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


HST mean time between TLEs by year

  • 2
    $\begingroup$ Can you post a graph with the x-axis labelled by date? $\endgroup$
    – Steve
    Commented Apr 20, 2016 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
    Commented Apr 20, 2016 at 17:47
  • $\begingroup$ Does this happen only for Hubble, or for other satellites in the database as well? $\endgroup$ Commented Apr 22, 2016 at 8:32
  • 2
    $\begingroup$ @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
    Commented May 6, 2016 at 1:12
  • 1
    $\begingroup$ 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
    Commented Apr 23, 2020 at 16:49

1 Answer 1


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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.