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Re: [Orekit Developers] Ephemeris Writer Interface



On 10/26/2016 08:30 AM, MAISONOBE Luc wrote:
For the most part it seems that the OrekitFixedStepHandler could be used 
as the interface for an EphemerisWriter.
Do you imply that EphemerisWriter would extend OrekitFixedStepHandler? What would be the additional methods in this case? So do you intend something like: 

  OEMWriter writer = new OEMWriter(file, metaData1, metaData2, ...);
  writer.method1();
  writer.method2();
  propagator.setMasterMode(step, writer);
  propagator.propagate();
  writer.close();
For the most part, yes. To provide a couple more details of what I'm thinking: 

Appendable out = ...;
Frame frame = ...;
TimeScale scale = ...;
OEMWriter writer = new OEMWriter(out, frame, scale, metadata);
Propagator propagator = ...;
propagator.setMasterMode(step, writer);
propagator.propagate();
// close out if necessary
The EphemerisWriter interface could provide methods such as getFrame(), and getTimeScale(), but these methods don't seem especially necessary. I think the writer can use the isLast parameter of handleStep(...) to figure out when to write the footer, if the file format has one. Do you see a case where we would need an explicit close() or flush() method for the EphemerisWriter? If it turns out that we don't need any additional methods beyond those in OrekitFixedStepHandler then we can just use that interface, with some documentation explaining the intended usage. 




One piece of information that is
missing is the step size, but that could easily be added to the init(...) 
method call.
Yes, but if we use constructor for other meta-data, we could also provide the 
step size this way.
I pushed up a commit that added the step size to the init(...) method. I think this helps reduce the duplicated information that could become mismatched. 




Additional meta data could be passed in through the
constructor of the EphemerisWriter, such as frame, time scale, and center name. Given the wide variety of metadata accepted by different formats I'm 
not planning on standardizing an interface for setting the metadata. I
think covariance output could also be supported by providing an initial
covariance and a suitable mapper to extract the STM from the
SpacecraftState.
Hank, are you still interested in contributing your OEM writer and would 
it fit with the described interface?

Are there any other formats that would need to be supported by the
EphemerisWriter interface?
I guess the interface should be designed so we provide a few implementations 
for well-known formats, but also to let people implement it really easily
for their own mission-specific formats. That is it should be something
closer to OrekitFixedStepHandler, OrekitStepHandler (user-oriented interfaces) rather than IAUPole or PropagatorConverter (interfaces mainly intended for internal use) 



Agreed.

Best Regards,
Evan

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