A New Way to Obtain GP Data (aka TLEs)

by Dr. T.S. Kelso
2020 May 27

Background

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.

The Solution

CelesTrak—working closely with Space Track—has begun making the GP data available via standard queries using the Orbit Mean-Elements Message (OMM) that is part of the Orbit Data Messages (ODM) Recommended Standard CCSDS 502.0-B-2 developed by The Consultative Committee for Space Data Systems (CCSDS) in November 2009. We are recommending the XML format of Version 2.0 of the OMM, as defined in XML Specification for Navigation Data Messages (CCSDS 505.0-B-1) to ensure future compatibility and interoperability.

There are XML and KVN (key-value notation) versions of the OMM standard and CelesTrak will provide all mandatory elements of those formats. Some elements may be blank (KVN) or null (XML), if not available via the current TLE format. An example might be that an object in the current analyst sat range (80000-series) typically will not have a name (OBJECT_NAME) or International Designator (OBJECT_ID).

Use of the new data queries is NOT required for most users at this time, since CelesTrak will continue to provide data for RSOs with 5-digit catalog numbers in the TLE/3LE or 2LE formats. The current focus is to provide software developers a way to test modifications to their code to support using the new OMM format and 9-digit catalog numbers. Legacy links to fixed .txt files will continue indefinitely, although links on web pages will eventually be transitioned to use the new GP query (as we already do in many places using the tle.php query) and allow users to define their default format. Of course, TLE formats will not support objects with catalog numbers above 99999.

Additionally, data will be provided in both JSON and CSV formats, using the same keywords and definitions as provided in the OMM standard (CCSDS 502.0-B-2, Table 4-1), although null/blank or redundant (e.g., CENTER_NAME = EARTH, REF_FRAME = TEME, TIME_SYSTEM = UTC, MEAN_ELEMENT_THEORY = SGP4) mandatory fields will not be included.

CelesTrak will work to ensure that all GP data received via 18 SPCS and Space Track will be ingested in a way that supports users requesting GP data in any of the TLE or OMM formats.

The Implementation

All GP queries on CelesTrak will take the form:

where {QUERY} is:

Allowed formats are:

The FORMAT specification is optional, but defaults to XML.

Examples:

For Software Developers

Software developers looking for code to input or output these formats in a variety of languages are invited to check out the Space Data Standards web site developed by our partners at Digital Arsenal.

There is code to support C++, Kotlin, Java, C#, Go, Python, JavaScript, TypeScript, PHP, Dart, Lua, Lobster, Swift, and JSON Schema. There are also examples converting TLEs to OMMs in XML, KVN, JSON, CSV, and FlatBuffers. And there is a GitHub repository for the code that allows users to submit suggested changes.

Summary

Providing GP data in the OMM format provides a way forward for all software developers to continue to support using SGP4 in their applications and add support for 9-digit catalog numbers. In addition, it elimates the Y2K problem still coming NLT 2057 by using the ISO 8601-1 (WD) date and time standard and also supports the use of Unicode characters for satellite names from non-English languages.

FAQs

Q: Why don't we just modify the current 5-digit catalog numbers to include letters to increase the range of objects that can be represented?

A: There have been numbering schemes suggested that would extend the range of catalog IDs that would fit in a 5-character field of a TLE. The easiest would be to extend the current 'numbering' by allowing the leading character to go from 0-9 and then A-Z. At best, this would allow for tracking 360,000 objects (assuming you don't discard I and O, as has been suggested by some). That could work for the Space Fence, but ignores other potential large increases in objects tracked due to the development of large constellations (which currently propose as many as 100,000 new satellites) or additional large debris events that might occur due to the collision of large uncontrolled rocket bodies.

Even allowing all 5 characters to go from 0-9 and then A-Z would only allow 60,466,176 catalog IDs, but 18 SPCS is already assigning catalog numbers in their new analyst sat range of 7995xxxxx (or over 799,500,000). So, there would be no way to map these 9-digit catalog numbers to 5-character IDs.

And the ability to fit new characters into a field does not make the problem of interpreting the change in software go away, any more than the change in interpretation of the 2-digit year did for Y2K. All the code using these catalog numbers will have to be updated to change their interpretation, with a cascading set of implications. Including letters means the field will no longer be able to be simply validated by verifying that it is an integer and integer comparisons will no longer be possible. For example, some TLEs use leading zeros in the 5-digit field while others do not. But a value of 00964 parses as an integer the same way as 964.

Every software developer will need to update their code to adapt, so this is an opportunity to do that in a way that provides future flexibility and no longer relies on the limitations of fixed formats.

Q: Why do we have to use formats that are so verbose?

A: While it is possible to use formats that are less verbose, they do not provide the framework to ensure future interoperability that an international standard like the OMM provides. Software developers—particularly those developing to support systems for use in satellite operations, ensuring safety of flight, or national security—are strongly encouraged to use the recommended OMM XML standard. Others that do not support critical functions like these may choose the JSON or CSV formats based on the OMM standard (but not currently part of that standard), although there is always the possibility that these could change. CelesTrak has worked hard to ensure that doesn't happen, going back almost 35 years now, and will endeavor to make changes in a way that maintain backward compatibility whenever possible.

But the reality is that using a full XML version of the OMM for a single object takes about 1,200 characters (including all the overhead XML formatting for a set of OMMs) compared to the 168 characters of a three-line element set. That is a factor of 7 larger. When I first started CelesTrak in 1985, I had a 1200 baud (120 Bps) modem on the system for a single user at a time. Today, CelesTrak has a 1 Gbps connection that supports over 400,000 unique users a day and my home Internet service allows up to 350 Mbps—that's a factor of almost 300,000 times faster. And my hard disk at the time was a whopping 5 MB—I have several 10-TB drives on my home system (including 5-TB external drives that cost a tiny fraction of that 5-MB one) and CelesTrak has 400 MB of SSD storage. That's a factor of 80 to 1,000 times as much storage. And we will continue to see similar advancements in bandwidth and storage that make these differences irrelevant.

And the long-established (February 1998) XML format has extensive software support to allow easily ingesting the OMM XML data.