1. TAI
   1. computed
      1. by
         1. weighted average of
            1. 300 atomic clocks
         2. International Bureau of Weights and Measures (BIPM)
      2. at
         1. 60 laboritories
            1. around the world
   2. no leap seconds
   3. ahead of
      1. GPS
         1. by 19 seconds
      2. UTC
         1. by 36 seconds
   4. origin
      1. 1JAN1958
2. UTC
   1. based on
      1. TAI
   2. adjusted by
      1. leap seconds
         1. keep
            1. UTC
            2. in conjunction with
            3. Sun and the stars
3. GPS
   1. time scale
      1. atomic clocks
         1. on Satellites
         2. steered by
            1. time scale
            2. @GPS Master Control Station
            3. compared to
            4. UTC (USNO)
      2. same as
         1. UTC
         2. TAI
   2. no leap seconds
   3. origin
      1. 6JAN1980
   4. ahead of
      1. UTC(USNO)
         1. by
            1. leap seconds
            2. since
            3. 1980
            4. 16 seconds
   5. satellites
      1. broadcast
         1. offset
            1. from
            2. UTC
            3. automatically
            4. applied by GPS receiver
   6. TAI -19 seconds
   7. measured from
      1. SAT/SUN midnight
         1. at the beginning of the week
   8. roll over
      1. after
         1. 1024 = 2^10 weeks
            1. <-->
            2. 10bits
      2. first
         1. 21AUG1999
         2. next
            1. 25MAY2019
4. Paper Clocks
   1. because
      1. no clock keeps official UTC
      2. can only be calculated after all the data from the international contributors are received.
   2. however
      1. NIST and other laboratories
         1. maintain
            1. real-time
            2. version of UTC
            3. within a few nanoseconds of UTC
5. UTC (USNO)
   1. time reference
      1. maintained by
         1. US naval observatory
      2. contributes to
         1. UTC
         2. TAI
      3. for
         1. US DoD
6. Leap second
   1. added to
      1. UTC
         1. to
            1. synchronize
            2. with
            3. astronomical time (UT1) or Mean Solar Time
            4. keep
            5. (UTC-UT1) < 0.9s
         2. based on atomic clocks
            1. more stable
            2. than
            3. Earth rotation rate
            4. sometimes
            5. speeds up
            6. slows down
            7. gradually
            8. slowing
            9. making mean solar second bigger wrt atomic second