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Re: [Orekit Users] how to calculate iss passes


MAISONOBE Luc <luc.maisonobe@c-s.fr> a écrit :


Matteo <appdeveloper80@gmail.com> a écrit :


I thank you again for your answer, I'm trying to use formula in this post (
http://stackoverflow.com/questions/19739831/is-there-any-way-to-calculate-the-visual-magnitude-of-a-satellite-iss)
for reflection model.

When I use ElevationExtremumDetector, I'm actually filtering for positive
elevation: in this case regarding number 2) (the fact ISS is illuminated by
Sun or in Earth shadow) is ISS always illuminated by Sun?
No. ISS is in Low Earth Orbit (very low, indeed) so Earth as seen from ISS fills a very big part of the visible space. ISS is in eclipse roughly half of the time. Basically, if local time at the ground point is such that it is the middle of the 
night and if ISS flies directly at the zenith, it will also be in the night.



Is
ElevationExtremumDetector filtering for eclipsed transit?


No, because for classical operations, we need to be able to get satellite
telemetry and send telecommands even at night. We don't really "look" at
the satellite, we connect to it using ground stations antennas. So this
detector finds by default all passes.

Also rather than ElevationExtremumDetector (which will give you only one
point, the max of the pass), you can use ElevationDetector which will
give you the raising and setting times, and can take into account
atmospheric refraction, which distorts rays near horizon. Another
reason to use ElevationDetector is that if your ground point is slightly
after sunset (or before dawn) and therefore is in night, the max pass
will also be in night and you will miss it, despite near the horizon, the
ISS may be out of shadow and be visible (despite the observer is in the
night part of the Earth, near the terminator).


If not, how can I know if ISS is eclipsed in the "eventOccurred" of an
ElevationExtremumHandler since I'm out of a EclipseDetector event?


You can wrap your ElevationDetector within an EventEnablingPredicateFilter
object, together with your own implementation of EnablingPredicate based
on the g method of an EclipseDetector (not the eventOccurred method, which you 
don't need here). Something along these lines:


If you are using the development version of Orekit (not released yet), you
can also use BooleanDetector and the NegateDetector to combine an
ElevationDetector and an EclipseDetector (you want to be visible AND (NOT
in eclipse)). The advantages of BooleanDetector over EventEnablingPredicateFilter 
are that

 1) it is simpler
 2) it will detect for example the end of visibility due to entering eclipse
    while still being at positive elevation.

The second case mean an observer on ground would see ISS go dark in the
middle of the sky.

best regards,
Luc



 public class Illuminated implements EnablingPredicate<ElevationDetector> {

    private final EclipseDetector eclipseDetector;

    public Illuminated() throws OrekitException {
        this.eclipseDetector = new EclipseDetector(CelectialBody.getSun(),
                                                   696000000.,
CelestialBodyFactory.getEarth(), Constants.WGS84_EARTH_EQUATORIAL_RADIUS); 
    }
public boolean eventIsEnabled(SpacecraftState state, S eventDetector, double g) 
        throws OrekitException {
// we are in direct Sun light when the g function of the eclipse detector is positive 
        return eclipseDetector.g(state) >= 0;
    }

 }
Then, instead of using the ElevationDetector itself, you would use the EventEnablingPredicateFilter that wraps it, and it will perform the filtering logic for you, so you are not bothered with the passes over your ground point that happen when ISS is not illuminated at all.
Beware, I just wrote this code quickly in the mail, I did not even try to compile it, 
so it may well contain errors.

bes



Thanks again!

Il venerdì 24 febbraio 2017, MAISONOBE Luc <luc.maisonobe@c-s.fr> ha
scritto:


MAISONOBE Luc <luc.maisonobe@c-s.fr> a écrit :


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);


in fact last line should be:

  double angle = FastMath.PI - Vector3D.angle(sunPos.subtract(issPos),

issPos);


Luc



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