> ## Documentation Index > Fetch the complete documentation index at: https://docs.valar.space/llms.txt > Use this file to discover all available pages before exploring further. # OEM (Orbit Ephemeris Message) > Working with CCSDS Orbit Ephemeris Message format in VALAR > For the complete documentation index, see [llms.txt](/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 a `sigmas` 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](https://ccsds.org/Pubs/502x0b3e1.pdf) guidelines. The following illustrates OEM fields and covariance components. It is not an import-ready KVN fixture: OEM KVN covariance uses lower-triangular numeric rows. Observations without covariance use the assumed uncertainty described above. ``` 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 ``` ## 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. 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