Last week, authorities in Indonesia confirmed that rescue crews had located the tail from AirAsia flight QZ8501. Grainy images of the submerged plane soon appeared on Australian and then American television. The black boxes were quickly recovered. But how were they found?
The pictures that have emerged from early media reports — green monochrome images of the missing plane — come from what’s called side scan sonar. Whereas radar uses electrical pulses to locate objects, sonar sends sound pulses that reflect off of objects like submarines or the sea floor to provide the operator with a picture based on acoustic reflection.
At very low frequencies, in the 300 to 900 khz (kilohertz), range, sonar can send sound waves out to cover wide areas. At higher frequencies, 600 to 1800 khz, the sound waves create incredibly detailed pictures of the objects that they encounter.