Ground based remote sensing is an attractive form of providing wind data for the growing field of wind energy. The commonly used mono-static SODARs and LIDARs rely on a horizontally homogeneous wind field to accurately measure wind speeds. Bi-static SODARs present a promising alternative instrument for measuring inhomogeneous wind fields, since their geometry, with separated transmitter and receiver, allows probing of the wind field in a single volume. This geometry also makes it more challenging to retrieve wind pro les. In this thesis the performance of novel bi-static SODAR receivers, which allow measuring wind pro les, is investigated thoroughly on at and complex terrain. In total over 69000 10-minute averaged measurements from individual receivers and over 8000 10-minute averaged wind speeds derived from a combination of four receivers are analysed, which represents a significant increase in the amount of data available from bi-static SODARs. The SODAR receivers built and tested as a part of this thesis are based on phased arrays of microphones. Each array consists of 16 rows of microphones which are individually sampled. This allows scanning a column above a vertically transmitting antenna using frequency domain beamforming. The scanning capability of the receivers is demonstrated in field experiments, whose results motivated the development of a new theory about the effect of the displacement of sound by the mean wind field on bi-static SODAR measurements. It is found that the ratio of the beam widths of transmitter and receiver is crucial for the effect on measured Doppler shifts. The Doppler shifts of individual receivers are least effected if the beam width ratio is approximately one. However, it is predicted that multiple receivers, with beam widths different from the transmitter, can be suitably combined to give accurate horizontal wind speeds. Both assumptions are confirmed in experiments employing receivers with different beam widths. On at terrain a comparison between wind speeds measured by mast mounted instruments and wind speeds measured with a combination of four receivers lead to excellent results with a coefficient of determination of 0.986. In complex terrain, with a more compromised setup, the comparison yielded good agreement with a coefficient of determination of 0.956.
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