Abstract:

An investigation into multipath resolution for outdoor sound propagation is done using spread spectrum acoustic signals. A low-frequency sinusoid is bi-phase modulated using a pseudorandom sequence with period 2[sup M]-1. The ``chip rate'' fc in Hz is the number of bits per second used in the bi-phase modulation. The shape of the spread spectrum sinusoid is in the form of sin(x)/x, where the peak is at the sinusoid frequency and the adjacent nulls are at (plus or minus)fc in Hz. Faster chip rates result in broader spectra. Cross correlating the received acoustic signals with the transmitted signals provides a time delay and amplitude estimate for each path in the propagation channel. A field experiment in a strong 15-kn wind revealed a clear direct path and ground reflection appropriate for the profile conditions and confirmed by ray tracing. [Work supported by DARPA.]