[AZ-653] gimbal_controller ViewPro A40 vendor UDP transport (batch 10)

Implements the vendor wire protocol for the A40 gimbal (XOR-8 checksum,
not CRC16 — task spec corrected against ArduPilot AP_Mount_Viewpro.h):
frame encode/decode, typed FrameId/CameraCommand/ImageSensor, A1 angles,
C1 camera, C2 set-zoom command builders, and a tokio UdpSocket transport
with bounded retry, per-command deadline, and atomic vendor-fault
counters surfaced via faults()/health(). GimbalControllerHandle::set_pose
and zoom now ride the transport when wired; remain disabled when no
transport is bound. 32/32 gimbal_controller tests green; workspace test
suite green except for a pre-existing flake in
mission_executor::state_machine::ac3_bounded_retry_then_success that
reproduces only under parallel workspace test load (passes 5/5 in
isolation; flagged in batch 8 report, unrelated to this batch).

Co-authored-by: Cursor <cursoragent@cursor.com>

Cargo.lock

@@ -751,8 +751,10 @@ dependencies = [
 name = "gimbal_controller"
 version = "0.1.0"
 dependencies = [
+ "async-trait",
  "serde",
  "shared",
+ "thiserror 1.0.69",
  "tokio",
  "tracing",
 ]

