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    * Licensed to CS Systèmes d'Information (CS) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
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    * CS licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
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    *
    *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
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  package org.orekit.files.ccsds;
  
  /** Keywords for CCSDS Navigation Data Messages.<p>
   * Only these should be used.<p>
   *  The enumeration is split in 3 parts:<p>
   *  - Common NDM keywords;<p>
   *  - Orbit Data Messages (ODM) specific keywords;<p>
   *  - Tracking Data Messages (TDM) specific keywords.<p>
   * References:<p>
   * - <a href="https://public.ccsds.org/Pubs/502x0b2c1.pdf">CCSDS 502.0-B-2 recommended standard</a> ("Orbit Data Messages", Blue Book, Issue 2.0, November 2009).<p>
   * - <a href="https://public.ccsds.org/Pubs/503x0b1c1.pdf">CCSDS 503.0-B-1 recommended standard</a> ("Tracking Data Message", Blue Book, Issue 1.0, November 2007).
   * @author sports
   * @author Maxime Journot
   * @since 6.1
   */
  public enum Keyword {
  
      // ---------------------------------------------------
      // Common NDM (Navigation Data Message) CCSDS keywords
      // ---------------------------------------------------
  
      /** Comments specific to a ODM file. */
      COMMENT,
      /** CCSDS OEM format version. */
      CREATION_DATE,
      /** Creating agency or operator. */
      ORIGINATOR,
      /** Time system used for state vector, maneuver, and covariance data. */
      TIME_SYSTEM,
      /** Epoch of state vector and optional Keplerian elements.
       *  Or epoch of a TDM observation.
       */
      EPOCH,
      /** Start of total time span covered by: <p>
       * - Ephemerides data and covariance data;<p>
       * - Tracking data session.
       */
      START_TIME,
      /** End of total time span covered by: <p>
       * - Ephemerides data and covariance data;<p>
       * - Tracking data session.
       */
      STOP_TIME,
      /** User defined parameter, where X is replaced by a variable length user specified character
       *  string. Any number of user defined parameters may be included, if necessary to provide
       *  essential information that cannot be conveyed in COMMENT statements. */
      USER_DEFINED_X,
      /** Keyword used to delineate the start of a Meta-data block. */
      META_START,
      /** Keyword used to delineate the end of a Meta-data block. */
      META_STOP,
  
      // -------------------------------------------
      // Orbit Data Messages (ODM) specific keywords
      // -------------------------------------------
  
