USA-66, also known as GPS IIA-1, GPS II-10 and GPS SVN-23, was an American navigation satellite which formed part of the Global Positioning System. It was the first of nineteen Block IIA GPS satellites to be launched, and was the oldest GPS satellite still in operation until its decommissioning on 25 January 2016. It is located in slot 5 of plane E of the GPS constellation. The satellite has a mass of kilogram (lb). It had a design life of 7.5 years, but remained in service for over 25 years. On 25 January 2016, USA-66 was decommissioned, and removed from the GPS constellation.