For many years—until the early 1990s—the TLE data on CelesTrak was entered by hand from Part I of the NASA Prediction Bulletins, which were received via First Class Mail. If you have ever been curious about this data and how it was used to allow manual calculations back before computers were widely available, this document explains it all.
NORAD maintains general perturbation element sets on all resident space objects. These element sets are periodically refined so as to maintain a reasonable prediction capability on all space objects. In turn, these element sets are provided to users. The purpose of this report is to provide the user with a means of propagating these element sets in time to obtain a position and velocity of the space object.
The most important point to be noted is that not just any prediction model will suffice. The NORAD element sets are "mean" values obtained by removing periodic variations in a particular way. In order to obtain good predictions, these periodic variations must be reconstructed (by the prediction model) in exactly the same way they were removed by NORAD. Hence, inputting NORAD element sets into a different model (even though the model may be more accurate or even a numerical integrator) will result in degraded predictions. The NORAD element sets must be used with one of the models described in this report in order to retain maximum prediction accuracy.
Over a quarter century ago, the United States Department of Defense (DoD) released the equations and source code used to predict satellite positions through SpaceTrack Report Number 3 (STR#3). Because the DoD's two-line element sets (TLEs) were the only source of orbital data, widely available through NASA, this code became commonplace among users needing accurate results. However, end users made code changes to correct the implementation of the equations and to handle rare cases encountered in operations. These changes migrated into numerous new versions and compiled programs outside the DoD. Changes made to the original STR#3 code have not been released in a comprehensive form to the public, so the code available to the public no longer matches the code used by DoD to produce the TLEs. Fortunately, independent efforts, technical papers, and source code enabled us to synthesize a non-proprietary version which we believe is up-to-date and accurate. This paper provides source code, test cases, results, and analysis of a version of SGP4 theory designed to be highly compatible with recent DoD versions.
New ways to get GP data are now available! Up until now, we've used the TLE format to ingest data into SGP4. With the growth of the catalog soon to exceed the current range of 5-digit catalog numbers, you may be wondering how we will handle that. If so, be sure to read this article!
The SPACEWARN Bulletin is intended to serve as an international communication mechanism for the rapid distribution of information on satellites and space probes. The material it contains is based on guidelines in "COSPAR Guide to Rocket and Satellite Information and Data Exchange," COSPAR Transactions #8, December 1972, and various Committee on Space Research (COSPAR) resolutions.
