diff --git a/mode_s.c b/mode_s.c
index dbacfc6..fe68720 100644
--- a/mode_s.c
+++ b/mode_s.c
@@ -545,26 +545,9 @@ int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
mm->bFlags = 0;
- // CF (Control field)
- // done first so we can use it in AA.
- if (mm->msgtype == 18) {
- mm->cf = msg[0] & 7;
- }
-
// AA (Address announced)
if (mm->msgtype == 11 || mm->msgtype == 17 || mm->msgtype == 18) {
mm->addr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
-
- if (mm->msgtype == 18 && (mm->cf != 0 && mm->cf != 6))
- mm->addr |= MODES_NON_ICAO_ADDRESS; // don't confuse this with any ICAO address
-
- if (!mm->correctedbits && (mm->msgtype != 11 || mm->iid == 0)) {
- // No CRC errors seen, and either it was an DF17/18 extended squitter
- // or a DF11 acquisition squitter with II = 0. We probably have the right address.
-
- // NB this is the only place that adds addresses!
- icaoFilterAdd(mm->addr);
- }
}
// AC (Altitude Code)
@@ -590,6 +573,11 @@ int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
// CC (Cross-link capability) not decoded
+ // CF (Control field)
+ if (mm->msgtype == 18) {
+ mm->cf = msg[0] & 7;
+ }
+
// DR (Downlink Request) not decoded
// FS (Flight Status)
@@ -624,12 +612,8 @@ int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
// ME (message, extended squitter)
if (mm->msgtype == 17 || // Extended squitter
- (mm->msgtype == 18 && // Extended squitter/non-transponder:
- (mm->cf == 0 || // ADS-B ES/NT devices that report the ICAO 24-bit address in the AA field
- mm->cf == 1 || // Reserved for ADS-B for ES/NT devices that use other addressing techniques in the AA field
- mm->cf == 5 || // TIS-B messages that relay ADS-B Messages using anonymous 24-bit addresses
- mm->cf == 6))) { // ADS-B rebroadcast using the same type codes and message formats as defined for DF = 17 ADS-B messages
- decodeExtendedSquitter(mm);
+ mm->msgtype == 18) { // Extended squitter/non-transponder:
+ decodeExtendedSquitter(mm);
}
// MV (message, ACAS) not decoded
@@ -645,6 +629,17 @@ int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
mm->bFlags |= MODES_ACFLAGS_AOG;
}
+ if (!mm->correctedbits && (mm->msgtype == 17 || mm->msgtype == 18 || (mm->msgtype != 11 || mm->iid == 0))) {
+ // No CRC errors seen, and either it was an DF17/18 extended squitter
+ // or a DF11 acquisition squitter with II = 0. We probably have the right address.
+
+ // We wait until here to do this as we may have needed to decode an ES to note
+ // the type of address in DF18 messages.
+
+ // NB this is the only place that adds addresses!
+ icaoFilterAdd(mm->addr);
+ }
+
// all done
return 0;
}
@@ -684,6 +679,39 @@ static void decodeExtendedSquitter(struct modesMessage *mm)
int metype = mm->metype = msg[4] >> 3; // Extended squitter message type
int mesub = mm->mesub = (metype == 29 ? ((msg[4]&6)>>1) : (msg[4] & 7)); // Extended squitter message subtype
+ int check_imf = 0;
+
+ // Check CF on DF18 to work out the format of the ES and whether we need to look for an IMF bit
+ if (mm->msgtype == 18) {
+ switch (mm->cf) {
+ case 0: // ADS-B ES/NT devices that report the ICAO 24-bit address in the AA field
+ break;
+
+ case 1: // Reserved for ADS-B for ES/NT devices that use other addressing techniques in the AA field
+ case 5: // TIS-B messages that relay ADS-B Messages using anonymous 24-bit addresses (format not explicitly defined, but it seems to follow DF17)
+ mm->addr |= MODES_NON_ICAO_ADDRESS;
+ break;
+
+ case 2: // Fine TIS-B message (formats are close enough to DF17 for our purposes)
+ case 6: // ADS-B rebroadcast using the same type codes and message formats as defined for DF = 17 ADS-B messages
+ check_imf = 1;
+ break;
+
+ case 3: // Coarse TIS-B airborne position and velocity.
+ // TODO: decode me.
+ // For now we only look at the IMF bit.
+ if (msg[4] & 0x80)
+ mm->addr |= MODES_NON_ICAO_ADDRESS;
+ return;
+
+ default: // All others, we don't know the format.
+ mm->addr |= MODES_NON_ICAO_ADDRESS; // assume non-ICAO
+ return;
+ }
+ }
+
+
+
switch (metype) {
case 1: case 2: case 3: case 4: {
// Aircraft Identification and Category
@@ -714,6 +742,9 @@ static void decodeExtendedSquitter(struct modesMessage *mm)
}
case 19: { // Airborne Velocity Message
+ if (check_imf && (msg[5] & 0x80))
+ mm->addr |= MODES_NON_ICAO_ADDRESS;
+
// Presumably airborne if we get an Airborne Velocity Message
mm->bFlags |= MODES_ACFLAGS_AOG_VALID;
@@ -788,6 +819,9 @@ static void decodeExtendedSquitter(struct modesMessage *mm)
// Ground position
int movement;
+ if (check_imf && (msg[6] & 0x08))
+ mm->addr |= MODES_NON_ICAO_ADDRESS;
+
mm->bFlags |= MODES_ACFLAGS_AOG_VALID | MODES_ACFLAGS_AOG;
mm->raw_latitude = ((msg[6] & 3) << 15) | (msg[7] << 7) | (msg[8] >> 1);
mm->raw_longitude = ((msg[8] & 1) << 16) | (msg[9] << 8) | (msg[10]);
@@ -813,6 +847,9 @@ static void decodeExtendedSquitter(struct modesMessage *mm)
case 20: case 21: case 22: { // Airborne position, GNSS HAE
int AC12Field = ((msg[5] << 4) | (msg[6] >> 4)) & 0x0FFF;
+ if (check_imf && (msg[4] & 0x01))
+ mm->addr |= MODES_NON_ICAO_ADDRESS;
+
mm->bFlags |= MODES_ACFLAGS_AOG_VALID;
if (metype != 0) {
@@ -866,6 +903,9 @@ static void decodeExtendedSquitter(struct modesMessage *mm)
mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID;
mm->modeA = decodeID13Field(ID13Field);
}
+
+ if (check_imf && (msg[10] & 0x01))
+ mm->addr |= MODES_NON_ICAO_ADDRESS;
}
break;
}
@@ -877,6 +917,8 @@ static void decodeExtendedSquitter(struct modesMessage *mm)
break;
case 31: // Aircraft Operational Status
+ if (check_imf && (msg[10] & 0x01))
+ mm->addr |= MODES_NON_ICAO_ADDRESS;
break;
default: