diff --git a/libraries/AP_GPS/AP_GPS.cpp b/libraries/AP_GPS/AP_GPS.cpp
index 311e742460..f5c9552053 100644
--- a/libraries/AP_GPS/AP_GPS.cpp
+++ b/libraries/AP_GPS/AP_GPS.cpp
@@ -428,12 +428,11 @@ AP_GPS::setHIL(uint8_t instance, GPS_Status _status, uint64_t time_epoch_ms,
     istate.location = _location;
     istate.location.options = 0;
     istate.velocity = _velocity;
-    istate.have_vertical_velocity = true;
     istate.ground_speed = pythagorous2(istate.velocity.x, istate.velocity.y);
     istate.ground_course_cd = degrees(atan2f(istate.velocity.y, istate.velocity.x)) * 100UL;
     istate.hdop = hdop;
     istate.num_sats = _num_sats;
-    istate.have_vertical_velocity = _have_vertical_velocity;
+    istate.have_vertical_velocity |= _have_vertical_velocity;
     istate.last_gps_time_ms = tnow;
     uint64_t gps_time_ms = time_epoch_ms - (17000ULL*86400ULL + 52*10*7000ULL*86400ULL - 15000ULL);
     istate.time_week     = gps_time_ms / (86400*7*(uint64_t)1000);