      /** CCSDS OPM format version. */
      CCSDS_OPM_VERS,
      /** CCSDS OMM format version. */
      CCSDS_OMM_VERS,
      /** File creation date in UTC. */
      CCSDS_OEM_VERS,
      /** Spacecraft name for which the orbit state is provided. */
      OBJECT_NAME,
      /** Object identifier of the object for which the orbit state is provided. */
      OBJECT_ID,
      /** Origin of reference frame. */
      CENTER_NAME,
      /** Name of the reference frame in which the state vector and optional Keplerian element data are given. */
      REF_FRAME,
      /** Epoch of reference frame, if not intrinsic to the definition of the reference frame. */
      REF_FRAME_EPOCH,
      /** Mean element theory. */
      MEAN_ELEMENT_THEORY,
      /** Position vector X-component. */
      X,
      /** Position vector Y-component. */
      Y,
      /** Position vector Z-component. */
      Z,
      /** Velocity vector X-component. */
      X_DOT,
      /** Velocity vector Y-component. */
     Y_DOT,
     /** Velocity vector Z-component. */
     Z_DOT,
     /** Orbit semi-major axis. */
     SEMI_MAJOR_AXIS,
     /** Mean Motion. */
     MEAN_MOTION,
     /** Orbit eccentricity. */
     ECCENTRICITY,
     /** Orbit inclination. */
     INCLINATION,
     /** Orbit right ascension of ascending node. */
     RA_OF_ASC_NODE,
     /** Orbit argument of pericenter. */
     ARG_OF_PERICENTER,
     /** Orbit true anomaly. */
     TRUE_ANOMALY,
     /** Orbit mean anomaly.*/
     MEAN_ANOMALY,
     /** Gravitational coefficient. */
     GM,
     /** Spacecraft mass. */
     MASS,
     /** Solar radiation pressure area. */
     SOLAR_RAD_AREA,
     /** Solar radiation pressure coefficient. */
     SOLAR_RAD_COEFF,
     /** Drag area. */
     DRAG_AREA,
     /** Drag coefficient. */
     DRAG_COEFF,
     /** Ephemeris type. */
     EPHEMERIS_TYPE,
     /** Classification type. */
     CLASSIFICATION_TYPE,
     /** NORAD catalogue number. */
     NORAD_CAT_ID,
     /** Element set number of the satellite. */
     ELEMENT_SET_NO,
     /** Revolution Number. */
     REV_AT_EPOCH,
     /** SGP/SGP4 drag-like coefficient. */
     BSTAR,
     /** First Time Derivative of the Mean Motion. */
     MEAN_MOTION_DOT,
     /** Second Time Derivative of the Mean Motion. */
     MEAN_MOTION_DDOT,
     /** Coordinate system for covariance matrix. Its value can either be RSW, RTN (both indicating
     /* "Radial, Transverse, Normal") or TNW. */
     COV_REF_FRAME,
     /** Covariance matrix [1, 1] element. */
     CX_X,
     /** Covariance matrix [2, 1] element. */
     CY_X,
     /** Covariance matrix [2, 2] element. */
     CY_Y,
     /** Covariance matrix [3, 1] element. */
     CZ_X,
     /** Covariance matrix [3, 2] element. */
     CZ_Y,
     /** Covariance matrix [3, 3] element. */
     CZ_Z,
     /** Covariance matrix [4, 1] element. */
     CX_DOT_X,
     /** Covariance matrix [4, 2] element. */
     CX_DOT_Y,
     /** Covariance matrix [4, 3] element. */
     CX_DOT_Z,
     /** Covariance matrix [4, 4] element. */
     CX_DOT_X_DOT,
     /** Covariance matrix [5, 1] element. */
     CY_DOT_X,
     /** Covariance matrix [5, 2] element. */
     CY_DOT_Y,
     /** Covariance matrix [5, 3] element. */
     CY_DOT_Z,
     /** Covariance matrix [5, 4] element. */
     CY_DOT_X_DOT,
     /** Covariance matrix [5, 5] element. */
     CY_DOT_Y_DOT,
     /** Covariance matrix [6, 1] element. */
     CZ_DOT_X,
     /** Covariance matrix [6, 2] element. */
     CZ_DOT_Y,
     /** Covariance matrix [6, 3] element. */
     CZ_DOT_Z,
     /** Covariance matrix [6, 4] element. */
     CZ_DOT_X_DOT,
     /** Covariance matrix [6, 5] element. */
     CZ_DOT_Y_DOT,
     /** Covariance matrix [6, 6] element. */
     CZ_DOT_Z_DOT,
     /** Epoch of ignition. */
     MAN_EPOCH_IGNITION,
     /** Maneuver duration (If = 0, impulsive maneuver). */
     MAN_DURATION,
     /** Mass change during maneuver (value is &lt; 0). */
     MAN_DELTA_MASS,
     /** Coordinate system for velocity increment vector. Its value can either be RSW, RTN (both
      * indicating "Radial, Transverse, Normal") or TNW. */
     MAN_REF_FRAME,
     /** First component of the velocity increment. */
     MAN_DV_1,
     /** Second component of the velocity increment. */
     MAN_DV_2,
     /** Third component of the velocity increment. */
     MAN_DV_3,
     /** This keyword must appear before the first line of the covariance matrix data. */
     COVARIANCE_START,
     /** Start of useable time span covered by ephemerides data, it may be
      * necessary to allow for proper interpolation. */
     USEABLE_START_TIME,
     /** End of useable time span covered by ephemerides data, it may be
      * necessary to allow for proper interpolation. */
     USEABLE_STOP_TIME,
     /** The interpolation method to be used. */
     INTERPOLATION,
     /** The interpolation degree. */
     INTERPOLATION_DEGREE,
     /** This keyword must appear after the last line of the covariance matrix data. */
     COVARIANCE_STOP,
 
 
     // ----------------------------------------------
     // Tracking Data Messages (TDM) specific keywords
     // ----------------------------------------------
 
     // TDM Header section
     // ------------------
 
     /** Header: TDM format version in the form of ‘x.y’, where ‘y’ shall be incremented for
      *  corrections and minor changes, and ‘x’ shall be incremented for major changes.
      *  <p>Obligatory: YES
      */
     CCSDS_TDM_VERS,
 
     // TDM meta-data section
     // ---------------------
 
     /** Meta-data: PARTICIPANT_n, n = {1, 2, 3, 4, 5}.<p>
      *  Participants in a tracking data sessions (spacecraft(s), ground station(s)...)
      * <p>Obligatory: YES (at least 1)
      */
     PARTICIPANT_1,
     /** Participant 2. */
     PARTICIPANT_2,
     /** Participant 3. */
     PARTICIPANT_3,
     /** Participant 4. */
     PARTICIPANT_4,
     /** Participant 5. */
     PARTICIPANT_5,
     /** Meta-data: Tracking mode associated with data section of the segment.<p>
      *  - SEQUENTIAL: Applies only for range, Doppler, angles, and LOS ionosphere calibrations.
      *                The name implies a sequential signal path between tracking participants;<p>
      *  - SINGLE_DIFF: Applies for differenced data;<p>
      *  - In other cases, such as troposphere, weather, clocks, etc., use of the MODE keyword does not apply.
      *  <p>Obligatory: NO
      */
     MODE,
     /** Meta-data: The PATH keywords shall reflect the signal path by listing the index of each PARTICIPANT
      * in order, separated by commas, with no inserted white space.<p>
      * The first entry in the PATH shall be the transmit participant.<p>
      * The non-indexed ‘PATH’ keyword shall be used if the MODE is SEQUENTIAL.<p>
      * The indexed ‘PATH_1’ and ‘PATH_2’ keywords shall be used where the MODE is SINGLE_DIFF.<p>
      * Examples:<p>
      *  - 1,2 = one-way;<p>
      *  - 2,1,2 = two-way;<p>
      *  - 3,2,1 = three-way;<p>
      *  - 1,2,3,4 = four-way;<p>
      *  - 1,2,3,2,1 = turn-around range with 1=primary station, 2=satellite, 3=secondary station.
      *  <p>Obligatory: NO
      */
     PATH,
     /** Path 1. */
     PATH_1,
     /** Path 2. */
     PATH_2,
     /** Frequency band for transmitted frequencies.
      * <p>Obligatory: NO
      */
     TRANSMIT_BAND,
     /** Meta-data: Frequency band for received frequencies.
      * <p>Obligatory: NO
      */
     RECEIVE_BAND,
     /** Meta-data: Turn-around ratio numerator. <p>
      *  Numerator of the turn-around ratio that is necessary to calculate the coherent downlink from the uplink frequency.
      *  <p>Obligatory: NO
      */
     TURNAROUND_NUMERATOR,
     /** Meta-data: Turn-around ratio denominator.
      * <p>Obligatory: NO
      */
     TURNAROUND_DENOMINATOR,
     /** Meta-data: Timetag reference.<p>
      *  Provides a reference for time tags in the tracking data.<p>
      *  It indicates whether the timetag associated with the data is the transmit time or the receive time.
      *  <p>Obligatory: NO
      */
     TIMETAG_REF,
     /** Meta-data: Integration interval.<p>
      *  Provides the Doppler count time in seconds for Doppler data or for the creation
      *  of normal points.
      *  <p>Obligatory: NO
      */
     INTEGRATION_INTERVAL,
     /** Meta-data: Integration reference.<p>
      *  Used in conjunction with timetag reference and integration interval.<p>
      *  Indicates whether the timetag represents the start, middle or end of the integration interval.
      *  <p>Obligatory: NO
      */
     INTEGRATION_REF,
 
