diff --git a/libraries/AP_Compass/AP_Compass_AK09916.cpp b/libraries/AP_Compass/AP_Compass_AK09916.cpp
index 871606d22d3..ec5d2ab037b 100644
--- a/libraries/AP_Compass/AP_Compass_AK09916.cpp
+++ b/libraries/AP_Compass/AP_Compass_AK09916.cpp
@@ -252,16 +252,15 @@ bool AP_Compass_AK09916::init()
 
     /* register the compass instance in the frontend */
     _bus->set_device_type(_devtype);
-    if (!register_compass(_bus->get_bus_id(), _compass_instance)) {
+    if (!register_compass(_bus->get_bus_id())) {
         goto fail;
     }
-    set_dev_id(_compass_instance, _bus->get_bus_id());
 
     if (_force_external) {
-        set_external(_compass_instance, true);
+        set_external(true);
     }
 
-    set_rotation(_compass_instance, _rotation);
+    set_rotation(_rotation);
     
     bus_sem->give();
 
@@ -280,7 +279,7 @@ void AP_Compass_AK09916::read()
         return;
     }
 
-    drain_accumulated_samples(_compass_instance);
+    drain_accumulated_samples();
 }
 
 void AP_Compass_AK09916::_make_adc_sensitivity_adjustment(Vector3f& field) const
@@ -324,7 +323,7 @@ void AP_Compass_AK09916::_update()
     _make_adc_sensitivity_adjustment(raw_field);
     raw_field *= AK09916_MILLIGAUSS_SCALE;
 
-    accumulate_sample(raw_field, _compass_instance, 10);
+    accumulate_sample(raw_field, 10);
 
 check_registers:
     _bus->check_next_register();
diff --git a/libraries/AP_Compass/AP_Compass_AK09916.h b/libraries/AP_Compass/AP_Compass_AK09916.h
index 5c80cff9ef6..817c45297c5 100644
--- a/libraries/AP_Compass/AP_Compass_AK09916.h
+++ b/libraries/AP_Compass/AP_Compass_AK09916.h
@@ -101,7 +101,6 @@ private:
     AP_AK09916_BusDriver *_bus;
 
     bool _force_external;
-    uint8_t _compass_instance;
     bool _initialized;
     enum Rotation _rotation;
     enum AP_Compass_Backend::DevTypes _devtype;
diff --git a/libraries/AP_Compass/AP_Compass_AK8963.cpp b/libraries/AP_Compass/AP_Compass_AK8963.cpp
index c4dc7e66f88..a17b2fbdca8 100644
--- a/libraries/AP_Compass/AP_Compass_AK8963.cpp
+++ b/libraries/AP_Compass/AP_Compass_AK8963.cpp
@@ -162,12 +162,11 @@ bool AP_Compass_AK8963::init()
 
     /* register the compass instance in the frontend */
     _bus->set_device_type(DEVTYPE_AK8963);
-    if (!register_compass(_bus->get_bus_id(), _compass_instance)) {
+    if (!register_compass(_bus->get_bus_id())) {
         goto fail;
     }
-    set_dev_id(_compass_instance, _bus->get_bus_id());
 
-    set_rotation(_compass_instance, _rotation);
+    set_rotation(_rotation);
     bus_sem->give();
 
     _bus->register_periodic_callback(10000, FUNCTOR_BIND_MEMBER(&AP_Compass_AK8963::_update, void));
@@ -185,7 +184,7 @@ void AP_Compass_AK8963::read()
         return;
     }
 
-    drain_accumulated_samples(_compass_instance);
+    drain_accumulated_samples();
 }
 
 void AP_Compass_AK8963::_make_adc_sensitivity_adjustment(Vector3f& field) const
@@ -227,7 +226,7 @@ void AP_Compass_AK8963::_update()
     _make_adc_sensitivity_adjustment(raw_field);
     raw_field *= AK8963_MILLIGAUSS_SCALE;
 
-    accumulate_sample(raw_field, _compass_instance, 10);
+    accumulate_sample(raw_field, 10);
 }
 
 bool AP_Compass_AK8963::_check_id()
diff --git a/libraries/AP_Compass/AP_Compass_AK8963.h b/libraries/AP_Compass/AP_Compass_AK8963.h
index c0f02075657..b5d092d53b2 100644
--- a/libraries/AP_Compass/AP_Compass_AK8963.h
+++ b/libraries/AP_Compass/AP_Compass_AK8963.h
@@ -64,7 +64,6 @@ private:
 
     float _magnetometer_ASA[3] {0, 0, 0};
 
-    uint8_t _compass_instance;
     bool _initialized;
     enum Rotation _rotation;
 };
diff --git a/libraries/AP_Compass/AP_Compass_BMM150.cpp b/libraries/AP_Compass/AP_Compass_BMM150.cpp
index eb4dcd0b36d..11ae92b70a3 100644
--- a/libraries/AP_Compass/AP_Compass_BMM150.cpp
+++ b/libraries/AP_Compass/AP_Compass_BMM150.cpp
@@ -214,15 +214,14 @@ bool AP_Compass_BMM150::init()
 
     /* register the compass instance in the frontend */
     _dev->set_device_type(DEVTYPE_BMM150);
-    if (!register_compass(_dev->get_bus_id(), _compass_instance)) {
+    if (!register_compass(_dev->get_bus_id())) {
         return false;
     }
-    set_dev_id(_compass_instance, _dev->get_bus_id());
 
-    set_rotation(_compass_instance, _rotation);
+    set_rotation(_rotation);
 
     if (_force_external) {
-        set_external(_compass_instance, true);
+        set_external(true);
     }
 
     // 2 retries for run
@@ -318,13 +317,13 @@ void AP_Compass_BMM150::_update()
 
     _last_read_ms = AP_HAL::millis();
 
-    accumulate_sample(raw_field, _compass_instance);
+    accumulate_sample(raw_field);
     _dev->check_next_register();
 }
 
 void AP_Compass_BMM150::read()
 {
-    drain_accumulated_samples(_compass_instance);
+    drain_accumulated_samples();
 }
 
 
diff --git a/libraries/AP_Compass/AP_Compass_BMM150.h b/libraries/AP_Compass/AP_Compass_BMM150.h
index 2043f1b8902..56eb25827d6 100644
--- a/libraries/AP_Compass/AP_Compass_BMM150.h
+++ b/libraries/AP_Compass/AP_Compass_BMM150.h
@@ -54,8 +54,6 @@ private:
 
     AP_HAL::OwnPtr<AP_HAL::Device> _dev;
 
-    uint8_t _compass_instance;
-
     struct {
         int8_t x1;
         int8_t y1;
diff --git a/libraries/AP_Compass/AP_Compass_BMM350.cpp b/libraries/AP_Compass/AP_Compass_BMM350.cpp
index d5f83419b8a..d09a1babac7 100644
--- a/libraries/AP_Compass/AP_Compass_BMM350.cpp
+++ b/libraries/AP_Compass/AP_Compass_BMM350.cpp
@@ -406,17 +406,16 @@ bool AP_Compass_BMM350::init()
 
