DESCRIPTION
The file extension ".tle" commonly designates a list of elements of orbiting satellites in the twoline format of NORAD.
The positions and velocities of satellites are updated periodically by NORAD, and provided to users through their bulletin boards and anonymous ftp sites. A variety of models may be applied to these element sets in order to predict the future position and velocity of a particular satellite. However, it is important to note that the NORAD output data are mean values, i.e., periodic perturbations have been removed. Thus, any predictive model must be compatible with the NORAD models, in the sense that the same terms must be canceled. There are several models which accomplish this goal.
Data for each satellite consists of three lines in the following format:
000000000111111111122222222223333333333444444444455555555556666666666 123456789012345678901234567890123456789012345678901234567890123456789 AAAAAAAAAAAAAAAAAAAAAA 1 NNNNNU NNNNNAAA NNNNN.NNNNNNNN +.NNNNNNNN +NNNNNN +NNNNNN N NNNNN 2 NNNNN NNN.NNNN NNN.NNNN NNNNNNN NNN.NNNN NNN.NNNN NN.NNNNNNNNNNNNNN
These lines are encoded as follows:
LINE 0
A line containing a single 22character ASCII string giving the name of the satellite.
LINE 1
 Column Description
 0101
 Line Number of Element Data, in this case, 1.
 0307
 Satellite Number. Each time a satellite is launched NORAD assigns a number to that satellite. Vanguard 1 is the earliest satellite whose elements can currently be found (all earlier birds must have reentered by now). It was launched on 3/17/58 and carries "00005" as a NORAD Catalog number.
 1011
 International Designatorthe last two digits of the year the satellite was launched. This number is for reference only and is not used by tracking programs for predictions. Thus it may be omitted in some element sets.
 1214
 International Designatorthe number of the launch for that year. This number does not give any indication as to when during the year the bird went up just its ranking among its fellow launches for that year. This number is for reference only and is not used by tracking programs for predictions. Thus it may be omitted in some element sets.
 1517
 International Designatorpiece of launch. On many launches there are more than one payload. This number is for reference only and is not used by tracking programs for predictions. Thus it may be omitted in some element sets.
 1920
 Epoch YearThe last two digits of the year when the element set was measured.
 2132
 Epoch DayThe Julian Day and fractional portion of the day when the element set was measured.
 3443
 First Time Derivative of the Mean Motion or Ballistic Coefficient depending on ephemeris type.
 4552
 Second Time Derivative of Mean Motion (decimal point assumed; blank if N/A)
 5461
 BSTAR drag term if GP4 general perturbation theory was used. Otherwise, radiation pressure coefficient. (Decimal point assumed.) This number usually refers to atmospheric drag on a satellite. However, at times satellites are strongly affected by the gravitational pull of bodies other than the Earth (ie: Sun and Moon). While it seems unlikely, drag can actually be a negative number thus indicating an increase in orbital energy rather than a decrease. This happens when the Sun and Moon combine to pull the satellite's apogee to a higher altitude. However, this condition of negative drag is only valid for as long as the gravitational situation warrants it. So, some folks like to zero out negative drag factors for smoother orbital calculations.
 6363

Ephemeris type. This code indicates the type of model used to generate
the element set. Allowed values and their corresponding models are:
1 = SGP 2 = SGP4 3 = SDP4 4 = SGP8 5 = SDP8 The models designated "SG*" are used for nearearth satellites (i.e., those with periods less than 225 minutes), and the models designated "SD*" are used for deepspace satellites (those with periods equal to or greater than 225 minutes). Atmospheric drag is more important for nearearth satellites, while tidal effects from the sun and moon are more important for the deepspace satellites.
 6568
 Element number (modulo 1000). Each time a satellite's orbit is determined and an element set created the element set is assigned a number.
 6969

Checksum (Modulo 10). Letters, blanks, periods, plus signs = 0;
minus signs =1.
The last number in each of the 2 lines of an element set is a checksum.
This number is calculated by assigning the following values to each
character on the line. A number carries it's own value, a minus () sign
carries a value of one (1), and letters, blanks and periods (decimal points
(.)) carry a value of zero (0).
LINE 2
 0101
 Line Number of Element Data, in this case, 2.
 0307
 Satellite Number.
 0916
 Inclination (in degrees), i.e., the angle formed by the orbit to the equator. The inclination must be a positive number of degrees between 0 and 180. A zero angle of inclination indicates a satellite moving from west to east directly over the equator. An inclination of 28 degrees (most shuttle launches) would form an angle of 28 degrees between the equator and the orbit of the satellite. Also, that satellite will travel only as far north and south as + 28 degrees latitude. On it's ascending orbital crossing (moving from south to north) of the equator, the satellite will be moving from southwest to northeast. An inclination of 90 degrees would mean that the satellite is moving directly from south to north and will cross directly over the north and south poles. Any satellite with an inclination greater than 90 degrees is said to be in retrograde orbit. This means the satellite is moving in a direction opposite the rotation of the earth. A satellite with an inclination of 152 degrees will be moving from southeast to northwest as it cross the equator from south to north. This is opposite the rotation of the Earth. This satellite will move as far north and south of the equator as 28 degrees latitude and be in an orbital direction exactly opposite a satellite with an inclination of 28 degrees.
