LDEF Orbit and Attitude Description

The LDEF was gravity-gradient stabilized, so that the long axis was always directed at the sub-satellite point on the Earth's surface. Rotation about the long axis was inhibited by a magnetic damping system. Thus, one of the long faces was always the "leading", or "ram" edge, facing the velocity direction. The opposite edge was then the "trailing" or "wake" edge. One end of the cylinder was always facing space, the other end always towards the Earth. The faces perpendicular to both the long axis and the velocity vector were nominally "north" and "south", continuously facing +61.5 ° and -61.5 ° declination respectively. Post-flight analysis of LDEF surfaces indicated that the spacecraft was rotated (in "yaw") about 8.2 ° from the intended orientation about the long axis. Analysis of the IDE solar sensor data confirms this result independently for the first 11½ months of the mission. The normal to the leading edge was thus rotated about 8.2 ° away from the velocity vector, with the normal to the "north" edge rotated 8.2 ° towards the velocity vector. The scheduled return of LDEF to Earth was delayed, first by scheduling problems, and then by the loss of the Challenger. Exceptionally intense solar maximum activity accelerated the decay of its orbit and it became clear that if LDEF was not recovered, it would re-enter the atmosphere in early 1990, with the loss of all of its data. It was retrieved at an altitude of 330 km by the crew of the Columbia on 12 January 1990, and returned to Earth on 20 January, the same day that NORAD had predicted as its most probable atmospheric re-entry date.

The position of LDEF at any given time may be computed from the standard two-line element sets provided by the Air Force Space Command. A description of the format of these element sets is given below (description provided by T.S. Kelso):

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 0 is a eleven-character name.

Lines 1 and 2 are the standard Two-Line Orbital Element Set Format identical
to that used by NORAD and NASA.  The format description is:

Line 1
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 (Julian Day and fractional portion of the day)
 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)
 63-63     Ephemeris type
 65-68     Element number
 69-69     Check Sum (Modulo 10)
           (Letters, blanks, periods, plus signs = 0; minus signs = 1)

Line 2
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)

All other columns are blank or fixed.

Example:

NOAA 6
1 11416U          86 50.28438588 0.00000140           67960-4 0  5293
2 11416  98.5105  69.3305 0012788  63.2828 296.9658 14.24899292346978

LDEF TLE File