/// <summary>
/// Ensure that registered delegates receive the FrameAvailable event.
/// </summary>
/// <param name="e">Event argument.</param>
private void OnFrameAvailable(ModeSFrameAvailableEventArgs e)
{
// If there are subscriber(s), raises event.
ModeSFrameAvailable?.Invoke(this, e);
}
/// <summary>
/// Process the data. Will be invoked after preamble validation.
/// </summary>
private void ProcessSample()
{
// Reset the frame buffer.
Array.Clear(_frameBuffer, 0, _frameBuffer.Length);
// Set the initial helper variables.
var frameBitLength = 0;
var lastMagnitude = 0f;
var lastAverageMagnitude = 0f;
// Get the actual pointer and skips the preamble.
var pointer = (_candidateFramePointer + PreambleBitLength) % _candidateFrameBuffer.Length;
// Go through on the candidate frame, and creates a final one.
for (var i = 0; i < LongFrameBitLength * 2; i++)
{
// Get the magnitude from the action pointer and steps forward.
var magnitude = (float)_candidateFrameBuffer[pointer];
pointer = (pointer + 1) % _candidateFrameBuffer.Length;
// If the bit is even, does other tasks.
if (i % 2 != 0)
{
// Calculate the new average of magnitude.
var averageMagnitude = (magnitude + lastMagnitude) * 0.5f;
// Apply correction, if the last average is higher than 0.
if (lastAverageMagnitude > 0f)
{
// Calculate correction.
var correction = -CorrectionFactor * (averageMagnitude - lastAverageMagnitude) /
averageMagnitude;
// If the correction is higher than 0, apply correction on the magnitude value,
// and calculate the new average.
if (correction > 0)
{
magnitude += correction;
averageMagnitude = (magnitude + lastMagnitude) * 0.5f;
}
}
// Save the average as "previous" for further usage.
lastAverageMagnitude = averageMagnitude;
// Decide about the value of the bit based on the magnitude.
var bit = lastMagnitude > magnitude ? 1 : 0;
// Get bit position from sample position.
var frameBitPosition = i / 2;
// Store the bit, if the bit's value is 1,
// Remark: previously the frame buffer was filled with zeros.
if (bit == 1)
{
var index = frameBitPosition / 8;
var shift = 7 - (frameBitPosition % 8);
_frameBuffer[index] += (byte)(1 << shift);
}
// Decide about the downlink format (DF), if the frame's bit position is 7.
if (frameBitPosition == 7)
{
// Return, if the first byte is zero (the frame is not valid).
if (_frameBuffer[0] == 0)
{
return;
}
// Get the downlink format of the frame.
var df = ModeSHelper.GetDownlinkFormat(_frameBuffer);
// Assign the good length to the frame.
switch (df)
{
// Short DFs.
case ModeSDownlinkFormats.DF0:
case ModeSDownlinkFormats.DF4:
case ModeSDownlinkFormats.DF5:
case ModeSDownlinkFormats.DF11:
frameBitLength = ShortFrameBitLength;
break;
// Long DFs.
case ModeSDownlinkFormats.DF16:
case ModeSDownlinkFormats.DF17:
case ModeSDownlinkFormats.DF18:
case ModeSDownlinkFormats.DF20:
case ModeSDownlinkFormats.DF21:
case ModeSDownlinkFormats.DF24:
frameBitLength = LongFrameBitLength;
break;
// Any other number is not valid.
default:
return;
}
}
// Approach the final bit of the frame.
if (frameBitLength > 0 &&
frameBitPosition == frameBitLength - 1)
{
// Get length of the frame in byte.
var frameByteLength = frameBitLength / 8;
// If the last three bits are 0, the frame is not valid.
if (_frameBuffer[frameByteLength - 1] == 0 &&
_frameBuffer[frameByteLength - 2] == 0 &&
_frameBuffer[frameByteLength - 3] == 0)
{
return;
}
// Create the final frame buffer with valid length.
var finalFrameBuffer = _frameBuffer.Take(frameByteLength).ToArray();
// Local variable to store ICAO of the frame.
uint icao;
// Check the parity.
var df = ModeSHelper.GetDownlinkFormat(finalFrameBuffer);
switch (df)
{
// Check the frames which have Parity/Interrogator.
case ModeSDownlinkFormats.DF11:
case ModeSDownlinkFormats.DF17:
case ModeSDownlinkFormats.DF18:
// The syndrome must be zero, this means that the frame is valid.
if (ModeSParity.Syndrome(finalFrameBuffer) != 0)
{
return;
}
// Get the ICAO from the frame.
icao = ModeSHelper.GetICAOAddress(finalFrameBuffer);
// Break.
break;
// Check the frames which have Address/Parity.
case ModeSDownlinkFormats.DF0:
case ModeSDownlinkFormats.DF4:
case ModeSDownlinkFormats.DF5:
case ModeSDownlinkFormats.DF16:
case ModeSDownlinkFormats.DF20:
case ModeSDownlinkFormats.DF21:
case ModeSDownlinkFormats.DF24:
// Get the ICAO from the Address/Parity.
icao = ModeSHelper.GetICAOAddress(finalFrameBuffer);
// If this ICAO is in the trusted list, we assume that the frame is valid.
if (!_trustedICAOList.ContainsKey(icao))
{
return;
}
// Break.
break;
// Invalid DF.
default:
return;
}
// Return, if the the ICAO is not trusted and the candidate's confidence is not enough.
if (!_trustedICAOList.ContainsKey(icao) &&
!GetCandidateICAOConfidence(icao))
{
UpdateCandidateICAOList(icao);
return;
}
// Update trusted ICAOs "last seen" value and remove the ICAO from the candidate list.
_trustedICAOList[icao] = DateTime.UtcNow;
_candidateICAOList.TryRemove(icao, out _);
// Raise the ModeSFrameAvailable event.
var args = new ModeSFrameAvailableEventArgs(
new ModeSRawFrame(finalFrameBuffer, icao, df));
OnFrameAvailable(args);
// Return.
return;
}
}
// Save the current magnitude as "previous" for the future.
lastMagnitude = magnitude;
}
}
