AP_AHRS: implement get_velocity_NED() for DCM

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -1075,3 +1075,17 @@ bool AP_AHRS_DCM::healthy(void) const
     // consider ourselves healthy if there have been no failures for 5 seconds
     return (_last_failure_ms == 0 || AP_HAL::millis() - _last_failure_ms > 5000);
 }
+
+/*
+  return NED velocity if we have GPS lock
+ */
+bool AP_AHRS_DCM::get_velocity_NED(Vector3f &vec) const
+{
+    const AP_GPS &_gps = AP::gps();
+    if (_gps.status() < AP_GPS::GPS_OK_FIX_3D) {
+        return false;
+    }
+    vec = _gps.velocity();
+    return true;
+}
+

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -102,6 +102,8 @@ public:
     // is the AHRS subsystem healthy?
     bool healthy() const override;
 
+    bool get_velocity_NED(Vector3f &vec) const override;
+
 private:
     float _ki;
     float _ki_yaw;

libraries/AP_AHRS/AP_AHRS_NavEKF.cpp

@@ -643,7 +643,7 @@ bool AP_AHRS_NavEKF::get_velocity_NED(Vector3f &vec) const
 {
     switch (active_EKF_type()) {
     case EKF_TYPE_NONE:
-        return false;
+        return AP_AHRS_DCM::get_velocity_NED(vec);
 
     case EKF_TYPE2:
     default: