A Channel Sounder for Low-Frequency Sub-Surface Radio Paths

Abstract

In an application to measure subterranean radio propagation, a wide-band low-frequency channel sounder makes use of a modified pseudo-random binary sequence with an imperfect (non-impulsive) auto-correlation function (ACF). The transmitting antenna for this system is essentially an induction loop, and the extreme wideband nature of the system requires this to be operated untuned. The resultant low efficiency is countered by signal-averaging techniques at the receiver, requiring accurate synchronisation to the transmitter, which is achieved using a code- or delay-locked loop. Accurate system identification and synchronisation is possible because cross-correlation is carried out between the non-ideal transmitted sequence and its inverse reference. A simple algorithm is presented for calculating the inverse of an arbitrary sequence under the operation of cross-correlation, and some properties of inverse sequences are discussed.