Limiting the encoding of electronic missile testing data -- telemetry -- was among the touchiest and most contentious issues of the SALT II negotiations and among the last to be settled.

Both sides "listen" to electronic data that the other side's missiles send from space to monitors on Earth. This is particularly important to the United States as one of few sources of detailed technical information about Soviet military programs and a key means of verifying whether the Soviet Union is complying with treaty restrictions.

Because secret U.S. intelligence capabilities were at stake, the issue was so sensitive that for several years U.S. negotiators under Presidents Nixon, Ford and Carter were forbidden to mention the word "telemetry" to Soviet negotiators, even while trying hard to restrict "deliberate concealment measures." Finally a Soviet negotiator mentioned the word and opened up the subject.

Telemetry usually consists of signals transmitted over 40 to 60 electronic channels carrying a variety of information about the performance of a test missile. The Soviets had encoded some of these channels on their missile test flights since the mid-1970s, and U.S. officials were eager to eliminate or minimize the practice.

The United States did not propose that all encoding of telemetry be banned, primarily because negotiators argued that the Soviets would never accept such a restriction. Instead the U.S. proposed -- and the Soviets ultimately accepted -- a prohibition on telemetry encryption that "impedes verification of complicance with the provisions of the treaty."

U.S. negotiators conceded then -- and Reagan administration officials concede now -- that it is a "judgment call" to determine when encoding is permitted and when it is so extensive as to impede treaty compliance. During the last stages of the 1979 negotiations, strenuous efforts were made by the United States to describe impermissible encryption in order to strengthen the restriction.

The Soviet Union is reported to have sharply stepped up its telemetry encryption around 1981, prompting increasingly strong U.S. protests. Earle, the final chairman of the U.S. negotiating team for SALT II, said recently his "subjective view" is that the Soviets raised encryption levels when the United States made it clear it would not ratify the treaty, but would merely refrain from undercutting it so long as Moscow did the same.

The Soviets have insisted publicly and in diplomatic channels that their encryption of telemetry has been within the range permitted by SALT II. The Reagan administration has called the encryption, which according to officials has been at its most extensive on the SS25 missile, "deliberate impeding of verification" and thus a serious violation of SALT II.