This post is part of a series on understanding the meaning behind the values contained in a Two Line Element set, or Keplerian Elements. The main article is here.
Epoch Year and Day
The epoch is simply the point in time that the two line element set is valid for.
The position of the satellite is supplied as part of the two line element set – The position for a particular point in time can then be calculated from the difference between the Epoch and the time you want to know.
The epoch year is a two digit value for the Gregorian (standard calendar) year. Values from 00 to 56 are interpreted as 2000 to 2056, while values from 57 to 99 are for 1957 through to 1999.
The epoch day is a decimal day from the start of the epoch year. Day 1 is the 1st January. The decimal component of the day is simply a decimal value of 24 hours.
To convert a TLE epoch year and day to Julian Date, look at the function “getJulianDay_SatEpoch” in the C# code in this post.