Orbit Reference Frames

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The orbit reference frame defines the orientation of the orbital elements with respect to the central body. The reference frames available in FreeFlyer are described below. The PositionConvert, VelocityConvert, and PositionVelocityConvert functions can be used to convert spacecraft position and velocity vectors between the different frames. These reference frames can also be used when setting or getting a Spacecraft or CelestialObject's Keplerian or Cartesian state, using methods like Spacecraft.SetKeplerianState() and CelestialObject.GetCartesianStateAtEpoch().

 

FreeFlyer supports the following orbit reference frames, described on this page:

 

Earth Frames:

oMean of J2000 Earth Equator

oMean of J2000 Earth Ecliptic

oEarth Fixed

oTrue of Date Earth Equator

oTrue Equator Mean Equinox

oLocal Coordinates - Longitude

oLocal Coordinates - Longitude/Latitude

oMean of Date Earth Equator

oMean of 1950 Earth Equator

oTrue of Date Rotating

 

Moon Frames:

oMoon Fixed (Principal Axis Rotating)

oMoon Principle Axis - Inertial at J2000

oMoon Principle Axis - Inertial at Epoch

oMoon Mean Earth - Rotating

oMoon Mean Earth - Inertial at J2000

oMoon Mean Earth - Inertial at Epoch

 

Generic CelestialObject Frames:

oBody Fixed

oInertial Equatorial J2000

oInertial Equatorial At Epoch

 

For Spacecraft-referenced frames, see the Attitude Reference Frames page.

 

 

Mean of J2000 Earth Equator


Description:

Z-axis: Vector normal to the mean equatorial plane at Julian year 2000.0, pointing towards the Northern Hemisphere

X-axis: Vector pointing from the center of the Earth to the mean vernal equinox at Julian year 2000.0

Y-axis: Vector perpendicular to the x- and z-axes, forming a right-handed coordinate system

Origin: Center of the Earth

 

Notes:

Inertial frame.

This is the principal coordinate system used for computations in FreeFlyer.

Commonly referred to as Mean of J2000, ECI, or GCI.

 

 

Mean of J2000 Earth Ecliptic


Description:

Z-axis: Vector normal to the ecliptic plane at Julian year 2000.0, pointing toward the Northern Hemisphere

X-axis: Vector pointing from the center of the Earth to the mean vernal equinox at Julian year 2000.0

Y-axis: Vector perpendicular to the x- and z- axes, forming a right-handed coordinate system

Origin: Center of the Earth

 

Notes:

Inertial frame.

 

 

Earth Fixed


Description:

Z-axis: Points toward the true rotation axis at the current epoch

X-axis: Vector from the center of the Earth to the intersection of the prime meridian with the true equatorial plane

Y-axis: Vector perpendicular to the x- and z- axes, forming a right-handed coordinate system

Origin: Center of the Earth

 

Notes:

Rotating frame.

The Earth Fixed reference frame will also include polar motion effects if enabled:

 

// Turn on polar motion modeling

FF_SolarSystem.ComputePolarMotion = 1;

 

If polar motion modeling is not enabled, the Earth Fixed reference frame is equivalent to the True of Date Rotating reference frame.

 

 

True of Date Earth Equator


Description:

Z-axis: Points toward the true rotation axis at the current epoch

X-axis: Points toward the true vernal equinox at the current epoch

Y-axis: Vector perpendicular to the x- and z- axes, forming a right-handed coordinate system

Origin: Center of the Earth

 

Notes:

Directions of the coordinate system axes are time dependent

The "true" reference frames include nutation and precession effects. The user can control when the nutation and precession matrices are updated:

 

// Set to zero to update matrices with full available accuracy

// Default value for update is every 1800 seconds

FF_SolarSystem.NPUpdateInterval = 0;

 

 

True Equator Mean Equinox


Description:

Z-axis: Points toward the true rotation axis at the current epoch

X-axis: Points toward the mean vernal equinox at the current epoch

Y-axis: Vector perpendicular to the x- and z-axes, forming a right-handed coordinate system

Origin: Center of the Earth

 

 

Local Coordinates - Longitude


Description:

Z-axis: Points toward the true rotation axis at the reference epoch

X-axis: Lies in the true equatorial plane at the reference epoch, pointing from the center of the Earth to the reference longitude (usually the longitude of the launch site)

Y-axis: Vector perpendicular to the x- and z- axes, forming a right-handed coordinate system

Origin: Center of the Earth

 

Notes:

Launch vehicle trajectory can be designed without regard to launch time and date

In FreeFlyer 6.6 and earlier, this reference frame was referred to as "Launch Coordinates"

 

 

// Set the reference Epoch and Location of Launch Site

Spacecraft1.LCSEpoch = "Jan 01 2020 00:00:00.000".ParseCalendarDate();

Spacecraft1.LCSLongitude = GroundStation1.Longitude;

Spacecraft1.LCSLatitude = GroundStation1.Latitude;

 

 

Local Coordinates - Longitude/Latitude


Description:

Z-axis: Points from the center of the Earth to the reference longitude and geodetic latitude

X-axis: Lies in the true equatorial plane at the reference epoch

Y-axis: Vector perpendicular to the x- and z- axes, forming a right-handed coordinate system

Origin: Center of the Earth

 

Notes:

Launch vehicle trajectory can be designed without regard to launch time and date

In FreeFlyer 6.6 and earlier, this reference frame was referred to as "UVW Coordinates"

 

 

Mean of Date Earth Equator


Description:

