$LastChangedDate: 27-Oct-2009 $

# Reference Frames

Reference Frames

## Reference frames used in CelestLab

Several reference frames are used in CelestLab. Inertial reference frames are needed for the definition of orbits (integration of the equations of motion) whereas rotating frames (i.e. tied to planets) are convenient for many purposes (when visibility with ground locations is involved).

## Terrestrial Reference Frame (ITRF)

- Origin: Earth center of mass
- Z-axis : IRP (IERS Reference Pole). Z-axis is perpendicular to the Equatorial plane
- X-axis: IRM (IERS Reference Meridian) = Greenwich meridian
- Y completes the trihedron

## Inertial Geocentric Reference Frame: J2000 Reference Frame

This reference frame called J2000 in CelestLab follows the definition of the International Celestial Reference Frame (ICRF) in the IERS 1996 conventions. It is a standard inertial Earth centred, equatorial, mean of epoch Reference system. This inertial reference frame is defined as follows:

- The reference epoch is January 1st, 2000 at 12h00 (BDT time scale)
- The origin 0 is the Earth centre of mass
- Z2000 is parallel to the mean Earth rotation axis at epoch and is oriented towards the North pole
- (X2000, Y2000) lies in the mean equatorial plane of epoch
- X2000 axis points towards the mean vernal equinox of reference epoch.
- Y2000 axis completes the right handed co-ordinate system

## Transformation from ITRF to ICRF

In CelestLab, the transformation from ITRF to ICRF (J2000 in CelestLab) is achieved through a succession of rotations:

- Polar motion (PM) : (xp,yp) - no model, data available on IERS web site
- Sideral time (Theta) : IERS 1996
- Nutation (N) : Wahr IAU 1980
- Precession (P) : Lieske 1996

## G50 Veis

The advantage of this "quasi-inertial" reference frame is to allow a transformation from a Terrestrial Reference Frame (RF) to a Celestial Reference Frame using only a single rotation around the Z-axis (Veis Sidereal time). This reference frame can be very useful for mission analysis because of its simplicity.

The G50 Veis is defined as follows:

- The Z-axis is parallel to the rotation axis (and towards North)
- The X-axis is the projection of the mean vernal direction onto the true equator at epoch (01/01/1950 at 0h00). This definition makes the X-axis "as inertial as possible"
- The Y-axis is defined in order to obtain a direct trihedron

## H0-n frame

H0-n is a planetocentric inertial frame that is essentally a terrestrial frame frozen at some time. More specifically, it is defined by:

- Origin at the center of mass of the planet
- plane OXY is the planet's equatorial plane
- OZ axis is directed towards the planet's North pole
- OX axis is defined by its angle with the planet prime meridian (longitude)
- OY completes the orthonormal frame