Merge pull request 'feat: UWB anchor auto-calibration via inter-anchor ranging + MDS (Issue #602)' (#608) from sl-uwb/issue-602-anchor-calibration into main

- peer_range_once(): DS-TWR initiator role toward a peer anchor
  (POLL → RESP → FINAL, one-sided range estimate Ra - Da/2)
- AT+PEER_RANGE=<id>: returns +PEER_RANGE:<my>,<peer>,<mm>,<rssi>
  or +PEER_RANGE:ERR,<peer>,TIMEOUT

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

esp32/uwb_anchor/src/main.cpp

@@ -27,6 +27,8 @@
  *   AT+RANGE_ADDR=<hex_addr>   → pair with specific tag (filter others)
  *   AT+RANGE_ADDR=             → clear pairing (accept all tags)
  *   AT+ID?                     → returns +ID:<anchor_id>
+ *   AT+PEER_RANGE=<id>         → inter-anchor DS-TWR (for auto-calibration)
+ *                                → +PEER_RANGE:<my>,<peer>,<mm>,<rssi>
  *
  * Pin mapping — Makerfabs ESP32 UWB Pro
  * ──────────────────────────────────────
@@ -188,6 +190,116 @@ static float rx_power_dbm(void) {
     return 10.0f * log10f((f * f) / (n * n)) - 121.74f;
 }
 
+/* ── Peer-anchor ranging (initiator role, for auto-calibration) ─── */
+
+/* Timeout waiting for peer's RESP during inter-anchor ranging */
+#define PEER_RX_RESP_TIMEOUT_US   3500
+/* Block up to this many ms waiting for the interrupt flags */
+#define PEER_WAIT_MS              20
+
+/* Initiate a single DS-TWR exchange toward peer anchor `peer_id`.
+ * Returns range in mm (>=0) on success, or -1 on timeout/error.
+ * RSSI is stored in *rssi_out if non-null.
+ *
+ * Exchange:
+ *   This anchor  → peer  POLL
+ *   peer         → This  RESP  (carries T_poll_rx, T_resp_tx)
+ *   This anchor  → peer  FINAL (carries Ra, Da)
+ *   This side computes its own range estimate from Ra/Da.
+ */
+static int32_t peer_range_once(uint8_t peer_id, float *rssi_out) {
+    /* ── Reset interrupt flags ── */
+    g_tx_done = g_rx_ok = g_rx_err = g_rx_to = false;
+
+    /* ── Build POLL frame ── */
+    uint8_t poll_buf[FRAME_HDR + FCS_LEN] = {
+        FTYPE_POLL,
+        (uint8_t)ANCHOR_ID,
+        peer_id,
+    };
+
+    dwt_writetxdata(sizeof(poll_buf), poll_buf, 0);
+    dwt_writetxfctrl(sizeof(poll_buf), 0, 1);
+    dwt_setrxtimeout(PEER_RX_RESP_TIMEOUT_US);
+    dwt_rxenable(DWT_START_RX_IMMEDIATE);
+    if (dwt_starttx(DWT_START_TX_IMMEDIATE | DWT_RESPONSE_EXPECTED) != DWT_SUCCESS)
+        return -1;
+
+    /* Wait for TX done */
+    uint32_t t0 = millis();
+    while (!g_tx_done && !g_rx_err && !g_rx_to) {
+        if (millis() - t0 > (uint32_t)PEER_WAIT_MS) return -1;
+    }
+    if (g_rx_err || g_rx_to) return -1;
+    g_tx_done = false;
+
+    /* Capture T_poll_tx */
+    uint64_t T_poll_tx;
+    dwt_readtxtimestamp((uint8_t *)&T_poll_tx);
+    T_poll_tx &= DWT_TS_MASK;
+
+    /* Wait for RESP */
+    t0 = millis();
+    while (!g_rx_ok && !g_rx_err && !g_rx_to) {
+        if (millis() - t0 > (uint32_t)PEER_WAIT_MS) return -1;
+    }
+    if (!g_rx_ok) return -1;
+
+    /* Validate RESP */
+    if (g_rx_len < (uint32_t)(FRAME_HDR + RESP_PAYLOAD + FCS_LEN)) return -1;
+    if (g_rx_buf[0] != FTYPE_RESP)              return -1;
+    if (g_rx_buf[1] != peer_id)                  return -1;
+    if (g_rx_buf[2] != (uint8_t)ANCHOR_ID)       return -1;
+
+    uint64_t T_resp_rx;
+    dwt_readrxtimestamp((uint8_t *)&T_resp_rx);
+    T_resp_rx &= DWT_TS_MASK;
+
+    /* Extract peer timestamps from RESP payload */
+    const uint8_t *pl = g_rx_buf + FRAME_HDR;
+    uint64_t T_poll_rx_peer = ts_read(pl);
+    uint64_t T_resp_tx_peer = ts_read(pl + 5);
+
+    /* DS-TWR Ra, Da */
+    uint64_t Ra = ts_diff(T_resp_rx, T_poll_tx);
+    uint64_t Da = ts_diff(T_resp_tx_peer, T_poll_rx_peer);
+
+    g_rx_ok = g_rx_err = g_rx_to = false;
+
+    /* ── Build FINAL frame ── */
+    uint8_t final_buf[FRAME_HDR + FINAL_PAYLOAD + FCS_LEN];
+    final_buf[0] = FTYPE_FINAL;
+    final_buf[1] = (uint8_t)ANCHOR_ID;
+    final_buf[2] = peer_id;
+    ts_write(final_buf + FRAME_HDR,      Ra);
+    ts_write(final_buf + FRAME_HDR + 5,  Da);
+    ts_write(final_buf + FRAME_HDR + 10, (uint64_t)0);  /* Db placeholder */
+
+    dwt_setrxtimeout(0);
+    dwt_writetxdata(sizeof(final_buf), final_buf, 0);
+    dwt_writetxfctrl(sizeof(final_buf), 0, 1);
+    if (dwt_starttx(DWT_START_TX_IMMEDIATE) != DWT_SUCCESS) return -1;
+
+    t0 = millis();
+    while (!g_tx_done && !g_rx_err) {
+        if (millis() - t0 > (uint32_t)PEER_WAIT_MS) return -1;
+    }
+    g_tx_done = false;
+
+    /* Simplified one-sided range estimate: tof = Ra - Da/2 */
+    double tof_s = ticks_to_s(Ra) - ticks_to_s(Da) / 2.0;
+    if (tof_s < 0.0) tof_s = 0.0;
+    int32_t range_mm = (int32_t)(tof_s * SPEED_OF_LIGHT * 1000.0);
+
+    if (rssi_out) *rssi_out = rx_power_dbm();
+
+    /* Re-enable normal RX for tag ranging */
+    dwt_setrxtimeout(0);
+    dwt_rxenable(DWT_START_RX_IMMEDIATE);
+
+    return range_mm;
+}
+
 /* ── AT command handler ─────────────────────────────────────────── */
 
 static char g_at_buf[64];
@@ -212,6 +324,23 @@ static void at_dispatch(const char *cmd) {
             g_paired_tag_id = (uint8_t)strtoul(v, nullptr, 0);
             Serial.printf("+OK:PAIRED=0x%02X\r\n", g_paired_tag_id);
         }
+
+    } else if (strncmp(cmd, "AT+PEER_RANGE=", 14) == 0) {
+        /* Inter-anchor ranging for calibration.
+         * Usage: AT+PEER_RANGE=<peer_anchor_id>
+         * Response: +PEER_RANGE:<my_id>,<peer_id>,<range_mm>,<rssi_dbm>
+         *       or: +PEER_RANGE:ERR,<peer_id>,TIMEOUT
+         */
+        uint8_t peer_id = (uint8_t)strtoul(cmd + 14, nullptr, 0);
+        float rssi = 0.0f;
+        int32_t mm = peer_range_once(peer_id, &rssi);
+        if (mm >= 0) {
+            Serial.printf("+PEER_RANGE:%d,%d,%ld,%.1f\r\n",
+                          ANCHOR_ID, peer_id, mm, (double)rssi);
+        } else {
+            Serial.printf("+PEER_RANGE:ERR,%d,TIMEOUT\r\n", peer_id);
+        }
+
     } else {
         Serial.println("+ERR:UNKNOWN_CMD");
     }

include/balance.h

@@ -3,6 +3,7 @@
 
 #include <stdint.h>
 #include "mpu6000.h"
+#include "slope_estimator.h"
 
 /*
  * SaltyLab Balance Controller
@@ -36,6 +37,9 @@ typedef struct {
     /* Safety */
     float max_tilt;         /* Cutoff angle (degrees) */
     int16_t max_speed;      /* Speed limit */
+
+    /* Slope compensation (Issue #600) */
+    slope_estimator_t slope;
 } balance_t;
 
 void balance_init(balance_t *b);

include/jlink.h

@@ -87,6 +87,7 @@
 #define JLINK_TLM_SCHED         0x85u  /* jlink_tlm_sched_t (1+N*16 bytes, Issue #550) */
 #define JLINK_TLM_MOTOR_CURRENT 0x86u  /* jlink_tlm_motor_current_t (8 bytes, Issue #584) */
 #define JLINK_TLM_FAULT_LOG     0x87u  /* jlink_tlm_fault_log_t (20 bytes, Issue #565) */
+#define JLINK_TLM_SLOPE         0x88u  /* jlink_tlm_slope_t (4 bytes, Issue #600) */
 
 /* ---- Telemetry STATUS payload (20 bytes, packed) ---- */
 typedef struct __attribute__((packed)) {
@@ -164,6 +165,14 @@ typedef struct __attribute__((packed)) {
     uint8_t  _pad;         /* reserved                                               */
 } jlink_tlm_motor_current_t;  /* 8 bytes */
 
+/* ---- Telemetry SLOPE payload (4 bytes, packed) Issue #600 ---- */
+/* Published at SLOPE_TLM_HZ (1 Hz); reports slow-adapting terrain slope estimate. */
+typedef struct __attribute__((packed)) {
+    int16_t  slope_x100;  /* slope estimate degrees x100 (0.01° resolution) */
+    uint8_t  active;      /* 1 = slope compensation enabled                 */
+    uint8_t  _pad;        /* reserved                                        */
+} jlink_tlm_slope_t;  /* 4 bytes */
+
 /* ---- Telemetry FAULT_LOG payload (20 bytes, packed) Issue #565 ---- */
 /* Sent on boot (if last fault != NONE) and in response to FAULT_LOG_GET. */
 typedef struct __attribute__((packed)) {
@@ -272,6 +281,13 @@ JLinkSchedSetBuf *jlink_get_sched_set(void);
  */
 void jlink_send_motor_current_tlm(const jlink_tlm_motor_current_t *tlm);
 
+/*
+ * jlink_send_slope_tlm(tlm) - transmit JLINK_TLM_SLOPE (0x88) frame
+ * (10 bytes total) to Jetson.  Issue #600.
+ * Rate-limiting is handled by slope_estimator_send_tlm(); call from there only.
+ */
+void jlink_send_slope_tlm(const jlink_tlm_slope_t *tlm);
+
 /*
  * jlink_send_fault_log(fl) - transmit JLINK_TLM_FAULT_LOG (0x87) frame
  * (26 bytes) on boot (if fault log non-empty) and in response to

include/slope_estimator.h

@@ -0,0 +1,101 @@
+#ifndef SLOPE_ESTIMATOR_H
+#define SLOPE_ESTIMATOR_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+/*
+ * slope_estimator — slow-adapting terrain slope estimator for Issue #600.
+ *
+ * On a slope the robot must lean slightly into the hill to stay balanced.
+ * The IMU pitch reading therefore includes both the robot's balance offset
+ * and the ground incline.  This module decouples the two by tracking the
+ * slowly-changing DC component of the pitch signal with a first-order IIR
+ * low-pass filter (time constant SLOPE_TAU_S = 5 s).
+ *
+ * HOW IT WORKS:
+ *   Every call to slope_estimator_update(pitch_deg, dt):
+ *
+ *     alpha    = dt / (SLOPE_TAU_S + dt)       // ≈ 0.0002 at 1 kHz
+ *     raw      = slope * (1 - alpha) + pitch * alpha
+ *     slope    = clamp(raw, -SLOPE_MAX_DEG, +SLOPE_MAX_DEG)
+ *
+ *   The IIR converges to the steady-state pitch in ~5 s.  Fast tilt
+ *   transients (balance corrections, steps, bumps) are attenuated by
+ *   the long time constant and do not corrupt the estimate.
+ *
+ * INTEGRATION IN BALANCE PID:
+ *   Subtract slope_estimate from the measured pitch before computing
+ *   the PID error so the controller balances around the slope surface
+ *   rather than absolute vertical:
+ *
+ *     tilt_corrected = pitch_deg - slope_estimate_deg
+ *     error = setpoint - tilt_corrected
+ *
+ *   This is equivalent to continuously adjusting the balance setpoint
+ *   to track the incline.
+ *
+ * SAFETY:
+ *   - Estimate is clamped to ±SLOPE_MAX_DEG (15°) to prevent drift from
+ *     extreme falls being mistaken for genuine slopes.
+ *   - slope_estimator_reset() zeroes the state; call on disarm or after
+ *     a tilt fault so re-arming starts fresh.
+ *
+ * TELEMETRY:
+ *   JLINK_TLM_SLOPE (0x88) published at SLOPE_TLM_HZ (1 Hz):
+ *     jlink_tlm_slope_t { int16 slope_x100, uint8 active, uint8 _pad }
+ */
+
+/* ---- Configuration ---- */
+#define SLOPE_TAU_S         5.0f    /* IIR time constant (seconds)          */
+#define SLOPE_MAX_DEG      15.0f    /* Maximum estimate magnitude (degrees) */
+#define SLOPE_TLM_HZ        1u      /* JLINK_TLM_SLOPE publish rate (Hz)    */
+
+/* ---- State ---- */
+typedef struct {
+    float    estimate_deg;   /* current slope estimate (degrees, + = nose-up) */
+    bool     enabled;        /* compensation on/off; off = estimate frozen     */
+    uint32_t last_tlm_ms;   /* timestamp of last TLM transmission             */
+} slope_estimator_t;
+
+/* ---- API ---- */
+
+/*
+ * slope_estimator_init(se) — zero state, enable estimation.
+ * Call once during system init.
+ */
+void slope_estimator_init(slope_estimator_t *se);
+
+/*
+ * slope_estimator_reset(se) — zero estimate without changing enabled flag.
+ * Call on disarm or after BALANCE_TILT_FAULT to avoid stale state on rearm.
+ */
+void slope_estimator_reset(slope_estimator_t *se);
+
+/*
+ * slope_estimator_update(se, pitch_deg, dt) — advance the IIR filter.
+ *   pitch_deg : current fused pitch angle from IMU (degrees)
+ *   dt        : loop interval (seconds)
+ * No-op if se->enabled == false or dt <= 0.
+ */
+void slope_estimator_update(slope_estimator_t *se, float pitch_deg, float dt);
+
+/*
+ * slope_estimator_get_deg(se) — return current estimate (degrees).
+ * Returns 0 if disabled.
+ */
+float slope_estimator_get_deg(const slope_estimator_t *se);
+
+/*
+ * slope_estimator_set_enabled(se, en) — enable or disable compensation.
+ * Disabling freezes the estimate at its current value.
+ */
+void slope_estimator_set_enabled(slope_estimator_t *se, bool en);
+
+/*
+ * slope_estimator_send_tlm(se, now_ms) — transmit JLINK_TLM_SLOPE (0x88)
+ * frame to Jetson.  Rate-limited to SLOPE_TLM_HZ; safe to call every tick.
+ */
+void slope_estimator_send_tlm(const slope_estimator_t *se, uint32_t now_ms);
+
+#endif /* SLOPE_ESTIMATOR_H */

jetson/ros2_ws/src/saltybot_uwb_calibration/config/calibration_params.yaml

@@ -0,0 +1,17 @@
+uwb_anchor_calibration:
+  ros__parameters:
+    # Serial ports for each anchor, indexed by anchor ID
+    # udev rules create stable symlinks in /dev/uwb-anchor<N>
+    anchor_serials:
+      - /dev/uwb-anchor0
+      - /dev/uwb-anchor1
+      - /dev/uwb-anchor2
+      - /dev/uwb-anchor3
+
+    baud_rate: 115200
+
+    # Seconds to wait for a +PEER_RANGE response per attempt
+    peer_range_timeout_s: 2.0
+
+    # Range measurements to average per anchor pair (higher = more accurate)
+    default_n_samples: 5

