The White Cloud Naval Ocean Surveillance System (NOSS) performed wide area ocean surveillance, primarily for the Navy. White Cloud is used to determine the location of radio and radars transmissions, using triangulation. The identity of naval units can be deduced by analysis of the operating frequencies and transmission patterns of the emitters.
Each NOSS launch placed a cluster of one primary satellite and three smaller sub- satellites (that trail along at distances of several hundred kilometers) into low polar orbit. This satellite array can determine the location of radio and radars transmitters, using triangulation, and the identity of naval units, by analysis of the operating frequencies and transmission patterns.
NOSS used the ELINT technique called "time difference of arrival", TDOA, rather than true interferometry. Conceptually, TDOA and interferometry are very similar. though distinct, techniques. They may also use the frequency-domain version of TDOA, FDOA, which exploits doppler shifts somewhat in the way the COSPAS/SARSATs do.
Although there does not appear to be a definitely fixed constellation size for White Cloud, at the beginning of 1990 the constellation apparently consisted of two clusters of primary and secondary satellites, launched on 9 February 1986 and 15 May 1987. No launches under this program have been conducted since 1987.
Naval Ocean Surveillance System--2nd Generation Satellites