|Operator||US Air Force|
|Mission duration||7.5 years (planned)|
|Spacecraft type||GPS Block IIA|
|Launch mass||1,816 kilograms (4,004 lb)|
|Start of mission|
|Launch date||26 November 1990, 21:39:01UTC|
|Rocket||Delta II 7925-9.5, D201|
|Launch site||Cape Canaveral LC-17A|
|End of mission|
|Deactivated||25 January 2016|
|Perigee altitude||20,072 kilometres (12,472 mi)|
|Apogee altitude||20,293 kilometres (12,609 mi)|
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.
USA-66 was launched at 21:39:01 UTC on 26 November 1990, atop a Delta II carrier rocket, flight number D201, flying in the 7925-9.5 configuration. The launch took place from Launch Complex 17A at the Cape Canaveral Air Force Station, and placed USA-66 into a transfer orbit. The satellite raised itself into medium Earth orbit using a Star-37XFP apogee motor.
On 30 December 1990, USA-66 was in an orbit with a perigee of 20,072 kilometres (12,472 mi), an apogee of 20,293 kilometres (12,609 mi), a period of 717.98 minutes, and 54.9 degrees of inclination to the equator. It was initially given PRN 23, which it used until its retirement in February 2004; however, it was subsequently reactivated broadcasting PRN 32 and in February 2008 it rejoined the operational constellation.
It is located in slot 5 of plane E of the GPS constellation. The satellite has a mass of 1,816 kilograms (4,004 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. When engineers took it offline, its disappearance triggered a software bug that left the timing of 15 of the remaining GPS satellites off by 13.7 microseconds, causing widespread GPS disruptions. 
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- "The effects of the January 2016 UTC offset anomaly on GPS-controlled clocks monitored at NIST" (PDF). Retrieved 2 March 2018.
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