TLE Format Specification

Source attribution

This specification is extracted from Spacetrack Report No. 3, ADCOM/DO Form 12 (December 1978), page 91. It defines the column formats used by the Space Defense Center (SDC) and Space Computation Center (SCC) for distributing satellite orbital element sets. These are the authoritative column definitions for TLE data consumed by the SGP, SGP4, SDP4, SGP8, and SDP8 propagation models.

Column positions are 1-indexed throughout (column 1 is the first character).

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Visual anatomy

ISS (ZARYA), NORAD catalog #25544 — hover any field for a tooltip.

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

Identification Time Orbital elements Dynamics Metadata

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Transmission format (TLE)

This is the universally recognized “Two-Line Element” format distributed by NORAD and its successors (US Space Command, 18th Space Defense Squadron, space-track.org). Every TLE parser in the world implements this column specification.

The standard TLE line is 69 characters (columns 1—69). The ADCOM/DO Form 12 grid extends to column 80, but columns 70—79 are unused and column 69 carries the checksum.

Line 1

Columns	Width	Field	Description	Units	Symbol
1	1	LINE NO	Line number (always 1)	—	—
2	1	(blank)	Space delimiter	—	—
3—7	5	SATELLITE NUMBER	NORAD catalog number (right-justified)	—	—
8	1	CLASSIFICATION	U=Unclassified, C=Classified, S=Secret	—	—
9	1	(blank)	Space delimiter	—	—
10—11	2	INTL DES YEAR	International Designator: 2-digit launch year	—	—
12—14	3	INTL DES LAUNCH	International Designator: launch number (zero-padded)	—	—
15—17	3	INTL DES PIECE	International Designator: piece (left-justified)	—	—
18	1	(blank)	Space delimiter	—	—
19—20	2	EPOCH YEAR	Epoch year (2-digit)	—	—
21—32	12	EPOCH DAY	Day of year + fractional day (DDD.DDDDDDDD)	days	—
33	1	(blank)	Space delimiter	—	—
34—43	10	MEAN MOTION DOT/2	1st derivative of mean motion / 2	rev/day$^2$	$\dot{n}_o/2$
44	1	(blank)	Space delimiter	—	—
45—52	8	MEAN MOTION DDOT/6	2nd derivative of mean motion / 6 (implied decimal + exponent)	rev/day$^3$	$\ddot{n}_o/6$
53	1	(blank)	Space delimiter	—	—
54—61	8	BSTAR	$B^*$ drag term (implied decimal + exponent)	(ER)$^{-1}$	$B^*$
62	1	(blank)	Space delimiter	—	—
63	1	EPHEMERIS TYPE	0=SGP4/SDP4, 2=SGP/SDP	—	—
64	1	(blank)	Space delimiter	—	—
65—68	4	ELEMENT SET NO	Element set number (incremented per new elset)	—	—
69	1	CHECKSUM	Modulo-10 checksum	—	—

Line 2

Columns	Width	Field	Description	Units	Symbol
1	1	LINE NO	Line number (always 2)	—	—
2	1	(blank)	Space delimiter	—	—
3—7	5	SATELLITE NUMBER	NORAD catalog number (must match Line 1)	—	—
8	1	(blank)	Space delimiter	—	—
9—16	8	INCLINATION	Inclination at epoch	degrees	$i_o$
17	1	(blank)	Space delimiter	—	—
18—25	8	RAAN	Right ascension of ascending node	degrees	$\Omega_o$
26	1	(blank)	Space delimiter	—	—
27—33	7	ECCENTRICITY	Eccentricity (implied leading decimal)	—	$e_o$
34	1	(blank)	Space delimiter	—	—
35—42	8	ARG OF PERIGEE	Argument of perigee	degrees	$\omega_o$
43	1	(blank)	Space delimiter	—	—
44—51	8	MEAN ANOMALY	Mean anomaly	degrees	$M_o$
52	1	(blank)	Space delimiter	—	—
53—63	11	MEAN MOTION	Mean motion	rev/day	$n_o$
64—68	5	EPOCH REV	Revolution number at epoch	revs	—
69	1	CHECKSUM	Modulo-10 checksum	—	—

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Field encoding rules

TLE fields use three distinct encoding schemes. Getting these right is the difference between a working parser and garbage output.

Explicit decimal

Fields with an explicit decimal point parse as standard floating-point numbers. No special handling needed.

Applies to:

• $\dot{n}_o/2$ (Line 1, cols 34—43)
• Inclination (Line 2, cols 9—16), format NNN.NNNN
• RAAN (Line 2, cols 18—25), format NNN.NNNN
• Argument of perigee (Line 2, cols 35—42), format NNN.NNNN
• Mean anomaly (Line 2, cols 44—51), format NNN.NNNN
• Mean motion (Line 2, cols 53—63), format NN.NNNNNNNN

Example: -.00002182 parses to $-0.00002182$

Implied decimal

The field contains only digits with no decimal point. Prepend 0. to get the actual value.

