private static bool BruteForceAP(ref ModeSMessage message)
{
var aux = new byte[ModesLongMessageBytes];
int messageType = message.MessageType;
int messageBits = message.RawMessage.Length * 8;
if (messageType != 0 && messageType != 4 && messageType != 5 && messageType != 16 && messageType != 20 &&
messageType != 21 && messageType != 24)
{
return(false);
}
int lastByte = (messageBits / 8) - 1;
Array.Copy(message.RawMessage, aux, messageBits / 8);
// -- Compute the CRC of the message and XOR it with the AP field to recover the address.
// -- (ADDR xor CRC) xor CRC = ADDR
uint crc = Checksum(aux, messageBits);
aux[lastByte] ^= (byte)(crc & 0xff);
aux[lastByte - 1] ^= (byte)((crc >> 8) & 0xff);
aux[lastByte - 2] ^= (byte)((crc >> 16) & 0xff);
var address = (uint)(aux[lastByte] | (aux[lastByte - 1] << 8) | (aux[lastByte - 2] << 16));
if (!WasICAORecentlySeen(address))
{
return(false);
}
message.Aa1 = aux[lastByte - 2];
message.Aa2 = aux[lastByte - 1];
message.Aa3 = aux[lastByte];
return(true);
}
public static ModeSMessage DecodeMessage(byte[] message)
{
var result = new ModeSMessage();
// -- Get the message type first.
result.MessageType = message[0] >> 3; /* Downlink Format */
result.RawMessage = new byte[GetMessageLength(result.MessageType)];
result.Crc = Checksum1(message, result.RawMessage.Length * 8);
uint crc2 = Checksum(message, result.RawMessage.Length * 8);
Array.Copy(message, result.RawMessage, result.RawMessage.Length);
result.ErrorBit = -1;
result.IsCrcOk = (result.Crc == crc2);
// -- Error correction
if (!result.IsCrcOk && FixErrors && (result.MessageType == 11 || result.MessageType == 17))
{
result.ErrorBit = FixSingleBitErrors(ref result.RawMessage, result.RawMessage.Length * 8);
if (result.ErrorBit != -1)
{
result.Crc = Checksum(result.RawMessage, result.RawMessage.Length * 8);
result.IsCrcOk = true;
}
}
result.Ca = message[0] & 7; // -- Capabilities
result.Aa1 = message[1]; // -- ICAO address
result.Aa2 = message[2];
result.Aa3 = message[3];
result.ICAO = (uint)((result.Aa1 << 16) | (result.Aa2 << 8) | (result.Aa3));
// -- DF-17 Type
result.MessageTypeExtended = message[4] >> 3;
result.MessageSubType = message[4] & 7;
// -- Fields for DF4,5,20,21.
result.FlightStatus = message[0] & 7;
result.DownlinkRequest = message[1] >> 3 & 31;
result.UtilityMessage = ((message[1] & 7) << 3) | message[2] >> 5;
{ // -- squawk decoding.
int a, b, c, d;
a = ((message[3] & 0x80) >> 5) |
((message[2] & 0x02) >> 0) |
((message[2] & 0x08) >> 3);
b = ((message[3] & 0x02) << 1) |
((message[3] & 0x08) >> 2) |
((message[3] & 0x20) >> 5);
c = ((message[2] & 0x01) << 2) |
((message[2] & 0x04) >> 1) |
((message[2] & 0x10) >> 4);
d = ((message[3] & 0x01) << 2) |
((message[3] & 0x04) >> 1) |
((message[3] & 0x10) >> 4);
result.Identity = a * 1000 + b * 100 + c * 10 + d;
}
// -- DF 11 & 17; Populate ICAO Whitelist. For DFs with an AP field, try to decode it.
if (result.MessageType != 11 && result.MessageType != 17)
{
result.IsCrcOk = BruteForceAP(ref result);
}
else
{
// -- If this is DF11 or 17 and checksum is ok, we can add this to list of recently seen.
if (result.IsCrcOk && result.ErrorBit == -1)
{
var address = (uint)result.ICAO;
AddRecentlySeenICAOAddress(address);
}
}
// -- Decode 13 bit altitude for DF0,16,20
if (result.MessageType == 0 || result.MessageType == 4 || result.MessageType == 16 ||
result.MessageType == 20)
{
var altitudeUnit = 0;
result.Altitude = DecodeAC13Field(result.RawMessage, ref altitudeUnit);
result.Unit = altitudeUnit;
}
// -- Extended squitter specific stuff
if (result.MessageType == 17)
{
if (result.MessageTypeExtended >= 1 && result.MessageTypeExtended <= 4)
{
// -- Aircraft Flight Information
result.AircraftType = result.MessageTypeExtended - 1;
result.Flight = GetAircraftIdentifier(message);
}
else if (result.MessageTypeExtended >= 9 && result.MessageTypeExtended <= 18)
{
// -- Position message
var positionUnit = 0;
result.Fflag = message[6] & (1 << 2);
result.Tflag = message[6] & (1 << 3);
result.Altitude = DecodeAC12Field(message, ref positionUnit);
result.RawLatitude = ((message[6] & 3) << 15) |
(message[7] << 7) |
(message[8] >> 1);
result.RawLongitude = ((message[8] & 1) << 16) |
(message[9] << 8) |
message[10];
}
else if (result.MessageTypeExtended == 19 && result.MessageSubType >= 1 &&
result.MessageSubType <= 4)
{
// -- Velocity Message
}
else if (result.MessageSubType == 3 || result.MessageSubType == 4)
{
// -- Heading message.
result.HeadingIsValid = (message[5] & (1 << 2)) > 0;
result.Heading = (int)((360.0 / 128) * (((message[5] & 3) << 5) | (message[6] >> 3)));
}
}
result.IsPhaseCorrected = false;
return(result);
}
