Mlat synthetic message detection.

Notice synthetic mlat messages by looking for messages with a magic
timestamp value. If they arrive, tag the derived data as mlat-derived.

Don't include mlat-derived output in FATSV output to avoid loops.

debian/changelog

@@ -18,6 +18,9 @@ dump1090-mutability (1.15~dev) UNRELEASED; urgency=medium
   * Support DC filtering (quite expensive) via --dcfilter
   * Track age of heading/speed/altitude; use this when deciding what to emit
     in FATSV format
+  * Notice synthetic mlat messages by looking for messages with a magic
+    timestamp value. If they arrive, tag the derived data as mlat-derived.
+    Don't include mlat-derived output in FATSV output to avoid loops.
 
  -- Oliver Jowett <oliver@mutability.co.uk>  Thu, 19 Feb 2015 22:39:19 +0000
 

dump1090.h

@@ -160,6 +160,7 @@ typedef struct rtlsdr_dev rtlsdr_dev_t;
 #define MODES_ACFLAGS_LATLON_REL_OK  (1<<15) // Indicates it's OK to do a relative CPR
 #define MODES_ACFLAGS_REL_CPR_USED   (1<<16) // Lat/lon derived from relative CPR
 #define MODES_ACFLAGS_CATEGORY_VALID (1<<17) // Aircraft category is known
+#define MODES_ACFLAGS_FROM_MLAT      (1<<18) // Data was derived from multilateration
 
 #define MODES_ACFLAGS_LLEITHER_VALID (MODES_ACFLAGS_LLEVEN_VALID | MODES_ACFLAGS_LLODD_VALID)
 #define MODES_ACFLAGS_LLBOTH_VALID   (MODES_ACFLAGS_LLEVEN_VALID | MODES_ACFLAGS_LLODD_VALID)

mode_s.c

@@ -59,6 +59,11 @@
 //
 //
 
+/* A timestamp that indicates the data is synthetic, created from a
+ * multilateration result
+ */
+#define MAGIC_MLAT_TIMESTAMP 0xFF01A71A71A7ULL
+
 //=========================================================================
 //
 // Given the Downlink Format (DF) of the message, return the message length in bits.
@@ -563,6 +568,9 @@ int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
 
     mm->bFlags = 0;
 
+    if (mm->remote && mm->timestampMsg == MAGIC_MLAT_TIMESTAMP)
+        mm->bFlags |= MODES_ACFLAGS_FROM_MLAT;
+
     // AA (Address announced)
     if (mm->msgtype == 11 || mm->msgtype == 17 || mm->msgtype == 18) {
         mm->addr  = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);

net_io.c

@@ -1513,6 +1513,8 @@ static void writeFATSV()
 
         char *p, *end;
 
+        int flags;
+
         // skip non-ICAO
         if (a->addr & MODES_NON_ICAO_ADDRESS)
             continue;
@@ -1527,16 +1529,19 @@ static void writeFATSV()
 
         emittedAge = (now - a->fatsv_last_emitted);
 
-        if (a->bFlags & MODES_ACFLAGS_ALTITUDE_VALID) {
+        // ignore all mlat-derived data
+        flags = a->bFlags & ~a->mlatFlags;
+
+        if (flags & MODES_ACFLAGS_ALTITUDE_VALID) {
             alt = a->altitude;
             altAge = now - a->seenAltitude;
             altValid = (altAge <= 30000);
         }
         
-        if (a->bFlags & MODES_ACFLAGS_AOG_VALID) {
+        if (flags & MODES_ACFLAGS_AOG_VALID) {
             groundValid = 1;
 
-            if (a->bFlags & MODES_ACFLAGS_AOG) {
+            if (flags & MODES_ACFLAGS_AOG) {
                 // force zero altitude on ground
                 alt = 0;
                 altValid = 1;
@@ -1545,17 +1550,17 @@ static void writeFATSV()
             }
         }
 
-        if (a->bFlags & MODES_ACFLAGS_LATLON_VALID) {
+        if (flags & MODES_ACFLAGS_LATLON_VALID) {
             latlonAge = now - a->seenLatLon;
             latlonValid = (latlonAge <= 30000);
         }
 
-        if (a->bFlags & MODES_ACFLAGS_HEADING_VALID) {
+        if (flags & MODES_ACFLAGS_HEADING_VALID) {
             trackAge = now - a->seenTrack;
             trackValid = (trackAge <= 30000);
         }
 
-        if (a->bFlags & MODES_ACFLAGS_SPEED_VALID) {
+        if (flags & MODES_ACFLAGS_SPEED_VALID) {
             speedAge = now - a->seenSpeed;
             speedValid = (speedAge <= 30000);
         }
@@ -1604,7 +1609,7 @@ static void writeFATSV()
             p += snprintf(p, bufsize(p,end), "\tident\t%s", a->flight);
         }
 
-        if (a->bFlags & MODES_ACFLAGS_SQUAWK_VALID) {
+        if (flags & MODES_ACFLAGS_SQUAWK_VALID) {
             p += snprintf(p, bufsize(p,end), "\tsquawk\t%04x", a->modeA);
         }
 

track.c

@@ -535,6 +535,13 @@ struct aircraft *trackUpdateFromMessage(struct modesMessage *mm)
     // Update the aircrafts a->bFlags to reflect the newly received mm->bFlags;
     a->bFlags |= mm->bFlags;
 
+    // Update mlat flags. The mlat flags indicate which bits in bFlags
+    // were last set based on a mlat-derived message.
+    if (mm->bFlags & MODES_ACFLAGS_FROM_MLAT)
+        a->mlatFlags = (a->mlatFlags & a->bFlags) | mm->bFlags;
+    else
+        a->mlatFlags = (a->mlatFlags & a->bFlags) & ~mm->bFlags;
+
     if (mm->msgtype == 32) {
         int flags = a->modeACflags;
         if ((flags & (MODEAC_MSG_MODEC_HIT | MODEAC_MSG_MODEC_OLD)) == MODEAC_MSG_MODEC_OLD) {

track.h

@@ -75,6 +75,8 @@ struct aircraft {
     uint64_t      seenSpeed;      // Time (millis) at which speed was measured
     uint64_t      seenTrack;      // Time (millis) at which track was measured
 
+    int           mlatFlags;      // Data derived from mlat messages
+
     long          messages;       // Number of Mode S messages received
     int           modeA;          // Squawk
     int           modeC;          // Altitude