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.
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.