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 used by the US Department of Defense Joint Space Operations Center (JSpOC) to distribute satellite elements via space-track.org.

As commonly used today, 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.

TLE files look like this…

OSCAR 10
1 14129U          88230.56274695 0.00000042           10000-3 0  3478
2 14129  27.2218 308.9614 6028281 329.3891   6.4794  2.05877164 10960
GPS-0008
1 14189U          88230.24001475 0.00000013                   0  5423
2 14189  63.0801 108.8864 0128028 212.9347 146.3600  2.00555575 37348

Each number is in a specified fixed column. Spaces are significant. The last digit on each line is a mod-10 check digit, which is checked by the program. The program also checks the sequence numbers (first column), and checks each orbital element for reasonable range. This is a very good set of checks, so this format is very safe, and robust.

There is some inconsistency about how the “+” character is figured into the check digit. If you have trouble with checksum failures on element sets with “+” signs in them, try replacing all the “+” signs with spaces.

Data for each satellite consists of three lines in the following format:

AAAAAAAAAAA
1 NNNNNU NNNNNAAA NNNNN.NNNNNNNN +.NNNNNNNN +NNNNN-N +NNNNN-N N NNNNN
2 NNNNN NNN.NNNN NNN.NNNN NNNNNNN NNN.NNNN NNN.NNNN NN.NNNNNNNNNNNNNN

Line 1

Line 1 is a eleven-character name.

Actually, there is some disagreement about how wide the name may be.  Some programs allow 12 characters.  Others allow 24 characters.

Some sources encode additional information on this line, but this is not part of the standard format. One scheme for encoding visual magnitude information is described in Ted Molczan’s format description.

There is no checksum on this line.


Line 2

Column     Description
 01-01     Line Number of Element Data
 03-07     Satellite Number
 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     First Time Derivative of the Mean Motion divided by 2.
        or Ballistic Coefficient (Depending of ephemeris type)
 45-52     Second Time Derivative of Mean Motion divided by 6. (Blank if N/A)
 54-61     BSTAR drag term if GP4 general perturbation theory was used.
           Otherwise, radiation pressure coefficient.
 63-63     Ephemeris type
 65-68     Element number
 69-69     Check Sum (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. For each minus sign, add 1.
  4. For each plus sign, add 2 (or maybe 0, depending on who created the element set and when)
  5. For each letter, blank, or period, don’t add anything.
  6. 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.


Line 3

Column     Description
 01-01     Line Number of Element Data
 03-07     Satellite 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     Check Sum (Modulo 10)

The same checksum algorithm is used.

All other columns are blank or fixed.