a flight path as already indicated a variety of signal formats and signal processing methods can be used a send then listen system can be used that takes into account the spacing of the receivers from the transmitter this would give a maximum pulse length of 0 3 s and a minimum listen time ie interval between pulses of about 1 2 seconds for an echo path length of up to about 300 m alternatively if a listen while sending system was adopted transmit pulses of about 1 s could be used with listening times of about 1 9 s for a 300 m echo path length details of both such systems have been provided in our above referenced prior patent applications finally the graph shown fig6 has been included to indicate the type of results that can be obtained in the field using the system disclosed herein where only one loudspeaker and three opposing microphones are employed this graph shows data relating to a wake vortex shed by a boeing 737 during an approach to the main runway at melbourne airport using a send while listening system the phase response shows a spacing of 8 m between peaks of the doppler signal the peaks being indicative of the maximum core edge velocities as noted in the graph assuming that the circulation strength of the wake vortex of a boeing 737 was 70 m s 2 in line with published data at the time as the sodar measurements were taken and that the relationship between circulation strength and averaged sodar measured maximum core edge velocity is linear then the constant of proportionality can be calculated to be 16 6 giving this simple relationship needs to be calibrated for each sodar setting calibration can be achieved by first using.