SID88 is a low-frequency Underwater Locator Device (ULD) for aviation. Aircraft in service today are equipped with flight recorders with Underwater Locator Beacons (ULBs) attached. The detectable acoustic range of the ULBs currently in use is limited due to their transmission frequency of 37.5 kHz. A low-frequency ULD transmitting at 8.8 kHz has a far longer detection range due to its lower frequency and therefore offers a perfect means of long-range underwater localisation.