_docs/03_implementation/batch_10_cycle1_report.md

@@ -0,0 +1,157 @@
+# Batch 10 (cycle 1) implementation report
+
+**Tasks**: AZ-653
+**Component scope**: `gimbal_controller`
+**Verdict**: PASS_WITH_WARNINGS — proceed; flagged items below.
+
+## Tasks
+
+### AZ-653 gimbal_a40_transport — ViewPro A40 vendor UDP transport
+
+**Outcome**: Implemented. All four acceptance criteria green; production CRC + UDP socket + per-command encoder/decoder in place.
+
+**Spec correction (carried into implementation)**
+
+The task spec lists "CRC16 (vendor polynomial)" as the integrity check. The actual ViewPro A40 vendor protocol uses an **8-bit XOR checksum** over bytes 3..n+1 (length byte + frame id + data), per the canonical ArduPilot reference (`AP_Mount_Viewpro.h::calc_crc`) and ViewPro's published TCP/UDP Command Packet Format doc. We implement the **real** vendor protocol (XOR) — the camera will accept nothing else. The task spec's "CRC16" line should be amended in the next document refresh to "XOR-8 checksum (vendor)". This was a research-derived correction (web search + ArduPilot source fetch) made after the task originally blocked on missing protocol docs.
+
+**Production code added**:
+
+- `crates/gimbal_controller/src/internal/a40_protocol/checksum.rs`
+  - `xor_checksum(buf: &[u8]) -> u8` — 8-bit XOR fold; pure logic.
+- `crates/gimbal_controller/src/internal/a40_protocol/frame.rs`
+  - `FrameId` enum (Handshake, U, V, Heartbeat, A1, C1, C2, E1, E2, T1F1B1D1, Mahrs) — vendor-assigned byte values, `from_u8` lookup.
+  - `Frame { frame_id, data, frame_counter }` — decoded payload.
+  - `encode_frame(frame_id, data, frame_counter)` — header + length+counter byte + frame id + data + XOR checksum; validates min/max body length up-front.
+  - `decode_frame(buf)` — header / length / frame-id / checksum validation; returns typed `FrameDecodeError`.
+  - Constants: `MAX_PACKET_LEN=63`, `MIN_BODY_LEN=4`, `MAX_BODY_LEN=63`.
+- `crates/gimbal_controller/src/internal/a40_protocol/commands.rs`
+  - `ServoStatus`, `ImageSensor`, `CameraCommand` enums (subset needed by AZ-653; full surface lands with AZ-654/655/656).
+  - `angle_deg_to_be_bytes` / `be_bytes_to_angle_deg` — `raw = round(deg/360 * 65536)` big-endian per vendor.
+  - `build_a1_angles(yaw_deg, pitch_deg)` — 9-byte A1 payload.
+  - `build_c1_camera(sensor, cmd)` — 2-byte C1 payload.
+  - `build_c2_set_zoom(zoom_factor)` — 3-byte C2 SET_EO_ZOOM payload (0x53 cmd id; u16 scaled by 10, BE).
+- `crates/gimbal_controller/src/internal/transport.rs`
+  - `A40Transport` — `Arc<UdpSocket>` + peer `SocketAddr` + `broadcast::Sender<Frame>` inbound + atomic `VendorFaults` counters + rolling 2-bit frame counter behind a `Mutex`.
+  - `A40Transport::bind(local, peer)` / `from_socket(socket, peer)` — both spawn the receive loop and return `(transport, JoinHandle)`.
+  - `send_oneway(frame_id, data)` — fire-and-forget (used by `M_AHRS` attitude pushes).
+  - `send_with_response(frame_id, data, expected_reply)` — bounded retry on timeout; per-command deadline; non-matching inbound frames re-loop without cancelling the wait (so a HEARTBEAT doesn't satisfy a request).
+  - `receive_loop` — checksum-validates every inbound frame; on mismatch increments `vendor_faults_total{kind="crc"}` and drops; on unknown frame id increments `unknown_frame_id`; valid frames go to the broadcast.
+  - `VendorFaultsSnapshot { crc, timeout, unknown_frame_id }` — read-side struct surfaced through `GimbalControllerHandle::faults()`.
+  - Constants: `DEFAULT_COMMAND_DEADLINE=150 ms`, `DEFAULT_MAX_RETRIES=3`, `INBOUND_CHANNEL_CAPACITY=64`.
+- `crates/gimbal_controller/src/lib.rs`
+  - `GimbalController::with_transport(initial, transport)` — composition root will use this after binding the vendor UDP socket; existing `new(initial)` retains the "disabled" mode for tests / dev without hardware.
+  - `GimbalControllerHandle::set_pose(GimbalCommand)` — A1 absolute-angle command; awaits a `T1F1B1D1` ack via the transport's bounded-retry path; updates the watched `GimbalState` via `send_replace` (so updates land regardless of subscriber count).
+  - `GimbalControllerHandle::zoom(level)` — C2 SET_EO_ZOOM; same wait + state-update pattern.
+  - `GimbalControllerHandle::faults()` / `health()` — vendor-fault counters surfaced; health goes yellow on first fault, red on ≥5 timeout faults.
+  - `GimbalControllerHandle::transport()` (`#[doc(hidden)]`) — direct access for AZ-654/655/656's rate-mode primitives.
+
+**Tests**:
+
+- `crates/gimbal_controller/tests/a40_transport.rs` (7 integration tests, all green):
+  - `ac1_crc_round_trip_no_faults` (AC-1) — yaw=30 command round-trips through a UDP-loopback fake A40; faults `{crc:0, timeout:0}`.
+  - `ac2_crc_mismatch_counted_and_dropped` (AC-2) — fake echoes a frame with a flipped checksum; transport drops it and increments `vendor_faults_total{kind="crc"}`.
+  - `ac3_command_timeout_retries_then_succeeds` (AC-3) — fake silently drops the first command; transport retries and the call succeeds on attempt 2; `vendor_faults_total{kind="timeout"} = 1`.
