The methodology for accurately calibrating the Niobe resonant-mass gravitational wave detector is presented. The transducer is based on a low noise resonant microwave cavity transducer that converts the displacement of the resonating mass to microwave energy. The calibration technique consists of a one off measurement of the microwave frequency versus resonant-mass displacement characteristic. To measure this accurately, known static forces were applied to the resonant mass and the change in the transducer microwave frequency was recorded. With the aid of finite element analysis and accurate measurements of the resonant-mass characteristics, the deflection due to the known force was calculated. The calculated deflections were verified coarsely with measurements from a calibrated linear variable differential transformer. Typically, the detector operates with a 1 mK noise temperature. A best noise temperature of 890 muK between 1300 and 2000 Universal Time Coordinate (UTC) for day 60 in 1997 is reported. The transducer has been upgraded with a new microwave amplifier, which has a measured electronic noise floor 40 dB lower than the previous amplifier, which is only 10 dB above the quantum limit. (C) 2000 American Institute of Physics. [S0034-6748(00)00512-8].