Groundwork to track "reliable" messages & aircraft.

A "reliable" message is a DF17 or DF11 with good CRC.
A "reliable" aircraft is one that's probably real - we decide it's reliable when
we've seen enough reliable messages, or just enough messages in total, that it's unlikely
to be noise.

dump1090.h

@@ -410,6 +410,7 @@ struct modesMessage {
     int           remote;                         // If set this message is from a remote station
     double        signalLevel;                    // RSSI, in the range [0..1], as a fraction of full-scale power
     int           score;                          // Scoring from scoreModesMessage, if used
+    int           reliable;                       // is this a "reliable" message (uncorrected DF11/DF17/DF18)?
 
     datasource_t  source;                         // Characterizes the overall message source
 

mode_s.c

@@ -465,6 +465,7 @@ int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
             }
         }
         mm->source = SOURCE_MODE_S_CHECKED;
+        mm->reliable = (mm->IID == 0 && mm->correctedbits == 0);
         break;
 
     case 17:   // Extended squitter
@@ -493,6 +494,7 @@ int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
         }
 
         mm->source = SOURCE_ADSB; // TIS-B decoding will override this if needed
+        mm->reliable = (mm->correctedbits == 0);
         break;
     }
 
@@ -1442,6 +1444,8 @@ static void decodeExtendedSquitter(struct modesMessage *mm)
         break;
 
     default:
+        // Dubious.
+        mm->reliable = 0;
         break;
     }
 }
@@ -1878,6 +1882,9 @@ void displayModesMessage(struct modesMessage *mm) {
                    mm->metype);
         }
     }
+    if (mm->reliable) {
+        printf(" (reliable)");
+    }
     printf("\n");
 
     if (mm->msgtype == 20 || mm->msgtype == 21) {

track.c

@@ -927,6 +927,26 @@ struct aircraft *trackUpdateFromMessage(struct modesMessage *mm)
     a->seen      = messageNow();
     a->messages++;
 
+    // count reliable messages we receive; use them as a metric to
+    // decide when this is a real aircraft, not noise
+    if (mm->msgtype == 11 && mm->reliable) {
+        ++a->reliableDF11;
+    }
+
+    if (mm->msgtype == 17 && mm->reliable) {
+        ++a->reliableDF17;
+    }
+
+    if (a->reliableDF11 >= TRACK_RELIABLE_DF11_MESSAGES || a->reliableDF17 >= TRACK_RELIABLE_DF17_MESSAGES || a->messages >= TRACK_RELIABLE_ANY_MESSAGES) {
+        a->reliable = 1;
+    }
+
+    if (!mm->reliable && !a->reliable) {
+        // no further update from this message as we don't trust it
+        ++a->discarded;
+        return a;
+    }
+
     // update addrtype, we only ever go towards "more direct" types
     if (mm->addrtype < a->addrtype)
         a->addrtype = mm->addrtype;
@@ -1282,8 +1302,7 @@ static void trackRemoveStaleAircraft(uint64_t now)
     struct aircraft *prev = NULL;
 
     while(a) {
-        if ((now - a->seen) > TRACK_AIRCRAFT_TTL ||
-            (a->messages == 1 && (now - a->seen) > TRACK_AIRCRAFT_ONEHIT_TTL)) {
+        if ((now - a->seen) > TRACK_AIRCRAFT_TTL || (!a->reliable && (now - a->seen) > TRACK_AIRCRAFT_UNRELIABLE_TTL)) {
             // Count aircraft where we saw only one message before reaping them.
             // These are likely to be due to messages with bad addresses.
             if (a->messages == 1)

track.h

@@ -50,11 +50,11 @@
 #ifndef DUMP1090_TRACK_H
 #define DUMP1090_TRACK_H
 
-/* Maximum age of tracked aircraft in milliseconds */
+/* Maximum age of a reliable tracked aircraft in milliseconds */
 #define TRACK_AIRCRAFT_TTL 300000
 
-/* Maximum age of a tracked aircraft with only 1 message received, in milliseconds */
-#define TRACK_AIRCRAFT_ONEHIT_TTL 60000
+/* Maximum age of an unreliable tracked aircraft, in milliseconds */
+#define TRACK_AIRCRAFT_UNRELIABLE_TTL 60000
 
 /* Maximum validity of an aircraft position */
 #define TRACK_AIRCRAFT_POSITION_TTL 60000
@@ -64,6 +64,15 @@
  */
 #define TRACK_MODEAC_MIN_MESSAGES 4
 
+/* Minimum number of DF17 messages required to mark a track as reliable */
+#define TRACK_RELIABLE_DF17_MESSAGES 2
+
+/* Minimum number of DF11 messages required to mark a track as reliable */
+#define TRACK_RELIABLE_DF11_MESSAGES 3
+
+/* Minimum number of any sort of messages required to mark a track as reliable */
+#define TRACK_RELIABLE_ANY_MESSAGES 5
+
 /* Special value for Rc unknown */
 #define RC_UNKNOWN 0
 
@@ -89,6 +98,11 @@ struct aircraft {
     uint64_t      seen;           // Time (millis) at which the last packet was received
     long          messages;       // Number of Mode S messages received
 
+    int           reliable;       // Do we think this is a real aircraft, not noise?
+    long          reliableDF11;   // Number of "reliable" DF11s (no CRC errors corrected, IID = 0) received
+    long          reliableDF17;   // Number of "reliable" DF17s (no CRC errors corrected) received
+    long          discarded;      // Number of messages discarded as possibly-noise
+
     double        signalLevel[8]; // Last 8 Signal Amplitudes
     int           signalNext;     // next index of signalLevel to use