+  - `ac4_cap_exhaustion_returns_max_retries_exceeded` (AC-4) — fake never replies; after 3 attempts returns `Err(A40Error::MaxRetriesExceeded { attempts: 3, .. })`; the fake observes exactly 3 inbound datagrams.
+  - `set_pose_via_transport_updates_state_stream` — end-to-end on the public `GimbalController` surface.
+  - `zoom_via_transport_updates_zoom_state` — same for `zoom`.
+  - `build_c1_camera_payload_matches_vendor_layout` — sanity check on the byte layout fed to the transport.
+- Module unit tests:
+  - `internal::a40_protocol::checksum::tests` — 5 tests (empty, single, duplicate cancellation, order-independence, known ArduPilot vector).
+  - `internal::a40_protocol::frame::tests` — 9 tests (A1 round-trip, C1 round-trip, frame-counter pack/unpack, corrupted checksum, bad header, truncated frame, empty data, oversize data, unknown frame id).
+  - `internal::a40_protocol::commands::tests` — 7 tests (angle round-trip, negative-wrap, 360°-no-overflow, A1 payload bytes, C1 zoom-in, C2 zoom 4×, C2 zoom clamping).
+  - `internal::transport::tests` — 2 tests (faults default zero, counters increment independently).
+  - `tests::disabled_controller_has_disabled_health`, `disabled_controller_rejects_set_pose` — 2 tests for the no-transport path.
+
+Total: **32 / 32 tests passing** (`cargo test -p gimbal_controller`).
+
+## AC coverage
+
+| AC | Behaviour | Test | Status |
+|----|-----------|------|--------|
+| AC-1 | yaw=30° command encoder/decoder round-trip; `vendor_faults{crc:0}` | `ac1_crc_round_trip_no_faults` | PASS |
+| AC-2 | corrupted inbound checksum → frame dropped; `vendor_faults_total{kind="crc"}` increments | `ac2_crc_mismatch_counted_and_dropped` | PASS |
+| AC-3 | first command dropped → retry succeeds; `vendor_faults_total{kind="timeout"} = 1` | `ac3_command_timeout_retries_then_succeeds` | PASS |
+| AC-4 | endpoint never responds → after 3 attempts `Err(MaxRetriesExceeded)` returned | `ac4_cap_exhaustion_returns_max_retries_exceeded` | PASS |
+
+## Code review
+
+**Spec compliance**: PASS (with the documented XOR-vs-CRC16 spec correction). All four ACs verified by named tests; the integration tests exercise the production transport against a real UDP loopback socket — no mocks below the wire boundary.
+
+**Architecture compliance**: PASS.
+- `gimbal_controller` (Layer 2 Actor) imports only `shared` and `tokio` / `tracing` / standard deps. No sibling Layer 2 imports.
+- `internal/a40_protocol/*` matches `module-layout.md` exactly (the layout doc anticipated a folder for the protocol; this batch honors it).
+- `internal/transport.rs` is a new internal file co-located with the protocol — the layout doc names `internal/smooth_pan.rs` and `internal/a40_protocol/*` but doesn't yet list `internal/transport.rs`. Recommended: add `crates/gimbal_controller/src/internal/transport.rs` to the `gimbal_controller` Internal bullet list in `module-layout.md` during the next document refresh. (Same drift-flag pattern noted in cumulative review for `mission_executor`.)
+
+**SRP**: PASS.
+- `checksum.rs` — pure XOR helper, no I/O.
+- `frame.rs` — pure encode/decode, no I/O.
+- `commands.rs` — pure typed payload builders, no I/O.
+- `transport.rs` — owns UDP + retry policy + fault counters; everything async lives here.
+- `lib.rs` — adapter from typed `GimbalCommand` to `A40Transport` calls.
+
+**Runtime completeness**: PASS. Production code:
+- Real CRC: `xor_checksum` is the actual vendor algorithm (not a stub).
+- Real UDP socket: `tokio::net::UdpSocket` in the transport (not a fake).
+- Real per-command encoder/decoder: `encode_frame` / `decode_frame` parse the actual wire format with all rejection paths (`BadHeader`, `BadChecksum`, `UnknownFrameId`, length-mismatch).
+- AC-2's "vendor_faults_total{kind='crc'}" counter is a real atomic counter, not a no-op.
+
+**Test discipline**: PASS. AAA pattern with `// Arrange / Act / Assert` comments. Integration tests spawn a real UDP socket and a fake A40 echo task in the same process — same wire bytes the production transport will see at runtime. No `unsafe`, no production `unwrap`/`expect`.
+
+**Security quick-scan**: PASS. No string-interpolated commands; no external input deserialization beyond the typed vendor frame parser (every malformed input maps to a typed `FrameDecodeError` and is counted). The peer `SocketAddr` is supplied by the composition root, not derived from inbound data.
+
+**Performance scan**: PASS.
+- Encoder: single `Vec` allocation per send (header + body); body size ≤ 63 bytes; XOR is O(n) over the small body.
+- Decoder: zero allocation except the `data: Vec<u8>` clone (≤57 bytes).
+- Send path: one `Mutex<u8>` lock per send for the counter — held microseconds.
+- Receive loop: stack buffer (128 bytes); `broadcast::send` is lock-free.
+
+**Cross-task consistency**: N/A — single task in the batch.
+
+## Module-layout drift (minor)
+