     /** Meta-data: Frequency offset.<p>
      *  A frequency in Hz that must be added to every RECEIVE_FREQ to reconstruct it.
      *  <p>Obligatory: NO
      */
     FREQ_OFFSET,
     /** Meta-data: Range mode.<p>
      *  COHERENT, CONSTANT or ONE_WAY.
      *  <p>Obligatory: NO
      */
     RANGE_MODE,
     /** Meta-data: Range modulus.<p>
      *  Modulus of the range observable in the units as specified by the RANGE_UNITS keyword.
      *  <p>Obligatory: NO
      */
     RANGE_MODULUS,
     /** Meta-data: The RANGE_UNITS keyword specifies the units for the range observable.<p>
      * ‘km’ shall be used if the range is measured in kilometers.<p>
      * ‘s’ shall be used if the range is measured in seconds.<p>
      * 'RU' for "range units'
      * <p>Obligatory: NO
      */
     RANGE_UNITS,
     /** Meta-data: The ANGLE_TYPE keyword shall indicate the type of antenna geometry represented in the angle data (ANGLE_1 and ANGLE_2 keywords).<p>
      * The value shall be one of the following:<p>
      * - AZEL for azimuth, elevation (local horizontal);<p>
      * - RADEC for right ascension, declination or hour angle, declination (needs to be referenced to an inertial frame);<p>
      * - XEYN for x-east, y-north;<p>
      * - XSYE for x-south, y-east.
      * <p>Obligatory: NO
      */
     ANGLE_TYPE,
     /** Reference frame in which data are given: used in combination with ANGLE_TYPE=RADEC.
      * <p>Obligatory: NO
      */
     REFERENCE_FRAME,
     /** Meta-data: Transmit delays list (up to 5).<p>
      *  Specifies a fixed interval of time, in seconds, for the signal to travel from the transmitting
      *  electronics to the transmit point. Each item in the list corresponds to the each participants.
      *  <p>Obligatory: NO
      */
     TRANSMIT_DELAY_1,
     /** Second. */
     TRANSMIT_DELAY_2,
     /** Second. */
     TRANSMIT_DELAY_3,
     /** Second. */
     TRANSMIT_DELAY_4,
     /** Second. */
     TRANSMIT_DELAY_5,
     /** Meta-data: Receive delays list.<p>
      *  Specifies a fixed interval of time, in seconds, for the signal to travel from the tracking
      *  point to the receiving electronics. Each item in the list corresponds to the each participants.
      *  <p>Obligatory: NO
      */
     RECEIVE_DELAY_1,
     /** Second. */
     RECEIVE_DELAY_2,
     /** Second. */
     RECEIVE_DELAY_3,
     /** Second. */
     RECEIVE_DELAY_4,
     /** Second. */
     RECEIVE_DELAY_5,
     /** Meta-data: Data quality.<p>
      *  Estimate of the quality of the data: RAW, DEGRADED or VALIDATED.
      *  <p>Obligatory: NO
      */
     DATA_QUALITY,
     /** Meta-data: Correction angle 1.<p>
      *  Angle correction that has been added or should be added to the ANGLE_1 data.
      *  <p>Obligatory: NO
      */
     CORRECTION_ANGLE_1,
     /** Meta-data: Correction angle 2.<p>
      *  Angle correction that has been added or should be added to the ANGLE_2 data.
      *  <p>Obligatory: NO
      */
     CORRECTION_ANGLE_2,
     /** Meta-data: Correction Doppler.<p>
      *  Doppler correction that has been added or should be added to the DOPPLER data.
      *  <p>Obligatory: NO
      */
     CORRECTION_DOPPLER,
     /** Meta-data: Correction Range.<p>
      *  Range correction that has been added or should be added to the RANGE data.
      *  <p>Obligatory: NO
      */
     CORRECTION_RANGE,
     /** Meta-data: Correction receive.<p>
      *  Receive correction that has been added or should be added to the RECEIVE data.
      *  <p>Obligatory: NO
      */
     CORRECTION_RECEIVE,
     /** Meta-data: Correction transmit.<p>
      *  Transmit correction that has been added or should be added to the TRANSMIT data.
      *  <p>Obligatory: NO
      */
     CORRECTION_TRANSMIT,
     /** Meta-data: Correction applied ? YES/NO<p>
      *  Indicate whethers or not the values associated with the CORRECTION_* keywords have been
      *  applied to the tracking data.
      *  <p>Obligatory: NO
      */
     CORRECTIONS_APPLIED,
 
