In the days of the DME Descent procedure Australia had its own DME system operating in VHF frequencies (~200MHz), developed before the current internationally used system was in service and incompatible with it. Some of the earlier airborne equipment (eg VAN5) wasn't capable of displaying GS. Distance was displayed on an analogue scale so timing over a distance was necessary to manually derive closure rate. Timing a change in distance over 36 secs was easiest because closure rate would then be 100 x change-in-distance. Later equipment such as VAN X used an additional needle to show closure rate using a dual scale.