The trial of retired Navy communications expert Jerry Alfred Whitworth is to start in federal court here this week in a case that should provide the fullest public picture yet of the damage allegedly caused by the Walker spy ring and an unprecedented glimpse into the arcane, supersecret world of military communications and codes.

Whitworth, 46, the last of four Navy men charged in the Walker espionage ring to appear in court, is charged with 13 counts of espionage, conspiracy and federal income tax violations.

The government alleges that, from 1974 until John Anthony Walker Jr.'s arrest May 20, 1985, Whitworth conspired with Walker to pass classified defense documents and information. He received more than $332,000, according to the government.

A senior chief radioman when he retired from the Navy in 1983 after a 21-year career, Whitworth "received training in virtually all aspects of Navy communications and served both at sea and at Navy bases ashore in positions that permitted him access to a broad spectrum of sensitive military communications," according to a federal indictment.

The most sensitive of the information allegedly funneled to the Soviets was "cryptographic keylists and key cards," the daily-changing codes that are used to encrypt and read classified messages, along with technical manuals and design plans for the coding machines themselves. With the "logic diagrams" contained in the manuals, prosecutors said in court papers filed last month, "a sophisticated adversary having modern computer capabilities" would have been able "to re-create the encryption machine."

Armed with both pieces of the cryptographic puzzle, sources familiar with the technology said, the Soviets would have been able to listen freely to some sensitive Navy communications. The Whitworth trial is expected to disclose what channels of communications may have been compromised and how sensitive they were.

The star witness at Whitworth's trial, which is expected to last eight to 10 weeks, will be his Navy colleague and close friend Walker, 48, a retired Navy chief warrant officer and Norfolk private detective.

Walker masterminded the espionage ring that included his brother, retired Navy Lt. Cmdr. Arthur James Walker, 51, and John Walker's son, Navy Seaman Michael Lance Walker, 23. John Walker's agreement to testify against Whitworth provided a crucial link in the prosecution's case, which until then was largely circumstantial.

John Walker pleaded guilty in federal court in Baltimore Oct. 28 to conspiring to commit espionage with the two other Walkers and Whitworth. Under the plea agreement, he is to be sentenced to life in prison. He promised to testify against Whitworth in return for a reduced sentence for his son, who also pleaded guilty and will be sentenced to 25 years.

Arthur Walker was convicted Aug. 9 of giving John Walker two reports marked "confidential," the lowest category of classified information, from VSE Corp., a Chesapeake, Va., firm where Arthur Walker worked as an engineer.

If U.S. District Judge John P. Vukasin Jr. permits it, Arthur Walker and Michael Walker are also expected to be called on by prosecutors to corroborate John Walker's story -- marking the first time either will have detailed publicly his espionage activities. According to court documents, John Walker urged his brother to "operate like Jerry, who was making big bucks" photographing classified documents for John Walker.

John Walker's ex-wife, Barbara Joy Crowley Walker, and his daughter, Laura Walker Snyder, whom John Walker tried to recruit to spy when she was an Army communications specialist, are also on the government's list of potential witnesses, as is Pamela K. Carroll, a former girlfriend of John Walker.

In addition to the first public statements by Walker about the origins and operation of the spy ring, the trial will feature testimony by Earl Clark, the former deputy chief of communications security at the National Security Agency, who is to discuss the importance of secure military communications and explain to the jurors how the coding machines and cards work. Clark is expected to bring one of the coding machines into court to demonstrate its operation.

The government's witness list includes Bobby Ray Inman, former director of the National Security Agency and former deputy director of the CIA; Vice Adm. Robert E. Kirksey, the director of the Navy division that handles cryptography and communications; Rear Adm. Lawrence Layman, head of naval communications, and Gerald Richard, an FBI expert in Soviet spy methods, or "tradecraft."

On June 12, nine days after Whitworth's arrest and at the height of public attention to the Walker case, Chief of Naval Operations Adm. James D. Watkins provided the first official assessment of the potential damage done by the ring. He said that the loss appeared to be "very serious" but "not catastrophic," and that the biggest damage was in the area of communications.

Whitworth's trial will offer the first public damage assessment since then -- other than testimony at Arthur Walker's trial, which was limited to the two reports he passed to the Soviets -- and the first since Walker agreed to provide details about the operations of the spy network.

The defense case will focus primarily on attacking Walker, according to defense lawyer James Larson. "We think the central issue in the case really is the credibility of John Walker," Larson said. He said the defense will attempt to undermine Walker's story by "going into what he says very thoroughly and very carefully."

Larson said he planned to call some defense witnesses, unlike lawyers for Arthur Walker, who rested their case without presenting a defense. But, Larson said, "A lot of our defense will consist of cross-examination of their witnesses, not necessarily presenting alternative" witnesses.

