21cm Global Spectrum Measuring Instrument For Lunar-Orbit Satellite Array

The ionosphere and radio frequency interference (RFI) make low frequency radio observation on Earth difficult. Lunar orbit is an ideal site for making low frequency observations, to probe the redshifted global 21cm signal for understanding the Cosmic Dawn and Epoch of Reionization. We present a global spectrum measuring instrument specially designed for deploying on lunar orbit. The instrument consists of an electrically small frequency-independent antenna and a differential two-way cross correlation receiver. The system can make real-time vector network measurement for the antenna and receiver, and it makes absolute calibration and relative calibration using the noise wave parameter calculation. A prototype has undergone ground test in Xinjiang. Some preliminary analysis of the system performance and test data are presented, which verified the feasibility of system design.