Appendix B: Two Line Element Set Format

Source

AIAA 2006-6753-Rev1, Vallado et al. (2006/2008) — Pages 30—32

TLE Format Overview

The format for the TLE is shown in Figure 11 of the paper with sample data. Each TLE consists of two 69-character lines. The eccentricity, mean motion second derivative, and B* have implied decimal points before the first numerical value. The mean motion derivative is already divided by 2, and the second derivative is already divided by 6. Shaded cells (in the original figure) do not contain data. The signs may be blank, +, or -. A classification field is sometimes included after the satellite number.

Sample TLE

1 06609U 86017A   93352.53502934  .00007889  00000-0  10529-3 0   342
2 06609  51.6190  13.3340 0005770 102.5680 257.5950 15.59114070447869

Line 1 Field Specification

Field	Columns	Width	Name	Format	Example	Description
1.0	1	1	Line Number	1	1	Always 1 for line 1
1.1	3—7	5	Satellite Number	NNNNN	06609	NORAD Catalog Number (0—99999)
1.2	8	1	Classification	C	U	U = Unclassified, C = Classified, S = Secret
1.3	10—11	2	Intl Designator (Year)	YY	86	Last two digits of launch year
1.4	12—14	3	Intl Designator (Launch #)	NNN	017	Launch number of the year
1.5	15—17	3	Intl Designator (Piece)	AAA	A	Piece of the launch (left-justified preferred)
1.6	19—20	2	Epoch Year	YY	93	Last two digits of epoch year
1.7	21—32	12	Epoch Day of Year	DDD.DDDDDDDD	352.53502934	Day of year and fractional portion of the day
1.8	34—43	10	Mean Motion Derivative	sN.NNNNNNNN	.00007889	First derivative of mean motion / 2 (rev/day^2 / 2)
1.9	45—52	8	Mean Motion 2nd Derivative	sNNNNNsN	00000-0	Second derivative of mean motion / 6 (rev/day^3 / 6), implied decimal
1.10	54—61	8	B* Drag Term	sNNNNNsN	10529-3	BSTAR drag term, implied decimal
1.11	63	1	Ephemeris Type	N	0	Internal use only (always 0 for distributed TLEs)
1.12	65—68	4	Element Set Number	NNNN	34	Incremented on new element set generation
1.13	69	1	Checksum	N	2	Modulo-10 checksum

Line 2 Field Specification

Field	Columns	Width	Name	Format	Example	Description
2.0	1	1	Line Number	2	2	Always 2 for line 2
2.1	3—7	5	Satellite Number	NNNNN	06609	Must match line 1
2.2	9—16	8	Inclination	NNN.NNNN	51.6190	Degrees (0—180)
2.3	18—25	8	Right Ascension of Ascending Node	NNN.NNNN	13.3340	Degrees (0—360)
2.4	27—33	7	Eccentricity	NNNNNNN	0005770	Implied leading decimal point
2.5	35—42	8	Argument of Perigee	NNN.NNNN	102.5680	Degrees (0—360)
2.6	44—51	8	Mean Anomaly	NNN.NNNN	257.5950	Degrees (0—360)
2.7	53—63	11	Mean Motion	NN.NNNNNNNN	15.59114070	Revolutions per day
2.8	64—68	5	Revolution Number at Epoch	NNNNN	44786	Revolution count
2.9	69	1	Checksum	N	9	Modulo-10 checksum

Notes

1. Accuracy Limitations

The maximum accuracy for a TLE is limited by the number of decimal places in each field (Vallado, 2004:116). In general, TLE data is accurate to about a kilometer or so at epoch and it quickly degrades (Hartman, 1993). The SGP4 theory is capable of much better accuracy through additional modeling and sufficient observational data. Cefola and McClain (1987) noted that certain low-inclination geosynchronous orbits exhibited large discrepancies from numerical simulations due to oversimplifications in the node rate calculations. Cefola and Fonte (1996) showed that addition of additional terms to the theory could improve the overall accuracy by almost an order of magnitude.

2. Satellite Number

The satellite number consists of any numeric value 0—99999. Discussions have hinted at a lengthening of the field size to 7 or 9 characters to accommodate future satellites.

3. Sign Convention

Sometimes additional assignments are made: plus signs = 0; minus signs = 1.

4. International Designator

The International designator is broken up into the last two digits of the launch year, the launch number for that year (3 digits), and the piece of the launch (3 digits). Per Kelso (2004):

“[The] International Designator of the object is an additional unique designation assigned by the World Data Center-A for Rockets and Satellites (WDC-A-R&S) in accordance with international treaty (1975 Convention on Registration of Objects Launched into Outer Space). The WDC-A-R&S works together with NORAD and NASA’s National Space Science Data Center (NSSDC) in maintaining this registry.”

There are some significant differences between NORAD’s Catalog Number and the International Designator. NORAD assigns a catalog number based upon when the object was first observed, whereas the International Designator is always tied to the original launch. For example, the 81st launch of 1968 carried four payloads into orbit: OV2-5, ERS 21 and 28, and LES 6. Together with the Titan 3C transtage rocket body, these objects were assigned International Designators 1968-081A through E and Catalog Numbers 03428 through 03431. NORAD later cataloged two additional pieces associated with this launch as Catalog Numbers 25000 and 25001 — they have the International Designators 1968-081F and G.

5. Mean Motion Rates

The mean motion rates (fields 1.8 and 1.9) are not used by SGP4 and are only valid for the older SGP model.

6. B* (BSTAR) Drag Coefficient

B* is an SGP4 drag-like coefficient. Usually, ballistic coefficients ($BC$) are used in aerodynamic theory. The $BC$ is $m / c_D A$, or the reciprocal ($A$ is cross-sectional area, $c_D$ is the coefficient of drag, and $m$ is mass). B* is an adjusted value of $BC$ using the reference value of atmospheric density, $\rho_0 = 2.461 \times 10^{-5}\;\text{kg/m}^2$, at one Earth radius.

BC = \frac{R_e \, \rho_0}{2 \, B^*} \tag{B-1}

7. Ephemeris Type

The Ephemeris type is not used external to CMOC. All TLE data is generated by SGP4.

8. Element Set Number

Per Kelso (2004):

“The element set number. Normally, this number is incremented each time a new element set is generated. In practice, however, this doesn’t always happen. When operations switch between the primary and backup Space Control Centers, sometimes the element set numbers get out of sync, with some numbers being reused and others skipped. Unfortunately, this makes it difficult to tell if you have all the element sets for a particular object.”

9. Checksum

The last column on each line represents a modulo-10 checksum of the data on that line. To calculate the checksum, add the values of all the numbers on each line — ignoring all letters, spaces, periods, and plus signs — and assigning a value of 1 to all minus signs. The checksum is the last digit of that sum. Although this is a simple error-checking procedure, it should catch 90 percent of all errors.

10. Revolution Number

The final field on line 2, prior to the checksum, is the revolution number. In NORAD’s convention, a revolution begins when the satellite is at the ascending node of its orbit and a revolution is the period between successive ascending nodes. The period from launch to the first ascending node is considered to be Rev 0 and Rev 1 begins when the first ascending node is reached. Since many element sets are generated with epochs that place the satellite near its ascending node, it is important to note whether the satellite has reached the ascending node when calculating subsequent rev numbers.

Implied Decimal Point Format

Fields 1.9 (mean motion second derivative), 1.10 (B*), and 2.4 (eccentricity) use an implied decimal point format:

• Eccentricity (field 2.4): 0005770 means 0.0005770
• Mean motion 2nd derivative (field 1.9): 00000-0 means 0.00000 x 10^0 = 0.0
• B* (field 1.10): 10529-3 means 0.10529 x 10^-3 = 0.00010529

For fields 1.9 and 1.10 the format is sNNNNNsN where the first sign applies to the mantissa and the second sN is the power-of-ten exponent. A leading decimal point is implied before the five-digit mantissa.