     /* Register the compass instance in the frontend */
     _dev->set_device_type(DEVTYPE_BMM350);
-    if (!register_compass(_dev->get_bus_id(), _compass_instance)) {
+    if (!register_compass(_dev->get_bus_id())) {
         return false;
     }
-    set_dev_id(_compass_instance, _dev->get_bus_id());
 
     // printf("BMM350: Found at address 0x%x as compass %u\n", _dev->get_bus_address(), _compass_instance);
 
-    set_rotation(_compass_instance, _rotation);
+    set_rotation(_rotation);
 
     if (_force_external) {
-        set_external(_compass_instance, true);
+        set_external(true);
     }
     
     // Call timer() at 100Hz
@@ -481,12 +480,12 @@ void AP_Compass_BMM350::timer()
 
     // Store in field vector and convert uT to milligauss
     Vector3f field { cr_ax_comp_x * 10.0f, cr_ax_comp_y * 10.0f, cr_ax_comp_z * 10.0f };
-    accumulate_sample(field, _compass_instance);
+    accumulate_sample(field);
 }
 
 void AP_Compass_BMM350::read()
 {
-    drain_accumulated_samples(_compass_instance);
+    drain_accumulated_samples();
 }
 
 #endif  // AP_COMPASS_BMM350_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_BMM350.h b/libraries/AP_Compass/AP_Compass_BMM350.h
index 4cf5fdf5550..fd6878f8de0 100644
--- a/libraries/AP_Compass/AP_Compass_BMM350.h
+++ b/libraries/AP_Compass/AP_Compass_BMM350.h
@@ -103,7 +103,6 @@ private:
     bool set_power_mode(const enum power_mode mode);
     bool read_bytes(const uint8_t reg, uint8_t *out, const uint16_t read_len);
 
-    uint8_t _compass_instance;
     bool _force_external;
     enum Rotation _rotation;
     struct mag_compensate _mag_comp;  // Structure for mag compensate
diff --git a/libraries/AP_Compass/AP_Compass_Backend.cpp b/libraries/AP_Compass/AP_Compass_Backend.cpp
index 5325e71a2a9..0bdea722f02 100644
--- a/libraries/AP_Compass/AP_Compass_Backend.cpp
+++ b/libraries/AP_Compass/AP_Compass_Backend.cpp
@@ -16,7 +16,7 @@ AP_Compass_Backend::AP_Compass_Backend()
 {
 }
 
-void AP_Compass_Backend::rotate_field(Vector3f &mag, uint8_t instance)
+void AP_Compass_Backend::rotate_field(Vector3f &mag)
 {
     Compass::mag_state &state = _compass._state[Compass::StateIndex(instance)];
     if (MAG_BOARD_ORIENTATION != ROTATION_NONE) {
@@ -31,7 +31,7 @@ void AP_Compass_Backend::rotate_field(Vector3f &mag, uint8_t instance)
       before the board orientation so it is independent of
       AHRS_ORIENTATION
      */
-    if (!is_external(instance)) {
+    if (!is_external()) {
         const uint32_t dev_id = uint32_t(_compass._state[Compass::StateIndex(instance)].dev_id);
         static const struct offset {
             uint32_t dev_id;
@@ -56,7 +56,7 @@ void AP_Compass_Backend::rotate_field(Vector3f &mag, uint8_t instance)
     }
 }
 
-void AP_Compass_Backend::publish_raw_field(const Vector3f &mag, uint8_t instance)
+void AP_Compass_Backend::publish_raw_field(const Vector3f &mag)
 {
     // note that we do not set last_update_usec here as otherwise the
     // EKF and DCM would end up consuming compass data at the full
@@ -71,9 +71,9 @@ void AP_Compass_Backend::publish_raw_field(const Vector3f &mag, uint8_t instance
 #endif
 }
 
-void AP_Compass_Backend::correct_field(Vector3f &mag, uint8_t i)
+void AP_Compass_Backend::correct_field(Vector3f &mag)
 {
-    Compass::mag_state &state = _compass._state[Compass::StateIndex(i)];
+    Compass::mag_state &state = _compass._state[Compass::StateIndex(instance)];
 
     const Vector3f &offsets = state.offset.get();
 #if AP_COMPASS_DIAGONALS_ENABLED
@@ -86,7 +86,7 @@ void AP_Compass_Backend::correct_field(Vector3f &mag, uint8_t i)
 
     // add in scale factor, use a wide sanity check. The calibrator
     // uses a narrower check.
-    if (_compass.have_scale_factor(i)) {
+    if (_compass.have_scale_factor(instance)) {
         mag *= state.scale_factor;
     }
 
@@ -111,7 +111,7 @@ void AP_Compass_Backend::correct_field(Vector3f &mag, uint8_t i)
       being applied so it can be logged correctly
     */    
     state.motor_offset.zero();
-    if (_compass._per_motor.enabled() && i == 0) {
+    if (_compass._per_motor.enabled() && instance == 0) {
         // per-motor correction is only valid for first compass
         _compass._per_motor.compensate(state.motor_offset);
     } else if (_compass._motor_comp_type == AP_COMPASS_MOT_COMP_THROTTLE) {
@@ -134,17 +134,17 @@ void AP_Compass_Backend::correct_field(Vector3f &mag, uint8_t i)
 #endif // COMPASS_MOT_ENABLED
 }
 
-void AP_Compass_Backend::accumulate_sample(Vector3f &field, uint8_t instance,
+void AP_Compass_Backend::accumulate_sample(Vector3f &field,
                                            uint32_t max_samples)
 {
     /* rotate raw_field from sensor frame to body frame */
-    rotate_field(field, instance);
+    rotate_field(field);
 
     /* publish raw_field (uncorrected point sample) for calibration use */
-    publish_raw_field(field, instance);
+    publish_raw_field(field);
 
     /* correct raw_field for known errors */
-    correct_field(field, instance);
+    correct_field(field);
 
     if (!field_ok(field)) {
         return;
@@ -161,8 +161,7 @@ void AP_Compass_Backend::accumulate_sample(Vector3f &field, uint8_t instance,
     }
 }
 
-void AP_Compass_Backend::drain_accumulated_samples(uint8_t instance,
-                                                   const Vector3f *scaling)
+void AP_Compass_Backend::drain_accumulated_samples(const Vector3f *scaling)
 {
     WITH_SEMAPHORE(_sem);
 
@@ -177,7 +176,7 @@ void AP_Compass_Backend::drain_accumulated_samples(uint8_t instance,
     }
     state.accum /= state.accum_count;
 
-    publish_filtered_field(state.accum, instance);
+    publish_filtered_field(state.accum);
 
     state.accum.zero();
     state.accum_count = 0;
@@ -186,7 +185,7 @@ void AP_Compass_Backend::drain_accumulated_samples(uint8_t instance,
 /*
   copy latest data to the frontend from a backend
  */
-void AP_Compass_Backend::publish_filtered_field(const Vector3f &mag, uint8_t instance)
+void AP_Compass_Backend::publish_filtered_field(const Vector3f &mag)
 {
     Compass::mag_state &state = _compass._state[Compass::StateIndex(instance)];
 
@@ -196,7 +195,7 @@ void AP_Compass_Backend::publish_filtered_field(const Vector3f &mag, uint8_t ins
     state.last_update_usec = AP_HAL::micros();
 }
 
