diff --git a/libraries/AP_NavEKF2/AP_NavEKF2_core.h b/libraries/AP_NavEKF2/AP_NavEKF2_core.h
index c79a283036..1295b8f2c5 100644
--- a/libraries/AP_NavEKF2/AP_NavEKF2_core.h
+++ b/libraries/AP_NavEKF2/AP_NavEKF2_core.h
@@ -726,7 +726,7 @@ private:
     float gpsPosAccuracy;           // estimated position accuracy in m returned by the GPS receiver
     uint32_t lastGpsVelFail_ms;     // time of last GPS vertical velocity consistency check fail
     uint32_t lastGpsAidBadTime_ms;  // time in msec gps aiding was last detected to be bad
-    float posDownAtTakeoff;         // flight vehicle vertical position at arming used as a reference point
+    float posDownAtTakeoff;         // flight vehicle vertical position sampled at transition from on-ground to in-air and used as a reference (m)
     bool useGpsVertVel;             // true if GPS vertical velocity should be used
     float yawResetAngle;            // Change in yaw angle due to last in-flight yaw reset in radians. A positive value means the yaw angle has increased.
     uint32_t lastYawReset_ms;       // System time at which the last yaw reset occurred. Returned by getLastYawResetAngle