jetson/ros2_ws/src/saltybot_uwb_calibration/launch/uwb_calibration.launch.py

@@ -0,0 +1,32 @@
+"""
+uwb_calibration.launch.py — Launch the UWB anchor auto-calibration service.
+
+Usage:
+    ros2 launch saltybot_uwb_calibration uwb_calibration.launch.py
+
+Then trigger from command line:
+    ros2 service call /saltybot/uwb/calibrate_anchors \\
+        saltybot_uwb_calibration_msgs/srv/CalibrateAnchors \\
+        "{anchor_ids: [], n_samples: 5}"
+"""
+
+from launch import LaunchDescription
+from launch_ros.actions import Node
+from ament_index_python.packages import get_package_share_directory
+import os
+
+
+def generate_launch_description() -> LaunchDescription:
+    pkg_dir = get_package_share_directory("saltybot_uwb_calibration")
+    params_file = os.path.join(pkg_dir, "config", "calibration_params.yaml")
+
+    calibration_node = Node(
+        package="saltybot_uwb_calibration",
+        executable="uwb_calibration",
+        name="uwb_anchor_calibration",
+        parameters=[params_file],
+        output="screen",
+        emulate_tty=True,
+    )
+
+    return LaunchDescription([calibration_node])

jetson/ros2_ws/src/saltybot_uwb_calibration/package.xml

@@ -0,0 +1,29 @@
+<?xml version="1.0"?>
+<?xml-model href="http://download.ros.org/schema/package_format3.xsd" schematypens="http://www.w3.org/2001/XMLSchema"?>
+<package format="3">
+  <name>saltybot_uwb_calibration</name>
+  <version>0.1.0</version>
+  <description>
+    UWB anchor auto-calibration for SaltyBot (Issue #602).
+    Commands anchors to range against each other via AT+PEER_RANGE=,
+    builds a pairwise distance matrix, and uses classical MDS to recover
+    2-D anchor positions.  Exposes /saltybot/uwb/calibrate_anchors service.
+  </description>
+  <maintainer email="sl-uwb@saltylab.local">sl-uwb</maintainer>
+  <license>Apache-2.0</license>
+
+  <depend>rclpy</depend>
+  <depend>saltybot_uwb_calibration_msgs</depend>
+
+  <exec_depend>python3-numpy</exec_depend>
+  <exec_depend>python3-serial</exec_depend>
+
+  <test_depend>ament_copyright</test_depend>
+  <test_depend>ament_flake8</test_depend>
+  <test_depend>ament_pep257</test_depend>
+  <test_depend>python3-pytest</test_depend>
+
+  <export>
+    <build_type>ament_python</build_type>
+  </export>
+</package>

jetson/ros2_ws/src/saltybot_uwb_calibration/saltybot_uwb_calibration/calibration_node.py

@@ -0,0 +1,272 @@
+"""
+calibration_node.py — UWB anchor auto-calibration ROS2 service (Issue #602)
+
+Service:  /saltybot/uwb/calibrate_anchors  (CalibrateAnchors)
+
+Workflow
+────────
+  1. For every ordered pair (i, j) with i < j, send AT+PEER_RANGE=<j> over
+     anchor i's serial port, collect `n_samples` measurements, average them.
+  2. Build the symmetric N×N distance matrix D.
+  3. Run classical MDS (mds_math.mds_2d) to recover (x, y) positions.
+  4. Align the frame: anchor-0 at origin, anchor-1 on +X axis.
+  5. Return positions + residual RMS + JSON config string.
+
+Parameters (ROS2, see config/calibration_params.yaml)
+──────────────────────────────────────────────────────
+  anchor_serials   list of serial port paths, index = anchor ID
+                   e.g. ["/dev/uwb-anchor0", "/dev/uwb-anchor1", ...]
+  baud_rate        115200
+  peer_range_timeout_s  2.0   (per AT+PEER_RANGE= attempt)
+  default_n_samples     5
+"""
+
+from __future__ import annotations
+
+import json
+import re
+import time
+from typing import Optional
+
+import numpy as np
+import serial
+
+import rclpy
+from rclpy.node import Node
+
+from saltybot_uwb_calibration_msgs.srv import CalibrateAnchors
+from saltybot_uwb_calibration.mds_math import mds_2d, anchor_frame_align
+
+
+_PEER_RANGE_RE = re.compile(
+    r"\+PEER_RANGE:(\d+),(\d+),(\d+),([-\d.]+)"
+)
+_PEER_ERR_RE = re.compile(
+    r"\+PEER_RANGE:ERR,(\d+),(\w+)"
+)
+
+
+class CalibrationNode(Node):
+
+    def __init__(self) -> None:
+        super().__init__("uwb_anchor_calibration")
+
+        self.declare_parameter("anchor_serials", [
+            "/dev/uwb-anchor0",
+            "/dev/uwb-anchor1",
+            "/dev/uwb-anchor2",
+            "/dev/uwb-anchor3",
+        ])
+        self.declare_parameter("baud_rate", 115200)
+        self.declare_parameter("peer_range_timeout_s", 2.0)
+        self.declare_parameter("default_n_samples", 5)
+
+        self._serials: list[str] = (
+            self.get_parameter("anchor_serials").value
+        )
+        self._baud: int = self.get_parameter("baud_rate").value
+        self._timeout: float = self.get_parameter("peer_range_timeout_s").value
+        self._default_n: int = self.get_parameter("default_n_samples").value
+
+        self._srv = self.create_service(
+            CalibrateAnchors,
+            "/saltybot/uwb/calibrate_anchors",
+            self._handle_calibrate,
+        )
+
+        self.get_logger().info(
+            f"UWB calibration service ready — "
+            f"{len(self._serials)} configured anchors"
+        )
+
+    # ── Service handler ────────────────────────────────────────────────────
+
+    def _handle_calibrate(
+        self,
+        request: CalibrateAnchors.Request,
+        response: CalibrateAnchors.Response,
+    ) -> CalibrateAnchors.Response:
+
+        # Determine anchor IDs to calibrate
+        if request.anchor_ids:
+            ids = list(request.anchor_ids)
+        else:
+            ids = list(range(len(self._serials)))
+
+        n_anchors = len(ids)
+        if n_anchors < 4:
+            response.success = False
+            response.message = (
+                f"Need at least 4 anchors, got {n_anchors}"
+            )
+            return response
+
+        n_samples = int(request.n_samples) if request.n_samples > 0 \
+            else self._default_n
+
+        self.get_logger().info(
+            f"Starting calibration: anchors={ids}, n_samples={n_samples}"
+        )
+
+        # Open serial ports
+        ports: dict[int, serial.Serial] = {}
+        try:
+            for anchor_id in ids:
+                if anchor_id >= len(self._serials):
+                    raise RuntimeError(
+                        f"No serial port configured for anchor {anchor_id}"
+                    )
+                port_path = self._serials[anchor_id]
+                ports[anchor_id] = serial.Serial(
+                    port_path,
+                    baudrate=self._baud,
+                    timeout=self._timeout,
+                )
+                # Flush any stale data
+                ports[anchor_id].reset_input_buffer()
+                time.sleep(0.05)
+                self.get_logger().info(
+                    f"  Opened anchor {anchor_id} → {port_path}"
+                )
+        except Exception as exc:
+            _close_all(ports)
+            response.success = False
+            response.message = f"Serial open failed: {exc}"
+            return response
+
+        # Measure all unique pairs
+        D = np.zeros((n_anchors, n_anchors), dtype=float)
+        id_to_idx = {aid: idx for idx, aid in enumerate(ids)}
+
+        try:
+            for i_idx, i_id in enumerate(ids):
+                for j_idx, j_id in enumerate(ids):
+                    if j_idx <= i_idx:
+                        continue
+
+                    dist_m = self._measure_pair(
+                        ports[i_id], i_id, j_id, n_samples
+                    )
+                    if dist_m is None:
+                        raise RuntimeError(
+                            f"No valid range between anchor {i_id} "
+                            f"and anchor {j_id}"
+                        )
+                    D[i_idx, j_idx] = dist_m
+                    D[j_idx, i_idx] = dist_m
+                    self.get_logger().info(
+                        f"  [{i_id}↔{j_id}] = {dist_m:.3f} m"
+                    )
+
+        except Exception as exc:
+            _close_all(ports)
+            response.success = False
+            response.message = str(exc)
+            return response
+
+        _close_all(ports)
+
+        # MDS
+        try:
+            positions, residual = mds_2d(D)
+            positions = anchor_frame_align(positions)
+        except Exception as exc:
+            response.success = False
+            response.message = f"MDS failed: {exc}"
+            return response
+
+        # Build JSON config
+        pos_dict = {}
+        for idx, aid in enumerate(ids):
+            pos_dict[str(aid)] = [
+                round(float(positions[idx, 0]), 4),
+                round(float(positions[idx, 1]), 4),
+                0.0,
+            ]
+        json_str = json.dumps(pos_dict, indent=2)
+
+        self.get_logger().info(
+            f"Calibration complete — residual RMS {residual:.4f} m\n{json_str}"
+        )
+
+        # Fill response
+        response.success = True
+        response.message = (
+            f"OK — residual RMS {residual*1000:.1f} mm"
+        )
+        response.anchor_ids = [int(a) for a in ids]
+        response.positions_x = [float(positions[i, 0]) for i in range(n_anchors)]
+        response.positions_y = [float(positions[i, 1]) for i in range(n_anchors)]
+        response.positions_z = [0.0] * n_anchors
+        response.residual_rms_m = float(residual)
+        response.anchor_positions_json = json_str
+        return response
+
+    # ── Per-pair measurement ───────────────────────────────────────────────
+
+    def _measure_pair(
+        self,
+        port: serial.Serial,
+        my_id: int,
+        peer_id: int,
+        n_samples: int,
+    ) -> Optional[float]:
+        """
+        Send AT+PEER_RANGE=<peer_id> `n_samples` times over `port`.
+        Returns the mean range in metres, or None if all attempts fail.
+        """
+        measurements: list[float] = []
+
+        for _ in range(n_samples):
+            cmd = f"AT+PEER_RANGE={peer_id}\r\n"
+            port.write(cmd.encode())
+            port.flush()
+
+            deadline = time.monotonic() + self._timeout
+            while time.monotonic() < deadline:
+                line = port.readline().decode(errors="replace").strip()
+                if not line:
+                    continue
+
+                m = _PEER_RANGE_RE.match(line)
+                if m:
+                    # +PEER_RANGE:<my_id>,<peer_id>,<mm>,<rssi>
+                    if int(m.group(1)) == my_id and int(m.group(2)) == peer_id:
+                        mm = float(m.group(3))
+                        measurements.append(mm / 1000.0)  # mm → m
+                        break
+
+                if _PEER_ERR_RE.match(line):
+                    break  # timeout from firmware — try next sample
+
+            time.sleep(0.05)  # inter-command gap
+
+        if not measurements:
+            return None
+        return float(np.mean(measurements))
+
+
+# ── Helpers ────────────────────────────────────────────────────────────────
+
+def _close_all(ports: dict) -> None:
+    for p in ports.values():
+        try:
+            p.close()
+        except Exception:
+            pass
+
+
+def main(args=None) -> None:
+    rclpy.init(args=args)
+    node = CalibrationNode()
+    try:
+        rclpy.spin(node)
+    except KeyboardInterrupt:
+        pass
+    finally:
+        node.destroy_node()
+        rclpy.try_shutdown()
+
+
+if __name__ == "__main__":
+    main()

