitude, true)) {
+        if ((target_altitude.terrain_following || g.terrain_follow) && 
+            terrain.height_above_terrain(terrain_altitude, true)) {
             correction = terrain_altitude - rangefinder_state.height_estimate;
         }
 #endif    
 
-    return correction;
+        // remember the last correction
+        rangefinder_state.correction = correction;
+        rangefinder_state.last_correction_time_ms = hal.scheduler->millis();    
+    }
 }

ArduPlane/sensors.pde

@@ -20,35 +20,10 @@ static void read_rangefinder(void)
 {
     rangefinder.update();
 
-    uint16_t distance_cm = rangefinder.distance_cm();
-    int16_t max_distance_cm = rangefinder.max_distance_cm();
-    if (rangefinder.healthy() && distance_cm < max_distance_cm) {
-        // correct the range for attitude (multiply by DCM.c.z, which
-        // is cos(roll)*cos(pitch))
-        float corrected_range = distance_cm * 0.01f * ahrs.get_dcm_matrix().c.z;
-        if (rangefinder_state.in_range_count == 0) {
-            // we've just come back into range, start with this value
-            rangefinder_state.height_estimate = corrected_range;
-        } else {
-            // low pass filter to reduce noise. This runs at 50Hz, so
-            // converges fast. We don't want too much filtering
-            // though, as it would introduce lag which would delay flaring
-            rangefinder_state.height_estimate = 0.75f * rangefinder_state.height_estimate + 0.25f * corrected_range;
-        }
-        // we consider ourselves to be fully in range when we have 10
-        // good samples (0.2s)
-        if (rangefinder_state.in_range_count < 10) {
-            rangefinder_state.in_range_count++;
-        } else {
-            rangefinder_state.in_range = true;
-        }
-    } else {
-        rangefinder_state.in_range_count = 0;
-        rangefinder_state.in_range = false;
-    }
-
     if (should_log(MASK_LOG_SONAR))
         Log_Write_Sonar();
+
+    rangefinder_height_update();
 }
 
 /*