Understanding Satellite Two Line Element Sets

(This page is a work in progress. It will be complete once all of the individual values are described in full.)

The orbital elements of the satellites tracked by NORAD are supplied in a “Two Line Element” format. This data is sometimes referred to as “Keplerian Elements”.

Here is a sample for the International Space Station for today.

ISS (ZARYA)
1 25544U 98067A   09070.38837942  .00007697  00000-0  63677-4 0  4357
2 25544  51.6394  87.9530 0009582 111.8389  22.9968 15.71382598590567

My intention with this post is to not just tell you what each value is, but demonstrate exactly what they relate to. Click on the description field for an in-depth detailed description.

Title Line

Field Columns Description Example
1 01-24 Satellite Name ISS (ZARYA)

Line 1

Field Columns Description Example
1 01-01 Line number 1
2 03-07 Satellite number 25544
3 08-08 Classification U
4 10-11 International Designator (Year) 98
5 12-14 International Designator (Launch number) 067
6 15-17 International Designator (Piece) A
7 19-20 Epoch Year (Last two digits of year) 09
8 21-32 Epoch (Day of the year and fractional portion of the day) 070.38837942
9 34-43 First Time Derivative of the Mean Motion .00007697
10 45-52 Second Time Derivative of Mean Motion (decimal point assumed) 00000-0
11 54-61 BSTAR drag term (decimal point assumed) 63677-4
12 63-63 The number 0 (Originally this should have been “Ephemeris type”) 0
13 65-68 Element number 435
14 69-69 Checksum (Modulo 10) 7

Line 2

Field Columns Description Example
1 01-01 Line number 2
2 03-07 Satellite number 25544
3 09-16 Inclination [Degrees] 51.6394
4 18-25 Right Ascension of the Ascending Node [Degrees] 87.9530
5 27-33 Eccentricity (decimal point assumed) 0009582
6 35-42 Argument of Perigee [Degrees] 111.8389
7 44-51 Mean Anomaly [Degrees] 22.9968
8 53-63 Mean Motion [Revs per day] 15.71382598
9 64-68 Revolution number at epoch [Revs] 59056
10 69-69 Checksum (Modulo 10) 7
Posted in Astronomy
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  1. […] This post is part of a series on understanding the meaning behind the values contained in a Two Line Element set, or Keplerian Elements. The mail article is here. […]

  2. […] This post is part of a series on understanding the meaning behind the values contained in a Two Line Element set, or Keplerian Elements. The main article is here. […]

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