18.3. Other Debrief algorithms

18.3.1. Frequency algorithms

In the Single Sided Reconstruction extensions to Debrief (2009) the use of frequency data in track reconstruction, specifically involving the application of Doppler Offsets. The algorithms used have been loosely taken from the Royal Naval Submarine School training guide titled The Bearing Frequency Plot and Narrowband Analysis. The core doppler calculation is as follows:

	private static double calcDopplerComponent(final double theBearingRads,
      final double myCourseRads, 
      final double mySpeedKts,	final double observedFreq)
	{
		final double speedOfSoundKts = 2951;
		double relBearingRads = theBearingRads - myCourseRads;

		final double ownSpeedAlongKts = Math.abs(Math.cos(relBearingRads)
				* mySpeedKts);

		// put rel brg into +/- 180 domain
		while (relBearingRads > Math.PI)
			relBearingRads -= (2 * Math.PI);
		while (relBearingRads < -(Math.PI))
			relBearingRads += (2 * Math.PI);

		double dopplerOffset = (ownSpeedAlongKts * observedFreq) / speedOfSoundKts;

		if (Math.abs(relBearingRads) < (Math.PI / 2))
			dopplerOffset = -dopplerOffset;
		return dopplerOffset;
	}

The doppler calculation is used in calculation of the corrected frequency - which involves the removal of the ownship doppler component:

	public double getCorrectedFrequency()
	{
		double correctedFreq = 0;

		final double theBearingDegs = getBearingToTarget();
		final double theBearingRads = MWC.Algorithms.Conversions
				.Degs2Rads(theBearingDegs);
		final double myCourseRads = _hostFix.getCourse();

		final double mySpeedKts = _hostFix.getSpeed();
		double observedFreq = _sensor.getFrequency();
		final double dopplerComponent = calcDopplerComponent(theBearingRads,
				myCourseRads, mySpeedKts, observedFreq);

		correctedFreq = observedFreq + dopplerComponent;

		return correctedFreq;
	}

The next frequency calculation involves the addition of ownship and target dopplers to the base frequency:

	public double getPredictedFrequency()
	{
		double predictedFreq = 0;

		if (_targetTrack instanceof RelativeTMASegment)
		{
			RelativeTMASegment rt = (RelativeTMASegment) _targetTrack;
			final double theBearingDegs = getBearingToTarget();
			final double theBearingRads = MWC.Algorithms.Conversions
					.Degs2Rads(theBearingDegs);
			final double myCourseRads = _hostFix.getCourse();

			final double mySpeedKts = _hostFix.getSpeed();
			double baseFreq = rt.getBaseFrequency();
			final double myDopplerComponent = calcDopplerComponent(theBearingRads,
					myCourseRads, mySpeedKts, baseFreq);

			final double hisCourseRads = _targetFix.getCourse();
			final double hisSpeedKts = _targetFix.getSpeed();

			final double hisDopplerComponent = calcDopplerComponent(Math.PI
					+ theBearingRads, hisCourseRads, hisSpeedKts, baseFreq);

			predictedFreq = baseFreq - (myDopplerComponent + hisDopplerComponent);
		}
		return predictedFreq;
	}
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