Bruno Tilgner scribbles: |>Dan Boulet's book "Methods of Orbit Determination" contains algorithms |>to compute the state vector (position and velocity in cartesian coordinates) |>from observations. A computer program (in BASIC) is given as well as another |>program to improve initial state vectors by further observations. There are a number of other books around on astrodynamics (particularly ones providing at least pseudo code) which outline eg Gaussian or Laplacian methods of orbit determination whereby you can arive at a state vector in some reference frame given 3 observations of RA,dec,time (I have the code hidden somewhere). I need to dig around the library... |>Ken Ernandes' program VEC2TLE allows to perform the next step, calculation of |>TLE's from the state vectors, due consideration being given to the reference |>system and units of measure. Ken's program can then be used to compute an initial SGP tle, |>not taken into account. Mike McCants' program FITELEM allows refinement |>of initial TLE's either by manually changing parameters or by inputting |>the time delay between computed and observed positions. His program is not |>terribly user-friendly because everything has to be entered through a batch |>file in classical mainframe style. and then refined as above. |>A word of caution is in order, however. The Spacetrack Report No.3, "Models |>for Propagation of NORAD Element Sets", warns that NORAD element sets are |>mean values obtained by removing periodic variations in a particular way |>(not explained further) which must be re-introduced before using the elements. |>Using NORAD TLE's in a different model will result in degraded predictions |>even if the model is intrinsically more accurate. Conversely, it could be |>argued that using TLE's, constructed in the way outlined above, with a NORAD |>model such as SGP4 could lead to erroneous results. My understanding is that Ken's vec2tle accounts for this - it generates SGP[4] compatible tle's from position, velocity (parameters which have not been 'massaged' in any way). You've just got to get your reference frames right. best wishes, -- Neil Clifford <n.clifford@physics.oxford.ac.uk> http://www.physics.ox.ac.uk/sat/satintro.html