Passive interferometric millimeter-wave imaging: achieving big results with a constellation of small satellites

Los Alamos National Laboratory (LANL) and AeroAstro have recently investigated the feasibility of space-based passive interferometric millimeter wave imaging (PIMI). The goal of this study is to explore a new capability that can offer day/night, all-weather, passive imaging with a 1-meter resolution, by means of millimetric interferometry via a small constellation of microsatellites. According to our preliminary study, a system with four LEO satellites operating at multiple frequency channels within 95-150 GHz is capable of providing an imagery of 1-m spatial resolution. The corresponding temperature sensitivity is estimated to be ~20°K, enough to distinguish most artifacts from a variety of backgrounds. To achieve the stated resolution and sensitivity with only four satellites, we make use of ten frequency channels to synthesize ten effective baselines between any pair of satellites. In addition, the satellites will “stare” at a common target area off the track direction for about 2 minutes while they pass over the area. This type of observation will introduce much improved spatial frequency coverage due to the relative rotation of the baseline vectors. It also improves the imagery SNR with a longer viewing time, as compared to a downward looking system. To the target, the side-looking observation also has the advantage of near constant incident (zenith) angle. The satellites are required to perform a formation flight but a rigid formation is not necessary. Simultaneous interferometric measurement of GPS signals, together with inter-satellites ranging will allow us to monitor the baseline length and direction to an adequate accuracy. A tradeoff study has also been conducted between the system performance and the technology availability, i.e., the current state-of-the-art technologies for space-borne antenna, millimeter-wave receiver, high-speed digitizer, inter-satellites data communication, and so forth.