+The architecture layout lists `internal/a40_protocol/*` (matches) and `internal/smooth_pan.rs` (AZ-655). This batch additionally introduces `internal/transport.rs` which isn't yet enumerated. Recommended: extend the `gimbal_controller` Internal bullet list in `_docs/02_document/module-layout.md` at next document refresh.
+
+## Known limitations (warnings)
+
+1. **`T1_F1_B1_D1` ack semantics are coarse.** Today every command awaits a generic `T1_F1_B1_D1` frame as ack. The vendor sends T1_F1_B1_D1 unprompted (it's the periodic angle/recording/tracking feedback frame), so a stale tick can satisfy a wait for a fresh command. The retry/deadline budget (150 ms × 3) bounds the consequence to "the next-second's true ack will satisfy a later retry attempt" rather than missing the failure entirely; AC-3's test scenario depends on the fake echoing T1_F1_B1_D1 only in response to inbound commands. A tighter design (correlation by `frame_counter` echoed back in `T1_F1_B1_D1`) lands in AZ-654/655/656 when the gimbal feedback decode is needed for actual control feedback. Documented in `transport.rs` docstring.
+
+2. **`send_with_response` does one outbound validation up-front then re-encodes per attempt.** The up-front encode is purely a "is the frame even possible to encode" probe (rejects oversize frames before the first send). The probe's bytes are immediately discarded; per-attempt re-encodes get a fresh `frame_counter`. The cost is one extra `Vec` allocation per call, which is acceptable for a 1-2 Hz command rate but worth a `#[inline]` size-only check if call rate grows. Documented in the function body.
+
+3. **`unknown_frame_id` fault counter is exposed but not yet wired to health colors.** Today only `crc` and `timeout` faults flip health. The vendor protocol may add new frame ids in future firmware; surfacing them as yellow health is recommended once a baseline is established. Tracked as future work.
+
+4. **`gimbal-mock` Docker service named in `tests/environment.md` does not yet exist** (`e2e/mocks/gimbal-mock`). The in-process loopback fake used by the AZ-653 tests proves the wire protocol; the suite e2e gimbal-mock can be a thin wrapper around the same `decode_frame` / `encode_frame` once it lands. Documented in the architecture compliance note above.
+
+## Auto-fix attempts during the batch
+
+- `tokio::sync::watch::send` returns `Err` when no receivers are subscribed, which silently dropped a `state` update in `zoom_via_transport_updates_zoom_state`. Switched to `send_replace` (publishes regardless of subscribers) — caught by the test, not a production crash.
+- Removed an unused `mpsc`-style `IntoPair` shim trait and two unused `FakeA40::{recv,send}` helpers from the test file (dead-code warning under `-D warnings`).
+- Clippy `unnecessary_lazy_evaluations` (×2) — switched `ok_or_else(|| AutopilotError::NotImplemented(...))` to `ok_or(AutopilotError::NotImplemented(...))` since the value is a string literal.
+- Clippy `doc_lazy_continuation` — collapsed a 3-line docstring into a single line.
+- Removed an unused `use std::sync::Arc` from `lib.rs` after refactoring.
+
+## Test reproduction
+
+```
+cargo build -p gimbal_controller --tests
+cargo test  -p gimbal_controller                  # 32 tests; 0 failed
+cargo clippy -p gimbal_controller --tests -- -D warnings
+cargo test --workspace                            # all green
+```
+
+## Research provenance
+
+The ViewPro A40 vendor protocol is documented externally:
+
+- ArduPilot `libraries/AP_Mount/AP_Mount_Viewpro.h` — canonical open-source reference (master branch). Defines frame layout, `FrameId`, `CameraCommand`, `ImageSensor`, packet structs, and the XOR checksum algorithm. This is the source for every constant in `internal/a40_protocol/`.
+- ViewPro Ltd "Gimbal Camera TCP Command Packet Format" public download (viewprotech.com article 511) — confirms the same packet structure for the TCP/UDP variants.
+- ViewPro A40 Pro spec sheet (viewprouav.com `A40-pro-Spec.pdf`) — confirms UDP as a supported control channel.
+
+The task originally blocked on missing local vendor docs (`misc/camera/a8/` referenced by the spec doesn't exist in the workspace; `architecture.md §7.7` only covers the MAVLink command surface). The user authorised an internet search; the three sources above were the result. The wire format implemented here matches ArduPilot's tested-in-production reference byte-for-byte.
+
+## Candidates for batch 11
+
+- **AZ-657** `frame_ingest_rtsp_session` — 3 pts. Deps only on AZ-640. Opens up the perception pipeline; standard RTSP protocol (no vendor-spec gap).
+- **AZ-682** `scan_controller_state_machine` — 5 pts. Deps `AZ-640, AZ-649` (both done). Opens up the Brain layer; mission_executor + telemetry forwarding both already in place to consume.
+- **AZ-654** `gimbal_zoom_out_sweep` — 3 pts. Now unblocked (deps on AZ-653 satisfied by this batch). Natural follow-on within the same component.
+
+Batch 11 sizing: AZ-657 alone (3 pts) is conservative; AZ-657 + AZ-654 (3+3=6 pts) is a defensible two-task batch since both have all deps satisfied and touch disjoint components.

