Copter: inertial nav replaced with AHRS

Author: lthall

ArduCopter/AP_Arming_Copter.cpp

@@ -133,8 +133,10 @@ bool AP_Arming_Copter::barometer_checks(bool display_failure)
         // that may differ from the baro height due to baro drift.
         const auto &ahrs = AP::ahrs();
         const bool using_baro_ref = !ahrs.has_status(AP_AHRS::Status::PRED_HORIZ_POS_REL) && ahrs.has_status(AP_AHRS::Status::PRED_HORIZ_POS_ABS);
+        float pos_d_m = 0;
+        UNUSED_RESULT(AP::ahrs().get_relative_position_D_origin_float(pos_d_m));
         if (using_baro_ref) {
-            if (fabsf(copter.inertial_nav.get_position_z_up_cm() - copter.baro_alt) > PREARM_MAX_ALT_DISPARITY_CM) {
+            if (fabsf(pos_d_m) > PREARM_MAX_ALT_DISPARITY_CM) {
                 check_failed(Check::BARO, display_failure, "Altitude disparity");
                 ret = false;
             }