jetson/ros2_ws/src/saltybot_uwb_calibration/saltybot_uwb_calibration/mds_math.py

@@ -0,0 +1,140 @@
+"""
+mds_math.py — Classical Multi-Dimensional Scaling for UWB anchor calibration
+             (Issue #602)
+
+Given an N×N symmetric distance matrix D (in metres), recover the 2-D
+Cartesian positions of N anchors using classical (metric) MDS.
+
+Algorithm
+─────────
+  1. Compute the double-centred squared-distance matrix B = -½ H D² H
+     where H = I - (1/N) 11ᵀ  (centering matrix).
+  2. Eigen-decompose B = V Λ Vᵀ (symmetric).
+  3. Keep the 2 largest positive eigenvalues (λ₁, λ₂).
+  4. Positions X = V[:, :2] · diag(√λ₁, √λ₂).
+
+The recovered configuration is up to a global rigid transformation
+(rotation, reflection, translation).  The caller is responsible for
+anchoring the coordinate frame (e.g. fix anchor-0 at origin, anchor-1
+on the positive-X axis).
+
+Usage
+─────
+  from saltybot_uwb_calibration.mds_math import mds_2d, anchor_frame_align
+
+  D = np.array([[0, d01, d02, d03],
+                [d01, 0, d12, d13],
+                ...])
+  positions, residual = mds_2d(D)          # (N,2) array, float RMS (m)
+  positions = anchor_frame_align(positions) # align frame: anchor-0 at origin
+"""
+
+from __future__ import annotations
+
+import numpy as np
+
+
+def mds_2d(D: np.ndarray) -> tuple[np.ndarray, float]:
+    """
+    Recover 2-D anchor positions from pairwise distance matrix D.
+
+    Parameters
+    ----------
+    D : (N, N) symmetric ndarray of float
+        Pairwise distances in metres.  Diagonal must be 0.
+        N must be >= 3 (4+ recommended for overdetermined system).
+
+    Returns
+    -------
+    positions : (N, 2) ndarray
+        Recovered x, y coordinates in metres.
+    residual_rms : float
+        RMS of (D_measured - D_recovered) in metres across all unique pairs.
+
+    Raises
+    ------
+    ValueError
+        If D is not square, not symmetric, or N < 3.
+    """
+    D = np.asarray(D, dtype=float)
+    n = D.shape[0]
+    if D.ndim != 2 or D.shape[1] != n:
+        raise ValueError(f"D must be square, got shape {D.shape}")
+    if n < 3:
+        raise ValueError(f"Need at least 3 anchors, got {n}")
+    if not np.allclose(D, D.T, atol=1e-3):
+        raise ValueError("Distance matrix D is not symmetric")
+
+    # ── Step 1: double-centre D² ──────────────────────────────────────────
+    D2 = D ** 2
+    H = np.eye(n) - np.ones((n, n)) / n
+    B = -0.5 * H @ D2 @ H
+
+    # ── Step 2: eigendecomposition ────────────────────────────────────────
+    eigvals, eigvecs = np.linalg.eigh(B)      # eigvals in ascending order
+
+    # ── Step 3: keep top-2 positive eigenvalues ───────────────────────────
+    idx = np.argsort(eigvals)[::-1]           # descending
+    top2_idx = idx[:2]
+    lam = eigvals[top2_idx]
+    lam = np.maximum(lam, 0.0)               # clamp numerical negatives
+
+    # ── Step 4: recover positions ─────────────────────────────────────────
+    V = eigvecs[:, top2_idx]
+    positions = V * np.sqrt(lam)             # (N, 2)
+
+    # ── Residual RMS ──────────────────────────────────────────────────────
+    residual_rms = _residual(D, positions)
+
+    return positions, residual_rms
+
+
+def anchor_frame_align(positions: np.ndarray) -> np.ndarray:
+    """
+    Align recovered positions so that:
+      - anchor-0 is at the origin (0, 0)
+      - anchor-1 lies on the positive-X axis (y = 0)
+
+    This removes the MDS ambiguity (translation + rotation + reflection).
+
+    Parameters
+    ----------
+    positions : (N, 2) ndarray
+
+    Returns
+    -------
+    (N, 2) ndarray  — aligned positions
+    """
+    positions = np.array(positions, dtype=float)
+    if positions.shape[0] < 2:
+        return positions
+
+    # Translate so anchor-0 is at origin
+    positions -= positions[0]
+
+    # Rotate so anchor-1 is on positive-X axis
+    dx, dy = positions[1, 0], positions[1, 1]
+    angle = np.arctan2(dy, dx)
+    cos_a, sin_a = np.cos(-angle), np.sin(-angle)
+    R = np.array([[cos_a, -sin_a],
+                  [sin_a,  cos_a]])
+    positions = (R @ positions.T).T
+
+    # Ensure anchor-1 has positive X (handle reflection)
+    if positions[1, 0] < 0:
+        positions[:, 0] *= -1.0
+
+    return positions
+
+
+def _residual(D_meas: np.ndarray, positions: np.ndarray) -> float:
+    """Compute RMS of (measured - recovered) distances over all unique pairs."""
+    n = D_meas.shape[0]
+    errors = []
+    for i in range(n):
+        for j in range(i + 1, n):
+            d_rec = float(np.linalg.norm(positions[i] - positions[j]))
+            errors.append(D_meas[i, j] - d_rec)
+    if not errors:
+        return 0.0
+    return float(np.sqrt(np.mean(np.array(errors) ** 2)))