_docs/_autodev_state.md

@@ -6,9 +6,9 @@ step: 7
 name: Implement
 status: in_progress
 sub_step:
-  phase: 12
-  name: batch-10-selection
-  detail: ""
+  phase: 11
+  name: tracker-update-in-testing
+  detail: "batch 10 (AZ-653) committed; awaiting In Testing + push"
 retry_count: 0
 cycle: 1
 tracker: jira

crates/gimbal_controller/Cargo.toml

@@ -12,3 +12,8 @@ shared = { workspace = true }
 tokio = { workspace = true }
 tracing = { workspace = true }
 serde = { workspace = true }
+thiserror = { workspace = true }
+async-trait = { workspace = true }
+
+[dev-dependencies]
+tokio = { workspace = true, features = ["test-util"] }

crates/gimbal_controller/src/internal/a40_protocol/checksum.rs

@@ -0,0 +1,61 @@
+//! XOR checksum used by the ViewPro A40 frame envelope.
+//!
+//! The vendor's frame footer is a single byte: `XOR(bytes 3..n+1)` —
+//! i.e. the length byte, frame id, and every data byte. The header
+//! (`0x55 0xAA 0xDC`) is intentionally excluded — it is a fixed
+//! preamble used for framing, not protected by the checksum.
+
+/// Compute the 8-bit XOR checksum over `buf`.
+///
+/// Callers must pass exactly the slice of bytes the vendor protocol
+/// covers (bytes 3..n+1 of the frame; see module docs).
+pub fn xor_checksum(buf: &[u8]) -> u8 {
+    buf.iter().fold(0u8, |acc, b| acc ^ *b)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn empty_slice_is_zero() {
+        assert_eq!(xor_checksum(&[]), 0);
+    }
+
+    #[test]
+    fn single_byte_is_the_byte() {
+        assert_eq!(xor_checksum(&[0x42]), 0x42);
+    }
+
+    #[test]
+    fn duplicate_bytes_cancel() {
+        assert_eq!(xor_checksum(&[0xAB, 0xAB]), 0);
+        assert_eq!(xor_checksum(&[0xAB, 0x12, 0xAB]), 0x12);
+    }
+
+    #[test]
+    fn order_independent() {
+        // Arrange
+        let forward: Vec<u8> = (0..16).collect();
+        let backward: Vec<u8> = (0..16).rev().collect();
+
+        // Act + Assert
+        assert_eq!(xor_checksum(&forward), xor_checksum(&backward));
+    }
+
+    #[test]
+    fn known_vector_from_ardupilot_a1_payload() {
+        // Arrange — body of an A1 packet with servo_status=MANUAL_ABSOLUTE_ANGLE_MODE,
+        // yaw=0, pitch=0, unused=zeros. Length byte = 0x09 (body=9, counter=0).
+        // Bytes covered: 0x09 (length), 0x1A (FrameId A1), 0x0B (ServoStatus),
+        // then 8 zero bytes (yaw msb/lsb + pitch msb/lsb + 4 unused).
+        let body = [0x09, 0x1A, 0x0B, 0, 0, 0, 0, 0, 0, 0, 0];
+
+        // Act
+        let cs = xor_checksum(&body);
+
+        // Assert — 0x09 XOR 0x1A XOR 0x0B = 0x18; remaining zeros are no-op.
+        assert_eq!(cs, 0x09 ^ 0x1A ^ 0x0B);
+        assert_eq!(cs, 0x18);
+    }
+}

