fw pos ctrl: add option to fix the glide slope reference altitude

- new param, FW_LND_TER_REL
- fixing the glide slope helps keep the landing glide behavior steady (avoiding bumps in the altitude setpoint from e.g. trees)
- flare is still triggered via the distance sensor, if enabled by FW_LND_USETERR

src/modules/fw_pos_control_l1/FixedwingPositionControl.cpp

@@ -1681,9 +1681,15 @@ FixedwingPositionControl::control_auto_landing(const hrt_abstime &now, const flo
 	const float terrain_alt = getLandingTerrainAltitudeEstimate(now, pos_sp_curr.alt);
 	float altitude_setpoint;
 
-	if (_current_altitude > terrain_alt + glide_slope_rel_alt) {
+	// by default the landing waypoint altitude is used for the glide slope reference. this ensures a constant slope
+	// during the landing approach, despite any terrain bumps (think tall trees below the landing approach) disrupting
+	// the glide behavior. in this case, when FW_LND_USETER==1, the terrain estimate is only used to trigger the flare.
+	// however - the option still exists to make the glide slope terrain relative via FW_LND_TER_REL.
+	const float glide_slope_reference_alt = (_param_fw_lnd_ter_rel.get()) ? terrain_alt : pos_sp_curr.alt;
+
+	if (_current_altitude > glide_slope_reference_alt + glide_slope_rel_alt) {
 		// descend to the glide slope
-		altitude_setpoint = terrain_alt + glide_slope_rel_alt;
+		altitude_setpoint = glide_slope_reference_alt + glide_slope_rel_alt;
 
 	} else {
 		// continue horizontally

src/modules/fw_pos_control_l1/FixedwingPositionControl.hpp

@@ -849,7 +849,8 @@ private:
 		(ParamFloat<px4::params::FW_LND_FL_SINK>) _param_fw_lnd_fl_sink,
 		(ParamFloat<px4::params::FW_LND_TD_OFF>) _param_fw_lnd_td_off,
 		(ParamInt<px4::params::FW_LND_NUDGE>) _param_fw_lnd_nudge,
-		(ParamInt<px4::params::FW_LND_ABORT>) _param_fw_lnd_abort
+		(ParamInt<px4::params::FW_LND_ABORT>) _param_fw_lnd_abort,
+		(ParamBool<px4::params::FW_LND_TER_REL>) _param_fw_lnd_ter_rel
 	)
 
 };

src/modules/fw_pos_control_l1/fw_pos_control_l1_params.c

@@ -1068,3 +1068,17 @@ PARAM_DEFINE_INT32(FW_LND_NUDGE, 0);
  * @group FW L1 Control
  */
 PARAM_DEFINE_INT32(FW_LND_ABORT, 0);
+
+/**
+ * Calculate the landing glide slope relative to the terrain estimate.
+ *
+ * If enabled, the terrain estimate (e.g. via distance sensor) will be used as the glide slope reference altitude, following
+ * all bumps in the terrain below the landing approach.
+ *
+ * If disabled, the land waypoint altitude will be a fixed glide slope reference, and the distance sensor (if enabled via
+ * FW_LND_USETER) will only be used to trigger the flare.
+ *
+ * @boolean
+ * @group FW L1 Control
+ */
+PARAM_DEFINE_INT32(FW_LND_TER_REL, 0);