Analog Transmission for Space Very Long Baseline Interferometry

Studies of advanced space very long baseline interferometry (SVLBI) missions envision data rates as high as 8 Gb/s, but to accomplish such high-rate data transfer from the spacecraft to ground using digital downlinks requires high effective isotropic radiated power (EIRP), as well as complex and substantially advanced modulation and receiving techniques. For these reasons, analog signal transfer becomes an attractive alternative. This article addresses the use of a technique dubbed constant envelope conversion (CEC) that permits analog VLBI signals to be efficiently transmitted using a saturated power amplifier. It is established that CEC analog transmission, compared to bandwidth compressive digital techniques such as M -quadrature amplitude modulation (QAM), requires no innovative designs and components to implement. CEC analog transfer is shown to provide VLBI fringe sensitivities no more than 0.5 dB less than those obtained from digital transmission, and for the same or much less spacecraft EIRP.