Intrusion detection algorithm with wind rejection heuristic

A method for monitoring an optical fiber for movements of the optical fiber indicative of intrusion events comprising:

• introducing a monitoring optical signal into the optical fiber and receiving optical signals from the optical fiber which are modified by intrusion events on the perimeter security system which affect the optical fiber;
• in a series of sequential cycles, causing an analysis of the received optical signals in the cycle to detect intrusion events by:
• converting the received optical signals into an electrical signal;
• converting the electrical signal into a sequence of digital samples;
• using a transform function to convert the sequence of digital samples into a set of frequency dependent transform coefficients;
• comparing the set of transform coefficients against an envelope where the envelope is a block of coefficients the same size as the set of transform coefficients;
• and indicating an intrusion event on determination that a transform coefficient exceeds an envelope coefficient by a predetermined threshold value;
• and automatically changing a sensitivity of the analysis to accommodate environmental noise on the fiber by the steps of:
• in respect of at least some of the analyses, when the transform coefficient is greater than the envelope coefficient but by a value less than the threshold value, changing the envelope coefficients to make the analysis less sensitive to accommodate increasing environmental noise conditions by increasing the envelope coefficients to a larger value;
• and
• decaying the envelope coefficients over time by periodically reducing each envelope coefficient by a decay value so as to make the analysis more sensitive to accommodate decreasing environmental noise conditions by decreasing the envelope coefficients to a smaller value.