-void AP_Compass_Backend::set_last_update_usec(uint32_t last_update, uint8_t instance)
+void AP_Compass_Backend::set_last_update_usec(uint32_t last_update)
 {
     Compass::mag_state &state = _compass._state[Compass::StateIndex(instance)];
     state.last_update_usec = last_update;
@@ -206,16 +205,19 @@ void AP_Compass_Backend::set_last_update_usec(uint32_t last_update, uint8_t inst
   register a new backend with frontend, returning instance which
   should be used in publish_field()
  */
-bool AP_Compass_Backend::register_compass(int32_t dev_id, uint8_t& instance) const
-{ 
-    return _compass.register_compass(dev_id, instance);
+bool AP_Compass_Backend::register_compass(int32_t dev_id)
+{
+    if (!_compass.register_compass(dev_id, instance)) {
+        return false;
+    }
+    set_dev_id(dev_id);
+    return true;
 }
 
-
 /*
   set dev_id for an instance
 */
-void AP_Compass_Backend::set_dev_id(uint8_t instance, uint32_t dev_id)
+void AP_Compass_Backend::set_dev_id(uint32_t dev_id)
 {
     _compass._state[Compass::StateIndex(instance)].dev_id.set_and_notify(dev_id);
     _compass._state[Compass::StateIndex(instance)].detected_dev_id = dev_id;
@@ -224,7 +226,7 @@ void AP_Compass_Backend::set_dev_id(uint8_t instance, uint32_t dev_id)
 /*
   save dev_id, used by SITL
 */
-void AP_Compass_Backend::save_dev_id(uint8_t instance)
+void AP_Compass_Backend::save_dev_id()
 {
     _compass._state[Compass::StateIndex(instance)].dev_id.save();
 }
@@ -232,20 +234,20 @@ void AP_Compass_Backend::save_dev_id(uint8_t instance)
 /*
   set external for an instance
 */
-void AP_Compass_Backend::set_external(uint8_t instance, bool external)
+void AP_Compass_Backend::set_external(bool external)
 {
     if (_compass._state[Compass::StateIndex(instance)].external != 2) {
         _compass._state[Compass::StateIndex(instance)].external.set_and_notify(external);
     }
 }
 
-bool AP_Compass_Backend::is_external(uint8_t instance)
+bool AP_Compass_Backend::is_external()
 {
     return _compass._state[Compass::StateIndex(instance)].external;
 }
 
 // set rotation of an instance
-void AP_Compass_Backend::set_rotation(uint8_t instance, enum Rotation rotation)
+void AP_Compass_Backend::set_rotation(enum Rotation rotation)
 {
     _compass._state[Compass::StateIndex(instance)].rotation = rotation;
 }
diff --git a/libraries/AP_Compass/AP_Compass_Backend.h b/libraries/AP_Compass/AP_Compass_Backend.h
index b6067359fd1..e2806997cbd 100644
--- a/libraries/AP_Compass/AP_Compass_Backend.h
+++ b/libraries/AP_Compass/AP_Compass_Backend.h
@@ -88,6 +88,9 @@ public:
     
 protected:
 
+    // this backend's instance number
+    uint8_t instance;
+
     /*
      * A compass measurement is expected to pass through the following functions:
      * 1. rotate_field - this rotates the measurement in-place from sensor frame
@@ -101,33 +104,33 @@ protected:
      * All those functions expect the mag field to be in milligauss.
      */
 
-    void rotate_field(Vector3f &mag, uint8_t instance);
-    void publish_raw_field(const Vector3f &mag, uint8_t instance);
-    void correct_field(Vector3f &mag, uint8_t i);
-    void publish_filtered_field(const Vector3f &mag, uint8_t instance);
-    void set_last_update_usec(uint32_t last_update, uint8_t instance);
+    void rotate_field(Vector3f &mag);
+    void publish_raw_field(const Vector3f &mag);
+    void correct_field(Vector3f &mag);
+    void publish_filtered_field(const Vector3f &mag);
+    void set_last_update_usec(uint32_t last_update);
 
-    void accumulate_sample(Vector3f &field, uint8_t instance,
+    void accumulate_sample(Vector3f &field,
                            uint32_t max_samples = 10);
-    void drain_accumulated_samples(uint8_t instance, const Vector3f *scale = NULL);
+    void drain_accumulated_samples(const Vector3f *scale = NULL);
 
-    // register a new compass instance with the frontend
-    bool register_compass(int32_t dev_id, uint8_t& instance) const WARN_IF_UNUSED;
+    // register compass instance with the frontend
+    bool register_compass(int32_t dev_id) WARN_IF_UNUSED;
 
     // set dev_id for an instance
-    void set_dev_id(uint8_t instance, uint32_t dev_id);
+    void set_dev_id(uint32_t dev_id);
 
     // save dev_id, used by SITL
-    void save_dev_id(uint8_t instance);
+    void save_dev_id();
 
     // set external state for an instance
-    void set_external(uint8_t instance, bool external);
+    void set_external(bool external);
 
     // tell if instance is an external compass
-    bool is_external(uint8_t instance);
+    bool is_external();
 
     // set rotation of an instance
-    void set_rotation(uint8_t instance, enum Rotation rotation);
+    void set_rotation(enum Rotation rotation);
 
     // get board orientation (for SITL)
     enum Rotation get_board_orientation(void) const;
diff --git a/libraries/AP_Compass/AP_Compass_DroneCAN.cpp b/libraries/AP_Compass/AP_Compass_DroneCAN.cpp
index c155ee8ab30..fa078e230af 100644
--- a/libraries/AP_Compass/AP_Compass_DroneCAN.cpp
+++ b/libraries/AP_Compass/AP_Compass_DroneCAN.cpp
@@ -91,12 +91,10 @@ AP_Compass_Backend* AP_Compass_DroneCAN::probe(uint8_t index)
 bool AP_Compass_DroneCAN::init()
 {
     // Adding 1 is necessary to allow backward compatibility, where this field was set as 1 by default
-    if (!register_compass(_devid, _instance)) {
+    if (!register_compass(_devid)) {
         return false;
     }
-
-    set_dev_id(_instance, _devid);
-    set_external(_instance, true);
+    set_external(true);
 
     AP::can().log_text(AP_CANManager::LOG_INFO, LOG_TAG,  "AP_Compass_DroneCAN loaded\n\r");
     return true;
@@ -228,6 +226,6 @@ void AP_Compass_DroneCAN::handle_magnetic_field_hires(AP_DroneCAN *ap_dronecan,
 
 void AP_Compass_DroneCAN::read(void)
 {
-    drain_accumulated_samples(_instance);
+    drain_accumulated_samples();
 }
 #endif  // AP_COMPASS_DRONECAN_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_ExternalAHRS.cpp b/libraries/AP_Compass/AP_Compass_ExternalAHRS.cpp
index af305afeb85..f630be205e8 100644
--- a/libraries/AP_Compass/AP_Compass_ExternalAHRS.cpp
+++ b/libraries/AP_Compass/AP_Compass_ExternalAHRS.cpp
@@ -25,24 +25,23 @@ AP_Compass_Backend *AP_Compass_ExternalAHRS::probe(uint8_t port)
     if (ret == nullptr) {
         return nullptr;
     }
-    if (!ret->register_compass(devid, ret->instance)) {
+    if (!ret->register_compass(devid)) {
         delete ret;
         return nullptr;
     }
-    ret->set_dev_id(ret->instance, devid);
-    ret->set_external(ret->instance, true);
+    ret->set_external(true);
     return ret;
 }
 
 void AP_Compass_ExternalAHRS::handle_external(const AP_ExternalAHRS::mag_data_message_t &pkt)
 {
     Vector3f field = pkt.field;
-    accumulate_sample(field, instance);
+    accumulate_sample(field);
 }
 
 void AP_Compass_ExternalAHRS::read(void)
 {
-    drain_accumulated_samples(instance);
+    drain_accumulated_samples();
 }
 
