The PT9 C-Proof is a Seawater-resistant underwater locating device. It is especially designed for the data protective capsules (FRM) of Voyage Data Recorder (VDR/SVDR) systems. It withstands the rough environmental conditions of maritime navigation, high G-shock impact and deep water immersion.   (S)VDRs which are pre MSS.333(90), can be fitted with a PT-9 ULD.