3D ultrasonic anemometer - plane airspeed = true wind speed? - Discussions

Hi Recmaster,

Here is a method to determine air speed and wind speed from GPS, gyro, and accelerometer information, without the need for a pitot tube or any other sort of airflow measurements. It is presently incorporated in software of the UAV DevBoard, and works as follows:

1. Whenever the plane changes orientation, an accurate estimate of air speed magnitude is made by taking the ratio of the magnitude of the change in 3D ground speed vector, divided by the magnitude of the change in fuselage axis of the IMU, as seen in the earth frame.

2. From the airspeed and ground speed vector, wind speed vector is computed. There is stream of estimates, whenever the plane changes orientation, which are filtered.

3. Once the wind vector is known, MatrixPilot computes a high bandwidth estimate of the ground speed vector using "dead reckoning", which fuses gyro, accelerometer, and GPS information.

4. A high bandwidth estimate of airspeed vector is computed from ground speed and wind speed.

Attached is the track from a typical flight in windy conditions. The large yellow arrows indicate the wind vector field. The airplane icons indicate the orientation of the plane.

Best regards,
Bill Premerlani

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Doug Weibel June 2, 2011 at 3:33pm

"3D ultrasonic anemometer" - I'd like to know where to get one of those that doesn't cost a fortune and would fit in a small UAV...... I know the wind farm guys use such things but they look to cost $$$$$$ and be about the size of a microwave oven.
T3

William Premerlani September 24, 2010 at 7:03am

Hi Recmaster,

Here is a method to determine air speed and wind speed from GPS, gyro, and accelerometer information, without the need for a pitot tube or any other sort of airflow measurements. It is presently incorporated in software of the UAV DevBoard, and works as follows:

1. Whenever the plane changes orientation, an accurate estimate of air speed magnitude is made by taking the ratio of the magnitude of the change in 3D ground speed vector, divided by the magnitude of the change in fuselage axis of the IMU, as seen in the earth frame.

2. From the airspeed and ground speed vector, wind speed vector is computed. There is stream of estimates, whenever the plane changes orientation, which are filtered.

3. Once the wind vector is known, MatrixPilot computes a high bandwidth estimate of the ground speed vector using "dead reckoning", which fuses gyro, accelerometer, and GPS information.

4. A high bandwidth estimate of airspeed vector is computed from ground speed and wind speed.

Attached is the track from a typical flight in windy conditions. The large yellow arrows indicate the wind vector field. The airplane icons indicate the orientation of the plane.

Best regards,
Bill Premerlani
Tom Yochum September 23, 2010 at 8:28am

TAS (true airspeed) is the vector sum of groundspeed and wind. So wind = TAS - groundspeed. TAS is usually measured using a Pitot-static system and a computer to correct for barometric effects. I don't know anything about 3D ultrasonic anemometers, so I can't comment on those. Groundspeed of course comes from a GPS.

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