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


       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:


       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

       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 satellite:

       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