A two-line element set (TLE) is a data format encoding a list of orbital elements of an Earth-orbiting object for a given point in time, the epoch. Using a suitable prediction formula, the state (position and velocity) at any point in the past or future can be estimated to some accuracy. The TLE data representation is specific to the simplified perturbations models (SGP, SGP4, SDP4, SGP8 and SDP8), so any algorithm using a TLE as a data source must implement one of the SGP models to correctly compute the state at a time of interest. TLEs can describe the trajectories only of Earth-orbiting objects. TLEs are widely used as input for projecting the future orbital tracks of space debris for purposes of characterizing "future debris events to support risk analysis, close approach analysis, collision avoidance maneuvering" and forensic analysis.
The format was originally intended for punched cards, encoding a set of elements on two standard 80-column cards. This format was eventually replaced by text files as punch card systems became obsolete, with each set of elements written to two 69-column ASCII lines preceded by a title line. The United States Space Force tracks all detectable objects in Earth orbit, creating a corresponding TLE for each object, and makes publicly available TLEs for many of the space objects on the websites Space Track and Celestrak, holding back or obfuscating data on many military or classified objects. The TLE format is a de facto standard for distribution of an Earth-orbiting object's orbital elements.
A TLE set may include a title line preceding the element data, so each listing may take up three lines in the file. The title is not required, as each data line includes a unique object identifier code.
History
In the early 1960s, Max Lane developed mathematical models for predicting the locations of satellites based on a minimal set of data elements. His first paper on the topic, published in 1965, introduced the Analytical Drag Theory, which concerned itself primarily with the effects of drag caused by a spherically symmetric non-rotating atmosphere. Joined by K. Cranford, the two published an improved model in 1969 that added various harmonic effects due to Earth-Moon-Sun interactions and various other inputs.
Lane's models were widely used by the military and NASA starting in the late 1960s. The improved version became the standard model for NORAD in the early 1970s, which ultimately led to the creation of the TLE format. At the time there were two formats designed for punch cards, an "internal format" that used three cards encoding complete details for the satellite (including name and other data), and the two card "transmission format" that listed only those elements that were subject to change. The latter saved on cards and produced smaller decks when updating the databases.
Cranford continued to work on the modelling, eventually leading Lane to publish Spacetrack Report #2 detailing the Air Force General Perturbation theory, or AFGP4. The paper also described two simplified versions of the system, IGP4 which used a simplified drag model, and SGP4 (Simplified General Perturbations) which used IGP4's drag model along with a simplified gravity model. The differences between the three models were slight for most objects. One year later, Spacetrack Report #3 was released, included full FORTRAN source code for the SGP4 model. This quickly became the de facto standard model, both in the industry as well as the astronomy field.
Shortly after the publication of Report #3, NASA began posting elements for a variety of visible and other well known objects in their periodic NASA Prediction Bulletins, which consisted of the transmission format data in printed form. After trying for some time to convince NASA to release these in electronic form, T.S. Kelso took matters into his own hands and began manually copying the listings into text files which he distributed through his CelesTrak bulletin board system. This revealed a problem in NASA's checksum system, which traced back to the lack of the plus character (+) on the teletype machines used at NASA, which ultimately turned out to be a problem from the punch card era that occurred when NORAD updated from the BCD to EBCDIC character set on the computer sending out the updates. This problem went away when Kelso began to receive data directly from NORAD in 1989.