Don't use DF0/4/16/20 altitudes when DF17/18 altitudes are available.

The DF17/18 values are generally more trustworthy as they have full
CRC coverage. Errors in the CRC bits of a DF0/4/16/20 message can
result in the contained altitude being attributed to the wrong aircraft.

net_io.c

@@ -959,6 +959,7 @@ static char * appendStatsJson(char *p,
                       ",\"local_range\":%u"
                       ",\"local_speed\":%u"
                       ",\"filtered\":%u}"
+                      ",\"altitude_suppressed\":%u"
                       ",\"cpu\":{\"demod\":%llu,\"reader\":%llu,\"background\":%llu}"
                       ",\"tracks\":{\"all\":%u"
                       ",\"single_message\":%u}"
@@ -977,6 +978,7 @@ static char * appendStatsJson(char *p,
                       st->cpr_local_range_checks,
                       st->cpr_local_speed_checks,
                       st->cpr_filtered,
+                      st->suppressed_altitude_messages,
                       (unsigned long long)demod_cpu_millis,
                       (unsigned long long)reader_cpu_millis,
                       (unsigned long long)background_cpu_millis,

stats.c

@@ -151,6 +151,7 @@ void display_stats(struct stats *st) {
            st->cpr_local_speed_checks,
            st->cpr_filtered);
 
+    printf("%u non-ES altitude messages from ES-equipped aircraft ignored\n", st->suppressed_altitude_messages);
     printf("%u unique aircraft tracks\n", st->unique_aircraft);
     printf("%u aircraft tracks where only one message was seen\n", st->single_message_aircraft);
 
@@ -325,6 +326,8 @@ void add_stats(const struct stats *st1, const struct stats *st2, struct stats *t
     target->cpr_local_speed_checks = st1->cpr_local_speed_checks + st2->cpr_local_speed_checks;
     target->cpr_filtered = st1->cpr_filtered + st2->cpr_filtered;
 
+    target->suppressed_altitude_messages = st1->suppressed_altitude_messages + st2->suppressed_altitude_messages;
+
     // aircraft
     target->unique_aircraft = st1->unique_aircraft + st2->unique_aircraft;
     target->single_message_aircraft = st1->single_message_aircraft + st2->single_message_aircraft;

stats.h

@@ -114,6 +114,10 @@ struct stats {
     unsigned int cpr_local_receiver_relative;
     unsigned int cpr_filtered;
 
+    // number of altitude messages ignored because
+    // we had a recent DF17/18 altitude
+    unsigned int suppressed_altitude_messages;
+
     // aircraft:
     // total "new" aircraft (i.e. not seen in the last 30 or 300s)
     unsigned int unique_aircraft;

track.c

@@ -483,9 +483,22 @@ struct aircraft *trackUpdateFromMessage(struct modesMessage *mm)
             a->modeCcount   = 0;               //....zero the hit count
             a->modeACflags &= ~MODEAC_MSG_MODEC_HIT;
             }
-        a->altitude = mm->altitude;
+
-        a->modeC    = (mm->altitude + 49) / 100;
+        // If we received an altitude in a DF17/18 squitter recently, ignore
-        a->seenAltitude = now;
+        // DF0/4/16/20 altitudes as single-bit errors can attribute them to the wrong
+        // aircraft
+        if ((a->bFlags & MODES_ACFLAGS_ALTITUDE_VALID) &&
+            (now - a->seenAltitude) < 15000 &&
+            (a->bFlags & MODES_ACFLAGS_LATLON_VALID) &&
+            (now - a->seenLatLon) < 15000 &&
+            mm->msgtype != 17 &&
+            mm->msgtype != 18) {
+            Modes.stats_current.suppressed_altitude_messages++;
+        } else {
+            a->altitude = mm->altitude;
+            a->modeC    = (mm->altitude + 49) / 100;
+            a->seenAltitude = now;
+        }
     }
 
     // If a (new) SQUAWK has been received, copy it to the aircraft structure