Communications, Navigation, and Timing Constraints for the Solar Imaging Radio Array (SIRA)

The Solar Imaging Radio Array (SIRA) is a proposed NASA mission to measure solar radio emissions in the 30kHz to 30MHz region of the electromagnetic spectrum. The baseline design consists of 16 separated spacecraft in an irregular pattern several kilometers across. Each spacecraft is equipped with a pair of crossed dipole antennas that together form a 16-element radio interferometer for Fourier-type image reconstruction (120 baselines in the UV-plane). The required close coordination between this formation of spacecraft places many unique constraints on the SIRA communications, navigation, control, and timing architectures. Current specifications call for knowledge of the relative locations of the spacecraft to approximately meter-level accuracy in order to maintain primary instrument resolution. Knowledge of the relative timing differences between the clocks on the spacecraft must likewise be maintained to tens of nanoseconds or better. This in turn sets a minimum bound on the regularity of communications updates between spacecraft. Although the actual positions of the spacecraft are not tightly constrained, enough control authority and system autonomy must be present to keep the spacecraft from colliding due to orbital perturbations. Each of these constraints has an important effect on the design of the architecture for the entire array. This paper examines the engineering requirements and design tradeoffs for the communications, navigation, and timing architectures for SIRA. Topics include the choice of navigation sensor, communications methodology and modulation schemes, and clock type to meet the overall system performance goals while overcoming issues such as communications dynamic range, bandwidth limitations, power constraints, available antenna beam patterns, and processing limitations. In addition, this paper discusses how the projected use of smaller spacecraft buses with their corresponding payload and cost limits has important consequences for the overall system design.