 #endif // AP_COMPASS_EXTERNALAHRS_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_ExternalAHRS.h b/libraries/AP_Compass/AP_Compass_ExternalAHRS.h
index 29e3367b366..ec79d3cd188 100644
--- a/libraries/AP_Compass/AP_Compass_ExternalAHRS.h
+++ b/libraries/AP_Compass/AP_Compass_ExternalAHRS.h
@@ -19,7 +19,6 @@ public:
 
 private:
     void handle_external(const AP_ExternalAHRS::mag_data_message_t &pkt) override;
-    uint8_t instance;
 };
 
 #endif  // AP_COMPASS_EXTERNALAHRS_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_HMC5843.cpp b/libraries/AP_Compass/AP_Compass_HMC5843.cpp
index 1c2d969f34b..a1957367d82 100644
--- a/libraries/AP_Compass/AP_Compass_HMC5843.cpp
+++ b/libraries/AP_Compass/AP_Compass_HMC5843.cpp
@@ -196,15 +196,14 @@ bool AP_Compass_HMC5843::init()
 
     //register compass instance
     _bus->set_device_type(DEVTYPE_HMC5883);
-    if (!register_compass(_bus->get_bus_id(), _compass_instance)) {
+    if (!register_compass(_bus->get_bus_id())) {
         return false;
     }
-    set_dev_id(_compass_instance, _bus->get_bus_id());
 
-    set_rotation(_compass_instance, _rotation);
+    set_rotation(_rotation);
     
     if (_force_external) {
-        set_external(_compass_instance, true);
+        set_external(true);
     }
 
     // read from sensor at 75Hz
@@ -241,14 +240,14 @@ void AP_Compass_HMC5843::_timer()
     raw_field *= _gain_scale;
 
     // rotate to the desired orientation
-    if (is_external(_compass_instance)) {
+    if (is_external()) {
         raw_field.rotate(ROTATION_YAW_90);
     }
 
     // We expect to do reads at 10Hz, and  we get new data at most 75Hz, so we
     // don't expect to accumulate more than 8 before a read; let's make it
     // 14 to give more room for the initialization phase
-    accumulate_sample(raw_field, _compass_instance, 14);
+    accumulate_sample(raw_field, 14);
 }
 
 /*
@@ -266,7 +265,7 @@ void AP_Compass_HMC5843::read()
         return;
     }
 
-    drain_accumulated_samples(_compass_instance, &_scaling);
+    drain_accumulated_samples(&_scaling);
 }
 
 bool AP_Compass_HMC5843::_setup_sampling_mode()
diff --git a/libraries/AP_Compass/AP_Compass_HMC5843.h b/libraries/AP_Compass/AP_Compass_HMC5843.h
index a2750c6ceff..4e5b9b6f0f3 100644
--- a/libraries/AP_Compass/AP_Compass_HMC5843.h
+++ b/libraries/AP_Compass/AP_Compass_HMC5843.h
@@ -63,8 +63,6 @@ private:
     int16_t _mag_y;
     int16_t _mag_z;
 
-    uint8_t _compass_instance;
-
     enum Rotation _rotation;
     
     bool _initialised:1;
diff --git a/libraries/AP_Compass/AP_Compass_IIS2MDC.cpp b/libraries/AP_Compass/AP_Compass_IIS2MDC.cpp
index 0476c5d419e..8252c819cb2 100644
--- a/libraries/AP_Compass/AP_Compass_IIS2MDC.cpp
+++ b/libraries/AP_Compass/AP_Compass_IIS2MDC.cpp
@@ -98,16 +98,14 @@ bool AP_Compass_IIS2MDC::init()
     // register compass instance
     _dev->set_device_type(DEVTYPE_IIS2MDC);
 
-    if (!register_compass(_dev->get_bus_id(), _instance)) {
+    if (!register_compass(_dev->get_bus_id())) {
         return false;
     }
 
-    set_dev_id(_instance, _dev->get_bus_id());
-
-    set_rotation(_instance, _rotation);
+    set_rotation(_rotation);
 
     if (_force_external) {
-        set_external(_instance, true);
+        set_external(true);
     }
 
     // Enable 100HZ
@@ -160,12 +158,12 @@ void AP_Compass_IIS2MDC::timer()
 
     Vector3f field{ x * range_scale, y * range_scale, z * range_scale };
 
-    accumulate_sample(field, _instance);
+    accumulate_sample(field);
 }
 
 void AP_Compass_IIS2MDC::read()
 {
-    drain_accumulated_samples(_instance);
+    drain_accumulated_samples();
 }
 
 #endif //AP_COMPASS_IIS2MDC_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_IIS2MDC.h b/libraries/AP_Compass/AP_Compass_IIS2MDC.h
index cd4989eced0..27db5acf627 100644
--- a/libraries/AP_Compass/AP_Compass_IIS2MDC.h
+++ b/libraries/AP_Compass/AP_Compass_IIS2MDC.h
@@ -60,7 +60,6 @@ private:
     AP_HAL::OwnPtr<AP_HAL::Device> _dev;
 
     enum Rotation _rotation;
-    uint8_t _instance;
     bool _force_external;
 };
 
diff --git a/libraries/AP_Compass/AP_Compass_IST8308.cpp b/libraries/AP_Compass/AP_Compass_IST8308.cpp
index 722457dda39..57b41d0615f 100644
--- a/libraries/AP_Compass/AP_Compass_IST8308.cpp
+++ b/libraries/AP_Compass/AP_Compass_IST8308.cpp
@@ -162,18 +162,17 @@ bool AP_Compass_IST8308::init()
 
     //register compass instance
     _dev->set_device_type(DEVTYPE_IST8308);
-    if (!register_compass(_dev->get_bus_id(), _instance)) {
+    if (!register_compass(_dev->get_bus_id())) {
         return false;
     }
-    set_dev_id(_instance, _dev->get_bus_id());
 
     printf("%s found on bus %u id %u address 0x%02x\n", name,
            _dev->bus_num(), unsigned(_dev->get_bus_id()), _dev->get_bus_address());
 
-    set_rotation(_instance, _rotation);
+    set_rotation(_rotation);
 
     if (_force_external) {
-        set_external(_instance, true);
+        set_external(true);
     }
 
