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diff --git a/apps/systemlib/airspeed.h b/apps/systemlib/airspeed.h
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+/****************************************************************************
+ *
+ *   Copyright (C) 2012-2013 PX4 Development Team. All rights reserved.
+ *   Author: Lorenz Meier <lm@inf.ethz.ch>
+ *
+ * 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 PX4 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 airspeed.c
+ * Airspeed estimation
+ *
+ * @author Lorenz Meier <lm@inf.ethz.ch>
+ *
+ */
+
+#include "math.h"
+#include "conversions.h"
+
+ __BEGIN_DECLS
+ 
+/**
+ * Calculate indicated airspeed.
+ *
+ * Note that the indicated airspeed is not the true airspeed because it
+ * lacks the air density compensation. Use the calc_true_airspeed functions to get
+ * the true airspeed.
+ *
+ * @param pressure_front pressure inside the pitot/prandl tube
+ * @param pressure_ambient pressure at the side of the tube/airplane
+ * @param temperature air temperature in degrees celcius
+ * @return indicated airspeed in m/s
+ */
+__EXPORT float calc_indicated_airspeed(float pressure_front, float pressure_ambient, float temperature);
+ 
+/**
+ * Calculate true airspeed from indicated airspeed.
+ *
+ * Note that the true airspeed is NOT the groundspeed, because of the effects of wind
+ *
+ * @param speed current indicated airspeed
+ * @param pressure_ambient pressure at the side of the tube/airplane
+ * @param temperature air temperature in degrees celcius
+ * @return true airspeed in m/s
+ */
+__EXPORT float calc_true_airspeed_from_indicated(float speed, float pressure_ambient, float temperature);
+ 
+/**
+ * Directly calculate true airspeed
+ *
+ * Note that the true airspeed is NOT the groundspeed, because of the effects of wind
+ *
+ * @param pressure_front pressure inside the pitot/prandl tube
+ * @param pressure_ambient pressure at the side of the tube/airplane
+ * @param temperature air temperature in degrees celcius
+ * @return true airspeed in m/s
+ */
+__EXPORT float calc_true_airspeed(float pressure_front, float pressure_ambient, float temperature);
+
+__END_DECLS
+\ No newline at end of file