ekf: add range finder kinematic consistency check

At each new valid range measurement, the time derivative of the distance
to the ground is computed and compared with the estimated velocity.
The average of a normalized innovation squared statistic check is used
to detect a bias in the derivative of distance measurement,
indicating that the distance measurements are kinematically inconsistent
with the filter.

src/modules/ekf2/EKF/range_finder_consistency_check.cpp

@@ -0,0 +1,64 @@
+/****************************************************************************
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+
+/**
+ * @file range_finder_consistency_check.cpp
+ */
+
+#include "range_finder_consistency_check.hpp"
+
+void RangeFinderConsistencyCheck::update(float dist_bottom, float dist_bottom_var, float vz, float vz_var, float time_s)
+{
+	const float dt = time_s - _time_last_update_s;
+
+	if ((_time_last_update_s < FLT_EPSILON)
+	    || (dt < 0.001f) || (dt > 0.5f)) {
+		_time_last_update_s = time_s;
+		_dist_bottom_prev = dist_bottom;
+		return;
+	}
+
+	const float vel_bottom = (dist_bottom - _dist_bottom_prev) / dt;
+	const float innov = -vel_bottom - vz; // vel_bottom is +up while vz is +down
+	const float vel_bottom_var = 2.f * dist_bottom_var / (dt * dt);
+	const float innov_var = vel_bottom_var + vz_var;
+	const float normalized_innov_sq = (innov * innov) / innov_var;
+	_vel_bottom_test_ratio = normalized_innov_sq / (_vel_bottom_gate * _vel_bottom_gate);
+
+	_vel_bottom_signed_test_ratio_lpf.setParameters(dt, _vel_bottom_signed_test_ratio_tau);
+	const float signed_test_ratio = matrix::sign(innov) * normalized_innov_sq / (_vel_bottom_signed_gate * _vel_bottom_signed_gate);
+	_vel_bottom_signed_test_ratio_lpf.update(signed_test_ratio);
+
+	_time_last_update_s = time_s;
+	_dist_bottom_prev = dist_bottom;
+}