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
VALAR Measurement Import Requirements
Covariance is optional. For OEM observations without covariance, VALAR uses assumed uncertainty of 100 m position and 0.1 m/s velocity, one sigma per source-frame axis. These compatibility defaults are not an accuracy estimate supplied by the producer. The import preview identifies the affected observation count; the completed import carries asigmas substitution warning, also retained in its job summary.
If the file supplies covariance, VALAR uses it for the matching state epoch. An omitted COV_REF_FRAME inherits the center-aware OEM state frame. Supported distinct absolute frames and frozen local orbital frames such as RTN/TNW are transformed into the stored state’s frame; unknown, body, and rotating local orbital frames are rejected. The assumed uncertainty is transformed with the state when the storage frame changes.
Supplied covariance must be finite, symmetric, and positive semidefinite, with strictly positive diagonal variances. Rank deficiency alone is not a rejection. VALAR does not interpolate covariance, pair matrices by list order, repair invalid variances, or replace invalid supplied covariance with defaults. Sparse covariance is used at its matching epochs; other observations use the disclosed assumptions.
For covariance-frame or variance errors, correct the supplied covariance and retry. Provide covariance when source-specific uncertainty is available: the compatibility assumptions do not establish the accuracy of the observations or the resulting orbit determination.
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.
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