A microsatellite cluster is a set of small satellites that cooperate, measuring phenomena from precise position some number of kilometers apart, to collect information from the earth's surface. The most common application has been direction-finding for electronic intelligence. It is one technique to get, much less expensively, the equivalent of sensors with extremely large collecting apertures. A group of sensors, which are positioned precisely with respect to one another, has the potential to approximate an aperture equivalent to a circle drawn around the outermost satellites. As is done on earth, with systems of telescopes using the principle of very long baseline interferometry, the clusters also can correct for inaccuracies by using observations from multiple points in their orbit. In a sense, this is the reverse of synthetic aperture radar, which takes multiple observations along the flight path of an airplane and combines them into an image equivalent to a sensor whose dimensions are equivalent to the distance between the starting and ending measurement positions.
During the Cold War, the U.S. Navy WHITE CLOUD satellite system was made up of four sets of three "daughter" and one "mother" satellite. Also called PARCAE, after the daughters of Zeus and three daughters of Zeus and the goddess Themida,
One of the daughters spins the thread of fate for each mortal (one satellite has a wide observation swath, but cannot exactly determine the coordinates of radio emitters). The second daughter measures out a length of thread for each person (when two satellites get a fix on the shipborne emitters, the position is obtained, but with some ambiguity). The third sister (Atropos - "she from whom one may not flee") cuts the measured thread of life (the third satellite, getting a fix on the emitters' signals, enables their coordinates to be determined precisely and then transmitted to Navy ships for weapons employment.
While the exact status is not known, the U.S. National Reconnaissance Office has been evaluating the cluster technique. Reducing size of the satellites themselves would reduce the cost of launching as well. Lower overall cost would, in turn, give the opportunity to have more payloads in orbit or that could quickly be orbited, increasing flexibility of deployment.
- Andronov A (1993), "The U.S. Navy's "White Cloud" Spaceborne ELINT System", Zarubezhnoye Voyennoye Obozreniye [Foreign Military Review] (no. 7)pp.57-60
- Richelson, Jeffrey, ed. (July 1996), Small Satellite Review Panel, The NRO Declassified, vol. National Security Archive Electronic Briefing Book No. 225
- McDowell, Jonathan (25 December 2004), Jonathan's Space Report No. 541: Helios 2, McDowell 2004
- de Selding, Peter B. (21 March 2005), "ESSAIM, Micro-Satellites In Formation", Space News Business Report