Applies to:

• Eccentricity $e_o$ (Line 2, cols 27—33)

Example: 0006703 parses to $0.0006703$

Implied decimal + exponent

An 8-character field with the structure SNNNNNSE:

Position	Meaning
Char 1	Sign (blank or space = positive, - = negative)
Chars 2—6	5-digit mantissa (implied leading 0.)
Char 7	Exponent sign (+, -, or blank)
Char 8	Exponent digit (single digit, power of 10)

Decoding formula: sign * 0.NNNNN * 10^(exponent_sign * exponent_digit)

Applies to:

• $\ddot{n}_o/6$ (Line 1, cols 45—52)
• $B^*$ (Line 1, cols 54—61)

Examples:

Raw	Sign	Mantissa	Exponent	Decoded value
00000-0	+	0.00000	0	$0.0$
-11606-4	-	0.11606	$-4$	$-1.1606 \times 10^{-5}$
12345+3	+	0.12345	$+3$	$123.45$

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Checksum algorithm

Each line ends with a modulo-10 checksum in column 69, computed over columns 1—68.

Rules:

1. Digits 0—9 contribute their face value
2. The minus sign - contributes 1
3. All other characters (blanks, letters, +, .) contribute 0
4. Sum all contributions, take modulo 10

Quick validation

A valid TLE always has matching checksums. If your parser reads a TLE and the checksum does not verify, the data is corrupt — do not propagate it. A single bit flip in eccentricity or mean motion can put your predicted position off by thousands of kilometers.

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Epoch year interpretation

The 2-digit epoch year (Line 1, cols 19—20) maps to a 4-digit year:

2-digit value	4-digit year
57—99	1957—1999
00—56	2000—2056

The cutoff at 57 reflects Sputnik 1’s launch year (1957) — no tracked object predates the space age.

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Example TLE

ISS (ZARYA), catalog #25544:

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

Line 1 parsed

Col  1     : '1'              Line number
Col  2     : ' '              (blank)
Col  3-7   : '25544'          Satellite number
Col  8     : 'U'              Classification (Unclassified)
Col  9     : ' '              (blank)
Col 10-11  : '98'             Intl designator year (1998)
Col 12-14  : '067'            Intl designator launch number
Col 15-17  : 'A  '            Intl designator piece
Col 18     : ' '              (blank)
Col 19-20  : '08'             Epoch year (2008)
Col 21-32  : '264.51782528'   Epoch day (day 264.51782528)
Col 33     : ' '              (blank)
Col 34-43  : '-.00002182'     n-dot/2 = -0.00002182 rev/day^2
Col 44     : ' '              (blank)
Col 45-52  : ' 00000-0'       n-ddot/6 = +0.00000 x 10^0 = 0.0
Col 53     : ' '              (blank)
Col 54-61  : '-11606-4'       B* = -0.11606 x 10^-4 = -1.1606e-5
Col 62     : ' '              (blank)
Col 63     : '0'              Ephemeris type (SGP4/SDP4)
Col 64     : ' '              (blank)
Col 65-68  : ' 292'           Element set number
Col 69     : '7'              Checksum

Line 2 parsed

Col  1     : '2'              Line number
Col  2     : ' '              (blank)
Col  3-7   : '25544'          Satellite number
Col  8     : ' '              (blank)
Col  9-16  : ' 51.6416'       Inclination = 51.6416 deg
Col 17     : ' '              (blank)
Col 18-25  : '247.4627'       RAAN = 247.4627 deg
Col 26     : ' '              (blank)
Col 27-33  : '0006703'        Eccentricity = 0.0006703
Col 34     : ' '              (blank)
Col 35-42  : '130.5360'       Arg of perigee = 130.5360 deg
Col 43     : ' '              (blank)
Col 44-51  : '325.0288'       Mean anomaly = 325.0288 deg
Col 52     : ' '              (blank)
Col 53-63  : '15.72125391'    Mean motion = 15.72125391 rev/day
Col 64-68  : '56353'          Epoch revolution number
Col 69     : '7'              Checksum

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Symbol-to-column mapping

Quick reference linking the mathematical symbols from Spacetrack Report No. 3 to their TLE column positions:

Symbol	Name	Line	Columns	Units	Encoding
$n_o$	Mean motion at epoch	2	53—63	rev/day	explicit decimal
$e_o$	Eccentricity at epoch	2	27—33	dimensionless	implied decimal
$i_o$	Inclination at epoch	2	9—16	degrees	explicit decimal
$M_o$	Mean anomaly at epoch	2	44—51	degrees	explicit decimal
$\omega_o$	Argument of perigee at epoch	2	35—42	degrees	explicit decimal
$\Omega_o$	Right ascension of ascending node	2	18—25	degrees	explicit decimal
$\dot{n}_o/2$	1st time derivative of mean motion / 2	1	34—43	rev/day$^2$	explicit decimal
$\ddot{n}_o/6$	2nd time derivative of mean motion / 6	1	45—52	rev/day$^3$	implied decimal + exponent
$B^*$	BSTAR drag term	1	54—61	(earth radii)$^{-1}$	implied decimal + exponent

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Data sources

Source	Access	Notes
CelesTrak	Free, no account	Dr. T.S. Kelso’s mirror of the GP catalog. Bulk CSV, JSON, XML, and TLE downloads. Updated multiple times daily.
Space-Track	Free, US government account required	Official source from 18th Space Defense Squadron. REST API with full catalog history.
Craft API	GET /api/satellites	Craft ingests TLEs from CelesTrak and exposes parsed orbital elements via its REST API.

