Abstract: A method of determining flow speed in a low pressure environment. The method includes obtaining bi-directional acoustic data from a plurality of transducer pairs as a fluid moves relative to the plurality of transducer pairs. The method further includes measuring phase lags corresponding to the bi-directional acoustic data, and determining bi-directional flight times using the phase lags. Additionally, the method includes calculating a flow speed of the fluid corresponding to each transducer pair from the bi-directional flight times. The method includes correcting for at least one of temperature effects, pressure effects, and wake effects, using a system model. The method further includes generating a report including the flow speed of the fluid.