A two-line element set (TLE) is a
data format encoding a list of
orbital elements
Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are considered in two-body systems using a Kepler orbit. There are many different ways to mathematically describe the same ...
of an Earth-orbiting object for a given point in time, the ''epoch''. Using a suitable prediction formula, the
state
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States
* ''Our S ...
(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
Simplified perturbations models are a set of five mathematical models (SGP, SGP4, SDP4, SGP8 and SDP8) used to calculate orbital state vectors of satellites and space debris relative to the Earth-centered inertial coordinate system. This set of mod ...
, 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
Forensic science, also known as criminalistics, is the application of science to criminal and civil laws, mainly—on the criminal side—during criminal investigation, as governed by the legal standards of admissible evidence and criminal p ...
.
[
]
The format was originally intended for
punched card
A punched card (also punch card or punched-card) is a piece of stiff paper that holds digital data represented by the presence or absence of holes in predefined positions. Punched cards were once common in data processing applications or to di ...
s, encoding a set of elements on two
standard 80-column cards. This format was eventually replaced by
text file
A text file (sometimes spelled textfile; an old alternative name is flatfile) is a kind of computer file that is structured as a sequence of lines of electronic text. A text file exists stored as data within a computer file system. In operat ...
s as punch card systems became obsolete, with each set of elements written to two 69-column
ASCII
ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
lines preceded by a title line. The
United States Space Force
The United States Space Force (USSF) is the space service branch of the U.S. Armed Forces, one of the eight U.S. uniformed services, and the world's only independent space force. Along with its sister branch, the U.S. Air Force, the Space ...
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 website Space Track, 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 card
A punched card (also punch card or punched-card) is a piece of stiff paper that holds digital data represented by the presence or absence of holes in predefined positions. Punched cards were once common in data processing applications or to di ...
s, 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 hi
CelesTrak bulletin board system. This revealed a problem in NASA's
checksum
A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify data ...
system, which traced back to the lack of the plus character (+) on the
teletype
A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations. Init ...
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
Extended Binary Coded Decimal Interchange Code (EBCDIC; ) is an eight- bit character encoding used mainly on IBM mainframe and IBM midrange computer operating systems. It descended from the code used with punched cards and the corresponding ...
character set
Character encoding is the process of assigning numbers to graphical characters, especially the written characters of human language, allowing them to be stored, transmitted, and transformed using digital computers. The numerical values tha ...
on the computer sending out the updates. This problem went away when Kelso began to receive data directly from NORAD in 1989.
The SGP4 model was later extended with corrections for deep space objects, creating SDP4, which used the same TLE input data. Over the years a number of more advanced prediction models have been created, but these have not seen widespread use. This is due to the TLE not containing the additional information needed by some of these formats, which makes it difficult to find the elements needed to take advantages of the improved model. More subtly, the TLE data is massaged in a fashion to improve the results when used with the SGP series models, which may cause the predictions of other models to be less accurate than SGP when used with common TLEs. The only new model to see widespread use is SGP8/SDP8, which were designed to use the same data inputs and are relatively minor corrections to the SGP4 model.
Format
Originally there were two data formats used with the SGP models, one containing complete details on the object known as the "internal format", and a second known as the "transmission format" that was used to provide updates to that data.
The internal format used three 80-column punch cards. Each card started with a card number, 1, 2 or 3, and ended with the letter "G". For this reason, the system was often known as the "G-card format". In addition to the orbital elements, the G-card included various flags like the launching country and orbit type (geostationary, etc.), calculated values like the
perigee
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ell ...
altitude and visual magnitude, and a 38-character comments field.
The transmission format is essentially a cut-down version of the G-card format, removing any data that is not subject to change on a regular basis, or data that can be calculated using other values. For instance, the perigee altitude from the G-card is not included as this can be calculated from the other elements. What remains is the set of data needed to update the original G-card data as additional measurements are made. The data is fit into 69 columns and does not include a trailing character. TLEs are simply the transmission format data rendered as ASCII text.
An example TLE for the
International Space Station
The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ...
:
ISS (ZARYA)
1 25544U 98067A 08264.51782528 -.00002182 00000-0 -11606-4 0 2927
2 25544 51.6416 247.4627 0006703 130.5360 325.0288 15.72125391563537
The meaning of this data is as follows:
Title line
Line 1
Line 2
![Tle second row](https://upload.wikimedia.org/wikipedia/commons/4/43/Tle_second_row.jpg)
Where decimal points are assumed, they are leading decimal points. The last two symbols in Fields 10 and 11 of the first line give powers of 10 to apply to the preceding decimal. Thus, for example, Field 11 (-11606-4) translates to −0.11606E−4 (−0.11606×10
−4).
The checksums for each line are calculated by adding all numerical digits on that line, including the line number. One is added to the checksum for each negative sign (-) on that line. All other non-digit characters are ignored.
For a body in a typical
low Earth orbit
A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never mor ...
, the accuracy that can be obtained with the SGP4 orbit model is on the order of 1 km within a few days of the epoch of the element set. The term "low orbit" may refer to either the altitude (minimal or global) or orbital period of the body. Historically, the SGP algorithms defines low orbit as an orbit of less-than 225 minutes.
Two-digit Epoch Years from 57-99 correspond to 1957-1999 and those from 00-56 correspond to 2000-2056.
The maximum number of Satellite Catalog Numbers that can be encoded in a TLE is rapidly being approached with the recent commercialization of space and several key break-up events that have created a massive number of debris objects. Future adaptations of the TLE have been imagined to extend the number of encodable Satellites within the TLE.
References
{{DEFAULTSORT:Two-Line Element Set
Spaceflight concepts
Orbits
Computer file formats
de:Satellitenbahnelement#Das Two Line Elements Format TLE