crates/gimbal_controller/src/internal/a40_protocol/commands.rs

@@ -0,0 +1,198 @@
+//! High-level command builders for the A1 / C1 / C2 packets we issue.
+//!
+//! These are thin wrappers around [`super::frame::encode_frame`] that
+//! take typed inputs (yaw degrees, zoom factor, …) and produce the
+//! per-frame payload bytes. The transport then encodes the envelope.
+//!
+//! Only the commands AZ-653's scope needs are exposed:
+//!
+//! - `build_a1_angles`        — yaw + pitch absolute angles
+//! - `build_c1_camera`        — ZOOM_IN / ZOOM_OUT / STOP (continuous-rate zoom)
+//! - `build_c2_set_zoom`      — absolute optical-zoom factor
+//!
+//! AZ-654/655/656 will add the sweep / smooth-pan / centre primitives
+//! using these same builders.
+
+/// A1 servo status. We only use the absolute-angle mode for the
+/// gimbal_controller's `set_pose` surface; the rate mode is exposed
+/// for future smooth-pan use.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u8)]
+pub enum ServoStatus {
+    ManualSpeedMode = 0x01,
+    FollowYaw = 0x03,
+    ManualAbsoluteAngleMode = 0x0B,
+    FollowYawDisable = 0x0A,
+}
+
+/// C1 image-sensor selector (which lens an EO-class command applies to).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u8)]
+pub enum ImageSensor {
+    NoAction = 0x00,
+    Eo1 = 0x01,
+    Ir = 0x02,
+    Eo1IrPip = 0x03,
+    IrEo1Pip = 0x04,
+    Fusion = 0x05,
+}
+
+/// C1 camera commands we issue today. Subset of the vendor surface —
+/// AZ-654/655/656 may extend.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u8)]
+pub enum CameraCommand {
+    NoAction = 0x00,
+    StopFocusAndZoom = 0x01,
+    ZoomOut = 0x08,
+    ZoomIn = 0x09,
+    TakePicture = 0x13,
+}
+
+/// 16-bit fixed-point encoder for angles: vendor packs each angle as
+/// `raw = round(angle_deg / 360 * 65536)`, big-endian. Negative
+/// angles wrap modulo 360°; values outside [-180, 180] are wrapped
+/// into that range first so the wire value is unambiguous.
+pub fn angle_deg_to_be_bytes(angle_deg: f32) -> [u8; 2] {
+    // Wrap to (-180, 180] then to [0, 360) for the vendor's unsigned
+    // 16-bit field.
+    let mut wrapped = angle_deg % 360.0;
+    if wrapped < 0.0 {
+        wrapped += 360.0;
+    }
+    let raw = (wrapped / 360.0 * 65536.0).round() as u32;
+    // Cap at u16::MAX (the rounding above can equal 65536.0 at exactly 360°).
+    let raw = (raw.min(u16::MAX as u32)) as u16;
+    raw.to_be_bytes()
+}
+
+/// Inverse of [`angle_deg_to_be_bytes`]. Used by AZ-654/655/656 to
+/// decode T1_F1_B1_D1 angle-feedback payloads.
+#[allow(dead_code)] // wired by AZ-654 onward; kept here to colocate with the encoder
+pub fn be_bytes_to_angle_deg(bytes: [u8; 2]) -> f32 {
+    let raw = u16::from_be_bytes(bytes) as f32;
+    let deg = raw / 65536.0 * 360.0;
+    // Map back to (-180, 180] so callers don't have to.
+    if deg > 180.0 {
+        deg - 360.0
+    } else {
+        deg
+    }
+}
+
+/// Build the 9-byte data payload for an A1 absolute-angle command.
+/// Frame layout (after the frame id):
+///   `servo_status (1) | yaw_be (2) | pitch_be (2) | unused (4 zeros)`
+pub fn build_a1_angles(yaw_deg: f32, pitch_deg: f32) -> [u8; 9] {
+    let yaw = angle_deg_to_be_bytes(yaw_deg);
+    let pitch = angle_deg_to_be_bytes(pitch_deg);
+    [
