Preliminary Downlink Design and Performance Assessment for Advanced Radio Interferometry Between Space and Earth (ARISE)

Advanced Radio Interferometry Between Space and Earth (ARISE) is a space very long baseline interferometry (VLBI) mission with a nominal launch date of 2008. It consists of an in∞atable 25-m radio telescope circulating in a highly elliptical Earth orbit with a perigee of 5,000 km and an apogee of 40,000 km. The objective is to observe in conjunction with Earth-based telescopes to obtain high-resolution maps of quasars and active galactic nuclei for science investigations. ARISE requires an 8-Gb/s downlink of science data, which is a challenge using today’s technology. In this article, 8-Gb/s systems using both traditional radio frequency (RF) and laser communication are proposed with the goal of minimizing both the cost and the risk of the design. Either option requires appropriate technology investments. The RF system requires the use of dual polarization, high-order modulations such as 32-quadrature amplitude modulation (QAM), and spectrally e‐cient square-root raised-cosine (SRRC) fllters to meet the Federal Communications Commission (FCC) spectral allocation. If additional bandwidth is allocated by the FCC, constant-envelope modulations such as cross-correlated trellis-coded quadrature modulation (XTCQM) can be used in place of SRRC fllters and QAM to reduce the power required on the spacecraft. The proposed laser communication system uses on{ofi keying (OOK) and wavelength division multiplexing (WDM). The wavelength of 1550 nm has the advantage of lower background light subtended at the ground receiver for downlink communications. The critical components of the system are based on mature flber-optic technologies. The downlink transceiver terminal will be a modifled Optical Communications Demonstrator (OCD) that has been in development at JPL over the past 3 years.