jetson/ros2_ws/src/saltybot_uwb_calibration/setup.cfg

@@ -0,0 +1,4 @@
+[develop]
+script_dir=$base/lib/saltybot_uwb_calibration
+[install]
+install_scripts=$base/lib/saltybot_uwb_calibration

jetson/ros2_ws/src/saltybot_uwb_calibration/setup.py

@@ -0,0 +1,29 @@
+import os
+from glob import glob
+from setuptools import setup
+
+package_name = "saltybot_uwb_calibration"
+
+setup(
+    name=package_name,
+    version="0.1.0",
+    packages=[package_name],
+    data_files=[
+        ("share/ament_index/resource_index/packages",
+            [f"resource/{package_name}"]),
+        (f"share/{package_name}", ["package.xml"]),
+        (os.path.join("share", package_name, "launch"), glob("launch/*.py")),
+        (os.path.join("share", package_name, "config"), glob("config/*.yaml")),
+    ],
+    install_requires=["setuptools"],
+    zip_safe=True,
+    maintainer="sl-uwb",
+    maintainer_email="sl-uwb@saltylab.local",
+    description="UWB anchor auto-calibration via MDS (Issue #602)",
+    license="Apache-2.0",
+    entry_points={
+        "console_scripts": [
+            "uwb_calibration = saltybot_uwb_calibration.calibration_node:main",
+        ],
+    },
+)

jetson/ros2_ws/src/saltybot_uwb_calibration/test/test_mds_math.py

@@ -0,0 +1,142 @@
+"""
+Unit tests for saltybot_uwb_calibration.mds_math (Issue #602).
+No ROS2 or hardware required — pure numpy.
+"""
+
+import math
+import sys
+import os
+
+import numpy as np
+import pytest
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
+from saltybot_uwb_calibration.mds_math import mds_2d, anchor_frame_align, _residual
+
+
+def _dist_matrix(positions: np.ndarray) -> np.ndarray:
+    """Build exact pairwise distance matrix from ground-truth positions."""
+    n = len(positions)
+    D = np.zeros((n, n))
+    for i in range(n):
+        for j in range(n):
+            D[i, j] = np.linalg.norm(positions[i] - positions[j])
+    return D
+
+
+# ── Square configuration (4 anchors) ──────────────────────────────────────
+
+GT_SQUARE = np.array([
+    [0.0, 0.0],
+    [4.0, 0.0],
+    [4.0, 3.0],
+    [0.0, 3.0],
+])
+
+
+class TestMdsSquare:
+    def test_residual_near_zero(self):
+        D = _dist_matrix(GT_SQUARE)
+        positions, rms = mds_2d(D)
+        assert rms < 1e-6, f"Residual too large: {rms}"
+
+    def test_correct_distances_preserved(self):
+        D = _dist_matrix(GT_SQUARE)
+        positions, _ = mds_2d(D)
+        positions = anchor_frame_align(positions)
+        D_rec = _dist_matrix(positions)
+        # All pairwise distances should match to < 1 mm
+        assert np.max(np.abs(D - D_rec)) < 1e-3
+
+    def test_anchor_0_at_origin(self):
+        D = _dist_matrix(GT_SQUARE)
+        positions, _ = mds_2d(D)
+        positions = anchor_frame_align(positions)
+        assert abs(positions[0, 0]) < 1e-9
+        assert abs(positions[0, 1]) < 1e-9
+
+    def test_anchor_1_on_positive_x(self):
+        D = _dist_matrix(GT_SQUARE)
+        positions, _ = mds_2d(D)
+        positions = anchor_frame_align(positions)
+        assert positions[1, 0] > 0
+        assert abs(positions[1, 1]) < 1e-9
+
+
+# ── Non-uniform configuration (5 anchors) ─────────────────────────────────
+
+GT_5 = np.array([
+    [0.0, 0.0],
+    [5.0, 0.0],
+    [5.0, 4.0],
+    [2.5, 6.0],
+    [0.0, 4.0],
+])
+
+
+class TestMds5Anchors:
+    def test_residual_near_zero(self):
+        D = _dist_matrix(GT_5)
+        positions, rms = mds_2d(D)
+        assert rms < 1e-6
+
+    def test_shape(self):
+        D = _dist_matrix(GT_5)
+        positions, _ = mds_2d(D)
+        assert positions.shape == (5, 2)
+
+
+# ── Noisy distances (simulate UWB measurement noise) ──────────────────────
+
+class TestMdsNoisyInput:
+    def test_residual_acceptable_with_5cm_noise(self):
+        rng = np.random.default_rng(42)
+        D = _dist_matrix(GT_SQUARE)
+        noise = rng.normal(0, 0.05, D.shape)
+        noise = (noise + noise.T) / 2   # keep symmetric
+        np.fill_diagonal(noise, 0)
+        D_noisy = np.maximum(D + noise, 0)
+        positions, rms = mds_2d(D_noisy)
+        # With 5 cm noise the residual should be sub-10 cm
+        assert rms < 0.10, f"Residual {rms:.4f} m too large"
+
+
+# ── Error handling ─────────────────────────────────────────────────────────
+
+class TestMdsErrors:
+    def test_non_square_raises(self):
+        with pytest.raises(ValueError):
+            mds_2d(np.ones((3, 4)))
+
+    def test_too_few_anchors_raises(self):
+        with pytest.raises(ValueError):
+            mds_2d(np.array([[0.0, 1.0], [1.0, 0.0]]))
+
+    def test_asymmetric_raises(self):
+        D = _dist_matrix(GT_SQUARE)
+        D[0, 1] += 0.5  # break symmetry
+        with pytest.raises(ValueError):
+            mds_2d(D)
+
+
+# ── anchor_frame_align ─────────────────────────────────────────────────────
+
+class TestFrameAlign:
+    def test_translated_positions_aligned(self):
+        pts = GT_SQUARE.copy() + np.array([10.0, -5.0])
+        aligned = anchor_frame_align(pts)
+        assert abs(aligned[0, 0]) < 1e-9
+        assert abs(aligned[0, 1]) < 1e-9
+
+    def test_rotated_positions_aligned(self):
+        angle = math.pi / 3
+        R = np.array([[math.cos(angle), -math.sin(angle)],
+                      [math.sin(angle),  math.cos(angle)]])
+        pts = (R @ GT_SQUARE.T).T
+        aligned = anchor_frame_align(pts)
+        assert aligned[1, 0] > 0
+        assert abs(aligned[1, 1]) < 1e-9
+
+
+if __name__ == "__main__":
+    pytest.main([__file__, "-v"])

jetson/ros2_ws/src/saltybot_uwb_calibration_msgs/CMakeLists.txt

@@ -0,0 +1,12 @@
+cmake_minimum_required(VERSION 3.8)
+project(saltybot_uwb_calibration_msgs)
+
+find_package(ament_cmake REQUIRED)
+find_package(rosidl_default_generators REQUIRED)
+
+rosidl_generate_interfaces(${PROJECT_NAME}
+  "srv/CalibrateAnchors.srv"
+)
+
+ament_export_dependencies(rosidl_default_runtime)
+ament_package()

