The US Navy's spaceborne electronic intelligence (ELINT) system, White Cloud, is based on SSU (Subsatellite Unit) satellites and is intended for determining the location of warships of foreign states and following them by the method of taking bearings on the ships' onboard radioelectronic equipment from several positions. Official Pentagon representatives try not to attract attention to this system, since it is the principal means of over-the-horizon reconnaissance and target designation for the US Navy's weapons systems. In comparison with other American reconnaissance satellites, (for example, "Keyhole" and "Lacrosse") the SSU satellites are mentioned comparatively infrequently and have various names: White Cloud, Classic Wizard, and Parcae.
The last designation more precisely reflects the mission and operating principle of the system's satellites. The mythological Parcae were the three daughters of Zeus and the goddess Themida (the SSU satellites are launched in groups of three and fly relatively close together.) 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.
Experiments on space reconnaissance of radio signals in support of the Navy were carried out in the US from the early 1960s using small Air Force "Ferret" ELINT satellites. However, they could not determine the direction and speed of travel of naval targets, inasmuch as they were developed to fix the coordinates of immobile air defense complexes on the ground. Therefore, at the end of the 1960s, the Navy's White Cloud program began to develop the specialized SSU ELINT satellites. The first experimental Navy ELINT satellites were launched in 1971 using the Thorad-Agena booster and received the designation SSU-A1, -A2 and -A3. The Agena stage with a restartable engine dispensed the small, 123 kg, SSU satellites. Using the first experimental satellites, which were made by the Naval Research Laboratory, the principle of multi-site direction-finding of shipboard emitters from orbit was worked out, the on-board gravity-gradient stabilization apparatus was tested, as was signal intercept and transmission to Earth, and the optimal parameters of the working orbit were selected.
In 1976-1980 the Pentagon deployed on orbit an ELINT system consisting of three groups of first-generation SSU satellites. The satellites, mounted on a NOSS satellite-platform with a restartable liquid engine, were launched from the Western Test Range on an Atlas booster into circular orbits at an altitude of 1100 km and inclination of 63.5 degrees. Forming the orbital layout of the group was carried out in the process of multi-burn maneuvering of the satellite-platform and sequential separation from it of the three small SSU satellites. Beginning with the third group of satellites, launched in 1980, development and production of the satellites was carried out by the firm Martin Marietta, and the reconnaissance equipment was built by E- Systems. From 1983 to 1987 a total of five groups of modernized SSU-1A satellites with upgraded stabilization and data transmission systems were launched to replace failed satellites.
In external appearance, the SSU satellites (Fig.1) are reminiscent of the Navy's Transit navigational satellites. They have gravity gradient orientation booms 10-15 meters long. These ensure that the side of the satellite body where the signal intercept antennas are mounted is constantly oriented toward the Earth. The satellites maintain an assigned position in a group at distances of 30 to 240 km one from the other by using on-board low-thrust engines. As a result of ground processing of direction-finding data on the signals from target emitters provided by a group of satellites, and also sequentially by several groups of satellites, the coordinates, direction and speed of travel of ships are obtained.
Calculations indicate that in order to compute the direction and speed of ships using one group of satellites it is necessary to have fixes with a precision of the order of 2 to 3 km, or 8 to 10 km if four satellites are used. The task of determining the bearings of naval targets is made easier by the fact that practically all ships have continually operating emitters fulfilling various purposes: communications, navigation, surface and air search, and weapons control.
For determining the bearings of signals from different directions using the method of time difference of arrival, the intersatellite baselines (the imaginary straight line segments connecting the satellites) should form a right angle (or, at least, not be parallel). These conditions are fulfilled through the orbital parameters chosen for the satellites. As a group flies over the equator the baselines form a figure which is close to a right triangle (Fig. 2). However, in the polar regions, as the satellites go through latitudes which correspond to the maximum inclination of their orbits (around 63 deg.), the form of the group changes, and the satellites follow practically along one and the same trajectory one after the other. In order to avoid decreased signal bearing accuracy, the apogee portion of the orbit of one of the satellites is shifted relative to the apogee portions of the others. Thanks to this, in the polar regions one of the satellites moves 50 to 100 km lower [sic; this doesn't seem to agree with Figures 2 and 3] than the remaining ones, which lets the direction-finding baselines spread out and eliminates the "zone of inaccessability." (Fig. 3)
In its fully deployed form the Navy's White Cloud ELINT system has four groups of SSU satellites with orbital planes distributed at 60 to 120 degrees along the equator and a complex of ground stations for receiving and processing signals located in the US (Blossom Point, Maryland and Winter Harbor, Maine), Great Britain (Edsel, Scotland), on the islands Guam, Diego Garcia, Adak, and in other locations. Operational control of the system is carried out by the Navy Space Command, and processing of the reconnaissance signals is done in the Navy's information center in Suitland (Maryland) and regional Navy intelligence centers in Spain, Great Britain, Japan and Hawaii.
A satellite group is able to receive signals from a zone with a radius of about 3500 km (on the surface of the Earth) and under certain conditions can monitor the same object 108 minutes later. A system of four satellite groups enables any region at a latitude of 40 to 60 degrees to be monitored more than 30 times a day.