     _dev->register_periodic_callback(SAMPLING_PERIOD_USEC,
@@ -215,12 +214,12 @@ void AP_Compass_IST8308::timer()
     /* Resolution: 0.1515 µT/LSB - already convert to milligauss */
     Vector3f field = Vector3f{x * 1.515f, y * 1.515f, z * 1.515f};
 
-    accumulate_sample(field, _instance);
+    accumulate_sample(field);
 }
 
 void AP_Compass_IST8308::read()
 {
-    drain_accumulated_samples(_instance);
+    drain_accumulated_samples();
 }
 
 #endif  // AP_COMPASS_IST8308_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_IST8308.h b/libraries/AP_Compass/AP_Compass_IST8308.h
index 94d4899dd9a..a55295295d4 100644
--- a/libraries/AP_Compass/AP_Compass_IST8308.h
+++ b/libraries/AP_Compass/AP_Compass_IST8308.h
@@ -52,7 +52,6 @@ private:
     AP_HAL::OwnPtr<AP_HAL::Device> _dev;
 
     enum Rotation _rotation;
-    uint8_t _instance;
     bool _force_external;
 };
 
diff --git a/libraries/AP_Compass/AP_Compass_IST8310.cpp b/libraries/AP_Compass/AP_Compass_IST8310.cpp
index 75c0ab8e17d..27a411f0fd1 100644
--- a/libraries/AP_Compass/AP_Compass_IST8310.cpp
+++ b/libraries/AP_Compass/AP_Compass_IST8310.cpp
@@ -172,18 +172,17 @@ bool AP_Compass_IST8310::init()
 
     // register compass instance
     _dev->set_device_type(DEVTYPE_IST8310);
-    if (!register_compass(_dev->get_bus_id(), _instance)) {
+    if (!register_compass(_dev->get_bus_id())) {
         return false;
     }
-    set_dev_id(_instance, _dev->get_bus_id());
 
     printf("%s found on bus %u id %u address 0x%02x\n", name,
            _dev->bus_num(), unsigned(_dev->get_bus_id()), _dev->get_bus_address());
 
-    set_rotation(_instance, _rotation);
+    set_rotation(_rotation);
 
     if (_force_external) {
-        set_external(_instance, true);
+        set_external(true);
     }
     
     _periodic_handle = _dev->register_periodic_callback(SAMPLING_PERIOD_USEC,
@@ -247,12 +246,12 @@ void AP_Compass_IST8310::timer()
     /* Resolution: 0.3 µT/LSB - already convert to milligauss */
     Vector3f field = Vector3f{x * 3.0f, y * 3.0f, z * 3.0f};
 
-    accumulate_sample(field, _instance);
+    accumulate_sample(field);
 }
 
 void AP_Compass_IST8310::read()
 {
-    drain_accumulated_samples(_instance);
+    drain_accumulated_samples();
 }
 
 #endif  // AP_COMPASS_IST8310_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_IST8310.h b/libraries/AP_Compass/AP_Compass_IST8310.h
index 96d061a9c2d..2ef19de48e5 100644
--- a/libraries/AP_Compass/AP_Compass_IST8310.h
+++ b/libraries/AP_Compass/AP_Compass_IST8310.h
@@ -59,7 +59,6 @@ private:
     AP_HAL::Device::PeriodicHandle _periodic_handle;
 
     enum Rotation _rotation;
-    uint8_t _instance;
     bool _ignore_next_sample;
     bool _force_external;
 };
diff --git a/libraries/AP_Compass/AP_Compass_LIS3MDL.cpp b/libraries/AP_Compass/AP_Compass_LIS3MDL.cpp
index 392aa62f453..57b158b653d 100644
--- a/libraries/AP_Compass/AP_Compass_LIS3MDL.cpp
+++ b/libraries/AP_Compass/AP_Compass_LIS3MDL.cpp
@@ -106,17 +106,16 @@ bool AP_Compass_LIS3MDL::init()
 
     /* register the compass instance in the frontend */
     dev->set_device_type(DEVTYPE_LIS3MDL);
-    if (!register_compass(dev->get_bus_id(), compass_instance)) {
+    if (!register_compass(dev->get_bus_id())) {
         return false;
     }
-    set_dev_id(compass_instance, dev->get_bus_id());
 
-    printf("Found a LIS3MDL on 0x%x as compass %u\n", unsigned(dev->get_bus_id()), compass_instance);
-    
-    set_rotation(compass_instance, rotation);
+    printf("Found a LIS3MDL on 0x%x as compass %u\n", unsigned(dev->get_bus_id()), instance);
+
+    set_rotation(rotation);
 
     if (force_external) {
-        set_external(compass_instance, true);
+        set_external(true);
     }
     
     // call timer() at 80Hz
@@ -160,7 +159,7 @@ void AP_Compass_LIS3MDL::timer()
             data.magz * range_scale,
         };
 
-        accumulate_sample(field, compass_instance);
+        accumulate_sample(field);
     }
 
 check_registers:
@@ -169,7 +168,7 @@ check_registers:
 
 void AP_Compass_LIS3MDL::read()
 {
-    drain_accumulated_samples(compass_instance);
+    drain_accumulated_samples();
 }
 
 #endif  // AP_COMPASS_LIS3MDL_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_LIS3MDL.h b/libraries/AP_Compass/AP_Compass_LIS3MDL.h
index 71c5d4a32c7..39e8cd6c1a9 100644
--- a/libraries/AP_Compass/AP_Compass_LIS3MDL.h
+++ b/libraries/AP_Compass/AP_Compass_LIS3MDL.h
@@ -58,7 +58,6 @@ private:
     bool init();
     void timer();
 
-    uint8_t compass_instance;
     bool force_external;
     enum Rotation rotation;
 };
diff --git a/libraries/AP_Compass/AP_Compass_LSM303D.cpp b/libraries/AP_Compass/AP_Compass_LSM303D.cpp
index 7d0437d169d..fdd3107ed21 100644
--- a/libraries/AP_Compass/AP_Compass_LSM303D.cpp
+++ b/libraries/AP_Compass/AP_Compass_LSM303D.cpp
@@ -270,12 +270,11 @@ bool AP_Compass_LSM303D::init(enum Rotation rotation)
 
     /* register the compass instance in the frontend */
     _dev->set_device_type(DEVTYPE_LSM303D);
-    if (!register_compass(_dev->get_bus_id(), _compass_instance)) {
+    if (!register_compass(_dev->get_bus_id())) {
         return false;
     }
-    set_dev_id(_compass_instance, _dev->get_bus_id());
 
-    set_rotation(_compass_instance, rotation);
+    set_rotation(rotation);
 
     // read at 91Hz. We don't run at 100Hz as fetching data too fast can cause some very
     // odd periodic changes in the output data
@@ -343,7 +342,7 @@ void AP_Compass_LSM303D::_update()
 
     Vector3f raw_field = Vector3f(_mag_x, _mag_y, _mag_z) * _mag_range_scale;
 
-    accumulate_sample(raw_field, _compass_instance, 10);
+    accumulate_sample(raw_field, 10);
 }
 
 // Read Sensor data
@@ -353,7 +352,7 @@ void AP_Compass_LSM303D::read()
         return;
     }
 
