[amsat-bb] Decoding Keps

David A. Rush david at rushtone.com
Wed Sep 19 17:19:37 PDT 2012

I've been working on some code to decode the two-line Keplerian data 
like the ARRL sends out weekly (with a thank-you to AMSAT-NA).

A couple of the values are in a somewhat cryptic format, the "BSTAR" and 
"second derivative of the mean motion".  The strings tend to be either a 
space or a + sign, five digits, usually a minus sign (but seen at least 
once as a plus sign) and finally another digit, such as:


What I have found about this format is that it is "decimal assumed", and 
examples suggest that +89689-4 = 0.89689 * 10 ^-4 or 0.000089689, if I 
converted that correctly.
I'm guessing that the first character is the sign (+ or -) of the 
number, where a space is assumed positive.  I've only seen one example 
of the 7th character being a plus (and all the digits were zero), so I 
assume that + or - are valid values for the exponent.  I guess the 
format is fundamentally limited to an exponent between +9 and -9.

Am I interpreting it correctly?  Any other insight?

David, ky7dr

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