Space

Almaz - Radar non-acoustic anti-submarine warfare (NAASW)

In the 1980's a large synthetic aperture radar reconnaissance system was developed under the then military Almaz program. This System lost support from the Ministry of Defense and was eventually converted into a quasi-commercial Earth observation program with flights in 1987 and 1991. No comparable SAR system was known to be under development by the Russian Federation for dedicated national security applications.

Radar non-acoustic anti-submarine warfare (NAASW) became the subject of considerable scientific investigation and controversy in the West subsequent to the discovery by the Seasat satellite in 1978 that manifestations of underwater topography, thought to be hidden from the radar, were visible in synthetic aperture radar (SAR) images of the ocean. In addition, the Seasat radar produced images of ship wakes where the observed angle between the wake arms was much smaller than expected from classical Kelvin wake theory. These observations cast doubt on the radar oceanography community's ability to adequately explain these phenomena, and by extension on the ability of existing hydrodynamic and radar scattering models to accurately predict the observability of submarine-induced signatures. If one is of the opinion that radar NAASW is indeed a potentially significant tool in detecting submerged operational submarines, then the Soviet capability would be somewhat daunting.

The Soviets had extremely fine capabilities in both theoretical and experimental hydrodynamics, that Soviet researchers have been conducting at-sea radar remote sensing experiments on a scale comparable to those of the United States for several years longer than we have, and that they have both an airborne and spaceborne SAR capability. The only discipline that the Soviet Union appears to be lacking is in the area of digital radar signal processing. If one is of the opinion that radar NAASW can have at most a minimal impact on the detection of submerged submarines, then the Soviet effort is of little consequence and poses not threat.

In the very broadest of terms, Synthetic Aperture Radar (SAR) is a coherent imaging radar that takes advantage of platform motion to achieve very high resolution in the along track (crossrange or azimuth) direction. Complementary high resolution in the across track (range) direction is usually achieved by the utilization of conventional pulse compression techniques, although this is not an absolute requirement and an important exception to this general principle is cited by a specific example later. The technique of SAR is not new, in fact the principle was patented by Carl Wiley of the Goodyear Aircraft Corporation in August 1954, and he is credited as having first reported the SAR principle even earlier, in 1951. However, it was not until 1978 that an experimental spaceborne SAR was deployed, in the shape of the NASA/JPL SEASAT mission.

SAR is based on the generation of an effective long antenna by signal processing means rather than by the use of a physically long antenna. SAR achieves its high azimuth resolution by taking advantage of the motion of the radar platform to translate a single antenna element to take up sequential positions along a line. At each of these positions, a radar signal is transmitted, and the amplitude and phase of the returned radar signal in response to that transmission are stored. After the radiating element has traversed a distance, the signals that have been stored resemble the signals that would have been received by the elements of an actual linear array. Consequently, if the stored signals are processed in a similar way to those from an actual linear array, the performance of a long effective antenna aperture can be synthesised. For the spaceborne SAR case, the dimensions of the synthesised antenna can exceed the physical dimensions of the real antenna by many times, and can be as great as several kilometers.

Although the Soviet space agency, Glavkosmos, was active in the development of spaceborne SAR, it was with an instrument significantly different from its Western counterparts. The operational satellite is known as ALMAZ (diamond) and the first in the series was launched on 31 March 1991 from Baikonur Cosmodrome. It was preceeded by a prototype system, the enigmatic platform, COSMOS- 1870. COSMOS- 1870 was built and prepared for launch in 1981, so was very much of the SEASAT generation. However, the launch was cancelled and all further work on the project was suspended. The satellite was, however, saved from destruction and eventually launched into orbit on a PROTON booster from the Baikonur Cosmodrome launch site on 25 July 1987 Ill. It appears that the ALMAZ satellite system would have both civil and military applications.

The radar carried on COSMOS-1870 and its ALMAZ successors is significantly different from other spaceborne SARs to warrant some discussion. The principal difference is that the radar does not use pulse compression to achieve high range resolution. Rather, it uses a short duration, high power, constant frequency pulse. This has major implications on the design of the spacecraft. It requires more prime power for the radar, and hence it has larger solar panels (some 86 square meters in area, supplying 2.5 kW). This, in turn, means greater atmospheric drag at its orbital altitude of 265 km, requiring frequent manoeuvres to prolong platform lifetime. Consequently, the spacecraft itself had an in-orbit mass of 18.5 tonnes of which only 4 tonnes were payload (cf. a mass of 2.3 tonnes for ERS-! and a payload of 1000 kg) to provide sufficient fuel for the required manoeuvres over the satellite's two year design lifetime.

From published photographs, the spacecraft appeared to be based on a SALYUT space station module, although it is, of course, unmanned. The design vintage also has implications on the design of the SAR. As the Soviets were behind in coherent transmitter technology at the time it was designed, the SAR was conceived as a phase locked magnetron based design. In this design, the frequency stability requirements of the prime R.F. power source, the magnetron, can be relaxed, with the required phase coherence being supplied by a highly stable crystal oscillator. Return signals are down-converted and stored on an analogue magnetic tape recorder of 6 MHz bandwidth. A limited linearity, rapid reacting AGC system is used in the receiver to compress the dynamic range of the signals. The requirement for this process is a consequence of the ground station optical processor, which itself has a limited dynamic range. At an appropriate time, data are down linked over an analogue data link for processing 1141.

Another significant difference between ALMAZ and other spaceborne SARs is that it is equipped with two antennas enabling imaging on either side of the ground track, although not simultaneously. The radar antennas are of slotted waveguide design, with an overall size of 15 metres by 1.5 metres. They are divided into three hinged panels for launch requirements. Ir, terms of radar parameters, ALMAZ appears to be virtually identical to COSMOS-1870, the only real differences being wider swaths and slightly higher resolution.

Other differences between ALMAZ and its predecessor are in data handling. The data is processed digitally rather than optically as it was for COSMOS-1870, the satellite will be used in conjunction with geo-stationary data relay satellites for near real-time operation, and its data down link will be digital rather than analogue as it was for COSMOS-1870.

Maritime applications of ALMAZ are broadly as for other SARs, but it appears that monitoring the ocean is not its primary function. Specific objectives to be studied will be the distribution and dynamics of currents, the spatial structure of wave formations, the evaluation of surface winds, the topography of the ocean floor, the identification of oil spills, the state of ice cover and its seasonal fluctuations and the identification of navigational hazards.