Decoding TLE

The standard layout of Two Line Elements is shown below, indicating what type of character is valid for each column. Columns with a space or period can have no other character. Columns with an 'N' can have any number 0-9 or, in some cases, a space. Columns with an 'A' can have any character A-Z or a space. The column with a 'C' can only have a character representing the classification of the element set—normally either a 'U' for unclassified data or an 'S' for secret data (of course, only unclassified data are publicly available). Columns with a '+' can have either a plus sign, a minus sign, or a space and columns with a '-' can have either a plus or minus sign (if the rest of the field is not blank).  Both the second time derivative of mean motion and the B-star drag term use an exponential as the final unit after the '-'.

Generic
1 NNNNNC NNNNNAAA NNNNN.NNNNNNNN +.NNNNNNNN +NNNNN-N +NNNNN-N N NNNNN
2 NNNNN NNN.NNNN NNN.NNNN NNNNNNN NNN.NNNN NNN.NNNN NN.NNNNNNNNNNNNNN

for example:
ISS
1 25544U 98067A 03074.52258979 .00026001 00000-0 33596-3 0 7841
2 25544 51.6355 130.6661 0007946 331.6542 129.3073 15.58737682246391

Line 1 definition of Two-Line Element Set Format

Converting the epoch for some PC based systems can be a challenge.  An epoch of 03001.00000000 corresponds to 0000 UT on 2003 January 01 - that is midnight between 2002 December 31 and 2003 January 01. An epoch of 03000.00000000 would actually equate to the beginning of 2002 December 31 - which seems rather bizarre at first sight. The epoch day starts at UT midnight (not midday) and that all times are measured mean solar, rather than sidereal, time units.

Line 2 consists primarily of mean elements calculated using the SGP4/SDP4 orbital model. The definitions for third through to the eighth fields be seen in the table below. Fields 3, 4, 6, and 7 all have units of degrees and can range from 0 up to 360 degrees—field 3 (inclination) only goes up to 180 degrees. The eccentricity (field 5) is a unitless value with an assumed leading decimal point. For example, a value of 1234567 corresponds to an eccentricity of 0.1234567. The mean motion (field 8) is measured in revolutions per day.

Generic
1 NNNNNC NNNNNAAA NNNNN.NNNNNNNN +.NNNNNNNN +NNNNN-N +NNNNN-N N NNNNN
2 NNNNN NNN.NNNN NNN.NNNN NNNNNNN NNN.NNNN NNN.NNNN NN.NNNNNNNNNNNNNN

for example:
ISS
1 25544U 98067A 03074.52258979 .00026001 00000-0 33596-3 0 7841
2 25544 51.6355 130.6661 0007946 331.6542 129.3073 15.58737682246391

Line 2 definition of Two-Line Element Set Format

Loads of other great info is at:
http://www.hq.nasa.gov/osf/station/viewing/issvis.html
and current TLE data at:
http://www.celestrak.com/NORAD/elements/
and expanded info on decoding TLEs at:
http://www.celestrak.com/columns/v04n03/

Back