 
     // TDM Data section
     // ----------------
 
     // Signal related keywords.
     /** Data: Carrier power [dBW].<p>
      *  Strength of the radio signal transmitted by the spacecraft as received at the ground station or at another spacecraft.
      */
     CARRIER_POWER,
     /** Data: Doppler instantaneous [km/s].<p>
      *  Instantaneous range rate of the spacecraft.
      */
     DOPPLER_INSTANTANEOUS,
     /** Data: Doppler integrated [km/s].<p>
      *  Mean range rate of the spacecraft over the INTEGRATION_INTERVAL specified in the meta-data section.
      */
     DOPPLER_INTEGRATED,
     /** Data: Carrier power to noise spectral density ratio (Pc/No) [dBHz]. */
     PC_N0,
     /** Data: Ranging power to noise spectral density ratio (Pr/No) [dBHz]. */
     PR_N0,
     /** Data: Range value [km, s or RU].
      * @see #RANGE_UNITS
      */
     RANGE,
     /** Data: Received frequencies [Hz].<p>
      * The RECEIVE_FREQ keyword shall be used to indicate that the values represent measurements of the received frequency.<p>
      * The keyword is indexed to accommodate a scenario in which multiple downlinks are used.<p>
      * RECEIVE_FREQ_n (n = 1, 2, 3, 4, 5)
      */
     RECEIVE_FREQ_1,
     /** Received frequency 2. */
     RECEIVE_FREQ_2,
     /** Received frequency 3. */
     RECEIVE_FREQ_3,
     /** Received frequency 4. */
     RECEIVE_FREQ_4,
     /** Received frequency 5. */
     RECEIVE_FREQ_5,
     /** Data: Received frequency [Hz].<p>
      *  Case without an index; where the frequency cannot be associated with a particular participant.
      */
     RECEIVE_FREQ,
     /** Data: Transmitted frequencies [Hz].<p>
      * The TRANSMIT_FREQ keyword shall be used to indicate that the values represent measurements of a transmitted frequency, e.g., from an uplink operation.<p>
      * The TRANSMIT_FREQ keyword is indexed to accommodate scenarios in which multiple transmitters are used.<p>
      * TRANSMIT_FREQ_n (n = 1, 2, 3, 4, 5)
      */
     TRANSMIT_FREQ_1,
     /** Transmitted frequency 2. */
     TRANSMIT_FREQ_2,
     /** Transmitted frequency 3. */
     TRANSMIT_FREQ_3,
     /** Transmitted frequency 4. */
     TRANSMIT_FREQ_4,
     /** Transmitted frequency 5. */
     TRANSMIT_FREQ_5,
     /** Data: Transmitted frequencies rates [Hz/s].<p>
      * The value associated with the TRANSMIT_FREQ_RATE_n keyword is the linear rate of
      * change of the frequency TRANSMIT_FREQ_n starting at the timetag and continuing
      *  until the next TRANSMIT_FREQ_RATE_n timetag (or until the end of the data).<p>
      * TRANSMIT_FREQ_RATE_n (n = 1, 2, 3, 4, 5)
      */
     TRANSMIT_FREQ_RATE_1,
     /** Transmitted frequency rate 2. */
     TRANSMIT_FREQ_RATE_2,
     /** Transmitted frequency rate 3. */
     TRANSMIT_FREQ_RATE_3,
     /** Transmitted frequency rate 4. */
     TRANSMIT_FREQ_RATE_4,
     /** Transmitted frequency rate 5. */
     TRANSMIT_FREQ_RATE_5,
 
