Telemetry intelligence (TELINT) is a subset of foreign instrumentation signals intelligence. It has special significance in cooperative arms control as one of the national means of technical verification, because it can provide information not externally observable, such as engine thrust, guidance system performance, and weapons release parameters. Especially when confirmed by other sensors, such as spectroscopic MASINT on the exhaust plume, they can provide hard information on the capabilities of missiles.
The simplest way to counter TELINT is to encrypt it, rather than trying to jam TELINT receivers; jammers can interfere with one's own use of telemetry. Strategic arms agreements have variously included a provision that the telemetry not be encrypted, or, if the countries involved want to keep it secret to the participants in the agreement, they exchange information on the encryption method.
Since telemetry is usually line-of-sight, especially in the Cold War, a number of U.S. foreign policies were dictated by the need to have TELINT intercept stations in specific geographic locations. While the National Security Agency was usually responsible for the actual interception, the Central Intelligence Agency might be involved if the intercept location was in a country where the host nation wanted plausible deniability, or even if the intercept receivers were unmanned and clandestinely emplaced in hostile areas.
To intercept telemetry from Soviet launch centers in the southern part of the USSR, some bases were most feasible Iran, and that, in turn, caused the U.S. to make deals with the government of the Shah. Subsequently, the U.S. reluctantly turned to new stations in China, with the understanding the data were shared with the Chinese, who were also deeply interested in Soviet weapon. That sharing, however, revealed U.S. interception capabilities that could be used against Chinese programs.