Public outcry prompts Swedish Navy to get tough with intruders

The beleaguered Swedish Navy is taking more seriously the ''cat-and-mouse'' game it plays regularly with intruders in its waters. This new toughness is one result of the rash of foreign - and presumably Soviet - submarine incursions into Swedish territorial waters. Since 1962 there have been 143 violations of Sweden's territorial waters by foreign submarines, 40 of them in 1982. Tracks left by mini-subs that crawl along the bottom have even been found at the entrance to Stockholm's harbor.

In response to this ''invasion'' of underwater snoopers, the Swedish Navy is busy beefing up its detection hardware and refining its ''search and destroy'' tactics. According to the Swedish technical magazine ''Ny Teknik,'' any unwanted interlopers will now be met with an air-sea barrage of advanced electronic detection devices, backed up by an embarrassed Navy with instructions to shoot to kill.

The first line of defense consists of specially equipped navy planes crammed full of sophisticated radar designed for marine targets. The Ministry of Defense claims that the new radar systems being tested can pick out a periscope within a radius of 125 miles at an altitude of 300 feet.

In addition to radar, these sub hunters will have heat cameras - called FLIR for ''forward-looking infra-red'' - which are sensitive enough to detect the turbulence or displacement caused by a sub moving through the water.

Budget permitting, the Navy is also talking about installing equipment capable of registering the magnetic disturbances caused by submarines.

If this first ''trip line'' fails to deter and a submarine manages to slip into Sweden's near-shore waters, especially along its vulnerable 4,600-mile-long Baltic coastline, it will be greeted by yet a second line of detection devices - and presumably an even more embarrassed Navy.

Once a sub has been spotted, a hail of hydrophones will be dropped in the area by planes or helicopters. This detection package, manufactured by Ultra Electronics Communications of Britain, consists of a floating antenna and transmitter connected by cable to the subsurface hydrophone, which is really not much more than an underwater microphone: ''marine ears,'' as the Navy refers to them. They are said to be so sensitive that a skilled operator can identify the number of cylinders in a submarine's quiet-running diesel engine.

In the event that a sub's ''sound track'' is picked up, the information will be relayed automatically via the hydrophone's radio to a receiver on board a special sub-hunting plane or helicopter. Since hundreds of these hydrophones can be used in a single sub chase, the chances of pinpointing its location are greatly increased.

However, there are two drawbacks to using the British-made hydrophones in this way - cost and longevity. Each hydrophone costs between 2,000 and 3,000 Swedish kroner ($266-$400) and each one lasts a mere eight hours in the water before its powerful batteries wear out and it sinks to the bottom.

Contemplating whether to force the sub to surface or blow it out of the water causes other worries. The Swedes have always tried to avoid offending the Soviets, and have never provoked a direct confrontation either with foreign subs that have continually violated its territorial waters or with Soviet jet fighters, which also routinely violate Swedish airspace.

Apparently, public outrage and political pressures are forcing the navy to get tough. Whether this means they will ever ''get'' any Soviet subs remains to be seen. Their track record is dismal. The closest the Swedish Navy has come to a Soviet submarine was when the Whiskey-class sub conveniently ran aground nine miles south of the major Swedish naval base at Karlskrona in November 1981. Even then, a reluctant Navy patrol boat had to be led to the sub by local fishermen.

A somewhat sanguine Ministry of Defense feels that the new equipment, coupled with more aggressive anti-sub tactics, will discourage the wholesale violation of Sweden's territorial waters of the past 24 months.

Comments one Defense Ministry official: ''We may never be able to completely eliminate foreign subs in our near-shore waters, but at least we can try to discourage them from 'picnicking' in our waters.''