NORAD Tracking
Appendix K
NORAD Tracking
The 7200 ACU supports the utilization of "NORAD 2-line element sets", also
known as "NORAD TLE". The use of the name "NORAD" is historical, as the
NORAD organization no longer exists and has been replaced with Space Command.
The name "NORAD TLE" continues as people are used to it.
Space Command 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 the "NORAD tracking" function in the 7200 ACU is to use
the NORAD TLE sets to provide the user with a means of propagating these
element sets in real time and point the antenna under control of the ACU at the
space object.
It is important to clarify 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 SGP4
or SPD4 as implementing in the 7200 ACU in order to retain maximum prediction
accuracy.
Element set Format
A NORAD two-line element set actually consists of three lines of information. The
first line is the name of the target followed by two 69-character lines of data
which can be used together with the NORAD SGP4/SDP4 orbital model to
determine the position and velocity of the associated satellite. The only valid
characters in a two-line element set are the numbers 0-9, the capital letters A-Z,
the period, the space, and the plus and minus signs—no other characters are valid.
Of course, not all valid characters can be used in all columns within the element
set. Figure 1 shows what type of character is valid for each column. Columns with
a space or period can have no other character. Columns with an 'N' can have any
number 0-9 or, in some cases, a space. Columns with an 'A' can have any
character A-Z or a space. The column with a 'C' can only have a character
representing the classification of the element set—normally either a 'U' for
unclassified data or an 'S' for secret data (of course, only unclassified data are
K-1
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