Z-axis: Vector normal to the mean equatorial plane at the current epoch

X-axis: Vector pointing from the center of the Earth to the mean vernal equinox at the current epoch

Y-axis: Vector perpendicular to the x- and z-axes, forming a right-handed coordinate system

Origin: Center of the Earth

 

 

Mean of 1950 Earth Equator


Description:

Z-axis: Vector normal to the mean equatorial plane at 12:00:00 on January 1, 1950

X-axis: Vector pointing from the center of the Earth to the mean vernal equinox at 12:00:00 on January 1, 1950

Y-axis: Vector perpendicular to the x- and z-axes, forming a right-handed coordinate system

Origin: Center of the Earth

 

 

True of Date Rotating


Description:

Z-axis: Points toward the true rotation axis at the current epoch

X-axis: Vector from the center of the Earth to the intersection of the prime meridian with the true equatorial plane

Y-axis: Vector perpendicular to the x- and z- axes, forming a right-handed coordinate system

Origin: Center of the Earth

 

Notes:

Rotating frame.

This is nearly equivalent to the Earth Fixed reference frame, but does not include polar motion effects

 

 

Moon Fixed (Principal Axis Rotating)


Description:

The rotating Moon PA (Principal Axis) frame, as defined by the JPL planetary ephemeris.

Origin: Center of the Moon

 

Notes:

Rotating frame.

The Moon ME frame has a constant rotational offset with respect to the Moon PA frame. The rotational offset is defined in the JPL planetary ephemeris.

This frame is based on the lunar principal axes, and differs from the Moon ME frame by about 1 km at the lunar surface.

 

 

Moon Principle Axis - Inertial at J2000


Description:

The Moon PA frame evaluated at the J2000 epoch (Jan 01 2000 00:00:00.000). The frame is treated as stationary at the J2000 epoch and is not rotating.

Origin: Center of the Moon

 

Notes:

Inertial frame.

 

 

Moon Principle Axis - Inertial at Epoch


Description:

The Moon PA frame evaluated at a user-specified epoch. The frame is treated as stationary at the specified epoch and is not rotating.

Origin: Center of the Moon

 

Notes:

Inertial frame.

 

 

Moon Mean Earth - Rotating


Description:

The rotating Moon ME (Mean Earth) frame, as defined by the JPL planetary ephemeris.

Origin: Center of the Moon

 

Notes:

Rotating frame.

The Moon ME frame has a constant rotational offset with respect to the Moon PA frame. The rotational offset is defined in the JPL planetary ephemeris.

The Moon ME frame differs from the Moon PA frame by about 1 km at the lunar surface.

 

 

Moon Mean Earth - Inertial at J2000


Description:

The Moon ME frame evaluated at the J2000 epoch (Jan 01 2000 00:00:00.000). The frame is treated as stationary at the J2000 epoch and is not rotating.

Origin: Center of the Moon

 

Notes:

Inertial frame.

 

 

Moon Mean Earth - Inertial at Epoch


Description:

The Moon ME frame evaluated at a user-specified epoch. The frame is treated as stationary at the specified epoch and is not rotating.

Origin: Center of the Moon

 

Notes:

Inertial frame.

 

 

CelestialObject Body Fixed


Description:

The rotating CelestialObject body-fixed frame for the specified CelestialObject.

oFor the Earth, this maps to the Earth-Fixed frame.

oFor the Moon, this maps to the Moon-Fixed (Principal Axis rotating) frame.

oFor other CelestialObjects, this is defined based on the IAU/IAG Attitude model for that body. See the Modeling Solar System Objects section of the Solar System page for more information.

Origin: Center of the CelestialObject

 

Notes:

Rotating frame.

The identifier for this frame can be accessed using the syntax shown below. See the Celestial Object Reference Frame IDs summary for more information.

 

Mars.ReferenceFrameIDs.BodyFixed

 

 

CelestialObject Inertial Equatorial J2000


Description:

The CelestialObject body frame evaluated at the J2000 epoch (Jan 01 2000 00:00:00.000). The frame is treated as stationary at the J2000 epoch and is not rotating.

oFor the Earth, this maps to the MJ2000 Earth Equator frame.

oFor the Moon, this maps to the Moon PA - Inertial at J2000 frame.

oFor other CelestialObjects, this is defined based on the IAU/IAG Attitude model for that body.

Origin: Center of the CelestialObject

 

Notes:

Inertial frame.

The identifier for this frame can be accessed using the syntax shown below. See the Celestial Object Reference Frame IDs summary for more information.

 

Mars.ReferenceFrameIDs.InertialEquatorialJ2000

 

 

CelestialObject Inertial Equatorial At Epoch


Description:

The CelestialObject body frame evaluated at a user-specified epoch. The frame is treated as stationary at the specified epoch and is not rotating.

oFor the Earth, this frame is defined based on the Earth-Fixed frame, "inertialized" (not rotating) at the specified epoch.

oFor the Moon, this maps to the Moon PA - Inertial at Epoch frame.

oFor other CelestialObjects, this is defined based on the IAU/IAG Attitude model for that body.

Origin: Center of the CelestialObject

 

Notes:

Inertial frame.

The identifier for this frame can be accessed using the syntax shown below. See the Celestial Object Reference Frame IDs summary for more information.

 

Mars.ReferenceFrameIDs.InertialEquatorialAtEpoch

 

 

See Also


Orbit Element Types

Attitude Reference Frames

Central Body

CelestialObjects

oCelestial Object Reference Frame IDs

Solar System Modeling

Planetary Ephemerides

PositionConvert

VelocityConvert

PositionVelocityConvert