diff --git a/libraries/AP_Baro/AP_Baro_HIL.cpp b/libraries/AP_Baro/AP_Baro_HIL.cpp
index 1ff685a86f..f1427c8726 100644
--- a/libraries/AP_Baro/AP_Baro_HIL.cpp
+++ b/libraries/AP_Baro/AP_Baro_HIL.cpp
@@ -31,11 +31,20 @@ uint8_t AP_Baro_HIL::read()
     return result;
 }
 
-void AP_Baro_HIL::setHIL(float _Temp, float _Press)
+void AP_Baro_HIL::setHIL(float altitude_msl)
 {
+    // approximate a barometer. This uses the typical base pressure in
+    // Canberra, Australia
+    const float Temp = 312;
+
+    float y = (altitude_msl - 584.0) / 29.271267;
+    y /= (Temp / 10.0) + 273.15;
+    y = 1.0/exp(y);
+    y *= 95446.0;
+
     _count++;
-    _pressure_sum += _Press;
-    _temperature_sum += _Temp;
+    _pressure_sum += y;
+    _temperature_sum += Temp;
     if (_count == 128) {
         // we have summed 128 values. This only happens
         // when we stop reading the barometer for a long time
diff --git a/libraries/AP_Baro/AP_Baro_HIL.h b/libraries/AP_Baro/AP_Baro_HIL.h
index 4e30f1081d..4e65919409 100644
--- a/libraries/AP_Baro/AP_Baro_HIL.h
+++ b/libraries/AP_Baro/AP_Baro_HIL.h
@@ -22,7 +22,7 @@ public:
     float get_temperature();
     int32_t get_raw_pressure();
     int32_t get_raw_temp();
-    void setHIL(float Temp, float Press);
+    void setHIL(float altitude_msl);
 };
 
 #endif //  __AP_BARO__HIL_H__