Downhole Calliper Tool

A downhole calliper tool is for measuring distance between the calliper tool and an interface in a well, such as a petroleum well or a geothermal well. The downhole calliper tool has an impulse radar system (of the CTBV-type) with at least one impulse radar unit configured for: a) transmitting electromagnetic pulses with the impulse radar unit in a direction away from the downhole calliper tool, and for b) receiving reflections of said electromagnetic pulses with the impulse radar unit, and for c) analyzing said reflections to determine the distance between the impulse system and the interface. The impulse radar system is designed for carrying out at least one distance determination per second, but preferably at least ten distance determinations per second.