Project Orbital State to Ground

[gwbres]

Hello,

Is there a method in the Nyx or ANISE toolkit to project an orbital state to the ground ?

Is that what this returns ?

[ChristopherRabotin]

Hi Guillaume,

Yes, ANISE provides the latlongalt function, which returns the latitude in degrees, longitude in degrees, and altitude in kilometers. If you only want one of these three, you can call the respective function directly: latitude_deg, longitude_deg, or height_km. Note that the orbit state should be in a body fixed frame (e.g. the ECEF frame) for this computation to be valid. The frame also needs to be loaded from the Almanac with the planetary constants so that the tri-axial ellipsoid which defines the body can be correctly computed, cf. the shape field of Frame.

I hope this helps.

[gwbres]

I presume the Orbital state will be naturally expressed in ECEF if my coordinates already are.

Could you give me an example of correct (geodetic ?) frame definition ? (with ellipsoid parameters being defined).
Right now, here is pretty much everything I'm capable of:

// retrieve an Almanac
let almanac = Almanac::until_2035().unwrap();
// retrieve a reference frame
let mut earth_j2000 = almanac.from_frame_uid(EARTH_J2000).unwrap();

// not sure if that's how you intent to define an Ellispoid shape 
earth_j2000.shape = Some(Ellipsoid { $WGS84 });

// build an ECEF orbit
let (x_ecef_km, y_ecef_km, z_ecef_km) = (0.0, 0.0, 0.0);
let orbit = Orbit::from_position(x_ecef_km, y_ecef_km, z_ecef_km, t, earth_j2000);

no limitations on t and its timescale here? (with respect to the reference frame being used).
Building a reference Frame and an Orbit correctly are the only points I need to master, really.

[ChristopherRabotin]

So the until_2035 almanac comes with the 11th version of the planetary constants kernel (PCK) provided by JPL, which itself includes the shape definition of the Earth: https://github.com/nyx-space/anise/blob/master/data/pck00011.tpc#L3540 .

Therefore, the following should work:

let almanac = Almanac::until_2035().unwrap();
// retrieve a reference frame
let earth_cef = almanac.from_frame_uid(IAU_EARTH_FRAME).unwrap();
assert!(earth_cef.shape.is_some());
assert!(earth_cef.shape.to_string(), "eq. radius = 6378.1366 km, polar radius = 6356.7519 km, f = 0.0033528131084554717");

No limitation on t or the time scale, ANISE does all of the transformations as needed.