fw pos ctrl: absorb fw_lnd_rel_ter into fw_lnd_useter parameter

src/modules/fw_pos_control_l1/FixedwingPositionControl.cpp

@@ -1686,13 +1686,10 @@ FixedwingPositionControl::control_auto_landing(const hrt_abstime &now, const flo
 					  _landing_approach_entrance_rel_alt);
 
 	const float terrain_alt = getLandingTerrainAltitudeEstimate(now, pos_sp_curr.alt);
-	float altitude_setpoint;
+	const float glide_slope_reference_alt = (_param_fw_lnd_useter.get() ==
+						TerrainEstimateUseOnLanding::kFollowTerrainRelativeLandingGlideSlope) ? terrain_alt : pos_sp_curr.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;
+	float altitude_setpoint;
 
 	if (_current_altitude > glide_slope_reference_alt + glide_slope_rel_alt) {
 		// descend to the glide slope
@@ -1706,6 +1703,7 @@ FixedwingPositionControl::control_auto_landing(const hrt_abstime &now, const flo
 	// flare at the maximum of the altitude determined by the time before touchdown and a minimum flare altitude
 	const float flare_rel_alt = math::max(_param_fw_lnd_fl_time.get() * _local_pos.vz, _param_fw_lnd_flalt.get());
 
+	// the terrain estimate (if enabled) is always used to determine the flaring altitude
 	if ((_current_altitude < terrain_alt + flare_rel_alt) || _flaring) {
 		// flare and land with minimal speed
 
@@ -2655,7 +2653,7 @@ FixedwingPositionControl::calculateLandingApproachVector() const
 float
 FixedwingPositionControl::getLandingTerrainAltitudeEstimate(const hrt_abstime &now, const float land_point_altitude)
 {
-	if (_param_fw_lnd_useter.get()) {
+	if (_param_fw_lnd_useter.get() > TerrainEstimateUseOnLanding::kDisableTerrainEstimation) {
 
 		if (_local_pos.dist_bottom_valid) {
 

src/modules/fw_pos_control_l1/FixedwingPositionControl.hpp

@@ -350,6 +350,12 @@ private:
 	// [us] time at which we had last valid terrain alt
 	hrt_abstime _last_time_terrain_alt_was_valid{0};
 
+	enum TerrainEstimateUseOnLanding {
+		kDisableTerrainEstimation = 0,
+		kTriggerFlareWithTerrainEstimate,
+		kFollowTerrainRelativeLandingGlideSlope
+	};
+
 	// AIRSPEED
 
 	float _airspeed{0.0f};
@@ -794,7 +800,7 @@ private:
 		(ParamFloat<px4::params::FW_LND_FLALT>) _param_fw_lnd_flalt,
 		(ParamFloat<px4::params::FW_LND_THRTC_SC>) _param_fw_thrtc_sc,
 		(ParamBool<px4::params::FW_LND_EARLYCFG>) _param_fw_lnd_earlycfg,
-		(ParamBool<px4::params::FW_LND_USETER>) _param_fw_lnd_useter,
+		(ParamInt<px4::params::FW_LND_USETER>) _param_fw_lnd_useter,
 
 		(ParamFloat<px4::params::FW_P_LIM_MAX>) _param_fw_p_lim_max,
 		(ParamFloat<px4::params::FW_P_LIM_MIN>) _param_fw_p_lim_min,
@@ -856,8 +862,7 @@ 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,
-		(ParamBool<px4::params::FW_LND_TER_REL>) _param_fw_lnd_ter_rel
+		(ParamInt<px4::params::FW_LND_ABORT>) _param_fw_lnd_abort
 	)
 
 };

src/modules/fw_pos_control_l1/fw_pos_control_l1_params.c

@@ -411,14 +411,20 @@ PARAM_DEFINE_FLOAT(FW_TKO_PITCH_MIN, 10.0f);
 PARAM_DEFINE_FLOAT(FW_LND_FLALT, 0.5f);
 
 /**
- * Use terrain estimate during landing. This is critical for detecting when to flare, and should be enabled if possible.
+ * Use terrain estimation during landing. This is critical for detecting when to flare, and should be enabled if possible.
  *
- * If enabled and no measurement is found within a given timeout, the landing waypoint will be used or the
- * landing will be aborted, depending on the criteria set in FW_LND_ABORT.
+ * NOTE: terrain estimate is currently solely derived from a distance sensor.
+ *
+ * If enabled and no measurement is found within a given timeout, the landing waypoint altitude will be used OR the landing
+ * will be aborted, depending on the criteria set in FW_LND_ABORT.
  *
  * If disabled, FW_LND_ABORT terrain based criteria are ignored.
  *
- * @boolean
+ * @min 0
+ * @max 2
+ * @value 0 Disable the terrain estimate
+ * @value 1 Use the terrain estimate to trigger the flare (only)
+ * @value 2 Calculate landing glide slope relative to the terrain estimate
  * @group FW L1 Control
  */
 PARAM_DEFINE_INT32(FW_LND_USETER, 1);
@@ -1070,17 +1076,3 @@ PARAM_DEFINE_INT32(FW_LND_NUDGE, 2);
  * @group FW L1 Control
  */
 PARAM_DEFINE_INT32(FW_LND_ABORT, 3);
-
-/**
- * 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);