Keplerian element sets are generally distributed in one of two formats, usually called NASA format and AMSAT format. A third format, called one-line elements or “Charlie” elements, is sometimes seen in military circles.
NASA 2-Line Format
This is the format used by NASA to distribute satellite elements in their NASA Prediction Bulletin. The origin of the format is unknown. Some old NORAD reports refer to this as T-card format.
As used in the amateur community, the format consists of groups of 3 lines: One line containing the satellite’s name, followed by the standard Two-Line Orbital Element Set Format identical to that used by NASA and NORAD. Tracking programs are generally unforgiving of anything that doesn’t fit this format.
NASA format files look like this…
OSCAR 10 1 14129U 88230.56274695 0.00000042 10000-3 0 3478 2 14129 27.2218 308.9614 6028281 329.3891 6.4794 2.05877164 10960 GPS-0008 1 14189U 88230.24001475 0.00000013 0 5423 2 14189 63.0801 108.8864 0128028 212.9347 146.3600 2.00555575 37348
Each number is in a specified fixed column. Spaces are significant. The last digit on each line is a mod-10 check digit, which is checked by the program. The program also checks the sequence numbers (first column), and checks each orbital element for reasonable range. This is a very good set of checks, so this format is very safe, and robust.
There seems to be some disagreement about how the “+” character is figured into the check digit. If you have trouble with checksum failures on element sets with “+” signs in them, try replacing all the “+” signs with spaces.
Data for each satellite consists of three lines in the following format:
AAAAAAAAAAA 1 NNNNNU NNNNNAAA NNNNN.NNNNNNNN +.NNNNNNNN +NNNNN-N +NNNNN-N N NNNNN 2 NNNNN NNN.NNNN NNN.NNNN NNNNNNN NNN.NNNN NNN.NNNN NN.NNNNNNNNNNNNNN
Line 1 is a eleven-character name.
Actually, there is some disagreement about how wide the name may be. Some programs allow 12 characters. Others allow 24 characters, which is consistent with some NORAD documents.
Some sources encode additional information on this line, but this is not part of the standard format. One scheme for encoding visual magnitude information is described in Ted Molczan’s format description.
There is no checksum on this line.
Column Description 01-01 Line Number of Element Data 03-07 Satellite Number 10-11 International Designator (Last two digits of launch year) 12-14 International Designator (Launch number of the year) 15-17 International Designator (Piece of launch) 19-20 Epoch Year (Last two digits of year) 21-32 Epoch (Day number and fractional portion of the day) 34-43 First Time Derivative of the Mean Motion divided by 2. or Ballistic Coefficient (Depending of ephemeris type) 45-52 Second Time Derivative of Mean Motion divided by 6. (Blank if N/A) 54-61 BSTAR drag term if GP4 general perturbation theory was used. Otherwise, radiation pressure coefficient. 63-63 Ephemeris type 65-68 Element number 69-69 Check Sum (Modulo 10)
The checksum is computed as follows:
- Start with zero.
- For each digit in the line, add the value of the digit.
- For each minus sign, add 1.
- For each plus sign, add 2 (or maybe 0, depending on who created the element set and when)
- For each letter, blank, or period, don’t add anything.
- Take the last decimal digit of the result (that is, take the result modulo 10) as the check digit.
All other columns are blank or fixed.
Note that the International Designator fields are usually blank, as issued in the NASA Prediction Bulletins.
Column Description 01-01 Line Number of Element Data 03-07 Satellite Number 09-16 Inclination [Degrees] 18-25 Right Ascension of the Ascending Node [Degrees] 27-33 Eccentricity (decimal point assumed) 35-42 Argument of Perigee [Degrees] 44-51 Mean Anomaly [Degrees] 53-63 Mean Motion [Revs per day] 64-68 Revolution number at epoch [Revs] 69-69 Check Sum (Modulo 10)
The same checksum algorithm is used.
All other columns are blank or fixed.
There are several very similar formats generated by several different people that seem to be called “AMSAT” format. Tracking programs generally try to read all of them. This format is very user-friendly, and can be easily read and/or edited by humans. Spaces are not significant. Each orbital element must appear on a separate line. The order in which orbital elements appear is not significant, except that each element set should begin with a line containing the word “satellite”. A blank line is usually interpreted as ending the element set.
This file format does not contain any check digits, but an overall checksum is sometimes used.
AMSAT format elements as distributed by AMSAT look like this:
Satellite: AO-13 Catalog number: 19216 Epoch time: 94311.77313192 Element set: 994 Inclination: 57.6728 deg RA of node: 221.5174 deg Eccentricity: 0.7242728 Arg of perigee: 354.2960 deg Mean anomaly: 0.7033 deg Mean motion: 2.09727084 rev/day Decay rate: -5.78e-06 rev/day^2 Epoch rev: 4902 Checksum: 312
The checksum is the same computation as for the NASA 2-line format, except that the whole sum is used instead of just the last digit. Every character on the line is included, so the “2” in “rev/day^2” does count.
The One Line Element (OLE) format is a somewhat abbreviated set of data used by the Navy at the Naval Research Laboratory (and perhaps others). Some useful information which is included in the 2-Line Element format is omitted, such as the Revolution Number at Epoch. Other information, such as the International Designator, can often be obtained from other sources using the satellite number (NORAD catalog number). The only virtue to this format is its brevity.
1 2 3 4 5 6 123456789012345678901234567890123456789012345678901234567890 nnnnnyydddffffffddddddiiiiiinnnnnneeeeeeaaaaaammmmmmxxxxxxxx 206399019071772000014705251829684400765901146334880715202450
|1 – 5||NORAD catalog number||NNNNN|
|6 – 7||Year||NN||years|
|8 – 10||Day number||NNN||days|
|11 – 16||Fraction of a day||0.NNNNNN||days|
|17 – 22||Drag||0.NNNNNN||rev/day^2|
|23 – 28||Inclination||NNN.NNN||degrees|
|29 – 34||R.A.A.N.||NNN.NNN||degrees|
|35 – 40||Eccentricity||0.NNNNNN||dimensionless|
|41 – 46||Argument of Perigee||NNN.NNN||degrees|
|47 – 52||Mean Anomaly||NNN.NNN||degrees|
|53 – 60||Mean Motion||NN.NNNNNN||rev/day|
The following values are obtained:
20639 catalog number
190 day number
0.717720 fraction of a day
0.000147 drag term
296.844 ascending node
011.463 argument of perigee
348.807 mean anomaly
15.202450 mean motion
The input of elements in this form may be terminated by a line which contains a zero for the catalog number.
[One-Line Element format information courtesy Mike McCants.]