Does anyone know if the black box pingers stay phase stable? Its been mentioned before about folding data to get a better signal to noise. Its a common pulsar technique.
Given that the ultrasound signal exhibited very significant frequency deviation (about 33 kHz rather than the nominal 37.5 kHz) it is unlikely that the ultrasound signal itself is phase stable. Thus, digital signal processing techniques predicated on phase stability are unlikely to be of assistance. However, the ULB may have reasonable frequency stability over the short term as temperature and water pressure are static and battery voltage decays only slowly.
The situation is more promising with regard to the timing of the sound
pulses. According to Air Chief Marshall Angus Houston, pulse periods of about 1.1 second were observed, indicating that the signal originated from the FDR ULB.
Pulse width and duty cycle are most likely governed by a single crystal oscillator and are thus sufficiently stable for signal integration etc. Of course, as Ocean Shield moves relative to the ULB, pulse phase will change with the effective sound path length between the ULB and the TPL.
Careful evaluation of (i) pulse phase; (ii) sound intensity; and (iii) ultrasound doppler shift of recorded signals correlated with the time and precise TPL location should help narrow the scope of further searches.