The Bistatic Sodar ''Heimdall'', You blow, I listen

With the increased exploitation of wind energy, remote sensing of wind profiles in the atmospheric boundary layer has come in focus as an attractive alternative to the operation of tall instrumented masts for wind energy siting and assessment studies. Project “Heimdall” describes a recent feasibility study conducted at Risø in the peiod 2001-2003 of the investigation of a new continuous wave (CW) bistatic sodar, initiated with the aim of investigating and improving the measuring accuracy of sodar remote sensing at heights ranging between 50 and 150 meters, during the often strong wind conditions in a neutrally stratified atmospheric conditions, characteristic for wind energy applications. Here we first describe the theoretical developments for sound wave scattering theory within a bistatic configuration. We calculate the Doppler shifts and returned power for the proposed new Heimdall bistatic configuration, implemented for measurements at 60 meters height above the ground. The bistatic “Heimdall” sodar was subsequently built and set in operation adjacent to the 123 meter tall met tower at Risø National Laboratory, which we had equipped with a sonic reference anemometer installed at the 60 meters height. “Heimdall” measured CW Doppler shifts, subsequently processed into wind speed in real time, along with received power which have been compared with our predictions, and we found that: 1) Our suggested bi-static configuration enabled real-time tracing of the wind speeds with data rates of ~1 Hz from fast processing of the measured spectral measurements of the Doppler shift with an adequate signalto-noise ratio, and 2) the amplitudes of the measured return powers compared to the predictions within about -10 dB. We conclude that the proposed bi-static configuration provides significantly improved signal-to-noise advantages over similar mono-static configurations, and thereby also better data availability rates for wind speed measurements during strong wind conditions.