jetson/ros2_ws/src/saltybot_uwb_calibration_msgs/package.xml

@@ -0,0 +1,23 @@
+<?xml version="1.0"?>
+<?xml-model href="http://download.ros.org/schema/package_format3.xsd" schematypens="http://www.w3.org/2001/XMLSchema"?>
+<package format="3">
+  <name>saltybot_uwb_calibration_msgs</name>
+  <version>0.1.0</version>
+  <description>
+    Service definitions for UWB anchor auto-calibration (Issue #602).
+    Provides CalibrateAnchors.srv used by saltybot_uwb_calibration node.
+  </description>
+  <maintainer email="sl-uwb@saltylab.local">sl-uwb</maintainer>
+  <license>Apache-2.0</license>
+
+  <buildtool_depend>ament_cmake</buildtool_depend>
+  <buildtool_depend>rosidl_default_generators</buildtool_depend>
+
+  <exec_depend>rosidl_default_runtime</exec_depend>
+
+  <member_of_group>rosidl_interface_packages</member_of_group>
+
+  <export>
+    <build_type>ament_cmake</build_type>
+  </export>
+</package>

jetson/ros2_ws/src/saltybot_uwb_calibration_msgs/srv/CalibrateAnchors.srv

@@ -0,0 +1,29 @@
+# CalibrateAnchors.srv — UWB anchor auto-calibration service (Issue #602)
+#
+# Request: list of anchor IDs to calibrate (minimum 4).
+#          If anchor_ids is empty, the node uses all discovered anchors.
+#
+# Each anchor is commanded via AT+PEER_RANGE=<peer_id> to range against every
+# other anchor.  The resulting N×N distance matrix is passed through classical
+# MDS to recover 2-D positions.  Results are written to anchor_positions_json.
+#
+# Response: success flag, per-anchor [x, y, z] positions, residual RMS error,
+#           and JSON string suitable for dropping into fusion_params.yaml.
+
+# ── Request ────────────────────────────────────────────────────────────────
+uint8[] anchor_ids          # IDs of anchors to calibrate (empty = auto-discover)
+uint8   n_samples           # range measurements to average per pair (default 5)
+
+---
+
+# ── Response ───────────────────────────────────────────────────────────────
+bool    success
+string  message             # human-readable status / error description
+
+uint8[]  anchor_ids         # same order as positions below
+float64[] positions_x       # metres, MDS-recovered X (2-D plane)
+float64[] positions_y       # metres, MDS-recovered Y
+float64[] positions_z       # metres, always 0.0 (flat-floor assumption)
+
+float64 residual_rms_m      # RMS of |D_measured - D_recovered| across all pairs
+string  anchor_positions_json  # JSON: {"0": [x,y,z], "1": [x,y,z], ...}

src/balance.c

@@ -1,4 +1,5 @@
 #include "balance.h"
+#include "slope_estimator.h"
 #include "config.h"
 #include <math.h>
 
@@ -19,6 +20,9 @@ void balance_init(balance_t *b) {
     /* Safety defaults from config */
     b->max_tilt = MAX_TILT_DEG;
     b->max_speed = MAX_SPEED_LIMIT;
+
+    /* Slope estimator */
+    slope_estimator_init(&b->slope);
 }
 
 void balance_update(balance_t *b, const IMUData *imu, float dt) {
@@ -28,12 +32,16 @@ void balance_update(balance_t *b, const IMUData *imu, float dt) {
     b->pitch_deg  = imu->pitch;
     b->pitch_rate = imu->pitch_rate;
 
+    /* Advance slope estimator (no-op when not armed) */
+    slope_estimator_update(&b->slope, b->pitch_deg, dt);
+
     /* Safety: tilt cutoff */
     if (b->state == BALANCE_ARMED) {
         if (fabsf(b->pitch_deg) > b->max_tilt) {
             b->state = BALANCE_TILT_FAULT;
             b->motor_cmd = 0;
             b->integral = 0.0f;
+            slope_estimator_reset(&b->slope);
             return;
         }
     }
@@ -45,8 +53,9 @@ void balance_update(balance_t *b, const IMUData *imu, float dt) {
         return;
     }
 
-    /* PID */
+    /* PID — subtract slope estimate so controller balances on incline */
-    float error = b->setpoint - b->pitch_deg;
+    float tilt_corrected = b->pitch_deg - slope_estimator_get_deg(&b->slope);
+    float error = b->setpoint - tilt_corrected;
 
     /* Proportional */
     float p_term = b->kp * error;
@@ -85,4 +94,5 @@ void balance_disarm(balance_t *b) {
     b->state = BALANCE_DISARMED;
     b->motor_cmd = 0;
     b->integral = 0.0f;
+    slope_estimator_reset(&b->slope);
 }

src/jlink.c

@@ -541,6 +541,30 @@ void jlink_send_sched_telemetry(const jlink_tlm_sched_t *tlm)
     jlink_tx_locked(frame, (uint16_t)(3u + plen + 3u));
 }
 
+/* ---- jlink_send_slope_tlm() -- Issue #600 ---- */
+void jlink_send_slope_tlm(const jlink_tlm_slope_t *tlm)
+{
+    /*
+     * Frame: [STX][LEN][0x88][4 bytes SLOPE][CRC_hi][CRC_lo][ETX]
+     * Total: 1+1+1+4+2+1 = 10 bytes
+     */
+    static uint8_t frame[10];
+    const uint8_t  plen = (uint8_t)sizeof(jlink_tlm_slope_t);  /* 4 */
+    const uint8_t  len  = 1u + plen;  /* CMD byte + payload */
+
+    frame[0] = JLINK_STX;
+    frame[1] = len;
+    frame[2] = JLINK_TLM_SLOPE;
+    memcpy(&frame[3], tlm, plen);
+
+    uint16_t crc = crc16_xmodem(&frame[2], len);
+    frame[3 + plen]     = (uint8_t)(crc >> 8);
+    frame[3 + plen + 1] = (uint8_t)(crc & 0xFFu);
+    frame[3 + plen + 2] = JLINK_ETX;
+
+    jlink_tx_locked(frame, sizeof(frame));
+}
+
 /* ---- jlink_send_fault_log() -- Issue #565 ---- */
 void jlink_send_fault_log(const jlink_tlm_fault_log_t *fl)
 {

src/slope_estimator.c

@@ -0,0 +1,82 @@
+/*
+ * slope_estimator.c — slow-adapting terrain slope estimator (Issue #600).
+ *
+ * First-order IIR low-pass filter on fused IMU pitch with tau = SLOPE_TAU_S (5 s).
+ * The long time constant means fast balance corrections and transients are
+ * ignored; only sustained pitch bias (i.e., genuine slope) accumulates.
+ *
+ * Estimate is clamped to ±SLOPE_MAX_DEG (15°) per the safety requirement.
+ */
+
+#include "slope_estimator.h"
+#include "jlink.h"
+
+/* ---- slope_estimator_init() ---- */
+void slope_estimator_init(slope_estimator_t *se)
+{
+    se->estimate_deg  = 0.0f;
+    se->enabled       = true;
+    /* Initialize so first send_tlm call fires immediately */
+    se->last_tlm_ms   = (uint32_t)(-(uint32_t)(1000u / (SLOPE_TLM_HZ > 0u ? SLOPE_TLM_HZ : 1u)));
+}
+
+/* ---- slope_estimator_reset() ---- */
+void slope_estimator_reset(slope_estimator_t *se)
+{
+    se->estimate_deg = 0.0f;
+}
+
+/* ---- slope_estimator_update() ---- */
+void slope_estimator_update(slope_estimator_t *se, float pitch_deg, float dt)
+{
+    if (!se->enabled || dt <= 0.0f) return;
+
+    /*
+     * First-order IIR: alpha = dt / (tau + dt)
+     * At 1 kHz (dt=0.001): alpha ≈ 0.0002  → time to ~63% ≈ 5 s
+     * At 100 Hz (dt=0.01):  alpha ≈ 0.002   → time to ~63% ≈ 5 s (same tau)
+     */
+    float alpha = dt / (SLOPE_TAU_S + dt);
+    float raw   = se->estimate_deg * (1.0f - alpha) + pitch_deg * alpha;
+
+    /* Clamp to ±SLOPE_MAX_DEG */
+    if (raw >  SLOPE_MAX_DEG) raw =  SLOPE_MAX_DEG;
+    if (raw < -SLOPE_MAX_DEG) raw = -SLOPE_MAX_DEG;
+
+    se->estimate_deg = raw;
+}
+
+/* ---- slope_estimator_get_deg() ---- */
+float slope_estimator_get_deg(const slope_estimator_t *se)
+{
+    if (!se->enabled) return 0.0f;
+    return se->estimate_deg;
+}
+
+/* ---- slope_estimator_set_enabled() ---- */
+void slope_estimator_set_enabled(slope_estimator_t *se, bool en)
+{
+    se->enabled = en;
+}
+
+/* ---- slope_estimator_send_tlm() ---- */
+void slope_estimator_send_tlm(const slope_estimator_t *se, uint32_t now_ms)
+{
+    if (SLOPE_TLM_HZ == 0u) return;
+
+    uint32_t interval_ms = 1000u / SLOPE_TLM_HZ;
+    if ((now_ms - se->last_tlm_ms) < interval_ms) return;
+    /* Cast away const for timestamp update -- only mutable field */
+    ((slope_estimator_t *)se)->last_tlm_ms = now_ms;
+
+    jlink_tlm_slope_t tlm;
+    /* Scale to ×100 for 0.01° resolution, clamped to int16 range */
+    float scaled = se->estimate_deg * 100.0f;
+    if (scaled >  32767.0f) scaled =  32767.0f;
+    if (scaled < -32768.0f) scaled = -32768.0f;
+    tlm.slope_x100 = (int16_t)scaled;
+    tlm.active     = se->enabled ? 1u : 0u;
+    tlm._pad       = 0u;
+
+    jlink_send_slope_tlm(&tlm);
+}

