EKF: Fix bug in initialisation of height and magnetic field

This prevents zero data being used to form the initial height and magnetic field.
Do not start sampling initial values until non-zero time values are retrieved from the buffer.

EKF/ekf.cpp

@@ -339,13 +339,16 @@ bool Ekf::initialiseFilter(void)
 
 	// Sum the magnetometer measurements
 	if (_mag_buffer.pop_first_older_than(_imu_sample_delayed.time_us, &_mag_sample_delayed)) {
-		if (_mag_counter == 0) {
+		if (_mag_counter == 0 && _mag_sample_delayed.time_us !=0) {
+			// initialise the filter states and counter when we start getting valid data from the buffer
 			_mag_filt_state = _mag_sample_delayed.mag;
-		}
-
+			_mag_counter = 1;
+		} else if (_mag_counter != 0) {
+			// increment the sample count and apply a LPF to the measurement
 			_mag_counter ++;
 			_mag_filt_state = _mag_filt_state * 0.9f + _mag_sample_delayed.mag * 0.1f;
 		}
+	}
 
 	// set the default height source from the adjustable parameter
 	if (_hgt_counter == 0) {
@@ -354,29 +357,37 @@ bool Ekf::initialiseFilter(void)
 
 	if (_primary_hgt_source == VDIST_SENSOR_RANGE) {
 		if (_range_buffer.pop_first_older_than(_imu_sample_delayed.time_us, &_range_sample_delayed)) {
-			if (_hgt_counter == 0) {
+			if (_hgt_counter == 0 && _range_sample_delayed.time_us != 0) {
+				// initialise the filter states and counter when we start getting valid data from the buffer
 				_control_status.flags.baro_hgt = false;
 				_control_status.flags.gps_hgt = false;
 				_control_status.flags.rng_hgt = true;
 				_hgt_filt_state = _range_sample_delayed.rng;
-			}
-
+				_hgt_counter = 1;
+			} else if (_hgt_counter != 0) {
+				// increment the sample count and apply a LPF to the measurement
 				_hgt_counter ++;
 				_hgt_filt_state = 0.9f * _hgt_filt_state + 0.1f * _range_sample_delayed.rng;
 			}
+		}
 
 	} else if (_primary_hgt_source == VDIST_SENSOR_BARO || _primary_hgt_source == VDIST_SENSOR_GPS) {
+		// if the user parameter specifies use of GPS for height we use baro height initially and switch to GPS
+		// later when it passes checks.
 		if (_baro_buffer.pop_first_older_than(_imu_sample_delayed.time_us, &_baro_sample_delayed)) {
-			if (_hgt_counter == 0) {
+			if (_hgt_counter == 0 && _baro_sample_delayed.time_us != 0) {
+				// initialise the filter states and counter when we start getting valid data from the buffer
 				_control_status.flags.baro_hgt = true;
 				_control_status.flags.gps_hgt = false;
 				_control_status.flags.rng_hgt = false;
 				_hgt_filt_state = _baro_sample_delayed.hgt;
-			}
-
+				_hgt_counter = 1;
+			} else if (_hgt_counter != 0) {
+				// increment the sample count and apply a LPF to the measurement
 				_hgt_counter ++;
 				_hgt_filt_state = 0.9f * _hgt_filt_state + 0.1f * _baro_sample_delayed.hgt;
 			}
+		}
 
 	} else {
 		return false;