AP_NavEKF2: Reduce allowance for magnetometer timing errors

The previous gain from rate to magnetometer error was excessive. The revised value is equivalent to a magnetic field length of 0.5 with a timing uncertainty of 0.01 sec

libraries/AP_NavEKF2/AP_NavEKF2.cpp

@@ -389,7 +389,7 @@ NavEKF2::NavEKF2(const AP_AHRS *ahrs, AP_Baro &baro, const RangeFinder &rng) :
     hgtRetryTimeMode12_ms(5000),    // Height retry time without vertical velocity measurement (msec)
     tasRetryTime_ms(5000),          // True airspeed timeout and retry interval (msec)
     magFailTimeLimit_ms(10000),     // number of msec before a magnetometer failing innovation consistency checks is declared failed (msec)
-    magVarRateScale(0.05f),         // scale factor applied to magnetometer variance due to angular rate
+    magVarRateScale(0.005f),        // scale factor applied to magnetometer variance due to angular rate and measurement timing jitter. Assume timing jitter of 10msec
     gyroBiasNoiseScaler(2.0f),      // scale factor applied to imu gyro bias learning before the vehicle is armed
     hgtAvg_ms(100),                 // average number of msec between height measurements
     betaAvg_ms(100),                // average number of msec between synthetic sideslip measurements

libraries/AP_NavEKF2/AP_NavEKF2_MagFusion.cpp

@@ -236,7 +236,7 @@ void NavEKF2_core::FuseMagnetometer()
         MagPred[1] = DCM[1][0]*magN + DCM[1][1]*magE  + DCM[1][2]*magD + magYbias;
         MagPred[2] = DCM[2][0]*magN + DCM[2][1]*magE  + DCM[2][2]*magD + magZbias;
 
-        // scale magnetometer observation error with total angular rate
+        // scale magnetometer observation error with total angular rate to allow for timing errors
         R_MAG = sq(constrain_float(frontend._magNoise, 0.01f, 0.5f)) + sq(frontend.magVarRateScale*imuDataDelayed.delAng.length() / dtIMUavg);
 
         // calculate observation jacobians