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Internal format (G-Card)

Historical reference only

The G-Card internal format was used within the Space Defense Center and Space Computation Center. External consumers — including all modern TLE distributors like space-track.org — use the transmission format above. This section is included for completeness and historical fidelity to the original Spacetrack Report No. 3 documentation.

The internal format uses three 80-column cards with card type G in column 80. Cards 1 and 2 contain orbital elements; Card 3 contains administrative and catalog metadata not used by propagators.

G-Card 1

Columns	Width	Field	Description	Units
1	1	CARD NO	Card number (always 1)	—
2—7	6	SATNO	Satellite catalog number	—
8	1	CARD NO	Card number (redundant)	—
9—12	4	ELEMENT NO	Element set number	—
13—14	2	YEAR	International Designator: 2-digit launch year	—
15—17	3	LAUNCH NO	International Designator: launch number of year	—
18—20	3	PIECE	International Designator: piece of launch	—
21	1	EL SOURCE	Element source code	—
22	1	EPHTYPE	Ephemeris type	—
23—24	2	ACC NO	Accuracy code (internal use)	—
25—26	2	EL LIFE	Element set lifetime	days
27—40	14	EPOCH TIME	Epoch as YYDDD.DDDDDDDD	days
41—45	5	EPOCH REV	Revolution number at epoch	revs
46—56	11	$M_o$	Mean anomaly (implied decimal)	degrees
57—68	12	RAAN	Right ascension of ascending node (implied decimal)	degrees
69—75	7	ARG PERIGEE	Argument of perigee	degrees
76	1	DREV	Delta revolution	—
77—78	2	SPX ID / DDC	Sign + exponent	—
79	1	PERIOD	Period indicator	minutes
80	1	CARD TYPE	Always G	—

G-Card 2

Columns	Width	Field	Description	Units
1	1	CARD NO	Card number (always 2)	—
2—7	6	SATNO	Satellite catalog number	—
8—16	9	$e_o$	Eccentricity (implied leading decimal)	—
17—24	8	$i_o$	Inclination (implied decimal)	degrees
25—33	9	$n_o$	Mean motion (implied decimal)	rev/day
34	1	S	Sign of $\dot{n}/2$	—
35—45	11	$\dot{n}/2$	First derivative of mean motion / 2 (implied decimal)	rev/day$^2$
46—50	5	$\ddot{n}/6$	Second derivative of mean motion / 6	rev/day$^3$
51—54	4	S E	Sign and exponent for $\ddot{n}/6$	—
55—60	6	$B^*$/AGOM	$B^*$ drag term or area-to-mass ratio	(ER)$^{-1}$
61—64	4	S E	Sign and exponent for $B^*$	—
65—72	8	PERIGEE HT	Perigee height	km
73	1	ISTOP	Stop flag	—
74—79	6	(unused)	—	—
80	1	CARD TYPE	Always G	—

G-Card 3 (administrative metadata)

Card 3 carries catalog metadata not used by propagators. Most external consumers of TLE data never receive Card 3.

Columns	Width	Field	Description
1	1	CARD NO	Card number (always 3)
2—7	6	SATNO	Satellite catalog number
8	1	COUNTRY	Country code
9	1	PAYLOAD	Payload flag
10	1	MOVER	Mover flag
11	1	X-SECTION	Cross-section flag
12	1	ORBIT TYPE	Orbit type code
13	1	SAT CLASS	Satellite classification
14—15	2	REBMC	Reserved/administrative
16—17	2	FREQ	Frequency code
18	1	VMAG LIM	Visual magnitude limit
19—20	2	OBSEL	Observation selectivity
21	1	OBESPAN	Observation span
22	1	FTAG	Flag tag
23	1	HPRI	High priority flag
24	1	HIPR	Reserved
25	1	DELTA MULT	Delta multiplier
26	1	BETA MULT	Beta multiplier
27	1	PHANG	Phase angle flag
28—35	8	SMAG	Standard magnitude
36—43	8	FPS-85 TASK	FPS-85 radar task assignment
44—79	36	COMMENTS	Free-form comments
80	1	CARD TYPE	Always G

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Extracted from ADCOM/DO Form 12, December 1978 — ELEMENT SET FORMATS