In-orbit measurement of noise figure of on-board transponder Investigation of measurement method

This paper deals with measurement methods of a transponder noise figure of an in-orbit satellite. Some measurement methods used for ground test are investigated from a viewpoint of suitability for an in-orbit measurement. As a result of the investigation, conventional methods that are the signal generator and Y factor methods are not suitable for an in-orbit measurement. The former is apt to increase the measurement error over an acceptable value. The latter is difficult to connect the input of transponder with noise sources at known temperature. Therefore, a new method which produces less measurement error as compared with the others is proposed. In this method, the noise figure is obtained from the measured values of signal power at a transponder input and of signal-to-noise power ratio at a transponder output. This method is applicable for a transponder which has an AGC function or an input-to-output nonlinearity. However, in case of a nonlinear transponder, a measured signal-to-noise power ratio is affected with nonlinearity. From the analysis of this effect, it is found that a few corrections are necessary and that the correction factor is expressed as functions of AM-AM and AM-PM conversion factors.