I found this DME indicator on the Cessna 210. Its mode switch has four positions - NAV1, NAV2, RNAV and HOLD. NAV1 and 2 are clear - it uses the NAV1 and 2 freq respectively. I'm wondering what the other two do, especially HOLD ?
RNAV is also quite logical - it uses the position selected in the RNAV selector. But how does the DME indicator determine the distance/speed to the RNAV position ? By using the GPS ?