Several more advanced satellites replaced the early VELAs, and the function exists today as the Integrated Operational Nuclear Detection System (IONDS), as an additional function on the NAVSTAR satellites used for GPS navigation information. Space-based Staring Infrared Sensors The US, in 1970, launched the first of a series of space-based staring array sensors that detected and located infrared heat signatures. Such signatures, which are associated with measurement of energy and location, are not pictures in the IMINT sense. Currently called the Satellite Early Warning System (SEWS), the program is the descendant of several generations of Defense Support Program (DSP) spacecraft. Originally intended to detect the intense heat of an ICBM launch, this system proved useful at a theater level in 1990–1991. It detected the launch of Iraqi Scud missiles in time to give early warning to potential targets. When an arms control agreement, such as the MCTR, limits missile technology transfer, this system can detect missile launches that may have been a result of inappropriate transfer, or independent development by a nation that has not imported rocket motors. Geophysical intelligence defines geophysical intelligence as a branch of MASINT. "it involves phenomena transmitted through the earth (ground, water, atmosphere) and manmade structures including emitted or reflected sounds, pressure waves, vibrations, and magnetic field or ionosphere disturbances." Seismic Intelligence defines seismic intelligence as "The passive collection and measurement of seismic waves or vibrations in the earth surface." In the context of verification, seismic intelligence makes use of the science of seismology to locate and characterize nuclear testing, especially underground testing. Seismic sensors also can characterize large conventional explosions that are used in testing the high-explosive components of nuclear weapons. In 1960, George Kistiakowsky introduced the "threshold principle" that balances the needs of arms control with the realities of seismic verification. He cited the difficulty in monitoring missile submarines, and proposed that the arms control strategy focus on disarmament rather than inspections into verification, which accepts that nations may do nuclear, or simulated nuclear, testing of an explosive yield below the energy level that seismic intelligence sensors can detect. All nuclear testing, of any level, was forbidden under the Comprehensive Test Ban Treaty (CTBT) (which has not entered into force), but there is controversy over whether the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) or its preparatory commission will be able to detect sufficiently small events. It is possible to gain valuable data from a