Three‐Phase Switching with m‐Sequences for Sideband Separation in Radio Interferometry
Eric Keto · Publications of the Astronomical Society of the Pacific · 2000
Orthogonal sequences known as m‐sequences can be used in place of Walsh functions in phase switching and sideband separation in cross‐correlation interferometers. Functions based on three‐character m‐sequences may be advantageous because they may provide a larger set of mutually orthogonal modulation and demodulation functions, and hence support a larger number of antennas for a given sequence length, than allowed by other orthogonal sequences such as Walsh functions. The reason for this advantage is that if the demodulation functions are formed from the differences of the three‐character m‐sequence modulation functions, then because the m‐sequences obey an addition rule whereby sums or differences of m‐sequences are also m‐sequences, the demodulation functions are also members of the original orthogonal set. In a complete set of sequences, all the differences are of course duplicates of the original sequences. However, certain subsets of sequences have differences which are not members of the subset, and these subsets can be used to form modulation and demodulation functions which have both the desired uniqueness and orthogonality properties. While it is not obvious how to select the subsets, heuristic methods seem reasonably successful.