Copter: increase sonar timeout to 1sec

Contributed by Julien Dubois.

ArduCopter/Attitude.pde

@@ -234,7 +234,7 @@ static float get_throttle_surface_tracking(int16_t target_rate, float current_al
     uint32_t now = millis();
 
     // reset target altitude if this controller has just been engaged
-    if( now - last_call_ms > 200 ) {
+    if (now - last_call_ms > SONAR_TIMEOUT_MS) {
         target_sonar_alt = sonar_alt + current_alt_target - current_loc.alt;
     }
     last_call_ms = now;

ArduCopter/config.h

@@ -199,6 +199,10 @@
  # define THR_SURFACE_TRACKING_VELZ_MAX 150 // max vertical speed change while surface tracking with sonar
 #endif
 
+#ifndef SONAR_TIMEOUT_MS
+ # define SONAR_TIMEOUT_MS  1000            // desired sonar alt will reset to current sonar alt after this many milliseconds without a good sonar alt
+#endif
+
 //////////////////////////////////////////////////////////////////////////////
 // Channel 7 and 8 default options
 //