Distributed correlators for interferometry in space

New and interesting science drivers have triggered a renewed interest in radio astronomy at ultra long wavelengths. However, at longer wavelengths (beyond 10 meters) ground-based radio astronomy is severely limited by earths ionosphere, in addition to man-made interferences and solar flares. An unequivocal solution to the problem is to establish a space based observatory for ultra low frequency (0.3MHz-30MHz) observations. In ground-based radio astronomy, interferometers comprising of widely spaced antennas are employed to enhance the sensitivity and angular resolution of the observations. The signals received from the antennas are pre-processed, phase corrected independently and then cross correlated with one another using a centralized correlator to estimate the coherence function. However, a space based array, in addition to several other obstacles, presents new challenges for both communication and processing. In this paper, we discuss various conventional correlator architectures, such as XF, FX and HFX. In addition, the importance of a distributed correlator is emphasized for a space based array, in particular Frequency distributed correlator. We compute transmission, reception and processing requirements for both centralized and distributed architecture. Finally, as a demonstration, we present 2 projects were these signal processing estimates are applied.