SITL: get_wall_time_us() to use a monotonic clock on non-Windows systems

libraries/SITL/SIM_Aircraft.cpp

@@ -447,9 +447,9 @@ uint64_t Aircraft::get_wall_time_us() const
     tPrev = now;
     return last_ret_us;
 #else
-    struct timeval tp;
-    gettimeofday(&tp, nullptr);
-    return static_cast<uint64_t>(tp.tv_sec * 1.0e6 + tp.tv_usec);
+    struct timespec ts;
+    clock_gettime(CLOCK_MONOTONIC, &ts);
+    return uint64_t(ts.tv_sec * 1000000ULL + ts.tv_nsec / 1000ULL);
 #endif
 }
 

libraries/SITL/SIM_Aircraft.h

@@ -248,7 +248,7 @@ protected:
     /* add noise based on throttle level (from 0..1) */
     void add_noise(float throttle);
 
-    /* return wall clock time in microseconds since 1970 */
+    /* return a monotonic wall clock time in microseconds */
     uint64_t get_wall_time_us(void) const;
 
     // update attitude and relative position