tle/lib/tle.js

Two-line element set (TLE) data format parser

class TLE {

  /**
   * Two Line Element Set (TLE)
   * @constructor
   * @returns {TLE}
   */
  constructor() {

    // Line 0
    this.name = ''
    // Line 1
    this.number = 0
    this.class = 'U'
    this.id = '00 000 A'
    this.date = new Date()
    this.fdmm = 0.0
    this.sdmm = 0.0
    this.drag = 0.0
    this.ephemeris = 0
    this.esn = 0
    // Line 2
    this.inclination = 0.0
    this.ascension = 0.0
    this.eccentricity = 0
    this.perigee = 0.0
    this.anomaly = 0.0
    this.motion = 0.0
    this.revolution = 0

  }

  /**
   * Create a streaming TLE parser
   * @param {Object} [options]
   * @returns {TLE.Parser}
   */
  static createParser( options ) {
    return new TLE.Parser( options )
  }

  /**
   * Parse a TLE's string representation
   * @see TLE.parse()
   * @param {String} value
   * @returns {TLE}
   */
  static parse( value ) {
    return new TLE().parse( value )
  }

  /**
   * Create a TLE instance from JSON data
   * @param {String|Object} value
   * @returns {TLE}
   */
  static fromJSON( value ) {

    var tle = new TLE()
    var data = typeof value === 'string' ?
      JSON.parse( value ) : value

    var keys = Object.keys( data )

    for( var i = 0; i < keys.length; i++ ) {
      if( tle.hasOwnProperty( keys[i] ) ) {
        tle[ keys[i] ] = keys[i] === 'date' ?
          new Date( Date.parse( data[ keys[i] ] ) ) :
          data[ keys[i] ]
      }
    }

    return tle

  }

  /**
   * Parse a floating point number from TLE format
   * @param {String} value
   * @returns {Number}
   */
  static parseFloat( value ) {

    var pattern = /([-])?([\.\d]+)([+-]\d+)?/
    var match = null

    if( match = pattern.exec( value ) ) {
      var sign = match[1] === '-' ? -1 : 1
      var power = match[3] ? 'e'+match[3] : 'e0'
      var value = match[2]
      return sign * parseFloat( value + power )
    }

    return NaN

  }

  /**
   * Parse the floating point number for the radiation pressure drag coefficient
   * @param {String} value
   * @returns {Number}
   */
  static parseDrag( value ) {

    var pattern = /([-])?([\.\d]+)([+-]\d+)?/
    var match = null

    if( match = pattern.exec( value ) ) {
      var sign = match[1] === '-' ? -1 : 1
      var power = match[3] ? 'e'+match[3] : 'e0'
      var value = match[2].indexOf( '.' ) === -1 ?
        '0.' + match[2] : match[2]
      return sign * parseFloat( value + power )
    }

    return NaN

  }

  /**
   * Parse a date from TLE format
   * @param {String} value
   * @returns {Date} utc
   */
  static parseDate( value ) {

    value = ( value + '' )
      .replace( /^\s+|\s+$/, '' )

    var epoch = parseInt( value.substr( 0, 2 ), 10 )
    var days  = parseFloat( value.substr( 2 ) )

    var year = new Date().getFullYear()
    var currentEpoch = year % 100
    var century = year - currentEpoch

    year = ( epoch > currentEpoch + 1 ) ?
      century - 100 + epoch :
      century + epoch

    var day = Math.floor( days )
    var hours = 24 * ( days - day )
    var hour = Math.floor( hours )
    var minutes = 60 * ( hours - hour )
    var minute = Math.floor( minutes )
    var seconds = 60 * ( minutes - minute )
    var second = Math.floor( seconds )
    var millisecond = 1000 * ( seconds - second )

    var utc = Date.UTC(
      year, 0, day,
      hour, minute, second,
      millisecond
    )

    return new Date( utc )

  }

  /**
   * Calculate the checksum of a TLE's line
   * @param {String} line
   * @returns {Number} checksum
   */
  static check( line ) {

    var sum = 0

    line.substring( 0, 68 ).replace( /[\d-]/g, function( digit ) {
      sum += digit === '-' ? 1 : +digit
    })

    return sum % 10

  }

  /**
   * Trim excessive whitespace from beginning and end of a string
   * @param {String} str
   * @returns {String}
   */
  static trim( str ) {
    return str.replace( /^[\s\uFEFF\xA0]+|[\s\uFEFF\xA0]+$/g, '' )
  }

  /**
   * Parse a TLE's string representation
   * @param {String} value
   * @returns {TLE}
   */
  parse( value ) {

    var lines = TLE.trim( value + '' ).split( /\r?\n/g )
    var line, checksum

    // Line 0
    if (lines.length === 3) {
      this.name = TLE.trim( lines.shift() )
    }

    // Line 1
    line = lines.shift()
    checksum = TLE.check( line )

    if( checksum != line.substring( 68, 69 ) ) {
      throw new Error(
        'Line 1 checksum mismatch: ' + checksum + ' != ' +
        line.substring( 68, 69 ) + ': ' + line
      )
    }

    this.number = TLE.parseFloat( line.substring( 2, 7 ) )
    this.class = TLE.trim( line.substring( 7, 9 ) )
    this.id = TLE.trim( line.substring( 9, 18 ) )
    this.date = TLE.parseDate( line.substring( 18, 33 ) )
    this.fdmm = TLE.parseFloat( line.substring( 33, 44 ) )
    this.sdmm = TLE.parseFloat( line.substring( 44, 53 ) )
    this.drag = TLE.parseDrag( line.substring( 53, 62 ) )
    this.ephemeris = TLE.parseFloat( line.substring( 62, 64 ) )
    this.esn = TLE.parseFloat( line.substring( 64, 68 ) )

    // Line 2
    line = lines.shift()
    checksum = TLE.check( line )

    if( checksum != line.substring( 68, 69 ) ) {
      throw new Error(
        'Line 2 checksum mismatch: ' + checksum + ' != ' +
        line.substring( 68, 69 ) + ': ' + line
      )
    }

    this.inclination = TLE.parseFloat( line.substring( 8, 17 ) )
    this.ascension = TLE.parseFloat( line.substring( 17, 26 ) )
    this.eccentricity = TLE.parseFloat( '0.' + line.substring( 26, 34 ) )
    this.perigee = TLE.parseFloat( line.substring( 34, 43 ) )
    this.anomaly = TLE.parseFloat( line.substring( 43, 52 ) )
    this.motion = TLE.parseFloat( line.substring( 52, 63 ) )
    this.revolution = TLE.parseFloat( line.substring( 63, 68 ) )

    return this

  }

}

// Export
module.exports = TLE

TLE.Parser = require( './stream' )