Here is a brief chronology of recent espionage cases:

June 13, 1988 - A Canadian, Stephen Joseph Ratkai, was charged with gathering secrets about U.S. submarine-monitoring operations at a naval base in Newfoundland during three periods in 1987 and 1988.

June 3, 1988 - A military jury sentenced Army Sgt. Daniel Walter Richardson to 10 years in prison for passing unclassified documents to an FBI agent who was posing as a Russian spy.

June 2, 1988 - A U.S. district judge in Baltimore ruled that former U.S. intelligence analyst Samuel Loring Morison would be allowed to stay out of prison at least temporarily while he appeals his October 1985 conviction for giving secret satellite photographs of a Soviet nuclear aircraft carrier to a British magazine.

May 20, 1988 - The 30-year prison sentence imposed on former Marine Sgt. Clayton J. Lonetree was reduced by five years, and a $5,000 fine was lifted. Lonetree was convicted in an August 1987 court martial on 13 counts, including allegations that he turned over CIA identities and floor plans of the U.S. embassies in Moscow and Vienna to the Soviet Union.

Dec. 21, 1987 - Ronald W. Pelton, a former National Security Agency employee, lost an appeal of his June 1986 espionage conviction for selling electronic surveillance secrets to Soviet agents. He was sentenced to three life terms plus 10 years.

Dec. 18, 1987 - Anne Henderson-Pollard, the wife of convicted spy Jonathan Jay Pollard, lost her bid for a reduction of her five-year sentence for being an accessory after the fact to possession of secret government documents and to a charge of receiving embezzled government property.

Jonathan Pollard, a former civilian intelligence analyst for the Navy, is serving a life term for his guilty plea to selling hundreds of pages of classified military documents to Israel.

Nov. 6, 1985 - Vitaly Yurchenko, a colonel in the Soviet KGB, returned to Moscow after "redefecting." On Aug. 1 he had sought political asylum at the U.S. Embassy in Rome. Yurchenko subsequently traveled to the United States, where he briefed American intelligence agents on Soviet operations, fingering former CIA employee Edward L. Howard and Pelton as Russian spies.

Oct. 28, 1985 - John Anthony Walker, retired Navy communications specialist, pleaded guilty to espionage charges for dropping off secret documents for his Soviet contact near Washingtion. He is serving a life sentence. He was arrested May 20.

Oct. 28, 1985 - Walker's son, Michael Lance Walker, a Navy seaman, pleaded guilty to passing secrets to the Soviets through his father. Now serving a 25-year prison term, the younger Walker was arrested May 21.

Aug. 7, 1985 - Arthur James Walker, John Walker's brother, was convicted of selling secrets to the Soviets. Walker, 51, said his younger brother enlisted him to pass confidential documents from the VSE Corp., a defense contractor for whom he worked as an engineer. Walker was sentenced to life imprisonment and fined $250,000.