test/test_slope_estimator.c

@@ -0,0 +1,421 @@
+/*
+ * test_slope_estimator.c — Unit tests for slope estimator (Issue #600).
+ *
+ * Build (host, no hardware):
+ *   gcc -I include -I test/stubs -DTEST_HOST -lm \
+ *       -o /tmp/test_slope src/slope_estimator.c test/test_slope_estimator.c
+ *
+ * All tests are self-contained; no HAL, no ADC, no UART required.
+ * slope_estimator.c calls jlink_send_slope_tlm() which is stubbed below.
+ */
+
+/* ---- Stubs ---- */
+
+/* Prevent jlink.h from pulling in HAL / pid_flash types */
+#define JLINK_H
+#include <stdint.h>
+#include <stdbool.h>
+
+/* Minimal jlink_tlm_slope_t matching the real definition */
+typedef struct __attribute__((packed)) {
+    int16_t  slope_x100;
+    uint8_t  active;
+    uint8_t  _pad;
+} jlink_tlm_slope_t;
+
+/* Capture last transmitted tlm for inspection */
+static jlink_tlm_slope_t g_last_tlm;
+static int g_tlm_count = 0;
+
+void jlink_send_slope_tlm(const jlink_tlm_slope_t *tlm)
+{
+    g_last_tlm = *tlm;
+    g_tlm_count++;
+}
+
+/* ---- Include implementation directly ---- */
+#include "../src/slope_estimator.c"
+
+/* ---- Test framework ---- */
+#include <stdio.h>
+#include <math.h>
+#include <string.h>
+
+static int g_pass = 0;
+static int g_fail = 0;
+
+#define ASSERT(cond, msg) do { \
+    if (cond) { g_pass++; } \
+    else { g_fail++; printf("FAIL [%s:%d] %s\n", __FILE__, __LINE__, msg); } \
+} while(0)
+
+#define ASSERT_NEAR(a, b, tol, msg) \
+    ASSERT(fabsf((a)-(b)) <= (tol), msg)
+
+/* ---- Tests ---- */
+
+static void test_init_zeroes_state(void)
+{
+    slope_estimator_t se;
+    /* dirty the memory first */
+    memset(&se, 0xAB, sizeof(se));
+    slope_estimator_init(&se);
+    ASSERT(se.estimate_deg == 0.0f,  "init: estimate_deg == 0");
+    ASSERT(se.enabled == true,        "init: enabled == true");
+}
+
+static void test_get_when_disabled_returns_zero(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    /* Force a non-zero estimate by running a few ticks, then disable */
+    slope_estimator_update(&se, 10.0f, 0.1f);
+    slope_estimator_set_enabled(&se, false);
+    ASSERT(slope_estimator_get_deg(&se) == 0.0f,
+           "get while disabled must return 0");
+}
+
+static void test_update_when_disabled_no_change(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    slope_estimator_set_enabled(&se, false);
+    slope_estimator_update(&se, 10.0f, 0.01f);
+    ASSERT(se.estimate_deg == 0.0f,
+           "update while disabled must not change estimate");
+}
+
+static void test_update_zero_dt_no_change(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    slope_estimator_update(&se, 5.0f, 0.0f);
+    ASSERT(se.estimate_deg == 0.0f,
+           "update with dt=0 must not change estimate");
+}
+
+static void test_update_negative_dt_no_change(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    slope_estimator_update(&se, 5.0f, -0.001f);
+    ASSERT(se.estimate_deg == 0.0f,
+           "update with dt<0 must not change estimate");
+}
+
+static void test_single_step_converges_toward_input(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    /* One update with dt=0.01s, pitch=10 deg */
+    slope_estimator_update(&se, 10.0f, 0.01f);
+    /* alpha = 0.01 / (5.0 + 0.01) ≈ 0.002; new estimate ≈ 0 + 0.002*10 = 0.02 */
+    ASSERT(se.estimate_deg > 0.0f,
+           "estimate should move toward positive pitch");
+    ASSERT(se.estimate_deg < 10.0f,
+           "estimate should not jump to full pitch in one step");
+}
+
+static void test_iir_reaches_63pct_in_one_tau(void)
+{
+    /* Run estimator at 1 kHz for exactly SLOPE_TAU_S seconds */
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    const float pitch   = 10.0f;
+    const float dt      = 0.001f;
+    const int   steps   = (int)(SLOPE_TAU_S / dt);  /* 5000 steps */
+    for (int i = 0; i < steps; i++) {
+        slope_estimator_update(&se, pitch, dt);
+    }
+    /* IIR should be at ≈63.2% of the step */
+    float expected_lo = pitch * 0.60f;
+    float expected_hi = pitch * 0.66f;
+    ASSERT(se.estimate_deg >= expected_lo && se.estimate_deg <= expected_hi,
+           "IIR should reach ~63% of step at t=tau");
+}
+
+static void test_iir_reaches_63pct_at_100hz(void)
+{
+    /* Same tau, but at 100 Hz */
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    const float pitch   = 8.0f;
+    const float dt      = 0.01f;
+    const int   steps   = (int)(SLOPE_TAU_S / dt);  /* 500 steps */
+    for (int i = 0; i < steps; i++) {
+        slope_estimator_update(&se, pitch, dt);
+    }
+    float expected_lo = pitch * 0.60f;
+    float expected_hi = pitch * 0.66f;
+    ASSERT(se.estimate_deg >= expected_lo && se.estimate_deg <= expected_hi,
+           "IIR 100 Hz: should reach ~63% at t=tau");
+}
+
+static void test_positive_clamp(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    /* Drive with a huge pitch for a long time — estimate must clamp at SLOPE_MAX_DEG */
+    for (int i = 0; i < 100000; i++) {
+        slope_estimator_update(&se, 90.0f, 0.001f);
+    }
+    ASSERT_NEAR(se.estimate_deg, SLOPE_MAX_DEG, 0.001f,
+                "estimate must clamp at +SLOPE_MAX_DEG");
+}
+
+static void test_negative_clamp(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    for (int i = 0; i < 100000; i++) {
+        slope_estimator_update(&se, -90.0f, 0.001f);
+    }
+    ASSERT_NEAR(se.estimate_deg, -SLOPE_MAX_DEG, 0.001f,
+                "estimate must clamp at -SLOPE_MAX_DEG");
+}
+
+static void test_reset_zeroes_estimate(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    /* Build up some estimate */
+    for (int i = 0; i < 1000; i++) {
+        slope_estimator_update(&se, 12.0f, 0.001f);
+    }
+    ASSERT(se.estimate_deg > 0.1f, "pre-reset: estimate should be non-zero");
+    slope_estimator_reset(&se);
+    ASSERT(se.estimate_deg == 0.0f, "post-reset: estimate should be zero");
+    /* enabled state must be preserved */
+    ASSERT(se.enabled == true, "reset must not change enabled flag");
+}
+
+static void test_reset_preserves_disabled_state(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    slope_estimator_set_enabled(&se, false);
+    slope_estimator_reset(&se);
+    ASSERT(se.enabled == false, "reset must not re-enable disabled estimator");
+}
+
+static void test_balance_setpoint_compensation(void)
+{
+    /*
+     * Simulate: robot on 5-deg slope, pitch = 5 deg, slope estimate converges.
+     * After convergence, tilt_corrected = pitch - slope ≈ 0.
+     * Verify that the effective PID error (setpoint=0, tilt_corrected≈0) is near 0.
+     */
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    const float slope_deg = 5.0f;
+    const float dt        = 0.001f;
+    const int   steps     = (int)(3.0f * SLOPE_TAU_S / dt);  /* 3 tau = ~95% converged */