One potential defense witness is Whitworth himself. In papers filed Feb. 7, defense lawyers argued that the case against Whitworth should be split in two, with the espionage charges tried separately from the tax counts. Although they did not explain why, defense lawyers Larson and Tony Tamburello said Whitworth "wishes to testify concerning the espionage charges but not the tax and fraud allegations." The motion to sever the charges is pending.

Vukasin is to hear arguments today on a renewed bid by Assistant U.S. Attorneys William Farmer Jr. and Leida B. Schoggen to introduce a series of letters to the FBI from "RUS" offering to expose a "significant espionage system." Prosecutors contend that Whitworth wrote the letters, but Vukasin has ruled against their introduction.

Jury selection, which is expected to begin tomorrow, is expected to consume a week because of the publicity the Walker cases have generated.

Whitworth was sitting at the personal computer in his Davis, Calif., mobile home on the morning of May 20, 1985, writing a letter to John Walker, when two FBI agents rang the doorbell.

Walker, they informed him, had been arrested and charged with espionage. "I was dumbfounded and didn't respond immediately," Whitworth wrote in an affidavit . . . . "I don't exactly recall my response, but I think it was something like, 'I don't know what to think.' "

Hours earlier, FBI agents had arrested Walker in a hallway of the Rockville Ramada Inn. Agents trailing Walker had seen him near a secluded site in Poolesville, in western Montgomery County, where they later found a bag disguised as trash and filled with classified documents from the aircraft carrier USS Nimitz, where Michael Walker was working as a clerk in the ship's operations department.

Also contained in the bag were two "Dear Friend" letters from Walker to his Soviet handler. " 'D' continues to be a puzzle," Walker wrote. "He is not happy, but is still not ready to continue our 'cooperation' . . . . My guess . . . he is going to flop in the stockbroker field and can probably make a modest living in computer sales." Walker included two "Dear Johnny" letters from "D" himself, which discussed, among other things, "news about Brenda's job prospects."

Whitworth's wife is Brenda Reis; Whitworth, who had retired from the Navy in October 1983, was studying to be a stockbroker, having decided to abandon the idea of computer sales.

FBI agents had already been alerted to Whitworth's possible involvement by two "confidential informants" later identified as Barbara Walker and Laura Walker Snyder, who told them of West Coast man named "Jerry Wentworth" who was allegedly part of the spy ring. In a search of Walker's Norfolk house, agents found -- among other things -- papers that identified "D" as "Jer," and handwritten notes that dealt with secure Navy communications systems and that contained one of Whitworth's fingerprints, according to court papers.

Two weeks after they first knocked on his door, the FBI issued an arrest warrant for Whitworth, who turned himself in at the FBI's San Francisco office.

As portrayed in the indictment, the espionage conspiracy between Whitworth and Walker started in 1974 at a meeting in Boom Trenchard's Flare Path restaurant and bar in San Diego.

The Navy colleagues had met a few years earlier when Whitworth was a communications instructor at the Service School Command in San Diego and Walker was assistant director of the Radioman "A" school there. Walker had been spying for the Soviets since 1968, but by 1974 -- two years before his retirement from the Navy -- he had apparently decided to expand his operations.

At Boom Trenchard's, the indictment alleges, the two men "formed an espionage partnership whereby Walker would eventually be responsible for the transportation and sale of classified information and Whitworth would be responsible for obtaining such information, the profits from the enterprise to be split equally between them."

The indictment details a series of more than 20 meetings, in California, Norfolk, Hong Kong and the Philippines, at which Whitworth allegedly passed classified information to Walker. The meetings were often followed shortly by meetings betwen Walker and his Soviet contact, according to the indictment.

In addition to the charge that he conspired with Walker to commit espionage, Whitworth faces eight counts of espionage for allegedly passing Walker classified information from the aircraft carrier USS Constellation, the USS Niagara Falls, the Naval Telecommunications Center at Alameda, Calif., and the nuclear aircraft carrier USS Enterprise. At those postings Whitworth held increasingly responsible jobs in communications, with access to intelligence messages and coding material.

Whitworth, a balding, bearded, studious-looking man who has been held without bond since his arrest, grew up on a 600-acre wheat and soybean farm in Muldrow, Okla., near the Arkansas border. He was voted "class clown" at Muldrow High and left home at age 17. He joined the Navy in 1962, and he specialized in communications in a career that took him across the globe.

Whitworth's uncle, Willard Owens, said Whitworth "sounds great" despite nine months in jail and remains optimistic about his chances for acquittal. "He believes that he's going to come free of the thing," Owens said.

His twice-weekly conversations with Whitworth, he said, touch on the monotony of the jail food and the newspapers and magazines Whitworth has been reading, but they mostly focus on life in Muldrow.

"We talk about things here at home mostly," he said, "about the farm and the way it used to be and the way it will be when he comes out."