A fundamental problem in performing multiposition signal intercept by the time difference of arrival method is the necessity of synchronizing the on-board radio receivers in an SSU satellite and determining the intersatellite (baseline) distances. In the opinion of American specialists, the problem of receiver synchronization and ranging can be solved by placing millimeter wave communications apparatus on the SSU satellites.
Operations of the ELINT satellites have demonstrated that they have a quite high reliability, with a mean operating lifetime in orbit of seven to eight years. The course of deployment of the Navy ELINT system and replacement of satellites is shown in Table 1. It is easy to see that only two SSU satellite groups (the fifth and eighth) gave out on orbit after two or three years. This might be connected with problems with a satellite: if even one satellite fails, operational utility and emitter location capability worsen significantly.
The routine replacement of the SSU-1A satellites, which were launched in 1983-1987, with second-generation SSU-2 satellites began in 1990. Originally, they were intended to be launched on the Shuttle, but after the catastrophe of 1986 it was decided to use the Titan-4 heavy booster as the principal launch vehicle.
SSU-2 has a new design and updated reconnaissance and data communication equipment. It lacks, in particular, the transmitters which worked in the 1427-1434 MHz band and interfered with radioastronomical observations. The configuration of the SSU group remained as before, but the dimensions of the direction-finding triangle of the new satellites were almost two times less than those of the earlier satellites. This could be connected with an expansion of the band of intercepted signals into the centimetric part of the SHF band, in which shipboard emitters use antennas with narrow main beams.
Completion of the deployment of the White Cloud ELINT system consisting of four groups of the new SSU-2 satellites is expected in 1995, and their employment will continue to the end of the 1990s. The characteristics of the SSU system is given in Table 2.
The space-based ELINT system is one of the basic means for over- the-horizon targeting for warships which are equipped with Tomahawk cruise missiles. The information from space and other reconnaissance systems is transmitted from data processing centers to ships by the Fleetsatcom and Leasesat satellites through the channels of the TADIX subsystem for tactical data interchange. In the early 1990s serial production of this system's receivers was begun; they are intended for installation on ships of various classes (command, control, major combatants) and nuclear attack submarines.
The foreign press did not report information on the successful employment of the White Cloud Navy ELINT system during combat operations in the Persian Gulf, obviously because of the secrecy of the program and the limited nature of Iraqi naval operations. However, it cannot be excluded that the system operated in conjunction with the Air Force Ferret ELINT satellites to seek out the Iraqi air defense system, since the frequencies of ground-based and shipborne emitters are basically the same. In this case the White Cloud Naval ELINT system could have supported a more frequent scrutiny of Iraq and a more precise location of the signals of ground-based and airborne emitters than could the Ferret satellites.
****** Table 1 Data on the Satellites and Their Operating Lifetimes Orbital Serial number Type of Lifetime in plane of group satellite orbit A 1 SSU-1 1976-1983 A 4 SSU-1A 1983-1992 A 10 SSU-2 Since 1992 A 2 SSU-1 1978-1984 B 5 SSU-1A 1984-1986 B 7 SSU-2 [sic] Since 1986 B 3 SSU-1 1980-1988 C 8 SSU-1A 1988-1990 C 9 SSU-2 Since 1990 [C?] Launch failure SSU-1 D 6 SSU-1A Since 1984 ****** Table 2 Characteristics of the White Cloud ELINT System Characteristic SSU-1 SSU-1A Second (original (modernized generation model) model) SSU-2 Years launched 1976-1980 1983-1987 Since 1990 Number of successfully launched groups (satellites) 3(9) 5(15) 2(6) Booster type Atlas-F Atlas-F Titan-4 Range WTR WTR WTR and ETR Orbital insertion method direct direct multi-impulse Days to make group operational 20-25 15-25 30-40 Average distance between satellites in a group, km 50-240 50-240 30-110 Mean lifetime in orbit, years 6-7 7-9 > 7 Range of intercepted frequencies, GHz (presumed) 0.5-4 0.5-4 0.5-10 Mass in orbit, tonnes Cluster [?-svyazka] (NOSS and SSU) 1-1.5 1-1.5 4-8 SSU satellite 0.196 0.2 > 0.2 NOSS satellite 0.548 0.6 3-7 Stabilization system Gravity-gradient Dimensions of SSU body, m. 0.3x0.9x0.4 0.3x0.9x0.4 1-3 (length) ****** Title of Figure 1: Sketch of the external appearance of an SSU satellite. ****** Title of Figure 2: Sketch of the mutual relationship of satellites in a group: 1a - when crossing the equator; 1b - at the maximum latitude. ****** Title of Figure 3: Sketch of the influence of displaced lines of apsides of the orbits of a group's satellites on the orbital make-up of the group in polar regions: 2a - displacement of the line of apsides of the satellites' orbits (angle J equals 20 to 40 deg); 2b - arrangement of the satellites in a group without displacement of the line of apsides; 2c - arrangement of the satellites in a group with displacement of the line of apsides.