The North American Aerospace Defense Command (NORAD) developed the Two-Line Element (TLE) format for transmitting satellite Keplerian elements.  This is a structured format intended to be very compact. It is, therefore, difficult to read unless you are familiar with the structure. Hence, the purpose of this page is to define the components of this structure as defined below.

United States Space Command (USSPACECOM) computes most of the publicly-available TLEs. These data are released by NASA Goddard Space Flight Center (GSFC).  In order to get accurate orbital predictions, you need to use the either USSPACECOM Simplified General Perturbations (SGP) or the SGP Version 4 (SGP4) orbit propagator.  Using even an inherently more accurate prediction model will get degraded predictions because the TLEs released by GSFC are "mean" Keplerian elements produced by removing the long- and short-term periodic variations in a particular way.  Complete details of the SGP/SGP4 orbit propagators are in Project Space Track, Models for the Propagation of NORAD Element Sets, Felix R. Hoots and Ronald L Roehrich, Spacetrack Report No. 3, December 1980. This report is in Adobe Acrobat Portable Document Format (PDF).  You can download the free Acrobat Reader software from the Adobe Web site.

The physical significance of each Keplerian orbital parameter is illustrated on the Orbital Description page.

Two-Line Element (TLE) Format:

This page illustrates the structure of the Two-Line Element (TLE) format

The Project Space Track report is the definitive source for SGP, SGP4, and the correct propagation of NORAD TLEs

• [1] - Line #1 label
• [2] - Line #2 label
• [AAAAA] - Catalog Number assigned sequentially (5-digit integer from 1 to 99999)
• [U] - Security Classification (U = Unclassified)
• [YYLLLPPP] - International Designator (YY = 2-digit Launch Year; LLL = 3-digit Sequential Launch of the Year; PPP = up to 3 letter Sequential Piece ID for that launch)
• [BBBBB.BBBBBBBB] - Epoch Time -- 2-digit year, followed by 3-digit sequential day of the year, followed by the time represented as the fractional portion of one day
• [.CCCCCCCC] - ndot/2 Drag Parameter (rev/day²) -- one half the first time derivative of the mean motion.  This drag term is used by the SGP orbit propagator.
• [DDDDD-D] - n double dot/6 Drag Parameter (rev/day³) -- one sixth the second time derivative of the mean motion. The "-D" is the tens exponent (10^-D). This drag term is used by the SGP orbit propagator.
• [EEEEE-E] - Bstar Drag Parameter (1/Earth Radii) -- Pseudo Ballistic Coefficient. The "-E" is the tens exponent (10^-E). This drag term is used by the SGP4 orbit propagator.
• [F] - Ephemeris Type -- 1-digit integer (zero value uses SGP or SGP4 as provided in the Project Spacetrack report.
• [GGGG] - Element Set Number assigned sequentially (up to a 4-digit integer from 1 to 9999). This number recycles back to "1" on the update following element set number "9999."
• [HHH.HHHH] - Orbital Inclination (from 0 to 180 degrees).
• [III.IIII] - Right Ascension of the Ascending Node (from 0 to 360 degrees).
• [JJJJJJJ] - Orbital Eccentricity -- there is an implied leading decimal point (between 0.0 and 1.0).
• [KKK.KKKK] - Argument of Perigee (from 0 to 360 degrees).
• [MMM.MMMM] - Mean Anomaly (from 0 to 360 degrees).
• [NN.NNNNNNNN] - Mean Motion (revolutions per day).
• [RRRRR] - Revolution Number (up to a 5-digit integer from 1 to 99999). This number recycles following revolution nymber 99999.
• [Z] - Check Sum (1-digit integer). Both lines have a check sum that is computed from the sum of all integer characters on that line plus a "1" for each negative (-) sign on that line. The check sum is the modulo-10 (or ones digit) of the sum of the digits and negative signs.

More details on the TLE structure, including column positions, ranges of legal values, and units are on the TLE Structure page.  This information  is in a format useful for software engineers.