TWTAs (Traveling Xhuna Tube Amplifiers) are presently the primary life-limiting element of a communications satellite transponder. Satellite operators rely on spare ‘TWTAs to ensure that most of the transponders will be operating at the spacecraft end·of-life. We wish to track the °health° of the operating TWTAs in order to accurately predict the remaining useful lifetime of the satellite transponder. This information is very useful for system planning and risk assessment. The primary failure mechanism of a TWTA is cathode deactivation, symptoms of which are measurable from an earth station. Present testing methods require the interruption of the communications traffic signal, which leads to a loss in revenue generated by the satellite. We present new methods to test TWTAs without significant interference to the communications signal. These methods include the use of test—tone signals and spread spectrum techniques. The test—tone method is readily implemented using commercially available hardware. The spread-spectrum technique requires customized hardware, and so is more expensive. Finally, the data from these tests can be inserted into a transponder reliability model. The model is then used to predict the availability of the satellite transponder, up to and including the end-ef-life of the satellite. / Master of Science