     // VLBI/Delta-DOR Related Keywords
     /** Data: DOR [s].<p>
      * the DOR keyword represents the range measured via PATH_2 minus the range measured via PATH_1.
      */
     DOR,
     /** Data: VLBI delay [s].<p>
      * The observable associated with the VLBI_DELAY keyword represents the time of signal
      * arrival via PATH_2 minus the time of signal arrival via PATH_1.
      */
     VLBI_DELAY,
 
     // Angle Related Keywords
     /** Data: ANGLE_1 in degrees and in [-180, +360[ [deg].<p>
      * The value assigned to the ANGLE_1 keyword represents the azimuth, right ascension, or ‘X’
      * angle of the measurement, depending on the value of the ANGLE_TYPE keyword.<p>
      * The angle measurement shall be a double precision value as follows: -180.0 &le; ANGLE_1 &lt; 360.0<p>
      * Units shall be degrees.<p>
      * See meta-data keyword ANGLE_TYPE for the definition of the angles.
      */
     ANGLE_1,
     /** Data: ANGLE_2 in degrees and in [-180, +360[ [deg].<p>
      * The value assigned to the ANGLE_2 keyword represents the elevation, declination, or ‘Y’
      * angle of the measurement, depending on the value of the ANGLE_TYPE keyword.<p>
      * The angle measurement shall be a double precision value as follows: -180.0 &le; ANGLE_2 &lt; 360.0.<p>
      * Units shall be degrees.<p>
      * See meta-data keyword ANGLE_TYPE for the definition of the angles.
      */
     ANGLE_2,
 
     // Time Related Keywords
     /** Data: Clock bias [s].<p>
      * The CLOCK_BIAS keyword can be used by the message recipient to adjust timetag
      * measurements by a specified amount with respect to a common reference.
      */
     CLOCK_BIAS,
     /** Data: Clock drift [s/s].<p>
      * The CLOCK_DRIFT keyword should be used to adjust timetag measurements by an amount that is a function of time with
      * respect to a common reference, normally UTC (as opposed to the CLOCK_BIAS, which is meant to be a constant adjustment).
      */
     CLOCK_DRIFT,
 
     // Media Related Keywords
     /** Data: STEC - Slant Total Electron Count [TECU].
      * The STEC keyword shall be used to convey the line of sight,
      * one way charged particle delay or total electron count (TEC) at the timetag associated with a
      * tracking measurement, which is calculated by integrating the electron density along the
      * propagation path (electrons/m2).
      */
     STEC,
     /** Data: TROPO DRY [m].<p>
      * Dry zenith delay through the troposphere measured at the timetag.
      */
     TROPO_DRY,
     /** Data: TROPO WET [m].<p>
      * Wet zenith delay through the troposphere measured at the timetag.
      */
     TROPO_WET,
 
     // Meteorological Related Keywords
     /** Data: Pressure [hPa].<p>
      * Atmospheric pressure observable as measured at the tracking participant.
      */
     PRESSURE,
     /** Data: Relative humidity [%].<p>
      * Relative humidity observable as measured at the tracking participant.
      */
     RHUMIDITY,
     /** Data: Temperature [K].<p>
      * Temperature observable as measured at the tracking participant.
      */
     TEMPERATURE,
 
     // Miscellaneous KEYVALUE keywords
     /** Keyword used to delineate the start of a Data block in Keyvalue files. */
     DATA_START,
     /** Keyword used to delineate the end of a Data block in Keyvalue files.. */
     DATA_STOP,
 
     // XML TDM start/end keywords
     /** TDM first keyword. */
     tdm,
     /** Header keyword. */
     header,
     /** Body keyword. */
     body,
     /** Segment keyword. */
     segment,
     /** Meta-data keyword. */
     metadata,
     /** Data keyword. */
     data,
     /** Observation keyword. */
     observation;
 }