Patent ID: 8665997

Claim:
A method of demodulating an Automatic Identification System signal having an unknown carrier frequency and carrier phase in the presence of interference, comprising the steps of: a) receiving on a computing device an Automatic Identification System signal, having an unknown carrier frequency, unknown carrier phase, information bits transmitted at a known baud rate, and an unknown preamble selected from a set of known preambles, where each preamble has a length equal to that of the other preambles, where the unknown preamble immediately precedes the information bits; b) sampling on the computing device the Automatic Identification System signal at a user-definable sampling rate; c) estimating on the computing device the carrier frequency in Hertz; d) estimating on the computing device a complex conjugate of the carrier frequency contribution to each signal sample using the estimated carrier frequency; e) multiplying on the computing device each signal sample by the corresponding result of step (d) and designating the collective results as a base-banded sampled signal; f) generating a GMSK signal without carrier frequency or carrier phase for each known preamble for the Automatic Identification System; g) sampling the results of step (f); h) computing on the computing device a correlation of the base-banded sampled signal with each result of step (g); i) identifying on the computing device a maximum magnitude of the results of step (h) and its location and designating the known preamble associated with the maximum magnitude as the preamble of the received Automatic Identification System signal; j) computing on the computing device a complex inner product of the preamble identified in step (i) with a segment of the base-banded sampled signal starting at the location identified in step (i); k) estimating on the computing device the carrier phase as an argument function of the result of step (j); l) estimating on the computing device a complex conjugate of the carrier phase contribution to the base-banded sampled signal; m) multiplying on the computing device each result of step (e) starting at the location identified in step (i) by the result of step (k); and n) demodulating the results of step (m) starting at the location identified in step (i) using a GMSK demodulator appropriate for a sampled signal with an interference contribution but no carrier frequency contribution or carrier phase contribution.