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I noticed that the TLEs for the Hubble Space Telescope (HST) are posted almost twice a day. Then I looked back in time and saw that this rate varies quite a lot!

The plot shows the time interval between consecutive TLEs (in days) for the HST in space-track.org. The blue line is all points and the black line is a rolling average of 100 points. It seems to suddenly jump from about 0.7 day to 2 days between TLEs in May 2009 (JD ~ 2,454,960) , then slowly increase in frequency, until August 2015 when it drops back down to about 0.7 days (JD ~ 2,457,285).

Do these changes (and this pattern) reflect anything of significance? Budget cuts? Shifts in Observing Program requirements? USSTRATCOM busy?

Data from 2000-01-01 to present:

These plots are intervals (days) between successive TLEs, not TLEs/day

HST mean time between TLEs

CLICK TO EXPAND

HST mean time between TLEs by year

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    $\begingroup$ Can you post a graph with the x-axis labelled by date? $\endgroup$ – Steve Apr 20 '16 at 16:11
  • $\begingroup$ @Steve python is supposed to be easy - so I'll give it a try... but plotting in python is not so pleasant. There is a tick every month, and you can see that the transitions are in the months already indicated in the text of the question. The terms of space-track don't allow me to post the data (which would be easier for everyone). $\endgroup$ – uhoh Apr 20 '16 at 17:47
  • $\begingroup$ Does this happen only for Hubble, or for other satellites in the database as well? $\endgroup$ – 2012rcampion Apr 22 '16 at 8:32
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    $\begingroup$ @uhoh, I'd use the ISS (roughly similar orbit, and likely to be heavily tracked), USA-132 (the oldest operational GPS satellite, likely to be heavily tracked), and a randomly-selected upper stage (to get a baseline for "minimal resources spent tracking something"). $\endgroup$ – Mark May 6 '16 at 1:12
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    $\begingroup$ I'm stumped by this: the decrease in frequency coincides with the last to servicing missions. Maybe they were more confident after the reboost? $\endgroup$ – Ludo Apr 23 at 16:49
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There were 5 servicing missions to HST:

  • Servicing Mission 1 in December 1993
  • Servicing Mission 2 in February 1997
  • Servicing Mission 3A in December 1999
  • Servicing Mission 3B in March 2002
  • Servicing Mission 4 in May 2009

The decrease in tracking frequencies in your plot coincide with SM 3B and SM4. At the end of each mission HST was boosted up to a higher orbit to counter orbital decay. Perhaps the confidence in the orbital parameters after the reboost was sufficiently high to warrant a lower update frequency for a while.

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