Keplerian “Two Line Element” Set Format

Keplerian element sets are usually distributed in Two Line Element Set format.   These are sometimes referred to as “TLEs” “Keps” “el-sets”

Two Line Format

This is the format defined by the United States Space Force (USSF) 18th Space Defense Squadron to distribute satellite elements via the space-track.org web site.   AMSAT uses this format for distribution of TLEs.

As presently used in most tracking software, each spacecraft gets three lines – one line containing the satellite’s name, followed by the standard two lines of elements. Tracking programs are usually unforgiving of anything that doesn’t fit this format.   Some USSF documentation refers to this as 3LE format.

TLE files look like a series of three line sets as shown in below:

ISS (ZARYA)
1 25544U 98067A   04236.56031392  .00020137  00000-0  16538-3 0  9993
2 25544  51.6335 344.7760 0007976 126.2523 325.9359 15.70406856328906
----------------------------------------------------------------------
1234567890123456789012345678901234567890123456789012345678901234567890   reference number line
         1         2         3         4         5         6         7

Each number is in a specified fixed column. Spaces are significant. The last digit on each line is a modulo 10 check digit giving recipients a means for rudimentary error checking.   The check digit was important when these data were distributed by noisy teletype lines, and is practically irrelevant when modern communication protocols are in use.

Line 0

Line 0 contains the object name up to 24 characters

There is no checksum on this line.

Line 1

Column     Description
 01-01     Line Number of Element Data
 03-07     Satellite Number
 08        Classification.   Almost always "U" for unclassified
 10-11     International Designator (Last two digits of launch year)
 12-14     International Designator (Launch number of the year)
 15-17     International Designator (Piece of launch)
 19-20     Epoch Year (Last two digits of year)
 21-32     Epoch (Day number and fractional portion of the day)
 34-43     1st Derivative of the Mean Motion with respect to Time *
 45-52     2nd Derivative of the Mean Motion with respect to Time (decimal point assumed) *
 54-61     BSTAR drag term 
 63-63     Ephemeris type
 65-68     Element set number (modern TLEs from USSF always use 999)
 69-69     Checksum (Modulo 10)

The checksum is computed as follows:

  1. Start with zero.
  2. For each digit in the line, add the value of the digit.
  3. Blanks, periods, letters, ‘+’ signs add zero
  4. For’-‘ signs add 1
  5. Take the last decimal digit of the result (that is, take the result modulo 10) as the check digit.

All other columns are blank or fixed.

* Note that the 1st and 2nd derivatives of mean motion are not used in amateur tracking software, however they may be useful to help predict re-entry of satellites in the weeks immediately before their demise.

Line 2

Column     Description
 01-01     Line Number of Element Data
 03-07     Satellite Catalog Number
 09-16     Inclination [Degrees]
 18-25     Right Ascension of the Ascending Node [Degrees]
 27-33     Eccentricity (decimal point assumed)
 35-42     Argument of Perigee [Degrees]
 44-51     Mean Anomaly [Degrees]
 53-63     Mean Motion [Revs per day]
 64-68     Revolution number at epoch [Revs] 
 69-69     Checksum (Modulo 10)

The same checksum algorithm as described in line 1 is used here.

All other columns are blank or fixed.