Skip to main content
For the complete documentation index, see llms.txt.
The Orbit Ephemeris Message (OEM) is a standardized ASCII text format for exchanging spacecraft position and velocity data over time. Each OEM file contains a time-ordered series of state vectors (ephemeris points) that define a spacecraft’s trajectory, enabling precise orbit propagation and prediction.

​
Key Components

An OEM file contains several distinct sections:
  • Header: File-level metadata including format version, creation date, and originator information
  • Metadata: Orbital context information such as object identification, reference frame, time system, and interpolation specifications
  • Ephemeris Data: Time-ordered state vectors containing position (X, Y, Z) and velocity (VX, VY, VZ) at specific epochs
  • Covariance (Optional): Position and velocity uncertainty matrices for each ephemeris point

​
Critical Fields

​
Metadata Section

  • OBJECT_NAME / OBJECT_ID: Spacecraft identification
  • REF_FRAME: Coordinate reference frame (e.g., EME2000, ITRF2000)
  • TIME_SYSTEM: Time system used (e.g., UTC, TAI)
  • START_TIME / STOP_TIME: Temporal bounds of ephemeris data
  • INTERPOLATION: Method for interpolating between points (e.g., Hermite, Lagrange)
  • INTERPOLATION_DEGREE: Polynomial degree for interpolation (typically 7-8)

​
Ephemeris Data Format

Each line contains: Epoch X Y Z VX VY VZ
  • Epoch: UTC timestamp
  • X, Y, Z: Position components in kilometers
  • VX, VY, VZ: Velocity components in km/s

​
Common Use Cases

  • Conjunction Assessment: Sharing precise trajectories for collision avoidance analysis
  • Maneuver Planning: Baseline trajectories for delta-V calculations
  • Mission Coordination: Exchanging orbit information between control centers
  • Orbit Propagation: High-fidelity trajectory propagation using interpolation
  • Ground Track Visualization: Displaying spacecraft position over time
Complete definition of the OEM standard on CCSDS 502.0-B-3 guidelines.
Here is a sample OEM file in KVN format:

​
Interpolation Methods

OEM files support multiple interpolation methods for computing state vectors between ephemeris points:
  • Hermite: Uses both position and velocity for smooth interpolation (most common)
  • Lagrange: Position-only polynomial interpolation
  • Linear: Simple linear interpolation between points
The interpolation degree (typically 7-8) determines the polynomial order and affects accuracy versus computational cost.