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.
Ever since the first proposal that tidal heating of Europa by Jupiter might lead to liquid water oceans below Europa's ice cover, scientists have speculated over the exobiological implications of such an ocean. Liquid water is thought to be an essential ingredient for life, so the existence of a second water ocean in the Solar System would be of paramount importance in any search for life beyond Earth. We present here a Discovery-class mission concept (Europa Ocean Discovery) to determine the existence of a liquid water ocean on Europa and to characterize Europa's surface structure. The technical goal of the Europa Ocean Discovery mission is to study Europa with an orbiting spacecraft. This goal is challenging but entirely feasible within the Discovery envelope. There are four key challenges: entering Europan orbit, generating power, surviving long enough in the radiation environment to return valuable science, and completing the mission within the Discovery program's constraints on launch vehicle (Delta II or smaller) and budget (approximately $DOL250M plus launch). Europa Ocean Discovery will carry four scientific instruments to study Europa: (1) an ice-penetrating radar sounder to probe tens of kilometers below Europa's surface; (2) a laser altimeter, to determine the height and phase of Europa's time-varying tidal bulge; (3) an X-band transponder to determine Europa's gravity field; and (4) a solid-state optical imager. These instruments will provide important information about Europa's surface, subsurface, and will provide definitive evidence about the existence of a Europan ocean.