In an alternative to a prior technique of time-domain- reflectometry (TDR) in which very short excitation pulses are used, the pulses have very short rise and fall times and the pulse duration is varied continuously between a minimum and a maximum value. In both the present and prior techniques, the basic idea is to (1) measure the times between the generation of excitation pulses and the reception of reflections of the pulses as indications of the locations of one or more defects along a cable and (2) measure the amplitudes of the reflections as indication of the magnitudes of the defects.
In general, an excitation pulse has a duration T. Each leading and trailing edge of an excitation pulse generates a reflection from a defect, so that a unique pair of reflections is associated with each defect. In the present alternative technique, the processing of the measured reflection signal includes computation of the autocorrelation function
where t is time, x(t) is the measured reflection signal at time t, and τ is the correlation interval. The integration is performed over a measurement time interval short enough to enable identification and location of a defect within the corresponding spatial interval along the cable. Typically, where there is a defect, R(τ) exhibits a negative peak having maximum magnitude for τ in the vicinity of T. This peak can be used as a means of identifying a leading-edge/trailing-edge reflection pair.
For a given spatial interval, measurements are made and R(τ) computed, as described above, for pulse durations T ranging from the minimum to the maximum value. The advantage of doing this is that the effective signal-to-noise ratio may be significantly increased over that attainable by use of a fixed pulse duration T.
This work was done by Angel Lucena, Pam Mullinex, PoTien Huang, and Josephine Santiago of Kennedy Space Center and Pedro Medelius, Carlos Mata, Carlos Zavala, and John Lane of ASRC Aerospace Corp.