+
+    for (int i = 0; i < steps; i++) {
+        /* Robot holds steady on slope — pitch stays constant */
+        slope_estimator_update(&se, slope_deg, dt);
+    }
+
+    float est   = slope_estimator_get_deg(&se);
+    float tilt_corrected = slope_deg - est;
+
+    /* After 3 tau, estimate should be within 5% of slope */
+    ASSERT(est >= slope_deg * 0.90f, "3-tau estimate >= 90% of slope");
+    /* tilt_corrected should be close to zero — minimal PID error */
+    ASSERT(fabsf(tilt_corrected) < slope_deg * 0.10f,
+           "corrected tilt error < 10% of slope after 3 tau");
+}
+
+static void test_fast_tilt_not_tracked(void)
+{
+    /*
+     * Simulate: robot starts balanced (pitch≈0), then tips to 20 deg suddenly
+     * for 100 ms (balance intervention), then returns to 0.
+     * The slope estimate should not move significantly.
+     */
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    const float dt = 0.001f;
+
+    /* 2 s of stable balancing at pitch ≈ 0 */
+    for (int i = 0; i < 2000; i++) {
+        slope_estimator_update(&se, 0.0f, dt);
+    }
+    float before = se.estimate_deg;
+
+    /* 100 ms spike to 20 deg (fast tilt) */
+    for (int i = 0; i < 100; i++) {
+        slope_estimator_update(&se, 20.0f, dt);
+    }
+    /* Return to 0 for another 100 ms */
+    for (int i = 0; i < 100; i++) {
+        slope_estimator_update(&se, 0.0f, dt);
+    }
+    float after = se.estimate_deg;
+
+    /* 200ms / 5000ms = 4% of tau — estimate should move < 1 deg */
+    ASSERT(fabsf(after - before) < 1.0f,
+           "fast tilt transient should not significantly move slope estimate");
+}
+
+static void test_slope_change_tracks_slowly(void)
+{
+    /* Robot transitions from flat (0°) to 10° slope at t=0.
+     * After 1 tau, estimate should be ~63% of 10 = ~6.3 deg. */
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    const float dt     = 0.001f;
+    const int   steps  = (int)(SLOPE_TAU_S / dt);
+
+    for (int i = 0; i < steps; i++) {
+        slope_estimator_update(&se, 10.0f, dt);
+    }
+    ASSERT(se.estimate_deg >= 6.0f && se.estimate_deg <= 6.5f,
+           "slope change: 1-tau estimate should be 6.0-6.5 deg");
+}
+
+static void test_symmetry_negative_slope(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    const float dt    = 0.001f;
+    const int   steps = (int)(SLOPE_TAU_S / dt);
+
+    for (int i = 0; i < steps; i++) {
+        slope_estimator_update(&se, -8.0f, dt);
+    }
+    /* Should be ~63% of -8 */
+    ASSERT(se.estimate_deg <= -4.0f && se.estimate_deg >= -5.5f,
+           "negative slope: estimate should converge symmetrically");
+}
+
+static void test_enable_disable_toggle(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+
+    /* Build estimate */
+    for (int i = 0; i < 2000; i++) {
+        slope_estimator_update(&se, 10.0f, 0.001f);
+    }
+    float val = se.estimate_deg;
+    ASSERT(val > 0.1f, "estimate built before disable");
+
+    /* Disable: estimate frozen */
+    slope_estimator_set_enabled(&se, false);
+    slope_estimator_update(&se, 10.0f, 0.001f);
+    ASSERT(se.estimate_deg == val, "estimate frozen while disabled");
+
+    /* get returns 0 when disabled */
+    ASSERT(slope_estimator_get_deg(&se) == 0.0f, "get returns 0 while disabled");
+
+    /* Re-enable: estimate resumes */
+    slope_estimator_set_enabled(&se, true);
+    slope_estimator_update(&se, 10.0f, 0.001f);
+    ASSERT(se.estimate_deg > val, "estimate resumes after re-enable");
+}
+
+static void test_tlm_rate_limiting(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+
+    g_tlm_count = 0;
+
+    /* Call at 1000 Hz for 3 seconds → should send 3 TLMs (at 1 Hz rate limit) */
+    for (uint32_t ms = 0; ms < 3000; ms++) {
+        slope_estimator_send_tlm(&se, ms);
+    }
+    /* First call at ms=0 sends, then 1000ms, 2000ms → 3 total */
+    ASSERT(g_tlm_count == 3, "TLM rate-limited to SLOPE_TLM_HZ (1 Hz)");
+}
+
+static void test_tlm_payload_encoding(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+
+    /* Force a known estimate */
+    se.estimate_deg = 7.5f;
+    se.enabled = true;
+
+    g_tlm_count = 0;
+    g_last_tlm.slope_x100 = 0;
+
+    /* Trigger a TLM by advancing time enough */
+    slope_estimator_send_tlm(&se, 0u);
+
+    ASSERT(g_tlm_count == 1, "TLM sent");
+    ASSERT(g_last_tlm.slope_x100 == 750, "7.5 deg -> slope_x100 = 750");
+    ASSERT(g_last_tlm.active == 1u, "active flag set when enabled");
+}
+
+static void test_tlm_disabled_active_flag(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    slope_estimator_set_enabled(&se, false);
+    se.estimate_deg = 3.0f;
+
+    g_tlm_count = 0;
+    slope_estimator_send_tlm(&se, 0u);
+
+    ASSERT(g_tlm_count == 1, "TLM sent when disabled");
+    ASSERT(g_last_tlm.active == 0u, "active flag clear when disabled");
+    /* slope_x100 reflects stored estimate but get() returns 0 */
+    ASSERT(g_last_tlm.slope_x100 == 300, "slope_x100 encodes stored estimate");
+}
+
+static void test_tlm_clamped_to_int16(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    /* Manually set beyond int16 range for test */
+    se.estimate_deg = 400.0f;  /* 400 * 100 = 40000, within int16 */
+    se.enabled = true;
+    g_tlm_count = 0;
+    slope_estimator_send_tlm(&se, 0u);
+    ASSERT(g_last_tlm.slope_x100 == 32767, "large estimate clamped to INT16_MAX");
+}
+
+static void test_multiple_resets_idempotent(void)
+{
+    slope_estimator_t se;
+    slope_estimator_init(&se);
+    slope_estimator_reset(&se);
+    slope_estimator_reset(&se);
+    ASSERT(se.estimate_deg == 0.0f, "multiple resets: still zero");
+    ASSERT(se.enabled == true, "multiple resets: still enabled");
+}
+
+/* ---- main ---- */
+int main(void)
+{
+    printf("=== test_slope_estimator ===\n");
+
+    test_init_zeroes_state();
+    test_get_when_disabled_returns_zero();
+    test_update_when_disabled_no_change();
+    test_update_zero_dt_no_change();
+    test_update_negative_dt_no_change();
+    test_single_step_converges_toward_input();
+    test_iir_reaches_63pct_in_one_tau();
+    test_iir_reaches_63pct_at_100hz();
+    test_positive_clamp();
+    test_negative_clamp();
+    test_reset_zeroes_estimate();
+    test_reset_preserves_disabled_state();
+    test_balance_setpoint_compensation();
+    test_fast_tilt_not_tracked();
+    test_slope_change_tracks_slowly();
+    test_symmetry_negative_slope();
+    test_enable_disable_toggle();
+    test_tlm_rate_limiting();
+    test_tlm_payload_encoding();
+    test_tlm_disabled_active_flag();
+    test_tlm_clamped_to_int16();
+    test_multiple_resets_idempotent();
+
+    printf("\nResults: %d passed, %d failed\n", g_pass, g_fail);
+    return g_fail ? 1 : 0;
+}