Reading Ephemeris Data

This tutorials will walk you through the basic features and interfaces that allow you to compute translation and orientation data from binary ephemeris kernels.

Computing state vectors

Ephemerides.jl allows the computation of a relative position between two points and its higher order derivatives up to order 3 (i.e., velocity, acceleration and jerk). All these computations are natively thread-safe and compatible with Automatic Differentiation (AD) with respect to time via ForwardDiff.jl.

In particular, the following methods are available to compute translation data:

ephem_vector3(eph, from, to, time)
ephem_vector6(eph, from, to, time)
ephem_vector9(eph, from, to, time)
ephem_vector12(eph, from, to, time)

They all share the same interface, requiring an EphemerisProvider object as the first input. from and to are integer numbers representing the ID of the center and target points that we desired. The time argument is expressed in TDB seconds since J2000.0.

An example to compute the position of the Moon (399) with respect to the Earth-Moon Barycenter (3) at J2000 (time = 0), is the following:

using Ephemerides

# Load an ephemeris kernel containing the requested data
kernel = "path_to_de440"
eph = EphemerisProvider(kernel)

# Compute the position
pos = ephem_vector3(eph, 3, 399, 0)

Instead, if one desires the whole state vector, up to the jerk components, the functions become:

# Compute position and velocity 
pv = ephem_vector6(eph, 3, 399, 0)

# Compute position, velocity and acceleration
pva = ephem_vector9(eph, 3, 399, 0)

# Compute position, velocity, acceleration and jerk
pvaj = ephem_vector12(eph, 3, 399, 0)

In all these examples, the returned data is always in the form of a StaticArray in order to minimise memory allocations.

Computing orientation angles

Similarly to position components, Ephemerides.jl also allows the computation of orientation angles and their derivatives up to order 3. All these computations are natively thread-safe and compatible with Automatic Differentiation (AD) with respect to time via ForwardDiff.jl.

In particular, the following methods are available to compute orientation data:

ephem_rotation3(eph, from, to, time)
ephem_rotation6(eph, from, to, time)
ephem_rotation9(eph, from, to, time)
ephem_rotation12(eph, from, to, time)

Again, they all share the same interface, requiring an EphemerisProvider object as the first input. from and to are integer numbers representing the ID of the reference and target axes that we desired. The time argument is expressed in TDB seconds since J2000.0.

An example to compute the Euler angles of the PA440 axes (31008) with respect to the ICRF (1) at J2000 (time = 0), is the following:

using Ephemerides

# Load an ephemeris kernel containing the requested data
kernel = "path_to_pa440"
eph = EphemerisProvider(kernel)

# Compute the position
ang = ephem_rotation3(eph, 1, 31008, 0)

Instead, if one desires the whole vector, up to the 3rd order derivative, the functions become:

# Compute angles and derivatives 
pv = ephem_rotation6(eph, 1, 31008, 0)
pva = ephem_rotation9(eph, 1, 31008, 0)
pvaj = ephem_rotation12(eph, 1, 31008, 0)

The returned orientation data is always in the form of a StaticArray in order to minimise memory allocations.