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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.

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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

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Critical Fields

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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)

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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

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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: 
CCSDS_OEM_VERS             = 2.0
CREATION_DATE              = 2024-06-15T08:30:00.000
ORIGINATOR                 = VALAR

META_START
OBJECT_NAME                = SPACECRAFT-1
OBJECT_ID                  = 2023-001A
CENTER_NAME                = EARTH
REF_FRAME                  = EME2000
TIME_SYSTEM                = UTC
START_TIME                 = 2024-06-15T00:00:00.000
STOP_TIME                  = 2024-06-16T00:00:00.000
USEABLE_START_TIME         = 2024-06-15T00:00:00.000
USEABLE_STOP_TIME          = 2024-06-16T00:00:00.000
INTERPOLATION              = HERMITE
INTERPOLATION_DEGREE       = 7
META_STOP

COMMENT Ephemeris generated from OD solution
COMMENT State vectors in EME2000 reference frame

2024-06-15T00:00:00.000  6655.994 -40218.575 -82.918  3.115482  0.470426 -0.001015
2024-06-15T01:00:00.000  6892.441 -39845.123 -76.234  3.087234  0.501287 -0.000923
2024-06-15T02:00:00.000  7112.873 -39458.892 -69.445  3.058012  0.531456 -0.000831
2024-06-15T03:00:00.000  7316.234 -39060.234 -62.567  3.027856  0.560847 -0.000738
2024-06-15T04:00:00.000  7501.567 -38649.512 -55.612  2.996812  0.589382 -0.000645
2024-06-15T05:00:00.000  7668.012 -38227.123 -48.593  2.964923  0.616989 -0.000551
2024-06-15T06:00:00.000  7814.789 -37793.478 -41.523  2.932234  0.643598 -0.000457
2024-06-15T07:00:00.000  7941.234 -37349.012 -34.415  2.898789  0.669142 -0.000362
2024-06-15T08:00:00.000  8046.678 -36894.189 -27.281  2.864634  0.693559 -0.000267

COVARIANCE_START
EPOCH                      = 2024-06-15T00:00:00.000
COV_REF_FRAME              = EME2000
CX_X                       = 4.142e+01
CY_X                       = -8.579e+00
CY_Y                       = 5.189e+01
CZ_X                       = -4.788e+01
CZ_Y                       = -4.472e+01
CZ_Z                       = 5.960e+01
CX_DOT_X                   = 3.523e-03
CX_DOT_Y                   = 1.045e-03
CX_DOT_Z                   = -1.234e-03
CX_DOT_X_DOT               = 8.945e-07
CY_DOT_X                   = -2.678e-03
CY_DOT_Y                   = -5.234e-04
CY_DOT_Z                   = 1.876e-03
CY_DOT_X_DOT               = -3.456e-07
CY_DOT_Y_DOT               = 7.234e-07
CZ_DOT_X                   = -1.234e-03
CZ_DOT_Y                   = 2.345e-03
CZ_DOT_Z                   = -8.765e-04
CZ_DOT_X_DOT               = 2.134e-07
CZ_DOT_Y_DOT               = -4.567e-07
CZ_DOT_Z_DOT               = 6.789e-07
COVARIANCE_STOP

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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.