Gates Learjet Corp., which claims to have sold one fourth of the world's business jets, is expanding into the lucrative field of military aircraft.

Beginning in January, a special mission 35A jet for surveillance and armament transportation will be marketed to foreign governments according to Winn Ekeland, Learjet's western region vice president.

The craft's uses vary from sea patrol and submarine detection to ambulance service. It can take "electronic counter-measures" to confuse enemy radar, and it can scar a country's shoreline to spot fishing vessels trespassing in territorial waters.

The aircraft has a range of more than 2,500 miles, a selling point for countries that recently have proclaimed 200-mile territorial limits. The United States and two dozen other countries have established such limits.

The planes are equipped with infrared scanners, low light level television and other video equipment, and many computers to process and display surveillance information.

Ekeland said that while major firms like Lockheed, North American, Douglas Aircraft and Boeing offer military aircraft, Gates Learjet is the smallest firm to enter that market.

Ekeland said he was "certain" that federal government contracts will be sought for the new military aircraft. Currently, Cessna and Learjet are the biggest sellers of business aircraft in the corporate market, he said.

The special mission craft will be produced in Wichita, Learjet's headquarters, then flown to its Tucson plant to be electronically outfitted.

Its sale price is about $3 million. Business aircraft prices vary from $1.5 million to $3 million.

Maximum altitude is 45,000 feet, and maximum speed, more than 500 miles an hour. The twin-engine craft carries a crew of two and up to 10 passengers.

Fitted under the wing are dispensers that may carry armaments or equipment used in submarine warfare to povide light and give positions.

Gates Rubber Co. in Denver is a majority stockholder of Gates Learjet, which it purchased in 1967.

Gates Learjet exports 27 percent of its planes, and its largest market outside the United States is Germany.Learjets also have been sold to France, Yugoslavia, the Middle East and virtually the "whole free world," Ekeland said.

The National Aeronauties and Space Administration employs Learjets for high altitude photography. Cameras built into the aircraft's underbelly are used in California, for instance, to study atmospheric pollution.

In addition to the military aircraft, Learjet plans to begin assembling a "50 series," a new generation of business aircraft with stand-up cabin space in 1980.