Re: [Orekit Users] how to calculate iss passes

[MAISONOBE Luc <luc.maisonobe@c-s.fr>]

Matteo <appdeveloper80@gmail.com> a écrit :

> Hi Luc,

Hi Matteo,

> thanks for your fast reply. I've used TLEPropagator and now it's all right!
> I'd want to ask if there is a way to calculate also brightness
> (magnitude) of ISS.

Orekit will help you to compute

 1) the distance between ISS and your ground point,
 2) the fact ISS is illuminated by Sun or in Earth shadow
 3) the Sun-ISS-ground point angle

From these, you will have to set up by yourself a reflection
model.

For item 1, as I assume you already have a TopocentricFrame
object associated to your ground point. So at any date, you
get the ISS SpacecraftState and just have to compute:

 distance = state.getPVCoordinates(topoFrame).getPosition().getNorm();

For item 2, you can set up an EclipseDetector and call its g function
directly with your SpacecraftState, the function will be positive
when spacecraft is illuminated and negative if shadowed.

For item 3, you have to compute:

  CelestialBody sun = CelestialBodyFactory.getSun();
  Vector3D sunPos   = sun.getPVCoordinates(topoFrame).getPosition();
  Vector3D issPos   = state.getPVCoordinates(topoFrame).getPosition();
  double angle      = Vector3D.angle(sunPos.subtract(issPos), issPos);

You may also take into account the directions of lines-of-sight in ISS
frame for both the incoming and reflected rays, taking ISS attitude (can
be assumed to be a LOFOffset attitude with LOFType.VNC local orbital
frame and all offset angles set to 0), if you want a reflection model
more realistic than something simply based on the Sun-Iss-ground point
angle.

best regards,
Luc

> Thanks again!
>
> Matteo
>
> 2017-02-23 13:23 GMT+01:00 MAISONOBE Luc <luc.maisonobe@c-s.fr>:
> > appdeveloper80@gmail.com a écrit :
> >
> > > Hi,
> >
> >
> > Hi Matteo,
> >
> > > I'm trying to calculate iss passes over Rome and I start from propagation
> > > tutorial and I write this code
> > > (https://gist.github.com/anonymous/fb939c0df8e6a72282eb27cc76840ac9).
> > > But if I compare the rise and set date with the Heavens Above prevision
> > >
> > > (http://heavens-above.com/PassSummary.aspx?satid=25544&lat=41.9028&lng=12.4964&loc=Roma&alt=53&tz=CET)
> > > I don't find a match:
> > >
> > > My code finds:
> > >
> > > Visibility on station1 begins at 2017-02-23T14:06:33.974
> > > Visibility on station1 ends at 2017-02-23T14:12:28.528
> > >
> > > Heavens Above calculates:
> > >
> > > 23 Feb  -       12:48:03        10°     NNW     12:50:12        16°     N
> > > 12:52:20        10°     NE      daylight
> > > 23 Feb  -       14:24:14        10°     NW      14:27:21        37°
> > > NNE
> > > 14:30:26        10°     E       daylight
> > > 23 Feb  -       16:00:50        10°     WNW     16:03:44        29°     SW
> > > 16:06:36        10°     SSE     daylight
> > >
> > > Note that Heavens Above times are in CET timezone.
> > >
> > > Can you help me where my error is?
> >
> >
> > From a quick check, I see two things.
> >
> > First is that you created a Keplerian orbit from mean orbital elements and
> > propagated them as if they were osculating elements. You will get tens or
> > perhaps hundreds of kilometer differences after some propagation time.
> > As Heavens Above used TLE, you should use TLE too for propagation, so you
> > use the same semantic on parameters and the same model.
> >
> > Second, when copying the mean anomaly (131.4349), it seems you forgot to
> > convert from degrees to radians.
> >
> > Hope thi_s helps,
> > Luc
> >
> > > Thanks,
> > >
> > > Matteo