Portal perimeter monitoring has been both the most complex and expensive on-site verification tool under the INF Treaty. Its basic concepts and procedures were developed by the U.S. Initially rejected by the Soviets, it eventually was accepted in the "end game" to verify the INF Treaty's ban on the production of INF missiles systems after the Soviets confirmed, in October 1987 in Geneva, U.S. suspicions that the first stage of the SS-25 mobile ICBM is outwardly similar to the first stage of the banned SS-20. To meet U.S. verification concerns that banned SS-20s were not being produced in the guise of legal SS-25s, the Soviets accepted earlier U.S. proposals for continuous portal monitoring at the Votkinsk Machine Building Plant (VMBP), the missile final assembly facility for the SS-25 which previously also had produced the SS-20. On a reciprocal basis, the Soviets were permitted to inspect shipments from Hercules Plant #1 in West Valley (Magna), Utah, the U.S. missile production facility at which Pershing-II stages had been produced. By monitoring shipments from the Hercules facility, the Soviets could be assured that Pershing-II first-stage production had not resumed.
The basic components of the portal system are set out in the INF Treaty's Inspection Protocol. They consist of the main portal, through which all missiles, missile stages, and containers large enough to contain a banned