Finally, the SDP8 model is an extension of SGP8 to be used for deep-space satellites. The
deep-space effects are modeled in SDP8 with the same equations used in SDP4.
3 COMPATIBILITY WITH NORAD ELEMENT SETS
The NORAD element sets are currently generated with either SGP4 or SDP4 depending on whether
the satellite is near-Earth or deep-space. For element sets sent to external users, the value of mean
motion is altered sligh tly and a pseudo-drag term ( ˙n/2) is generated. These changes allow an SGP
user to make compatible predictions in the following manner. If the satellite is near-Earth, then
the pseudo-drag term used in SGP sim ulates the drag effect of the SGP4 model. If the satellite
is deep-space, then the pseudo-drag term used in SGP sim ulates the deep-space secular effects of
SDP4.
For SGP4 and SDP4 users, the mean motion is first recovered from its altered form and the
drag effect is obtained from the SGP4 drag term (B
∗
) with the pseudo-drag term being ignored.
The value of the mean motion can be used to determine whether the satellite is near-Earth or
deep-space (and hence whether SGP4 or SDP4 was used to generate the element set). From this
information the user can decide whether to use SGP4 or SDP4 for propagation and hence be assured
of agreement with NORAD predictions.
The SGP8 and SDP8 models have the same gravitational and atmospheric models as SGP4
and SDP4, although the form of the solution equations is quite differen t. Additionally, SGP8 and
SDP8 use a ballistic coefficient (B term) in the drag equations rather than the B
∗
drag term.
Ho wever, compatible predictions can be made with NORAD elemen t sets b y first calculating a B
term from the SGP4 B
∗
drag term.
At the present time consideration is being given to replacing SGP4 and SDP4 by SGP8 and
SDP8 as the NORAD satellite models. In such a case the new NORAD element sets would still
give compatible predictions for SGP, SGP4, and SDP4 users and, for SGP8 and SDP8 users, would
give agreement with NORAD predictions.
4 GENERAL PROGRAM DESCRIPTION
The five ephemeris packages cited in Section Two have each been programmed in FORTRAN IV
as stand-alone subroutines. They each access the two function subroutines ACTAN and FMOD2P
and the deep-space equations access the function subroutine THETAG. The function subroutine
ACTAN is a two argument (quadrant preserving) arctangen t subroutine which has been specifically
designed to return the angle within the range of 0 to 2π. The function subroutine FMOD2P takes
an angle and returns the modulo by 2π of that angle. The function subroutine THETA G calculates
the epoch time in days since 1950 Jan 0.0 UTC, stores this in COMMON, and returns the right
ascension of Greenwich at epoch.
One additional subroutine DEEP is accessed by SDP4 and SDP8 to obtain the deep-space
perturbations to b e added to the main equations of motion.
2
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