-    drain_accumulated_samples(_compass_instance);
+    drain_accumulated_samples();
 }
 
 void AP_Compass_LSM303D::_disable_i2c()
diff --git a/libraries/AP_Compass/AP_Compass_LSM303D.h b/libraries/AP_Compass/AP_Compass_LSM303D.h
index 328e0477e05..503d9255f40 100644
--- a/libraries/AP_Compass/AP_Compass_LSM303D.h
+++ b/libraries/AP_Compass/AP_Compass_LSM303D.h
@@ -50,7 +50,6 @@ private:
     int16_t _mag_y;
     int16_t _mag_z;
 
-    uint8_t _compass_instance;
     bool _initialised;
 
     uint8_t _mag_range_ga;
diff --git a/libraries/AP_Compass/AP_Compass_LSM9DS1.cpp b/libraries/AP_Compass/AP_Compass_LSM9DS1.cpp
index d423254347e..09dc4c3a770 100644
--- a/libraries/AP_Compass/AP_Compass_LSM9DS1.cpp
+++ b/libraries/AP_Compass/AP_Compass_LSM9DS1.cpp
@@ -98,12 +98,11 @@ bool AP_Compass_LSM9DS1::init()
 
     //register compass instance
     _dev->set_device_type(DEVTYPE_LSM9DS1);
-    if (!register_compass(_dev->get_bus_id(), _compass_instance)) {
+    if (!register_compass(_dev->get_bus_id())) {
         goto errout;
     }
-    set_dev_id(_compass_instance, _dev->get_bus_id());
 
-    set_rotation(_compass_instance, _rotation);
+    set_rotation(_rotation);
 
 
     _dev->register_periodic_callback(10000, FUNCTOR_BIND_MEMBER(&AP_Compass_LSM9DS1::_update, void));
@@ -150,12 +149,12 @@ void AP_Compass_LSM9DS1::_update(void)
 
     raw_field *= _scaling;
 
-    accumulate_sample(raw_field, _compass_instance);
+    accumulate_sample(raw_field);
 }
 
 void AP_Compass_LSM9DS1::read()
 {
-    drain_accumulated_samples(_compass_instance);
+    drain_accumulated_samples();
 }
 
 bool AP_Compass_LSM9DS1::_check_id(void)
diff --git a/libraries/AP_Compass/AP_Compass_LSM9DS1.h b/libraries/AP_Compass/AP_Compass_LSM9DS1.h
index 7017883bd62..5f3a0b4c2e9 100644
--- a/libraries/AP_Compass/AP_Compass_LSM9DS1.h
+++ b/libraries/AP_Compass/AP_Compass_LSM9DS1.h
@@ -39,7 +39,6 @@ private:
     void _dump_registers();
 
     AP_HAL::OwnPtr<AP_HAL::Device> _dev;
-    uint8_t _compass_instance;
     float _scaling;
     enum Rotation _rotation;
 
diff --git a/libraries/AP_Compass/AP_Compass_MAG3110.cpp b/libraries/AP_Compass/AP_Compass_MAG3110.cpp
index c486a0fac10..080b196a7d4 100644
--- a/libraries/AP_Compass/AP_Compass_MAG3110.cpp
+++ b/libraries/AP_Compass/AP_Compass_MAG3110.cpp
@@ -119,14 +119,13 @@ bool AP_Compass_MAG3110::init(enum Rotation rotation)
     
     /* register the compass instance in the frontend */
     _dev->set_device_type(DEVTYPE_MAG3110);
-    if (!register_compass(_dev->get_bus_id(), _compass_instance)) {
+    if (!register_compass(_dev->get_bus_id())) {
         return false;
     }
-    set_dev_id(_compass_instance, _dev->get_bus_id());
 
-    set_rotation(_compass_instance, rotation);
+    set_rotation(rotation);
 
-    set_external(_compass_instance, true);
+    set_external(true);
 
     // read at 75Hz
     _dev->register_periodic_callback(13333, FUNCTOR_BIND_MEMBER(&AP_Compass_MAG3110::_update, void)); 
@@ -208,7 +207,7 @@ void AP_Compass_MAG3110::_update()
 
     Vector3f raw_field = Vector3f((float)_mag_x, (float)_mag_y, (float)_mag_z) * MAG_SCALE;
 
-    accumulate_sample(raw_field, _compass_instance);
+    accumulate_sample(raw_field);
 }
 
 
@@ -219,7 +218,7 @@ void AP_Compass_MAG3110::read()
         return;
     }
 
-    drain_accumulated_samples(_compass_instance);
+    drain_accumulated_samples();
 }
 
 #endif  // AP_COMPASS_MAG3110_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_MAG3110.h b/libraries/AP_Compass/AP_Compass_MAG3110.h
index 80167ba38d6..1c504c89a59 100644
--- a/libraries/AP_Compass/AP_Compass_MAG3110.h
+++ b/libraries/AP_Compass/AP_Compass_MAG3110.h
@@ -45,7 +45,6 @@ private:
     int32_t _mag_y;
     int32_t _mag_z;
 
-    uint8_t _compass_instance;
     bool _initialised;
 };
 
diff --git a/libraries/AP_Compass/AP_Compass_MMC3416.cpp b/libraries/AP_Compass/AP_Compass_MMC3416.cpp
index f58b779e31e..f3aef33b01b 100644
--- a/libraries/AP_Compass/AP_Compass_MMC3416.cpp
+++ b/libraries/AP_Compass/AP_Compass_MMC3416.cpp
@@ -93,18 +93,16 @@ bool AP_Compass_MMC3416::init()
 
     /* register the compass instance in the frontend */
     dev->set_device_type(DEVTYPE_MMC3416);
-    if (!register_compass(dev->get_bus_id(), compass_instance)) {
+    if (!register_compass(dev->get_bus_id())) {
         return false;
     }
-    
-    set_dev_id(compass_instance, dev->get_bus_id());
 
-    printf("Found a MMC3416 on 0x%x as compass %u\n", unsigned(dev->get_bus_id()), compass_instance);
-    
-    set_rotation(compass_instance, rotation);
+    printf("Found a MMC3416 on 0x%x as compass %u\n", unsigned(dev->get_bus_id()), instance);
+
+    set_rotation(rotation);
 
     if (force_external) {
-        set_external(compass_instance, true);
+        set_external(true);
     }
     
     dev->set_retries(1);
@@ -253,7 +251,7 @@ void AP_Compass_MMC3416::timer()
         // sensor is not FRD
         field.y = -field.y;
 
-        accumulate_sample(field, compass_instance);
+        accumulate_sample(field);
 
         if (!dev->write_register(REG_CONTROL0, REG_CONTROL0_TM)) {
             state = STATE_REFILL1;
@@ -283,7 +281,7 @@ void AP_Compass_MMC3416::timer()
         field.y = -field.y;
 
         last_sample_ms = AP_HAL::millis();
-        accumulate_sample(field, compass_instance);
+        accumulate_sample(field);
 
         // we stay in STATE_MEASURE_WAIT3 for measure_count_limit cycles
         if (measure_count++ >= measure_count_limit) {
@@ -301,7 +299,7 @@ void AP_Compass_MMC3416::timer()
 
 void AP_Compass_MMC3416::read()
 {
-    drain_accumulated_samples(compass_instance);
+    drain_accumulated_samples();
 }
 
