Re: TLE's from observations

Neil Clifford (n.clifford1@physics.oxford.ac.uk)
Wed, 4 Dec 1996 09:56:38 +0000 (GMT) 

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