NAME
tle - extension for files containing NORAD two-line orbital element
sets.
DESCRIPTION
The file extension ".tle" commonly designates a list of elements of
orbiting satellites in the two-line 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
These lines are encoded as follows:
LINE 0
A line containing a single 22-character ASCII string giving the name of
the satellite.
LINE 1
Column Description
01-01 Line Number of Element Data, in this case, 1.
03-07 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.
10-11 International Designator--the 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.
12-14 International Designator--the 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.
15-17 International Designator--piece 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.
19-20 Epoch Year--The last two digits of the year when the element set
was measured.
21-32 Epoch Day--The Julian Day and fractional portion of the day when
the element set was measured.
34-43 First Time Derivative of the Mean Motion or Ballistic
Coefficient-- depending on ephemeris type.
45-52 Second Time Derivative of Mean Motion (decimal point assumed;
blank if N/A)
54-61 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.
63-63 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 near-earth satellites
(i.e., those with periods less than 225 minutes), and the models
designated "SD*" are used for deep-space satellites (those with
periods equal to or greater than 225 minutes). Atmospheric drag
is more important for near-earth satellites, while tidal effects
from the sun and moon are more important for the deep-space
satellites.
65-68 Element number (modulo 1000). Each time a satellite’s orbit is
determined and an element set created the element set is
assigned a number.
69-69 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
01-01 Line Number of Element Data, in this case, 2.
03-07 Satellite Number.
09-16 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.
18-25 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.
27-33 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.
35-42 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.
44-51 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.
53-63 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.
64-68 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.
69-69 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 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?}
[Need more explanation here.]{about?}
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.
SEE ALSO
seesat5(1), seesat5(7), SEESAT5.INI(5), cr(1)
NOTES
Availability
NORAD two-line orbital element sets are available from:
Additional Information
"The Satellite Experimenter’s Handbook" by Martin Davidoff. Available from
"Fundamentals of Astrodynamics" by Roger Bate, Donald Mueller, and Jerry