airborne CPR: allow bigger time interval for lower speeds

nogrind.sh

@@ -7,6 +7,7 @@ source /etc/default/test
 cp -f readsb /tmp/test123
 
 #gdb -ex=run --args /tmp/test123 --quiet $RECEIVER_OPTIONS $DECODER_OPTIONS $NET_OPTIONS $JSON_OPTIONS $@
+#gdb -batch -ex 'set confirm off' -ex 'handle SIGTERM nostop print pass' -ex 'handle SIGINT nostop print pass' -ex run -ex 'bt full' --args \
 /tmp/test123 --quiet $RECEIVER_OPTIONS $DECODER_OPTIONS $NET_OPTIONS $JSON_OPTIONS $@
 #sudo -u readsb ./readsb --quiet $RECEIVER_OPTIONS $DECODER_OPTIONS $NET_OPTIONS $JSON_OPTIONS --net-connector 127.0.0.1,50006,beast_in $@
 

track.c

@@ -79,6 +79,19 @@ static uint16_t simpleHash(uint64_t receiverId) {
  return simpleHash;
 }
 
+static int64_t cpr_global_airborne_max_elapsed(int64_t now, struct aircraft *a) {
+ if (trackDataAge(now, &a->gs_valid) > 10 * SECONDS) {
+ return 10 * SECONDS;
+ }
+ int speed = imax((int64_t) a->gs, 1);
+ // max time for 500 knots gs
+ int64_t ref = 19 * SECONDS; // empirically tested
+ int64_t ival = (ref * 500) / speed;
+ // never return more than 30 seconds
+ ival = imin(30 * SECONDS, ival);
+ return ival;
+}
+
 static float knots_to_meterpersecond = (1852.0 / 3600.0);
 
 static void calculateMessageRateGlobal(int64_t now) {
@@ -1247,8 +1260,8 @@ static void updatePosition(struct aircraft *a, struct modesMessage *mm, int64_t
  if (mm->source != SOURCE_MLAT)
  Modes.stats_current.cpr_airborne++;
 
- // Airborne: 10 seconds
- max_elapsed = 10000;
+ // Airborne: determine depending on speed, fallback 10 seconds
+ max_elapsed = cpr_global_airborne_max_elapsed(now, a);
  }
 
  // If we have enough recent data, try global CPR