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       tle  -  extension  for  files containing NORAD two-line orbital element


       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


       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

       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

       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

       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

       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.


       This is an example using an element set for the Oscar 10 amateur  radio

       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.


       seesat5(1), seesat5(7), SEESAT5.INI(5), cr(1)



       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