# Two-Line Element Set (TLE): Format Overview

To calculate the position of the object around the earth, for example, satellites and space debris, we need information on the orbital parameters which describes the shape and size of the orbit. Two-Line Element Set is one of the data formats for describing the orbital parameters.

## What is Two-Line Element Set?

Two Line Element Set (TLE) is the data format to describe the orbital parameters of the earth-orbiting object. The example of TLE for ISS:

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

First line is Line 0, thus this is called two-line element set.

Then, for each element.

## Object Name

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

Line 0 includes only the object name (maximum 24 characters).

ISS means International Space Station, ZARYA is the first module of ISS, which has launched in 1998.

Type description follows this object name, i.e. R/B for rocket bodies, DEB for space debris.

The following TLE is about the upper stage of the H-IIA rocket in Japan:

H-2A R/B
1 38341U 12025E   21080.34754647  .00000316  00000-0  44249-4 0  9997
2 38341  98.5245 259.8111 0056444  63.1730 297.5239 14.84192015478432

China has conducted an anti-satellite missile test in 2007, which destroyed Fengyun-1C and generated a lot of fragment debris. This is one of the fragments of Fengyun-1C.

FENGYUN 1C DEB
1 29733U 99025X   21080.32325869  .00000063  00000-0  17212-3 0  9993
2 29733  99.2303  24.6693 0578834 210.4520 307.7773 12.92087092667824

We can tell the type of object.

## Line Number

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

The first column is line number.

## Satellite Catalog

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

This is the satellite catalog number, that is, consecutive assigned number for launched object.

The world’s first satellite, Sputnik 1 is assigned the catalog number 00002. The upper stage of the launch rocket is assigned 00002.

## Secutiry Classification

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

This field is security classification.

U means Unclassified, All of publicly available TLEs have U in this field.

It is said that S is set in this field for the non-public objects, such as military satellites, however, we cannot see it.

## International Designator

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

the two lower digits of launch year, the number of launch (3 numbers), identification of launched object (1-3 characters).

This TLE indicates that this object was launched in 1998, 67th, object A.

FENGYUN 1C DEB
1 47041U 99025FBT 21078.22908376  .00000829  00000-0  19377-3 0  9995
2 47041  98.1863  64.2148 0034562 278.0092  93.2342 14.56898177457720

Fragments generated from Fengyun 1C are identified by this.

## Epoch

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

TLE includes the orbital parameters of satellites, the reference time is needed.

The reference time for orbital parameters is called an epoch.

In order, the two lower digits of epoch year, the elapsed time from noon of January 1st in that year.

In this situation, 300.83097691 days in 2020.

Note that this is in UTC.

## First Time Derivative of the Mean Motion

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

This is the first time derivative of the mean motion divided by two (in rev/day^2).

## Second Time Derivative of the Mean Motion

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

This is the second time derivative of the mean motion divided by 6 (in rev/day^3).

This is a small number, almost all TLEs have 00000-0 in this field.

## BSTAR Drag Term

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

This is drag coefficient used in SGP4.

Ballistic coefficient B is described as

$B = \frac{C_D A}{m}$

where $$C_D$$ is drag coefficient, $$A$$ is cross sectional area, $$m$$ is mass.

Ballistic coefficient denotes how objects be affected by drag. The bigger this value, The more effect.

$$B*$$ is complement of B using atmospheric density $$rho_0$$,

$B* = \frac{B\rho_0}{2}$

## Ephemeris type

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

This number describes the ephemeris model used for the calculation of orbital elements.

1=SGP, 2=SGP4, 3=SDP4, 4=SGP8, 5=SDP8

However, this field is 0 in all public data, the orbital elements are calculated by using SGP4/SDP4.

## Element number

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

This number is incremented by updating this information.

However, this number is fixed at 999.

## Checksum

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

The last field of each line is the checksum for detecting data errors.

This number is the remainder after dividing the sum of all the numbers in a line by 10, ignoring characters, spaces, periods, and plus symbols. Only minus signs are treated as -1.

## Inclination

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

Inclination in degree．

## Right Ascension of the Ascending Node

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

Right ascnsion of the ascending node in degree．

## Eccentricity

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

Eccentricity. The decimal point is omitted. In this case, the eccentricity is 0.0001671.

## Argument of Perigee

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

Argument of perigee in degree．

## Mean Anomaly

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

Mean motion in degree．

## Mean Motion

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

Mean motion in Rev/day．

## Revolution number at epoch

ISS (ZARYA)
1 25544U 98067A   20300.83097691  .00001534  00000-0  35580-4 0  9996
2 25544  51.6453  57.0843 0001671  64.9808  73.0513 15.49338189252428

This is the revolution number at epoch. Note that this field shows only the five lower digits.

## Summary

These TLEs are managed by NORAD and published.