diff --git a/EKF/terrain_estimator.cpp b/EKF/terrain_estimator.cpp
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+/****************************************************************************
+ *
+ *   Copyright (c) 2015 Estimation and Control Library (ECL). All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name ECL nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+/**
+ * @file terrain_estimator.cpp
+ * Function for fusing rangefinder measurements to estimate terrain vertical position/
+ *
+ * @author Paul Riseborough <p_riseborough@live.com.au>
+ *
+ */
+
+#include "ekf.h"
+#include "mathlib.h"
+
+bool Ekf::initHagl()
+{
+	// get most recent range measurement from buffer
+	rangeSample latest_measurement = _range_buffer.get_newest();
+
+	if ((_time_last_imu - latest_measurement.time_us) < 2e5) {
+		// if we have a fresh measurement, use it to initialise the terrain estimator
+		_terrain_vpos = _state.pos(2) + latest_measurement.rng;
+		// initialise state variance to variance of measurement
+		_terrain_var = sq(_params.range_noise);
+		// success
+		return true;
+
+	} else if (!_in_air) {
+		// if on ground we assume a ground clearance
+		_terrain_vpos = _state.pos(2) + _params.rng_gnd_clearance;
+		// Use the ground clearance value as our uncertainty
+		_terrain_var = sq(_params.rng_gnd_clearance);
+		// ths is a guess
+		return false;
+
+	} else {
+		// no information - cannot initialise
+		return false;
+	}
+}
+
+void Ekf::predictHagl()
+{
+	// predict the state variance growth
+	// the state is the vertical position of the terrain underneath the vehicle
+	_terrain_var += sq(_imu_sample_delayed.delta_vel_dt * _params.terrain_p_noise);
+
+	// limit the variance to prevent it becoming badly conditioned
+	_terrain_var = math::constrain(_terrain_var, 0.0f, 1e4f);
+}
+
+void Ekf::fuseHagl()
+{
+	// If the vehicle is excessively tilted, do not try to fuse range finder observations
+	if (_R_prev(2, 2) > 0.7071f) {
+		// get a height above ground measurement from the range finder assuming a flat earth
+		float meas_hagl = _range_sample_delayed.rng * _R_prev(2, 2);
+
+		// predict the hagl from the vehicle position and terrain height
+		float pred_hagl = _terrain_vpos - _state.pos(2);
+
+		// calculate the innovation
+		_hagl_innov = pred_hagl - meas_hagl;
+
+		// calculate the observation variance adding the variance of the vehicles own height uncertainty and factoring in the effect of tilt on measurement error
+		float obs_variance = fmaxf(P[8][8], 0.0f) + sq(_params.range_noise / _R_prev(2, 2));
+
+		// calculate the innovation variance - limiting it to prevent a badly conditioned fusion
+		_hagl_innov_var = fmaxf(_terrain_var + obs_variance, obs_variance);
+
+		// perform an innovation consistency check and only fuse data if it passes
+		float gate_size = fmaxf(_params.range_innov_gate, 1.0f);
+		float test_ratio = sq(_hagl_innov) / (sq(gate_size) * _hagl_innov_var);
+
+		if (test_ratio <= 1.0f) {
+			// calculate the Kalman gain
+			float gain = obs_variance / _hagl_innov_var;
+			// correct the state
+			_terrain_vpos -= gain * _hagl_innov;
+			// correct the variance
+			_terrain_var = fmaxf(_terrain_var * (1.0f - gain), 0.0f);
+			// record last successful fusion time
+			_time_last_hagl_fuse = _time_last_imu;
+		}
+
+
+	} else {
+		return;
+	}
+}
+
+// return true if the estimate is fresh
+// return the estimated vertical position of the terrain relative to the NED origin
+bool Ekf::get_terrain_vert_pos(float *ret)
+{
+	memcpy(ret, &_terrain_vpos, sizeof(float));
+
+	// The height is useful if the uncertainty in terrain height is significantly smaller than than the estimated height above terrain
+	bool accuracy_useful = (sqrtf(_terrain_var) < 0.2f * fmaxf((_terrain_vpos - _state.pos(2)), _params.rng_gnd_clearance));
+
+	if (_time_last_imu - _time_last_hagl_fuse < 1e6 || accuracy_useful) {
+		return true;
+
+	} else {
+		return false;
+	}
+}
+
+void Ekf::get_hagl_innov(float *hagl_innov)
+{
+	memcpy(hagl_innov, &_hagl_innov, sizeof(_hagl_innov));
+}
+
+
+void Ekf::get_hagl_innov_var(float *hagl_innov_var)
+{
+	memcpy(hagl_innov_var, &_hagl_innov_var, sizeof(_hagl_innov_var));
+}