💻Working with Data

Each OEM is made up of one or more segments of state and optional covariance data. The OrbitEphemerisMessage class provides iterables for both.

for segment in ephemeris:
    for state in segment:
        print(state.epoch, state.position, state.velocity)

        # Acceleration is not available in every OEM, but is supported
        # print(state.epoch, state.position, state.velocity, state.acceleration)

    for covariance in segment.covariances:
        print(covariance.epoch, covariance.matrix)

Epochs are represented as astropy.time.Time objects if the time system is supported by astropy or datetime otherwise. Positions, velocities, and accelerations are numpy arrays.

Address specific segments through the .segments attribute

segment = ephemeris.segments[0]
for state in segment:
    ...

Both the ephemeris and segment objects support interpolation of states. If the requested epoch is not within the useable date range specified in the OEM file headers, this action will raise an exception.

from astropy.time import Time

epoch = Time('2024-03-09T00:00:00.000', format='isot', scale='utc')

# Sample OEM directly
state_at_epoch = oem(epoch)

# Sample specific OEM segment
segment = oem.segments[0]
state_at_epoch = segment(epoch)

For easy iterative sampling with fixed step sizes, use the .steps method. The sampled states will span from the useable start time to the useable end time of the ephemeris, stepping according to the provided step_size in seconds. Individual segments also support the .steps method.

for state in ephemeris.steps(60):
    print(state.epoch, state.position, state.velocity)

It is also possible to retrieve a complete list of states and covariances through the .states and .covariances properties.

for state in ephemeris.states:
    print(state.epoch, state.position, state.velocity)

for covariance in ephemeris.covariances:
    print(covariance.epoch, covariance.matrix)

To read OEM metadata, access the .header attribute. The header acts as a dictionary and supports basic getting and setting operations. Similarly, segments expose metadata through the .metadata attribute. Both headers and metadata support the standard items attribute.

>>> print(oem.header['CCSDS_OEM_VERS'])
2.0

>>> print(oem.segments[0].metadata['CENTER_NAME'])
Earth