Fraunhofer Institute of Optronics, System Technologies and Image Exploitation IOSB
Fraunhofer Institute of Optronics, System Technologies and Image Exploitation IOSB

Measurement of optical turbulence with an AUS

from an unmanned aerial system

© Fraunhofer IOSB, Erik Sucher
© Fraunhofer IOSB
Comparison of UAS "hover" -2 m "fixed" mode
© Fraunhofer IOSB, Erik Sucher

Influence of turbulence

For the conception and use of electro-optical systems, the expected turbulence effects must be taken into account. One cause of the turbulence in the atmospheric boundary layer (up to 1-2 km height) is the temperature difference between the ground and the air layer above. During the day, warmer air bubbles detach from the ground, rise upwards and cause turbulent mixing.

The turbulence intensity is strongly influenced by the weather, i.e. solar radiation, temperature fluctuations and wind force. It varies with the time of day and season in the prevailing climate. The turbulence strength is described by the structural parameter of the refractive index fluctuation Cn2.

Since the turbulence intensity decreases with increasing height above the ground, knowledge of the height profile of Cn2 is also necessary for non-horizontal propagation distances. In order to determine the elevation profile of Cn2, the elevation profiles of the following meteorological quantities at different altitudes (h) must be known: air temperature T (h), air pressure p (h) and structural parameters of temperature fluctuation CT2 (h).
 

Motivation

  • set up a flexible System
  • easy measurements over area of interest
  • easy measurements of vertical and horizontal profiles
     

Challenge

  • turbulence limits optical sensor performance
  • optical turbulence is depending on height, surface, climate
  • need for over-water measurements
  • fixed measurements with towers are not flexible
  • regulation and permission procedure working with UAS
  • battery capacity and weight limits flight time
© Fraunhofer IOSB
Comparison of Cn² UAS - 2 m measurements 5 min averaging (3 flights)

Solution approach

For the determination of turbulence effects with an unmanned flight system 3D ultrasonic anemometers are used. These measure time series of the sound velocity between 3 opposite pairs of transmitter and receiver units in 3 directions with a high temporal resolution. The horizontal wind speed components u and v, as well as the vertical wind speed w can be derived from the transit time differences of the sound velocity. From the dependence of the sound velocity on the temperature, the time series of temperature variations can be determined.

For wavelengths in the visible and near infrared, temperature variations are mainly responsible for variations in the refractive index. From the spectral analysis of the time series of the temperature variations, the structural function constant of the temperature CT2 can be determined by a Fourier transformation. Cn2 is proportional to CT2 .

© Fraunhofer IOSB, Erik Sucher

It is important to take into account or compensate for the inherent movement of the mobile platform. Due to our extensive expertise in this area, the measurement techniques are continuously refined.
 

Conclusion

  • turbulence measurements with an UAS is possible
  • turbulence measurements with the small »TriSonica«sensor is possible
  • good agreement between »hover«and »fixed«point measurements
  • reduced averaging time down to 1 min (desirable for »hover«measurements) is possible
     

Outlook

  • further investigations are necessary, especially for vertical profiling
  • proof of weight reduction for longer flight time
  • turbulence measurements in vertical and horizontal flying mode

Further Informations

Unmanned systems

Technical data and equipment of unmanned systems

 

To the systems
 

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