Background:
Alpha5 is a stopgap numbering scheme for identifying space objects using integers in Two Line Element (TLE) formatted data. The TLE format only allocates 5 characters for the identifier and has typically only used digits 0-9, with 99999 being the maximum number that could be used. Alpha5 formatting uses its first character (a capital letter) as a prefix indicating additional digits: 'A' -> 10, 'B'-> 11, etc. so that C9876 indicates the integer 129876, thus allowing the TLE format to specify objects whose integer value is greater than 99999. With the letters ‘I’ and ‘O’ being skipped, the maximum number that can be used is 339999.
See also:
https://en.wikipedia.org/wiki/Two-line_element_set
https://www.space-track.org/documentation#/tle-alpha5
Timeline of Alpha5 support in STK:
STK 12.1 (2021).
• First version to support alpha-5 and also 9 digit SSC numbers.
• Undocumented as it wasn't an official format at the time.
STK 13.1 (2026R1)
• First version to support STK export or creation of alpha-5
• Generate TLE tool updated to output results in the TLE format for SSC numbers up to 339999. Numbers greater than 99999 are now formatted using the USAF alpha5 format which uses a letter prefix and 4-digit number.
Example workflows:
1) Single TLE:
Given attached notional .tle file with an Alpa-5 ssc shown below…
1 T0446U 99025AFH 26053.70081522 .00006030 00000-0 17033-2 0 9998
2 T0446 98.6054 142.2060 0034204 236.9955 122.7948 14.45692492992164
Do Insert new > Satellite > From TLE File > browse to file.
2) Local Satellite Database Search:
Note: By default the local Satellite Database Search will check online and local file location. The local file location STK uses is located in C:ProgramData which may be a hidden by Windows File Explorer settings. For example.
C:\ProgramData\AGI\STK_ODTK 13\Databases\Satellite
C:\ProgramData\AGI\STK 12\Databases\Satellite
In this folder STK stores a TLE/TCE files obtained from CelesTrak and corresponding satellite database (.sd) file used to search on corresponding parameters. See link below for specific details of this format.
https://help.agi.com/stk/#stk/importfiles-11.htm
As an example you
a)can add the notional TLE from above to the STK default TCE file.
• Open stkAllTLE.tce
• Add…
1 T0446U 99025AFH 26053.70081522 .00006030 00000-0 17033-2 0 9998
2 T0446 98.6054 142.2060 0034204 236.9955 122.7948 14.45692492992164
• Open stkAllTLE.sd
• Add…
T0446USASAT SATSATSAT 1958-006A US Earth Sci AFE195803171215 1.5 3822 649 132.6 34.3 Unknown 20251023
Do Insert new > Satellite > From Standard Object Database > Search for T0446, select the hit and click Insert.
You may see this error click Advanced button at the bottom...
...go to Propagation TLE Source to Import from File and verify the .tce file is the one you want to use. For this example we edited the default database so no file change is required.
3) Export an Alpha-5 TLE from STK 13.1:
- Create a new satellite. Insert > New > Default (non SGP4).
- From Satellite Right-click menu select Generate TLE...
- Enter an integer between 100000 and 339000 in the SSC # field.
- In Output area provide name, click create and then run the Element Set Report to see the Alpha-5 notation.
Alpha-5 can be reported in the Element set or TLE report.
