Osa Class

A common Russian craft, the Osa [Wasp] is a simple design that any nation can build, and many did.

Some accounts credit the Osa with sinking the Israeli Destroyer INS Eilat duirng the 1967 war, but this appears to be in error [most authoritative sources say the missiles were fired from Komars]. The small sizes and low profiles of the Osas and Komars provide difficult targets for enemy radars. The boats rely on their speed and maneuverability to evade attacks by enemy surface forces . They carry antiaircraft weapons but are vulnerable to aircraft attack.

The Osa—class guided-missile patrol boat was designed specifically to launch the SS—N—2 missile. Each Osa carries four missiles in covered launchers. Its 9.5—foot draft and speed of 40 knots gives the boat great flex- ibility in operating from protected coves, from which it can attack ships as far away as 150 miles. The Osa boats, with a displacement of 208 tons, probably are capable of launching their missiles in fairly heavy seas and in winds up to 30 knots.

The radar aboard the boats is used to locate the target, provide firing data to the SS-N-2, and assess damage after an attack. The effective range of the system probably could be extended — and the element of surprise enhanced — through the use of a coastal radar to provide target detection and location data. For maximum effect, however, the missile must be aimed and fired on the basis of data provided either by the ship—borne radar or by an optical system carried on the patrol boats.

After the necessary informa— tion has been fed into the missile's guidance system, a rocket booster lifts it from its launcher and carries it to cruise altitude and speed. The missile flies toward the target at a speed of Mach 0.9 and an altitude between 300 and 1,000 feet. Because of the low altitude and high speed, defending forces have only about a minute to react.

The best defense against the system is to attack the patrol boats before they fire their missiles. The missile's speed, low flight altitude, small radar cross-section, and built-in defense against electronic countermeasures made it difficult to counter. The system, nevertheless, has limitations. Targets closer than five miles cannot be engaged because the SS—N-2's homing and arming mechanisms are not activated until the missile has flown that far. Moreover, if two targets are detected, the homing radar cannot be relied upon to seek out the preferred one. The homing radar also may be susceptible to some types of electronic countermeasures.

Fixings apparently cannot be made if the air temperature falls below 4°F or rises above 104°F. A final shortcoming of the system is that targets within four miles of shore cannot be attacked from their seaward side, because "clutter" reflected by the terrain prevents the homing radar from operating properly. In such cases, the attacking patrol boat has to maneuver to fire parallel to or away from the beach.