Celetrak's A New Way to Obtain GP Data (aka TLEs) by Dr. T.S. Kelso 2020 May 27 explains the arrival a new infoblob format that in several ways is "better" than the ancient TLE, and contains the following:

The US government has provided GP or general perturbations orbital data to the rest of the world since the 1970s. These data are produced by fitting observations from the US Space Surveillance Network (SSN) to produce Brouwer mean elements using the SGP4 or Simplified General Perturbations 4 orbit propagator.

Many of you are familiar with this data in the form of TLEs or Two-Line Element Sets. TLEs were designed to provide the minimum data necessary to propagate the orbit of a resident space object (RSO) at a time when both bandwidth for transmission or digital storage were extremely limited. In fact, at the time, transmission might be via fax, hard copy (postal delivery), or even read over the phone and storage was handled using punch cards or magnetic tape.

While this format has served us well for many decades, it has not been without its share of problems. For example, the choice of a two-digit year caused many problems approaching Y2K—problems that were side-stepped by redefining what those two digits represented—but that Y2K problem persists fully 20 years into the 21st century. And now we are approaching another milestone where we will no longer be able to catalog all the objects we track within the 5-digit catalog number limitation of the TLE format.

One of the key drivers forcing us to consider tracking more than 100,000 objects is the activation of the Space Fence on Kwajalein Atoll. The Space Fence reached initial operational capability (IOC) on 2020 Mar 27 and is expected to track far more than the ~26,000 objects currently tracked by the SSN—perhaps by as much as an order of magnitude.

And we are expecting to see public availability of data from the Space Fence starting some time this summer (2020). The 18th Space Control Squadron (18 SPCS) has already transitioned internally to using 9-digit catalog numbers in support of these changes and we expect 18 SPCS to release data from the Space Fence using 9-digit catalog numbers.

Question: Certainly the housekeeping improvements are critical, but are there potentially orbital mechanical improvements over SGP4 propagation of traditional TLEs as well? I don't really understand what it is that is happening; is it strictly a formatting and indexing improvement, or is the amount of orbital information contained going to be modified and/or improved as well?

  • $\begingroup$ I think the bulk of this question reads well - that you are primarily interested in accuracy of the data, though this can be read more than one way. The "accuracy of data" could mean a) of a single ephemeris (e.g. a TLE) has better measurements or b) a single ephemeris has a better model, (but measurements no better) or c) that of better accuracy because more frequent updates could be available. The answers so far seem to indicate outcome (b). $\endgroup$ – Puffin Nov 21 '20 at 0:40
  • $\begingroup$ @Puffin No,I'm interested in any improvements at all, in any way, relating to orbital mechanics. I've asked exactly the question I intended to ask: "...but are there potentially orbital mechanical improvements over SGP4 propagation of traditional TLEs as well?" The question is wide open on purpose, so as not to pre-constrain answers, and I think that that instructional classification would make for an excellent answer! $\endgroup$ – uhoh Nov 21 '20 at 1:10
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    $\begingroup$ Now I think about it, I've already heard that the new spacefence is a higher frequency radar, which lowers its minimum object size threshold, but I suspect all sorts of other parameters come into play before you can determine if objects are localised any better. BTW, I don't really know what you meant by your bit in italics. $\endgroup$ – Puffin Nov 21 '20 at 19:49
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    $\begingroup$ @Puffin Oh the bit in italics notes that you've classified potential improvements in three enumerated categories, (1, 2 and 3), that I found that classification instructional and feel others will as well, and think that it would be great if you'd expanded on it a bit as an answer post. $\endgroup$ – uhoh Nov 21 '20 at 22:46

Changes due to the new fence:

  • The new space fence will most likely lead to better orbit estimates for most objects currently in the catalog, as they will be observed more often.
  • It will also detect more and smaller pieces of debris. They might only be detected if they pass at high elevation and less likely at low elevation. Consequently, their accuracy might end up to be poor.

Changes due to the new format:

  • The main reason for the new format is that it allows specifying more than 5 digits for the catalog number. It is expected that the new fence observes more than 100000 objects and hence this change was required.
  • The format (OMM) allows specifying any kind of theory and not only the ones used in the TLEs. TLEs have a flag for the ephemeris type, which is linked to a theory. Recently a new updated version of SGP4, called SGP4-XP has been published (a compiled library unfortunately not the code or equations behind). If the new provided orbital elements are generated using SGP4-XP, then you will also need this theory to propagate them. It is not clear when the new theory is started to be used and when/if elements with it are published. Also the accuracy is not yet clear, but it is supposed to be closer to high-fidelity models.

Skimming CCSDS RECOMMENDED STANDARD FOR ORBIT DATA MESSAGES, it looks to me like three different message types are defined: Orbit Parameter Message (OPM), Orbit Mean Elements Message (OMM), and Orbit Ephemeris Message (OEM).

OPM gives position and velocity and optionally "osculating Keplerian elements". It is "suited to exchanges that "do not require high-fidelity dynamic modeling".

OMM is essentially the same info as a TLE in a more readable (by both humans and modern software) format.

