Off-Line Signal Processing for Weak-Signal Detection and Orbit and Attitude Determination

Communication with spacecraft is essential to every space mission. However, when the estimation or control of the motion of a spacecraft cannot be performed precisely, it is extremely difficult to communicate with the spacecraft in real-time operations. To enable communication in this situation, this paper proposes an off-line signal-processing method to detect the weak signal transmitted from a spacecraft under poor communication conditions. This is achieved by the following process. First, the Doppler shift effect caused by the motion of the spacecraft is removed from the received signal. The signal power is then enhanced by integrating the signal spectrum over a long time period. Moreover, the estimated frequency model, which contains the Doppler shift information, can be used for orbit and attitude determination as well. A coarse-ranging technique based on the detected weak signal is then introduced as a method of improving the orbit-estimation accuracy. The validity and effectiveness of the proposed method are demonstrated by applying these algorithms to actual radio waves transmitted from the IKAROS.