 #endif  // AP_COMPASS_MMC3416_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_MMC3416.h b/libraries/AP_Compass/AP_Compass_MMC3416.h
index 005a8840d1c..c858a99116c 100644
--- a/libraries/AP_Compass/AP_Compass_MMC3416.h
+++ b/libraries/AP_Compass/AP_Compass_MMC3416.h
@@ -64,7 +64,6 @@ private:
     void timer();
     void accumulate_field(Vector3f &field);
 
-    uint8_t compass_instance;
     bool force_external;
     Vector3f offset;
     uint16_t measure_count;
diff --git a/libraries/AP_Compass/AP_Compass_MMC5xx3.cpp b/libraries/AP_Compass/AP_Compass_MMC5xx3.cpp
index f932865ab32..43d51495399 100644
--- a/libraries/AP_Compass/AP_Compass_MMC5xx3.cpp
+++ b/libraries/AP_Compass/AP_Compass_MMC5xx3.cpp
@@ -109,18 +109,16 @@ bool AP_Compass_MMC5XX3::init()
 
     /* register the compass instance in the frontend */
     dev->set_device_type(DEVTYPE_MMC5983);
-    if (!register_compass(dev->get_bus_id(), compass_instance)) {
+    if (!register_compass(dev->get_bus_id())) {
         return false;
     }
 
-    set_dev_id(compass_instance, dev->get_bus_id());
+    printf("Found a MMC5983 on 0x%x as compass %u\n", unsigned(dev->get_bus_id()), instance);
 
-    printf("Found a MMC5983 on 0x%x as compass %u\n", unsigned(dev->get_bus_id()), compass_instance);
-
-    set_rotation(compass_instance, rotation);
+    set_rotation(rotation);
 
     if (force_external) {
-        set_external(compass_instance, true);
+        set_external(true);
     }
 
     dev->set_retries(1);
@@ -250,7 +248,7 @@ void AP_Compass_MMC5XX3::timer()
             offset = offset * 0.5f + new_offset * 0.5f;
         }
 
-        accumulate_sample(field, compass_instance);
+        accumulate_sample(field);
 
         if (!dev->write_register(REG_CONTROL0, REG_CONTROL0_TMM)) {
             printf("failed to initiate measurement\n");
@@ -288,7 +286,7 @@ void AP_Compass_MMC5XX3::timer()
                        float((data1[4] << 8) + data1[5]) - zero_offset};
         field *= counts_to_milliGauss;
         field -= offset;
-        accumulate_sample(field, compass_instance);
+        accumulate_sample(field);
 
         // we stay in STATE_MEASURE for measure_count_limit cycles
         if (measure_count++ >= measure_count_limit) {
@@ -306,7 +304,7 @@ void AP_Compass_MMC5XX3::timer()
 
 void AP_Compass_MMC5XX3::read()
 {
-    drain_accumulated_samples(compass_instance);
+    drain_accumulated_samples();
 }
 
 #endif  // AP_COMPASS_MMC5XX3_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_MMC5xx3.h b/libraries/AP_Compass/AP_Compass_MMC5xx3.h
index 8acd6e56336..7c3ba8228b9 100644
--- a/libraries/AP_Compass/AP_Compass_MMC5xx3.h
+++ b/libraries/AP_Compass/AP_Compass_MMC5xx3.h
@@ -64,7 +64,6 @@ private:
     void timer();
     void accumulate_field(Vector3f &field);
 
-    uint8_t compass_instance;
     bool force_external;
     Vector3f offset;
     uint16_t measure_count;
diff --git a/libraries/AP_Compass/AP_Compass_MSP.cpp b/libraries/AP_Compass/AP_Compass_MSP.cpp
index c764c8ccfca..2dc06bba0a5 100644
--- a/libraries/AP_Compass/AP_Compass_MSP.cpp
+++ b/libraries/AP_Compass/AP_Compass_MSP.cpp
@@ -35,12 +35,11 @@ AP_Compass_Backend *AP_Compass_MSP::probe(uint8_t _msp_instance)
 bool AP_Compass_MSP::init()
 {
     auto devid = AP_HAL::Device::make_bus_id(AP_HAL::Device::BUS_TYPE_MSP, 0, msp_instance, 0);
-    if (!register_compass(devid, instance)) {
+    if (!register_compass(devid)) {
         return false;
     }
 
-    set_dev_id(instance, devid);
-    set_external(instance, true);
+    set_external(true);
 
     return true;
 }
@@ -56,7 +55,7 @@ void AP_Compass_MSP::handle_msp(const MSP::msp_compass_data_message_t &pkt)
 
 void AP_Compass_MSP::read(void)
 {
-    drain_accumulated_samples(instance);
+    drain_accumulated_samples();
 }
 
 #endif // AP_COMPASS_MSP_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_QMC5883L.cpp b/libraries/AP_Compass/AP_Compass_QMC5883L.cpp
index c0627764bde..804731d7613 100644
--- a/libraries/AP_Compass/AP_Compass_QMC5883L.cpp
+++ b/libraries/AP_Compass/AP_Compass_QMC5883L.cpp
@@ -115,18 +115,17 @@ bool AP_Compass_QMC5883L::init()
 
     //register compass instance
     _dev->set_device_type(DEVTYPE_QMC5883L);
-    if (!register_compass(_dev->get_bus_id(), _instance)) {
+    if (!register_compass(_dev->get_bus_id())) {
         return false;
     }
-    set_dev_id(_instance, _dev->get_bus_id());
 
     printf("%s found on bus %u id %u address 0x%02x\n", name,
            _dev->bus_num(), unsigned(_dev->get_bus_id()), _dev->get_bus_address());
 
-    set_rotation(_instance, _rotation);
+    set_rotation(_rotation);
 
     if (_force_external) {
-        set_external(_instance, true);
+        set_external(true);
     }
 
     //Enable 100HZ
@@ -191,16 +190,16 @@ void AP_Compass_QMC5883L::timer()
     Vector3f field = Vector3f{x * range_scale , y * range_scale, z * range_scale };
 
     // rotate to the desired orientation
-    if (is_external(_instance)) {
+    if (is_external()) {
         field.rotate(ROTATION_YAW_90);
     }
 
-    accumulate_sample(field, _instance, 20);
+    accumulate_sample(field, 20);
 }
 
 void AP_Compass_QMC5883L::read()
 {
-    drain_accumulated_samples(_instance);
+    drain_accumulated_samples();
 }
 
 void AP_Compass_QMC5883L::_dump_registers()
diff --git a/libraries/AP_Compass/AP_Compass_QMC5883L.h b/libraries/AP_Compass/AP_Compass_QMC5883L.h
index 7343dd45fda..1fc1d7b4255 100644
--- a/libraries/AP_Compass/AP_Compass_QMC5883L.h
+++ b/libraries/AP_Compass/AP_Compass_QMC5883L.h
@@ -67,7 +67,6 @@ private:
     AP_HAL::OwnPtr<AP_HAL::Device> _dev;
 
     enum Rotation _rotation;
-    uint8_t _instance;
     bool _force_external;
 };
 
diff --git a/libraries/AP_Compass/AP_Compass_QMC5883P.cpp b/libraries/AP_Compass/AP_Compass_QMC5883P.cpp
index d4987f203e1..f30c9f7dfb5 100644
--- a/libraries/AP_Compass/AP_Compass_QMC5883P.cpp
+++ b/libraries/AP_Compass/AP_Compass_QMC5883P.cpp
@@ -126,18 +126,17 @@ bool AP_Compass_QMC5883P::init()
 
