This paper deals with the calibration of time-domain network analyzers operating in the bandwidth dc-20 GHz, After identification and experimental estimation of the error sources, an approach based on a standard TRI, (thru-reflect-line) calibration procedure, adapted to bidirectional (two generators and two samplers) time-domain measurement systems for the correction of systematic errors, is developed. Optimal conditions for the minimization of random errors are also carried out. Next, the measurement system dynamic range and repeatability are accurately characterized, Finally, experimental results obtained on coaxial attenuators and a mismatched lossy microstrip line demonstrate the accuracy of the method.