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Re: [Orekit Users] TLEPropagator Question
Thomas Neidhart <thomas.neidhart@spaceapplications.com> a écrit :
Dear all,
Hi Thomas,
I am using this very nice library (many kudos also for making it
available as open-source!) to calculate visibility events for a
satellite based on TLEs.
many thanks, your comments are appreciated
For this purpose I have used the already available example using a
KeplerianPropagator and a custom ElevationDetector to collect the events.
This works so far, but I was a bit concerned about the accuracy of the
resulting events, especially when comparing it with some available
tracking sites on the internet (e.g. http://www.heavens-above.com/), as
results were diverging in the range of minutes after some hours.
If I understand it correctly, the provided KeplerianPropagator is a very
simplistic propagator based on a keplerian orbit, while the
TLEPropagator is way more sophisticated and should yield much more
accurate results.
This is true, but I would not say that the TLE model is
"sophisticated" and yields "accurate" results. This model is known to
have many drawbacks and is clearly suited neither for long term
propoagation nor for accurate orbit propagation. Apart from the model
by itself, the published TLE data often has large errors. Several
studies have shown that you may get several hundreds or even thousands
of kilometers errors propagating TLE a few days (yes, these orders of
magnitude are in kilometers, not in meters).
Unfortunately, you cannot simply convert a TLE into another orbit type
using a single point position-velocity pair. The state representation
(the TLE) and the propagation model (SGP4/SDP4) are tightly bound
together. Doing such a conversion and using a more accurate numerical
propagator would therefore not improve things. This is a real problem.
Operational systems use TLE only for first assessment of debris
mitigation, simply because the available data is in this format. When
more accurate results are needed (for further analysis if a close
approach occurs or for control on known satellites), we use other
types of orbits and different propagation models.
Sadly the TLEPropagator does not implement the
Propagator interface, but I created mysself a simple wrapper for testing
purposes (updating the SpacecraftState orbit with propagated P/V
coordinates from the TLEPropagator instead of propagating the orbit itself).
We are aware of this missing feature and it has been fixed. If you
want to give it a try, checkout the current development version of
Orekit from the Git repository (it uses the newly released Apache
Commons Math 2.2). In this version, TLEPropagator does implement the
Propagator interface (which has also been improved) and can be used
with visibility events detectors.
The results are very promising and are almost the same compared to other
sites (~ 1-2s delta). Can someone confirm that my assumptions so far are
correct wrt orbit propagation?
Yes but it does not mean that the results are "better", it means the
results are consistent with the way the other sites compute them.
Unfortunately, if you don't have access to better orbit data, you
cannot have a better result.
best regards,
Luc
Best regards,
Thomas
--
Thomas Neidhart, Software Engineer
Space Applications Services, www.spaceapplications.com
Leuvensesteenweg 325, B-1932 Zaventem, Belgium
Phone: +32-(0)2-721.54.84, Fax: +32-(0)2-721.54.44
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