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OEM — Orbit Ephemeris Message

Parses and writes CCSDS Orbit Ephemeris Messages containing time-ordered state vectors with optional covariance data.

OEM 

OEM(originator: Any, format_version: Any = 3.0, classification: Any = None)

Python wrapper for CCSDS Orbit Ephemeris Message (OEM).

OEM messages contain time-ordered sequences of state vectors (position and velocity), optionally with accelerations and covariance matrices.

Header-level fields are properties with getters and setters. Segment data is accessed via the segments property which supports indexing, iteration, and mutation.

Example 
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from brahe.ccsds import OEM
from brahe import Epoch

# Parse existing
oem = OEM.from_file("ephemeris.oem")
print(oem.originator)
seg = oem.segments[0]
sv = seg.states[0]
print(sv.position)

# Construct from scratch
start = Epoch.from_datetime(2024, 1, 1, 0, 0, 0)
stop = Epoch.from_datetime(2024, 1, 1, 1, 0, 0)
oem = OEM(originator="MY_ORG")
oem.add_segment(
    object_name="SAT1", object_id="2024-001A",
    center_name="EARTH", ref_frame="GCRF", time_system="UTC",
    start_time=start, stop_time=stop,
)
oem.segments[0].add_state(
    epoch=start, position=[7000e3, 0, 0], velocity=[0, 7500, 0],
)

Initialize instance.

classification property 

classification: str

Classification of the message, or None.

Returns:

Name Type Description
str str

Classification string, or None

creation_date property 

creation_date: Epoch

Creation date of the message.

Returns:

Name Type Description
Epoch Epoch

Creation date

format_version property 

format_version: float

CCSDS format version.

Returns:

Name Type Description
float float

Format version

message_id property 

message_id: str

Message ID, or None.

Returns:

Name Type Description
str str

Message ID, or None

originator property 

originator: str

Originator of the message.

Returns:

Name Type Description
str str

Originator string

segments property 

segments: OEMSegments

Segment collection, supporting len/indexing/iteration.

Returns:

Name Type Description
OEMSegments OEMSegments

Collection of OEM segments

add_segment method descriptor 

add_segment(object_name: str, object_id: str, center_name: str, ref_frame: str, time_system: str, start_time: Epoch, stop_time: Epoch, interpolation: str | None = None, interpolation_degree: int | None = None, trajectory: OrbitTrajectory | None = None) -> int

Add a new segment to the OEM.

Parameters:

Name Type Description Default
object_name str

Spacecraft name

 required
object_id str

International designator

 required
center_name str

Central body name

 required
ref_frame str

Reference frame name

 required
time_system str

Time system name

 required
start_time Epoch

Start time of ephemeris data

 required
stop_time Epoch

Stop time of ephemeris data

 required
interpolation str | None

Interpolation method

 None
interpolation_degree int | None

Interpolation degree

 None
trajectory OrbitTrajectory | None

Optional trajectory to populate states from

 None

Returns:

Name Type Description
int int

Index of the new segment
Example 
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from brahe import Epoch
from brahe.ccsds import OEM
oem = OEM(originator="MY_ORG")
start = Epoch.from_datetime(2024, 1, 1, 0, 0, 0)
stop = Epoch.from_datetime(2024, 1, 1, 1, 0, 0)
idx = oem.add_segment(
    object_name="SAT1", object_id="2024-001A",
    center_name="EARTH", ref_frame="GCRF", time_system="UTC",
    start_time=start, stop_time=stop,
    trajectory=prop.trajectory,
)

from_file staticmethod 

from_file(path: str) -> OEM

Parse an OEM from a file, auto-detecting the format.

Parameters:

Name Type Description Default
path str

Path to the OEM file

 required

Returns:

Name Type Description
OEM OEM

Parsed OEM message

from_str staticmethod 

from_str(content: str) -> OEM

Parse an OEM from a string, auto-detecting the format (KVN, XML, or JSON).

Parameters:

Name Type Description Default
content str

String content of the OEM message

 required

Returns:

Name Type Description
OEM OEM

Parsed OEM message

remove_segment method descriptor 

remove_segment(idx: int) -> Any

Remove a segment by index.

Parameters:

Name Type Description Default
idx int

Index of the segment to remove

 required

segment_to_trajectory method descriptor 

segment_to_trajectory(segment_idx: int) -> SOrbitTrajectory

Convert a single OEM segment to an OrbitTrajectory.

The trajectory contains Cartesian state vectors (position/velocity) in the reference frame specified by the segment metadata.

Parameters:

Name Type Description Default
segment_idx int

Index of the segment to convert (0-based)

 required

Returns:

Name Type Description
OrbitalTrajectory SOrbitTrajectory

Trajectory containing the segment's state vectors

Raises:

Type Description
BraheError

If segment index is out of range or frame is unsupported
Example 
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from brahe.ccsds import OEM
oem = OEM.from_file("ephemeris.oem")
traj = oem.segment_to_trajectory(0)
print(f"States: {len(traj)}")

state method descriptor 

state(segment_idx: int, state_idx: int) -> OEMStateVector

Get a state vector from a segment by index (shortcut).

This is a convenience method equivalent to oem.segments[segment_idx].states[state_idx].

Parameters:

Name Type Description Default
segment_idx int

Segment index (0-based)

 required
state_idx int

State vector index (0-based)

 required

Returns:

Name Type Description
OEMStateVector OEMStateVector

State vector proxy

to_dict method descriptor 

to_dict() -> dict

Convert the OEM to a Python dictionary.

Epochs are serialized as CCSDS datetime strings for JSON/dict compatibility.

Returns:

Name Type Description
dict dict

Dictionary representation of the OEM

to_file method descriptor 

to_file(path: str, format: str) -> Any

Write the OEM to a file in the specified format.

Parameters:

Name Type Description Default
path str

Output file path

 required
format str

Output format - "KVN", "XML", or "JSON"

 required

to_json_string method descriptor 

to_json_string(uppercase_keys: bool = False) -> str

Write the OEM to JSON with explicit key case control.

Parameters:

Name Type Description Default
uppercase_keys bool

If True, use uppercase CCSDS keywords. Default: False.

 False

Returns:

Name Type Description
str str

Serialized JSON string

to_string method descriptor 

to_string(format: str) -> str

Write the OEM to a string in the specified format.

Parameters:

Name Type Description Default
format str

Output format - "KVN", "XML", or "JSON"

 required

Returns:

Name Type Description
str str

Serialized OEM string

to_trajectories method descriptor 

to_trajectories() -> list[OrbitalTrajectory]

Convert all OEM segments to OrbitTrajectory objects.

Returns one trajectory per segment, each containing the segment's state vectors in its reference frame.

Returns:

Type Description
list[OrbitalTrajectory]

list[OrbitalTrajectory]: One trajectory per segment

Raises:

Type Description
BraheError

If any segment's frame is unsupported
Example 
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from brahe.ccsds import OEM
oem = OEM.from_file("multi_segment.oem")
trajs = oem.to_trajectories()
for t in trajs:
    print(f"{t.name}: {len(t)} states")

See Also