Land detector: Measure total system flight time

This implementation is a baseline implementation and makes no attempt to be tamper-proof. A monotonic counter like the W25R64FV or a similar HW facility would be required to achieve this.

src/modules/land_detector/LandDetector.cpp

@@ -103,6 +103,7 @@ void LandDetector::_cycle()
 		_landDetected.freefall = false;
 		_landDetected.landed = false;
 		_landDetected.ground_contact = false;
+		_p_total_flight_time = param_find("LND_FLIGHT_TIME");
 
 		// Initialize uORB topics.
 		_initialize_topics();
@@ -117,6 +118,8 @@ void LandDetector::_cycle()
 
 	_update_topics();
 
+	hrt_abstime now = hrt_absolute_time();
+
 	_update_state();
 
 	bool freefallDetected = (_state == LandDetectionState::FREEFALL);
@@ -135,6 +138,16 @@ void LandDetector::_cycle()
 		_landDetected.landed = (_state == LandDetectionState::LANDED);
 		_landDetected.ground_contact = (_state == LandDetectionState::GROUND_CONTACT);
 
+		// We did take off
+		if (landDetected && !_landDetected.landed) {
+			_takeoff_time = now;
+
+		} else if (_takeoff_time != 0 && !landDetected && _landDetected.landed) {
+			_total_flight_time = now - _takeoff_time;
+			_takeoff_time = 0;
+			param_set(_p_total_flight_time, &_total_flight_time);
+		}
+
 		int instance;
 		orb_publish_auto(ORB_ID(vehicle_land_detected), &_landDetectedPub, &_landDetected,
 				 &instance, ORB_PRIO_DEFAULT);
@@ -164,6 +177,7 @@ void LandDetector::_check_params(const bool force)
 
 	if (updated || force) {
 		_update_params();
+		param_get(_p_total_flight_time, &_total_flight_time);
 	}
 }
 
@@ -191,8 +205,6 @@ void LandDetector::_update_state()
 	} else {
 		_state = LandDetectionState::FLYING;
 	}
-
-	return;
 }
 
 bool LandDetector::_orb_update(const struct orb_metadata *meta, int handle, void *buffer)

src/modules/land_detector/LandDetector.h

@@ -166,6 +166,10 @@ private:
 	bool _taskShouldExit;
 	bool _taskIsRunning;
 
+	param_t _p_total_flight_time;
+	int32_t _total_flight_time;
+	hrt_abstime _takeoff_time;
+
 	struct work_s	_work;
 };
 

src/modules/land_detector/land_detector_params.c

@@ -174,3 +174,14 @@ PARAM_DEFINE_FLOAT(LNDFW_VELI_MAX, 4.0f);
  * @group Land Detector
  */
 PARAM_DEFINE_FLOAT(LNDFW_AIRSPD_MAX, 8.00f);
+
+/**
+ * Total flight time in microseconds
+ *
+ * Total flight time of this autopilot.
+ *
+ * @min 0
+ * @group Land Detector
+ *
+ */
+PARAM_DEFINE_INT32(LND_FLIGHT_TIME, 0);