A Philadelphia man indicted in connection with a planned illegal sale of a sophisticated aircraft tracking system to Syria pleaded guilty yesterday in federal court here to conspiracy to violate the Arms Export Control Act.

Kevin Gilday, 39, was arrested on a Capitol Hill street in November during last-minute negotiations for the sale. He told U.S. District Judge Thomas P. Jackson yesterday that he was in "substantial" agreement with the 17-page factual summary filed by the government, which stated that he was being helped in the deal by a Syrian diplomat.

The diplomat, embassy Third Secretary Souha Jamali, left this country shortly after the arrests of Gilday and his business associate, Rosemary Loughery, 45, of Mount Holly, N.J. Loughery pleaded guilty to a similar conspiracy charge last month.

According to the factual summary submitted by Assistant U.S. Attorneys Mark J. Biros and Paul L. Knight, Gilday was known to Customs agents as a man who previously had been involved in efforts to ship equipment through Europe and the Middle East to the Soviet Union and East Bloc countries.

The conspiracy, which allegedly involved two international arms dealers from Ireland and England, was revealed during an undercover investigation by Donald Bludworth and Robert Fischer, customs agents who were pretending to be international high-technology and arms brokers.

During the investigation, Gilday inquired about buying a long list of arms, including spare parts for tanks, helicopters, F4, F5 and F14 aircraft as well as radar equipment, Tow, Dragon and Stinger missiles. He also undertook negotiations to sell a C130H transport plan to Libyan leader Moammar Gadhafi.

Gilday, who has been under house arrest at his parents' Wilmington, Del., home since shortly after his arrest, will remain free under the same conditions until his sentencing Oct. 16, Jackson said.

Gilday faces up to five years in prison and a fine of $250,000.