In 2020 the United States Space Force (USSF) released an implementation of the SGP4-XP propagator, an advanced version of the existing SGP4 propagator stated to deliver accuracy roughly equivalent to that of the USSF’s Special Perturbations Ephemeris propagator (SPEPH) with runtimes only 50% to 100% longer than SGP4. This paper presents an analysis of the performance characteristics of the SGP4-XP propagation algorithm and considers new and existing practical applications where it can be used more effectively than SGP4. To perform this analysis, two-line element sets (TLEs) are generated from high-precision reference ephemerides for a selection of objects across different orbital regimes using the USSF implementations of both the SGP4 and SGP4-XP propagators. The fit accuracy of each set of TLEs is assessed by evaluating how well the propagation of the generated TLEs compares to the initial reference trajectory. Runtime performance is assessed by measuring the elapsed wall-clock time to propagate a TLE over the fit span.
The SGP4-XP algorithm was found to have significantly improved position accuracy compared to SGP4 for objects in medium- and high-Earth orbits. Position accuracy for objects in low-Earth orbit was comparable to that of SGP4, but velocity accuracy proved to be better which improves accuracy when propagating beyond the fit-span of the TLE generation process. The significance of the improved performance characteristics of the SGP4-XP are evaluated in the context of the impact that they have on an example contact analysis and conjunction assessment scenario.
The Simplified General Perturbations 4 (SGP4) model was developed in 1970 for the propagation of near-Earth satellite trajectories. This algorithm was made publicly available in 1980 with the publication of Spacetrack Report No. 3 . The report contained the equations for the algorithm as well as FORTRAN source code. Various updates have been made over the years since the first publication, but the underlying algorithm has remained basically the same. The algorithm is widely used for the propagation of Two-Line Element (TLE) sets . Over the years there has been a proliferation of versions of the algorithm in the aerospace community in various programming languages, refactored in various ways, and updated at various intervals. Additionally, for most commonly used implementations, there is no mechanism to enforce or evaluate what version is being used and what updates have been applied. Ultimately, this has led to differences between the propagation used in the USSF’s TLE generation process and the propagated trajectory of a TLE state using a non-official implementation of the SGP4. This was confirmed in 1990 when a copy of the current operational SGP4 code used to generate publicly-distributed TLEs was made available to the public. In 2006 Vallado et al. used the definitive set of SGP4 equations published in  to develop a non-proprietary implementation of SGP4 that replicated the behavior of the operational implementation.
To attempt to remedy this problem, the United Space Air Force (USAF, now the USSF) began releasing an official implementation of the SGP4 algorithm that is used in operations as part of the AstroStds software library . Initially, this library was export controlled, but as of the release of version 8.x it is now available with access to the space-track.org website under the label of “non-restricted access.”
In addition to the official implementation of the SGP4 algorithm, the AstroStds library contains the Special Perturbations Ephemeris propagator (SPEPH). The SPEPH is an implementation of a Runge-Kutta numerical integrator with high-fidelity force models. This propagator is the best-available trajectory propagation algorithm used by the USSF, but is still export controlled.
While TLEs and associated SGP4 propagation are widely used for many applications, traditionally the intent has been for it to provide fast and/or computationally efficient trajectory propagation in domains where significant error is acceptable. The classic example is for use at ground sites performing antenna pointing with a wide field of view (FOV). The accuracy of TLE propagation using SGP4 is summarized in . In-track error dominates and can grow as large as 25 km after only a few days of propagation. Also,  found that the error has a significant bias, especially in the in-track direction, and the minimum error does not necessarily occur at the epoch defined in the TLE state.
As the use of TLEs and SGP4 propagation has become more ubiquitous, the requirements and problems in modern space mission operations have evolved. A few examples include tighter pointing requirements to support higher data transfer rates and the evolution of space-situational awareness (SSA) and space-domain awareness (SDA) priorities.
To address some of these issues, in December 2020 the USSF released an implementation of the SGP4-XP algorithm with version 8 of the SGP4 library as part of the AstroStds library. The SGP4-XP is a new algorithm designed for the propagation of TLEs featuring “extended perturbations”. The release notes associated with this release  claim that SGP4-XP is “appropriate for applications that require SPEPH-level accuracy”. The SGP4-XP algorithm continues to operate using the same basic set of information that is in the existing TLE format, meaning all users need to do to take advantage of the new algorithm is to upgrade to the new version of the SGP4 library.
The release notes associated with the SGP4-XP reference the following improvements:
Additional details regarding how the values in the TLE are different for an SGP4-XP-derived TLE can be found in the release notes for the AstroStds library . One point to note is that SGP4-XP-derived TLEs are signified by setting the “Ephemeris Type” field to a value of 4. This leads to the usage of the terms “type 4 ephemeris” or “type 4 TLE”.
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