asteRisk (version 1.4.3)

GCRFtoITRF: Convert coordinates from GCRF to ITRF

Description

The GCRF (Geocentric Celestial Reference Frame) frame of reference is an Earth-centered inertial coordinate frame, where the origin is placed at the center of mass of Earth and the coordinate frame is fixed with respect to the stars (and therefore not fixed with respect to the Earth surface in its rotation). The X-axis is aligned with the mean equinox of Earth at 12:00 Terrestrial Time on the 1st of January, 2000, and the Z-axis is aligned with the Earth´s rotation axis.

It is almost equivalent to the J2000 frame of reference (also called EME2000), and in some contexts it is also referred to as ICRF frame (although in its strict definition, the origin of coordinates is placed at the barycenter of the Solar System).

In the ITRF (International Terrestrial Reference Frame), the origin is also placed at the center of mass of Earth, but the frame rotates with respect to the stars to remain fixed with respect to the Earth surface as it rotates. The Z-axis extends along the true North as defined by the IERS reference pole, and the X-axis extends towards the intersection between the equator and the Greenwich meridian at any time.

The coordinates and velocities input and calculated with the high-precision orbital propagator (hpop) are in the GCRF frame of reference.

This function requires the asteRiskData package, which can be installed by running install.packages('asteRiskData', repos='https://rafael-ayala.github.io/drat/')

Usage

GCRFtoITRF(position_GCRF, velocity_GCRF, dateTime)

Value

A list with two elements representing the position and velocity of the satellite in the ITRF (International Terrestrial Reference Frame) frame of reference. Position values are in m, and velocity values are in m/s. Each of the two elements contains three values, corresponding to the X, Y and Z components of position and velocity in this order.

Arguments

position_GCRF

Vector with the X, Y and Z components of the position of an object in GCRF frame, in m.

velocity_GCRF

Vector with the X, Y and Z components of the velocity of an object in GCRF frame, in m/s.

dateTime

Date-time string with the date and time in UTC corresponding to the provided position and velocity vectors. This specifies the time for which the conversion from GCRF to ITRF coordinates will be performed. It is important to provide an accurate value, since the point over the surface of Earth to which a set of GCRF coordinates refers varies with time due to the motion of Earth.

References

https://celestrak.org/columns/v02n01/

Examples

if(requireNamespace("asteRiskData", quietly = TRUE)) {
# The following were the position and velocity of satellite MOLNIYA 1-83 
# the 25th of June, 2006 at 00:33:43 UTC in the GCRF frame (in m and m/s).

position_GCRF <- c(-14471729.582, -4677558.558, 9369.461)
velocity_GCRF <- c(-3251.691, -3276.008, 4009.228)

# Let´s convert them into the ITRF frame

coordinates_ITRF <- GCRFtoITRF(position_GCRF, velocity_GCRF, "2006-06-27 00:58:29.34")
}