GP (General Perturbation) data consists of mean elements and other metadata that is intended for use with the SGP4 propagator. US Space Force continuously tracks objects in space and regularly publishes updated GP data to spacetrack.org for dissemination to the public. Historically, this data has been formatted as Two-Line Element (TLE) files. For a long time, this has meant that "GP data" and "TLE data" were synonymous.
However, improvements in detection sensors are enabling them to track smaller and smaller space objects. This yields a larger catalog of tracked objects than have been tracked before. Space objects are identified by the integer SSC number in the metadata. In the TLE format, the SSC number is restricted to a maximum of five digits. Thus, as longer SSC numbers (nine digits) become necessary to keep up with the expanding catalog, the new SSC numbers cannot be formatted in a TLE.
In response, spacetrack.org — in close concert with celestrak.com — has begun publishing other formats for GP data. These formats are based on the CCSDS Orbit Mean element Message (OMM). GP Data is now made available in the following formats:
- TLE (when the object identifier is <= 99999)
- XML
- Keyword Value Notation (KVN)
- Comma Separated Value (CSV)
- Json
All formats other than TLE support nine-digit SSC numbers.
STK supports reading GP data in TLE, XML, KVN, and CSV formats.
- The XML and KVN formats follow version 2.0 of the CCSDS OMM specification.
- STK supports two kinds of CSV formats. One is based on version 2.0 of the OMM spec but is not an official CCSDS format. Only the variant of OMM pertaining to GP data is supported; the general OMM message is not. The second CSV format supported is the USSF Astrodynamics Standards TLE CSV format, which is not OMM based. It consists of 26 columns. The first 17 columns are in the order of the data as found in a TLE.
- The file extension *.omm is used to identify an OMM file in KVN format.
SSC number in STK
STK supports the use of nine-digit SSC numbers in GP data files. When the number is less than 99999, it is often formatted with leading zeros so that it is five digits long (e.g., 00506). Larger numbers are also often formatted with leading zeros to be nine digits long (e.g., 007012345).
While the SSC number is officially a number, STK has supported the use of alphabetic characters in the SSC number. This option was only found in certain tools that consumed TLE content, to satisfy the request of customers. The use of alphabetic characters in a GP data file is now allowed by all tools that read GP data files.
Normally, the numerical value associated with an SSC number in the files is the value of the number itself. For example, 00506 corresponds to item number 506. When alphabetic characters are used, the associated number for those characters is assigned the value zero (0), except in the special case in which the SSC number is formatted in alpha5 format.
The alpha5 format has a leading capital letter (other than I or O) with four trailing numbers. The leading letter indicates a prefix (e.g., A = 10, B = 11). The leading letter combines with the four digits to provide the numerical value (e.g., A2058 = 102058).
The alpha5 format was proposed by the USAF as an intermediary solution for extending the use of TLEs to a larger catalog size. Its use is now discouraged.