     //register compass instance
     _dev->set_device_type(DEVTYPE_QMC5883P);
-    if (!register_compass(_dev->get_bus_id(), _instance)) {
+    if (!register_compass(_dev->get_bus_id())) {
         return false;
     }
-    set_dev_id(_instance, _dev->get_bus_id());
 
     printf("%s found on bus %u id %u address 0x%02x\n", name,
            _dev->bus_num(), unsigned(_dev->get_bus_id()), _dev->get_bus_address());
 
-    set_rotation(_instance, _rotation);
+    set_rotation(_rotation);
 
     if (_force_external) {
-        set_external(_instance, true);
+        set_external(true);
     }
 
     //Enable 100HZ
@@ -195,12 +194,12 @@ void AP_Compass_QMC5883P::timer()
 
     Vector3f field = Vector3f{x * range_scale, y * range_scale, z * range_scale };
 
-    accumulate_sample(field, _instance, 20);
+    accumulate_sample(field, 20);
 }
 
 void AP_Compass_QMC5883P::read()
 {
-    drain_accumulated_samples(_instance);
+    drain_accumulated_samples();
 }
 
 void AP_Compass_QMC5883P::_dump_registers()
diff --git a/libraries/AP_Compass/AP_Compass_QMC5883P.h b/libraries/AP_Compass/AP_Compass_QMC5883P.h
index 2b30e2f3c46..9e68353080c 100644
--- a/libraries/AP_Compass/AP_Compass_QMC5883P.h
+++ b/libraries/AP_Compass/AP_Compass_QMC5883P.h
@@ -66,7 +66,6 @@ private:
     AP_HAL::OwnPtr<AP_HAL::Device> _dev;
 
     enum Rotation _rotation;
-    uint8_t _instance;
     bool _force_external;
 };
 
diff --git a/libraries/AP_Compass/AP_Compass_RM3100.cpp b/libraries/AP_Compass/AP_Compass_RM3100.cpp
index 6a64bd0d0b1..aae03971848 100644
--- a/libraries/AP_Compass/AP_Compass_RM3100.cpp
+++ b/libraries/AP_Compass/AP_Compass_RM3100.cpp
@@ -145,17 +145,16 @@ bool AP_Compass_RM3100::init()
 
     /* register the compass instance in the frontend */
     dev->set_device_type(DEVTYPE_RM3100);
-    if (!register_compass(dev->get_bus_id(), compass_instance)) {
+    if (!register_compass(dev->get_bus_id())) {
         return false;
     }
-    set_dev_id(compass_instance, dev->get_bus_id());
 
-    DEV_PRINTF("RM3100: Found at address 0x%x as compass %u\n", dev->get_bus_address(), compass_instance);
-    
-    set_rotation(compass_instance, rotation);
+    DEV_PRINTF("RM3100: Found at address 0x%x as compass %u\n", dev->get_bus_address(), instance);
+
+    set_rotation(rotation);
 
     if (force_external) {
-        set_external(compass_instance, true);
+        set_external(true);
     }
     
     // call timer() at 80Hz
@@ -232,7 +231,7 @@ void AP_Compass_RM3100::timer()
              magz * _scaler
          };
 
-        accumulate_sample(field, compass_instance);
+        accumulate_sample(field);
     }
 
 check_registers:
@@ -241,7 +240,7 @@ check_registers:
 
 void AP_Compass_RM3100::read()
 {
-	drain_accumulated_samples(compass_instance);
+	drain_accumulated_samples();
 }
 
 #endif  // AP_COMPASS_RM3100_ENABLED
diff --git a/libraries/AP_Compass/AP_Compass_RM3100.h b/libraries/AP_Compass/AP_Compass_RM3100.h
index 2b6419f03c9..25a0ee8cef8 100644
--- a/libraries/AP_Compass/AP_Compass_RM3100.h
+++ b/libraries/AP_Compass/AP_Compass_RM3100.h
@@ -57,7 +57,6 @@ private:
     bool init();
     void timer();
 
-    uint8_t compass_instance;
     bool force_external;
     enum Rotation rotation;
     float _scaler = 1.0;
diff --git a/libraries/AP_Compass/AP_Compass_SITL.cpp b/libraries/AP_Compass/AP_Compass_SITL.cpp
index e3171e8d76f..3d388a73b67 100644
--- a/libraries/AP_Compass/AP_Compass_SITL.cpp
+++ b/libraries/AP_Compass/AP_Compass_SITL.cpp
@@ -18,18 +18,18 @@ AP_Compass_SITL::AP_Compass_SITL(uint8_t _sitl_instance) :
             if (dev_id == 0) {
                 return;
             }
-            if (!register_compass(dev_id, _compass_instance)) {
+            if (!register_compass(dev_id)) {
                 return;
             }
-            set_dev_id(_compass_instance, dev_id);
                 if (_sitl->mag_save_ids) {
                     // save so the compass always comes up configured in SITL
-                    save_dev_id(_compass_instance);
+                    save_dev_id();
                 }
-                set_rotation(_compass_instance, ROTATION_NONE);
+                set_rotation(ROTATION_NONE);
 
-            if (_compass.get_offsets(_compass_instance).is_zero()) {
-                _compass.set_offsets(_compass_instance, _sitl->mag_ofs[sitl_instance]);
+        // Scroll through the registered compasses, and set the offsets
+            if (_compass.get_offsets(instance).is_zero()) {
+                _compass.set_offsets(instance, _sitl->mag_ofs[sitl_instance]);
             }
 
         // we want to simulate a calibrated compass by default, so set
@@ -40,7 +40,7 @@ AP_Compass_SITL::AP_Compass_SITL(uint8_t _sitl_instance) :
 
         // make first compass external
         if (sitl_instance == 0) {
-            set_external(_compass_instance, true);
+            set_external(true);
         }
 
         hal.scheduler->register_timer_process(FUNCTOR_BIND(this, &AP_Compass_SITL::_timer, void));
@@ -130,7 +130,7 @@ void AP_Compass_SITL::_timer()
 
         switch (_sitl->mag_fail[sitl_instance]) {
         case 0:
-            accumulate_sample(f, _compass_instance, 10);
+            accumulate_sample(f, 10);
             _last_data = f;
             break;
         case 1:
@@ -138,13 +138,13 @@ void AP_Compass_SITL::_timer()
             break;
         case 2:
             // frozen compass
-            accumulate_sample(_last_data, _compass_instance, 10);
+            accumulate_sample(_last_data, 10);
             break;
         }
 }
 
 void AP_Compass_SITL::read()
 {
-    drain_accumulated_samples(_compass_instance, nullptr);
+    drain_accumulated_samples(nullptr);
 }
 #endif  // AP_COMPASS_SITL_ENABLED