 1825
 Right ascension of ascending node (RAAN or RA of Node). In order to fix the position of an orbit in space it is necessary to refer to a coordinate system outside the earth coordinate system. Because the Earth rotates latitude and longitude coordinates do not indicate an absolute frame of reference. Therefore it was decided to use astronomical conventions to fix orbits relative to the celestial sphere which is delineated in degrees of Right Ascension and declination. Right ascension is similar to longitude and Declination is similar to latitude. When an element set is taken Right Ascension of the ascending Node is computed in the following manner. As a satellite moves about the center of the earth it crosses the equator twice. It is either in ascending node, moving from south to north or descending node moving from north to south. The RAAN is taken from the point at which the orbit crosses the equator moving from south to north. If you were to stand at the center of the planet and look directly at the location where the satellite crossed the equator you would be pointing to the ascending node. To give this line a value the angle is measured between this line and 0 degrees right ascension (RA). Again standing at the center of the earth 0 degrees RA will always point to the same location on the celestial sphere.
 2733
 Eccentricity. In general, satellites execute elliptical orbits about the Earth. The center of the ellipse is at one of the two foci of the ellipse. The eccentricity of the orbit is the ratio of the distance between the foci to the major axis of the ellipse, i.e., the longest line between any two points. Thus the ellipticity is 0 for a perfectly circular orbit and approaches 1.0 for orbits which are highly elongated.
 3542
 Argument of Perigee (degrees). The orbital position corresponding to closest approach of a satellite to the Earth is called perigee. The argument of perigee is the angle measured from the center of the Earth between the ascending node and the perigee along the plane of the orbit (inclination). If the Argument of perigee is zero (0) then the lowest point of the orbit of that satellite would be at the same location as the point where it crossed the equator in it's ascending node. If the argument of perigee is 180 then the lowest point of the orbit would be on the equator on the opposite side of the earth from the ascending node.
 4451
 Mean Anomaly (degrees). The mean anomaly fixes the position of the satellite in the orbit as described above. So far we have only talked about the shape and location of the orbit of the satellite. We haven't placed the satellite along that path and given it an exact location. That's what Mean Anomaly does. Mean Anomaly is measured from the point of perigee. In the Argument of perigee example above it was stated that an Arg of Perigee of zero would place perigee at the same location as the Ascending node. If in this case the MA were also zero then the satellite's position as of the taking of the element set would also located directly over the equator at the ascending node. If the Arg of Perigee was 0 degrees and the MA was 180 degrees then the satellite's position would have been on the other side of the earth just over the equator as it was headed from north to south.
 5363
 Mean Motion (revolutions per day). The mean motion of a satellite is simply the number of orbits the satellite makes in one solar day (regular day, common day, 24 hours, 1440 minutes, 86400 seconds etc.). This number also generally indicates the orbit altitude.
 6468
 Revolution number at epoch (revs). Theoretically, this number equals the number of orbits the satellite has completed since it's launch, modulo 100,000. Some satellites have incorrect epoch orbit numbers. Oscar 10 is just such a case. However, this number is provided more for reference purposes than orbital calculation. And so, its accuracy or lack thereof doesn't affect the accuracy of a prediction.
 6969
 Check Sum (modulo 10). As with Line 1, this number is provided to check the accuracy of the element set. It's calculation is described above.
EXAMPLES
This is an example using an element set for the Oscar 10 amateur radio satellite:
000000000111111111122222222223333333333444444444455555555556666666666 123456789012345678901234567890123456789012345678901234567890123456789 OSCAR 10 1 14129U 83 58 B 91312.44187316 .00000072 000000 999984 0 7762 2 14129 25.9057 115.4097 6067273 291.5986 16.1497 2.05882356 35213
Oscar 10 has the catalog number 14129, and was the 58th satellite launched in 1983. The element set given above corresponds to the second ('B') item deployed from the launcher. It was measured in 1991 on the 312th day of the year. The decimal portion of the number reflects the fraction of the day since midnight. If this decimal were .5 it would be noon UTC. If it were 10:36:17 UTC. Remember that all epoch times are in UTC (GMT) time.
{Does that do it for you?}
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In the Oscar 10 element set above the checksum calculation would start out like this for line one of the set. In column one is the number one (1). So, so far the checksum is one (1). In column two is a blank space. That carries a value of zero (0), so the checksum remains one (1). In column three is the number one (1). Add this to the accumulated checksum so far and the new checksum value is two (2). In column four is the number four (4). Add four to the checksum value and the new value is six (6). If you continue along through the entire line you will end up with a value of 172. Only the last digit of this number is used. So the checksum of this line is two "2". DO NOT ADD the last figure in column 69 as that is the actual checksum. When programs verify Checksums they perform the above calculations. If the value of the calculated checksum disagrees with the very last (69th column) number then the element set fails the checksum test and is considered a bad element set.
NOTES
 Availability
NORAD twoline orbital element sets are available from:
BBS Celestial BBS *(205) 9049280* updated several times weekly. FTP archive.afit.af.mil (129.92.1.66) pub/space updated weekly. FTP spacelink.msfc.nasa.gov various paths good source for shuttle tle.
 Additional Information
 IT.DOC  The doc file for Instant Track. Antonio describes these parameters in concise terms easily understandable to all. "The Satellite Experimenter's Handbook" by Martin Davidoff. Available from the Amateur Radio Relay League, 225 Main St, Newington, Connecticut 06111 and probably most stores that sell amateur radio gear. "Fundamentals of Astrodynamics" by Roger Bate, Donald Mueller, and Jerry White. Publisher: Dover Publications, NYC, NY Copyright 1971.