AP_NavEKF: Prevent Z accel bias changing during ground effect takeoff

The inconsistent baro data during ground effect takeoff combined with the larger variances in the Z accel bias state early in flight can cause unwanted changes in bias estimate and therefore changes in height estimation error.
This patch turns of the process noise and state updates for the Z accel bias state when takeoff in ground effect is expected.

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -1254,9 +1254,13 @@ void NavEKF::CovariancePrediction()
         }
     }
     // scale accel bias noise when disarmed to allow for faster bias estimation
-    processNoise[13] = dVelBiasSigma;
-    if (!vehicleArmed) {
-        processNoise[13] *= accelBiasNoiseScaler;
+    // inhibit bias estimation during takeoff with ground effect to prevent bad bias learning
+    if (takeoffExpected) {
+        processNoise[13] = 0.0f;
+    } else if (!vehicleArmed) {
+        processNoise[13] = dVelBiasSigma * accelBiasNoiseScaler;
+    } else {
+        processNoise[13] = dVelBiasSigma;
     }
     for (uint8_t i=14; i<=15; i++) processNoise[i] = windVelSigma;
     for (uint8_t i=16; i<=18; i++) processNoise[i] = magEarthSigma;
@@ -2172,7 +2176,8 @@ void NavEKF::FuseVelPosNED()
                 // Only height and height rate observations are used to update z accel bias estimate
                 // Protect Kalman gain from ill-conditioning
                 // Don't update Z accel bias if off-level by greater than 60 degrees to avoid scale factor error effects
-                if ((obsIndex == 5 || obsIndex == 2) && prevTnb.c.z > 0.5f) {
+                // Don't update if we are taking off with ground effect
+                if ((obsIndex == 5 || obsIndex == 2) && prevTnb.c.z > 0.5f && !getTakeoffExpected()) {
                     Kfusion[13] = constrain_float(P[13][stateIndex]*SK,-1.0f,0.0f);
                 } else {
                     Kfusion[13] = 0.0f;