The Generate TLE tool computes the best-fit GP data to be used with the SGP4 propagator whose ephemeris is the closest to the selected satellite in an RMS sense. To use the Generate TLE tool, you must first create the satellite and propagate ephemeris.
When the specified satellite number contains five digits or less, the GP data can be formatted as a TLE. As this has been the case historically, this tool uses the term "TLE" as being synonymous with "GP data," though SSC numbers are no longer restricted to five digits. If a SSC Number is specified that is larger than five digits, then the tool still finds the best-fit GP data even though it cannot be formatted as a TLE. Those elements may be reported by updating the TLE Solution summary report style to include the "GP Elements" data item.
To open the Generate TLE tool, highlight the satellite in the Object Browser and select Generate TLE... from the Satellite menu.
Once the Generate TLE tool is active, you can generate TLEs in one of two ways:
Trajectory Sampling Method
In the Method field, select Trajectory Sampling to generate a TLE based on a batch least squares algorithm. Samples used to compute element set data are based on the Start and Stop Times and Time Step entered. Using this algorithm, STK will generate TLEs by progressively minimizing the difference between the TLE-generated ephemeris and the original ephemeris by comparing the two at each step. The iterative process is completed when either convergence of the algorithm is achieved or the maximum number of iterations is reached. Convergence is detected based on the relative error in the predicted sum of squares of the residuals.
The atmospheric drag effects are modeled through the BSTAR element of the TLE when the propagation of the orbit is performed using SGP4. This is the drag parameter that is solved for by STK. There are two other drag related elements of the TLE that are not used by SGP4, but are used by its predecessor, SGP. These elements are the nDot/2 and nDotDot/6 terms which are used to model the evolution of the mean motion. These terms are not solved for by STK as part of the batch least squares process but the nDot/2 term is estimated from the variation of the mean motion which is observed when the computed TLE is propagated over the fit span:
nDot/2 = (n_end - n_begin) / 2 (t_end - t_begin)
|Start Time /Stop Time||Default values are the start and stop times of the satellite ephemeris. For more information on the options available for start and stop times, see Time Options.|
|Time Step||The interval between calculated ephemeris output points.|
|TLE Epoch||The epoch of the TLE data being generated must be within the time span of the satellite ephemeris.
The TLE Epoch can be entered in YYDDD.DDDD format by entering the string YYDDD after the date (for example, "99012.77874231 YYDDD". For more information on the options available for epoch times, see Time Options.
|SSC Number||The catalog number of the spacecraft.|
|Int'l Designation||Up to 8-character user-defined field.|
|Element Number||Up to 4-digit user-defined number.|
|Classification||Specifies classification as U (unclassified), C (classified), S (secret), or a user-defined character.|
|Solve for Drag||If checked, solves for BStar drag and does not use the value in the BStar field.|
|BStar||Uses the BStar value (between 1.0 and -1.0) and does not solve for it.|
|Max Iterations||The maximum number of successive iterations that can be computed to achieve convergence.|
|Convergence||, where SoS is the sum of squares of residuals.|
Solutions typically converge within four iterations.
Once the data converges, you can create a satellite using the TLE data generated. In the Create Satellite section, enter the Name of the satellite you wish to create in the text box to the right of the field, and then click Create.
If you choose to create a satellite using the same name as an existing satellite, existing data is overwritten with the TLE data generated using the Generate TLE tool.
You can also generate several useful reports and graphs that summarize key data using the options in the Reports or Graphs section.
|TEME Residuals...||Graphs the residual (that is, position difference) between the satellite's ephemeris and the ephemeris generated using the solved-for TLE over the time span.|
Single Point Method
Summary and Residual reports are not applicable when computing using the Single Point method.
The convergence of this algorithm is defined by specifying the maximum acceptable relative error in the position of the satellite at the specified epoch.
TLEs generated using the single point method may produce ephemeris that diverges quickly from the reference trajectory.