The Boston Ephemeris was an almanac written by Mather in 1686. It was originally titled "American Ephemeris and Nautical Almanac". Satellites broadcast a new ephemeris every two hours. Atmospheric delay and satellite ephemeris errors have been significantly reduced. NASA Jet Propulsion Laboratory HORIZONS System provides one of several ephemeris computation services. Also errors associated with atmospheric delay and satellite ephemeris have been significantly reduced. This type of calendar requires an ephemeris for calculating dates. Subsequent broadcasts from an updated satellite would contain its most recent ephemeris. Subframes 2 and 3 together contain the transmitting satellite's ephemeris data. The then-current JPL Export Ephemeris was DE19. The construction of a full, high-precision Solar System ephemeris is an onerous task. Successive definitions of the unit of ephemeris time are mentioned above (History). Eventually this led to the Jet Propulsion Laboratory Development Ephemeris series. January 0 refers to the day before January 1 in an annual ephemeris. During the currency of ephemeris time as a standard, the details were revised a little.