EKF: Improve protection against badly conditioned dVel bias covariances

Reduces likelihood of Z delta velocity bias learning to wrong value.

EKF/covariance.cpp

@@ -769,12 +769,12 @@ void Ekf::fixCovarianceErrors()
 
 		// To ensure stability of the covariance matrix operations, the ratio of a max and min variance must
 		// not exceed 100 and the minimum variance must not fall below the target minimum
-		// Also limit variance to a maximum equivalent to a 1g uncertainty
-		const float minStateVarTarget = 1E-8f;
+		// Also limit variance to a maximum equivalent to a 0.1g uncertainty
+		const float minStateVarTarget = 5E-8f;
 		float minAllowedStateVar = fmaxf(0.01f * maxStateVar, minStateVarTarget);
 
 		for (uint8_t stateIndex = 13; stateIndex <= 15; stateIndex++) {
-			P[stateIndex][stateIndex] = math::constrain(P[stateIndex][stateIndex], minAllowedStateVar, sq(CONSTANTS_ONE_G * _dt_ekf_avg));
+			P[stateIndex][stateIndex] = math::constrain(P[stateIndex][stateIndex], minAllowedStateVar, sq(0.1f * CONSTANTS_ONE_G * _dt_ekf_avg));
 		}
 
 		// If any one axis has fallen below the safe minimum, all delta velocity covariance terms must be reset to zero