AP_NavEKF: Fix bug allowing takeoff in GPS modes without aiding

The legacy EKF switches GPs aiding on on arming, whereas the new EKF switches it on based on GPS data quality.
This means the decision to arm and therefore the predicted solution flags must now reflect the actual status of the navigation solution as it will no longer change when motor arming occurs.

libraries/AP_NavEKF2/AP_NavEKF2_Control.cpp

@@ -201,7 +201,7 @@ bool NavEKF2_core::optFlowDataPresent(void) const
 // return true if the filter to be ready to use gps
 bool NavEKF2_core::readyToUseGPS(void) const
 {
-    return validOrigin && tiltAlignComplete && yawAlignComplete && gpsQualGood;
+    return validOrigin && tiltAlignComplete && yawAlignComplete && gpsGoodToAlign;
 }
 
 // return true if we should use the compass

libraries/AP_NavEKF2/AP_NavEKF2_Measurements.cpp

@@ -490,7 +490,7 @@ void NavEKF2_core::readGpsData()
             // Monitor quality of the GPS velocity data before and after alignment using separate checks
             if (PV_AidingMode != AID_ABSOLUTE) {
                 // Pre-alignment checks
-                gpsQualGood = calcGpsGoodToAlign();
+                gpsGoodToAlign = calcGpsGoodToAlign();
             } else {
                 // Post-alignment checks
                 calcGpsGoodForFlight();
@@ -501,7 +501,7 @@ void NavEKF2_core::readGpsData()
             const struct Location &gpsloc = _ahrs->get_gps().location();
             if (validOrigin) {
                 gpsDataNew.pos = location_diff(EKF_origin, gpsloc);
-            } else if (gpsQualGood) {
+            } else if (gpsGoodToAlign) {
                 // Set the NE origin to the current GPS position
                 setOrigin();
                 // Now we know the location we have an estimate for the magnetic field declination and adjust the earth field accordingly

libraries/AP_NavEKF2/AP_NavEKF2_Outputs.cpp

@@ -396,7 +396,7 @@ void  NavEKF2_core::getFilterStatus(nav_filter_status &status) const
     bool someVertRefData = (!velTimeout && useGpsVertVel) || !hgtTimeout;
     bool someHorizRefData = !(velTimeout && posTimeout && tasTimeout) || doingFlowNav;
     bool optFlowNavPossible = flowDataValid && (frontend._fusionModeGPS == 3);
-    bool gpsNavPossible = !gpsNotAvailable && (frontend._fusionModeGPS <= 2);
+    bool gpsNavPossible = !gpsNotAvailable && (PV_AidingMode == AID_ABSOLUTE) && gpsGoodToAlign;
     bool filterHealthy = healthy() && tiltAlignComplete && yawAlignComplete;
 
     // set individual flags

libraries/AP_NavEKF2/AP_NavEKF2_core.cpp

@@ -167,7 +167,7 @@ void NavEKF2_core::InitialiseVariables()
     delAngCorrection.zero();
     delVelCorrection.zero();
     velCorrection.zero();
-    gpsQualGood = false;
+    gpsGoodToAlign = false;
     gpsNotAvailable = true;
     motorsArmed = false;
     prevMotorsArmed = false;

libraries/AP_NavEKF2/AP_NavEKF2_core.h

@@ -753,8 +753,8 @@ private:
     Vector3f delVelCorrection;      // correction applied to earth frame delta velocities used by output observer to track the EKF
     Vector3f velCorrection;         // correction applied to velocities used by the output observer to track the EKF
     float innovYaw;                 // compass yaw angle innovation (rad)
-    uint32_t timeTasReceived_ms;    // tie last TAS data was received (msec)
-    bool gpsQualGood;               // true when the GPS quality can be used to initialise the navigation system
+    uint32_t timeTasReceived_ms;    // time last TAS data was received (msec)
+    bool gpsGoodToAlign;            // true when the GPS quality can be used to initialise the navigation system
     uint32_t magYawResetTimer_ms;   // timer in msec used to track how long good magnetometer data is failing innovation consistency checks
     bool consistentMagData;         // true when the magnetometers are passing consistency checks
     bool motorsArmed;               // true when the motors have been armed