Most accurate clock in space

Launched in June 2019, NASAs Deep Space Atomic Clock (DSAC) was activated on 23rd August 2019. The atomic clock is designed to keep time to within one-ten-millionth of a second over the span of a year. It is the first such accurate clock in space and is a forerunning of 'Space GPS' where instead of relying on information calculated on Earth, future space missions (such as to Mars) will use the position and timing of such accurate clocks to calculation their precise position in space; the same method used for our GPS satellite navigation systems on Earth. The clock is about the size of a 4-slice toaster and uses mercury ions to produce a clock 50 times more accurate than the clocks currently employed in GPS satellites.