AP_AHRS: don't allow get_velocity_NED() and get_relative_position_NED() without EKF

this avoids some linking issues, plus the functions are inaccurate
without EKF

libraries/AP_AHRS/AP_AHRS.h

@@ -204,12 +204,12 @@ public:
     // return a ground velocity in meters/second, North/East/Down
     // order. This will only be accurate if have_inertial_nav() is
     // true 
-    virtual Vector3f get_velocity_NED(void) = 0;
+    virtual bool get_velocity_NED(Vector3f &vec) const { return false; }
 
     // return a position relative to home in meters, North/East/Down
     // order. This will only be accurate if have_inertial_nav() is
     // true 
-    virtual Vector3f get_relative_position_NED(void) = 0;
+    virtual bool get_relative_position_NED(Vector3f &vec) const { return false; }
 
     // return ground speed estimate in meters/second. Used by ground vehicles.
     float groundspeed(void) const {

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -913,32 +913,3 @@ void AP_AHRS_DCM::set_home(int32_t lat, int32_t lng, int32_t alt_cm)
     _home.alt = alt_cm;
 }
 
-// return a ground velocity in meters/second, North/East/Down order
-Vector3f AP_AHRS_DCM::get_velocity_NED(void)
-{
-    Vector2f xy = groundspeed_vector();
-    Vector3f vec(xy.x, xy.y, 0);
-
-    // if we have GPS lock and GPS has raw velocities then use GPS
-    // vertical velocity for z
-    if (_gps && _gps->status() >= GPS::GPS_OK_FIX_3D) {
-        vec.z = _gps->velocity_down();
-    } else {
-        // otherwise we have no choice but the baro climb rate
-        vec.z = - _baro.get_climb_rate();
-    }
-    return vec;
-}
-
-// return a relative position in meters, NED order This is only
-// approximate. Use the NavEKF version for accurate positioning
-Vector3f AP_AHRS_DCM::get_relative_position_NED(void)
-{
-    struct Location loc;
-    if (!get_position(loc)) {
-        // not available
-        return Vector3f(0,0,0);
-    }
-    Vector2f xy = location_diff(loc, _home);
-    return Vector3f(xy.x, xy.y, (_home.alt - loc.alt) * 1.0e-2f);
-}

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -79,9 +79,6 @@ public:
     bool            use_compass(void);
 
     void set_home(int32_t lat, int32_t lng, int32_t alt_cm);
-
-    Vector3f get_velocity_NED(void);
-    Vector3f get_relative_position_NED(void);
     void estimate_wind(void);
 
 private:

libraries/AP_AHRS/AP_AHRS_NavEKF.cpp

@@ -205,24 +205,25 @@ bool AP_AHRS_NavEKF::have_inertial_nav(void) const
     return using_EKF();
 }
 
-// return a ground velocity in meters/second, North/East/Down order
+// return a ground velocity in meters/second, North/East/Down
-Vector3f AP_AHRS_NavEKF::get_velocity_NED(void)
+// order. Must only be called if have_inertial_nav() is true
+bool AP_AHRS_NavEKF::get_velocity_NED(Vector3f &vec) const
 {
     if (using_EKF()) {
-        Vector3f vec;
         EKF.getVelNED(vec);
-        return vec;
+        return true;
     }
-    return AP_AHRS_DCM::get_velocity_NED();
+    return false;
 }
 
-Vector3f AP_AHRS_NavEKF::get_relative_position_NED(void)
+// return a relative ground position in meters/second, North/East/Down
+// order. Must only be called if have_inertial_nav() is true
+bool AP_AHRS_NavEKF::get_relative_position_NED(Vector3f &vec) const
 {
-    Vector3f ret;
+    if (using_EKF()) {
-    if (using_EKF() && EKF.getPosNED(ret)) {
+        return EKF.getPosNED(vec);
-        return ret;
     }
-    return AP_AHRS_DCM::get_relative_position_NED();
+    return false;
 }
 
 #endif // AP_AHRS_NAVEKF_AVAILABLE

libraries/AP_AHRS/AP_AHRS_NavEKF.h

@@ -89,8 +89,8 @@ public:
 
     bool have_inertial_nav(void) const;
 
-    Vector3f get_velocity_NED(void);
+    bool get_velocity_NED(Vector3f &vec) const;
-    Vector3f get_relative_position_NED(void);
+    bool get_relative_position_NED(Vector3f &vec) const;
 
     void set_ekf_use(bool setting) { _ekf_use.set(setting); }