diff --git a/libraries/SITL/SIM_AirSim.cpp b/libraries/SITL/SIM_AirSim.cpp
index f6e575992a..bac76a8aae 100644
--- a/libraries/SITL/SIM_AirSim.cpp
+++ b/libraries/SITL/SIM_AirSim.cpp
@@ -262,8 +262,8 @@ void AirSim::recv_fdm()
 
     dcm.from_euler(state.pose.roll, state.pose.pitch, state.pose.yaw);
 
-    if (last_state.timestamp) {
-        double deltat = state.timestamp - last_state.timestamp;
+    if (last_timestamp) {
+        int deltat = state.timestamp - last_timestamp;
         time_now_us += deltat;
 
         if (deltat > 0 && deltat < 100000) {
@@ -312,7 +312,7 @@ void AirSim::recv_fdm()
                        velocity_ef.z);
 #endif
 
-    last_state = state;
+    last_timestamp = state.timestamp;
 }
 
 /*
diff --git a/libraries/SITL/SIM_AirSim.h b/libraries/SITL/SIM_AirSim.h
index 2555b6ec73..eebcfa9c26 100644
--- a/libraries/SITL/SIM_AirSim.h
+++ b/libraries/SITL/SIM_AirSim.h
@@ -66,6 +66,7 @@ private:
     double average_frame_time;
     uint64_t frame_counter;
     uint64_t last_frame_count;
+    uint64_t last_timestamp;
 
 	void send_servos(const struct sitl_input &input);
 	void recv_fdm();
@@ -107,7 +108,7 @@ private:
         struct {
             struct float_array rc_channels;
         } rc;
-    } state, last_state;
+    } state;
 
     // table to aid parsing of JSON sensor data
     struct keytable {