OEM "is suited to exchanges that require higher fidelity or higher precision dynamic modeling than is possible with the OPM" and "allows for dynamic modeling of any number of gravitational and non-gravitational accelerations." It includes optional spacecraft parameters like mass, solar radiation area and drag area terms, so it can be used with a fancy propagator that models those effects.

The Celestrak post you link says Celestrak will be providing OMMs, so it won't provide any functional improvement in propagation versus TLE, but OEMs potentially would.

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    $\begingroup$ This is a good summary of the formats themselves. To fully answer @uhoh's question, could you also add that an SPG4 propagation is not high fidelity at all. For high fidelity propagation of a LEO craft, one needs to account for Earth's spherical harmonics (e.g. EMG2008), SRP, Drag, and the motion of the Sun, Jupiter and the Moon (at the least). SPG4 might work well-enough for tracking of spacecraft over very short periods of time but isn't good enough for any mission design. $\endgroup$ – ChrisR Jul 1 '20 at 5:21
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    $\begingroup$ My goodness. Kilograms and kilometers! how will people cope? $\endgroup$ – JCRM Sep 11 '20 at 12:05
  • $\begingroup$ As someone who wrote a lot of software and astrodynamic simulation tools, I would love to chat with whomever created the OEM format. It seems like the worst of both worlds: hard to read for humans, and hard to read for machines. Not to mention that it's text based and doesn't use interpolation data, so it can't be as precise as a BSP kernel... $\endgroup$ – ChrisR Nov 19 '20 at 20:19
  • $\begingroup$ @RusselBorogove +1, interesting but what is the difference between an OPM and OMM as they both sound similar to TLEs. Is an OMM a subset of an OPM; can either output the old SGP4 style TLEs for backwards compatibility? $\endgroup$ – Puffin Nov 21 '20 at 0:46
  • $\begingroup$ @Puffin You're welcome to read the specification for more details. OMM and OPM are message formats, not tools; it doesn't make sense to ask if they output other formats. $\endgroup$ – Russell Borogove Nov 21 '20 at 0:57

Changing only the data wouldn't do much, if it were just fed to the same old SGP4 we've been using for decades. However, to my delighted surprise, at long last, the US government has provided a new propagator for public use! I haven't seen it announced anywhere, but if you go to https://www.space-track.org/documentation#/sgp4 and download the zip file, the release notes inside say:

The Simplified General Perturbations Version 4 (SGP4) propagator now includes major new capabilities in the form of extended perturbations (XP) producing significant accuracy improvements over prior versions of SGP4 for all orbit regimes. This new advanced version of the SGP4 propagator is referred to as “SGP4-XP”.

To ensure backward compatibility, there is a new Ephemeris Type 4 (XP) now accommodated by the TLE that allows users to use legacy SGP4 Ephemeris Types 0 or 2, or SGP4-XP Ephemeris Type 4 TLEs without changing inputs to the SGP4prop library. This keeps interfaces from changing due to the XP upgrade.

The propagation accuracy now available by SGP4-XP is vastly improved by inclusion of extended perturbations that can be tapped by using the new TLE ephemeris type 4, which replaces the legacy n ̈⁄6 term in columns 45-52 of Line 1 with the object’s solar radiation pressure model parameter, AGOM [m^2/kg]. The leading decimal point is assumed for the AGOM value in columns 45-52. As previously summarized in Figure 3, extended perturbations includes better lunar perturbation modeling, new and more resonance modeling, solar radiation pressure modelling (in addition to drag modelling), and the Geopotential model has been extended to include the J5 zonal term and legacy WGS-72 terms are replaced with EGM-96 terms to be consistent with SP propagation by the operational ASW. In addition, the static static atmosphere model is replaced with the Jacchia-70 model that uses a generic solar flux and geomagnetic index that predicts future flux based on flux periodics dating back more than 40 years. Therefore, it is no longer necessary to maintain F10.7 and Ap values for running the SGP4-XP propagator.

Given this is a new capability, out of an abundance of caution, it is extremely important to continue to use the most current version of the SGP4 library in the event the SGP4-XP changes. If changes are required for any reason, changes may not be backward compatible with prior versions of SGP4-XP. Future version updates could contain bug fixes as well as improvements and may not be backwards compatible with prior versions of SGP4-XP. Experimentation and parallel runs are encouraged to provide insight into the modeling improvements.

The release notes actually describe changes to every part of the Astrodynamics Standards library, which may be of interest to those of you who want to know more about the related software that is for US government use only. However, I now find that a lot less interesting than I used to, because we finally have a major improvement to SGP4. I haven't got any numbers on how much better it is, but the release notes claim "the new SGP4-XP accuracy is statistically equivalent" to the high-accuracy numerical Special Perturbations propagator they use for real operations. This may be the best thing they've done for the public since releasing TLEs in the first place.

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    $\begingroup$ This is great news, thanks for drilling down and then providing the news here. Now will we all have to switch to Windows XP to run SGP4-XP? (humor) $\endgroup$ – uhoh Nov 19 '20 at 22:58
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    $\begingroup$ @uhoh I like to think of it as a reference to roleplaying games --- SGP4 has finally earned enough XP to go up a level! $\endgroup$ – Ryan C Nov 19 '20 at 23:31

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