Last week AMSAT-BB had a good comment and exchange of comments on an issue concerning the data in the AMSAT Keplerian Distribution (TLEs). I thought that at the risk of being repetitive, I would elaborate on this topic as it does come up from time to time.

The comment concerns the number of satellite orbits on some older satellites shown in the 2-line (TLE) data set for each satellite. This would affect the data for older satellites such as FO-29, AO-07, ISS, and any other satellite that has been orbit a very long time. First, note that at the top of your weekly distribution there is a key that describes the format of the data in the 2-line set for each satellite. Look at line number 2 and using the key find ORBITNUM. ORBITNUM uses columns 64-68 near the end of line number 2 and it tells us the number of orbits that the satellite has made since it was launched. You will quickly realize that the formatting of satellite number of orbits is limited to five digits or 99,999 orbits. This may seem a little short sighted today, but on early computers memory was limited and this, no doubt, was a compromise that needed to be made at that time. Note also that this is a number that comes directly from Space-Track and is not an AMSAT generated number. Considering all the tracking programs in use that are programmed for the standard 2-line format provided by Space-Track, it would not be easy to change that format today. (Tracking programs themselves could be re-programmed to correct for this, but ORBITNUM is not used to locate the satellite in space, it is just interesting information. If your favorite long-lived satellite exceeds 99,999 orbits then you will need to add 100,000 to the number shown to correct the value of ORBITNUM. (Someday you may need to add 200,000 and so on.)