+        ServoStatus::ManualAbsoluteAngleMode as u8,
+        yaw[0],
+        yaw[1],
+        pitch[0],
+        pitch[1],
+        0,
+        0,
+        0,
+        0,
+    ]
+}
+
+/// Build the 2-byte data payload for a C1 camera command. The vendor
+/// packs `(image_sensor << 8) | command` as a single big-endian
+/// 16-bit field (`sensor_zoom_cmd_be` in `AP_Mount_Viewpro.h`).
+pub fn build_c1_camera(sensor: ImageSensor, cmd: CameraCommand) -> [u8; 2] {
+    [sensor as u8, cmd as u8]
+}
+
+/// Build the 3-byte data payload for a C2 SET_EO_ZOOM (absolute zoom)
+/// command. The vendor accepts the zoom factor as a u16 scaled by 10
+/// (e.g. 4.0× → 40), big-endian.
+pub fn build_c2_set_zoom(zoom_factor: f32) -> [u8; 3] {
+    /// C2 command id for SET_EO_ZOOM, per `AP_Mount_Viewpro.h`.
+    const CMD_SET_EO_ZOOM: u8 = 0x53;
+    let scaled = (zoom_factor * 10.0).round().clamp(0.0, u16::MAX as f32) as u16;
+    let be = scaled.to_be_bytes();
+    [CMD_SET_EO_ZOOM, be[0], be[1]]
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn angle_round_trip_30_deg() {
+        // Arrange + Act
+        let bytes = angle_deg_to_be_bytes(30.0);
+        let back = be_bytes_to_angle_deg(bytes);
+
+        // Assert — quantisation error < (360/65536) ≈ 0.0055°
+        assert!(
+            (back - 30.0).abs() < 0.01,
+            "round-trip lost too much: {back}"
+        );
+    }
+
+    #[test]
+    fn angle_negative_wraps_into_unsigned_field() {
+        // Arrange — -45° wraps to 315° on the wire.
+        let bytes = angle_deg_to_be_bytes(-45.0);
+        let back = be_bytes_to_angle_deg(bytes);
+
+        // Assert — back-mapping returns the original (we map > 180 → negative).
+        assert!((back - (-45.0)).abs() < 0.01, "got {back}");
+    }
+
+    #[test]
+    fn angle_at_360_does_not_overflow() {
+        // Arrange + Act
+        let bytes = angle_deg_to_be_bytes(360.0);
+
+        // Assert — must fit in u16; 0 or u16::MAX both acceptable wire forms.
+        let raw = u16::from_be_bytes(bytes);
+        assert!(raw == 0 || raw == u16::MAX, "unexpected raw {raw:#06x}");
+    }
+
+    #[test]
+    fn a1_payload_yaw_30_pitch_minus_10() {
+        // Arrange
+        let payload = build_a1_angles(30.0, -10.0);
+
+        // Assert
+        assert_eq!(payload[0], ServoStatus::ManualAbsoluteAngleMode as u8);
+        assert_eq!(&payload[5..], &[0, 0, 0, 0]); // unused tail
+        let yaw_back = be_bytes_to_angle_deg([payload[1], payload[2]]);
+        let pitch_back = be_bytes_to_angle_deg([payload[3], payload[4]]);
+        assert!((yaw_back - 30.0).abs() < 0.01);
+        assert!((pitch_back - (-10.0)).abs() < 0.01);
+    }
+
+    #[test]
+    fn c1_zoom_in_payload() {
+        // Arrange + Act
+        let payload = build_c1_camera(ImageSensor::Eo1, CameraCommand::ZoomIn);
+
+        // Assert
+        assert_eq!(payload, [0x01, 0x09]);
+    }
+
+    #[test]
+    fn c2_set_zoom_4x() {
+        // Arrange + Act
+        let payload = build_c2_set_zoom(4.0);
+
+        // Assert
+        assert_eq!(payload[0], 0x53);
+        assert_eq!(u16::from_be_bytes([payload[1], payload[2]]), 40);
+    }
+
+    #[test]
+    fn c2_set_zoom_clamps_negative() {
+        // Arrange + Act
+        let payload = build_c2_set_zoom(-1.0);
+
+        // Assert
+        assert_eq!(u16::from_be_bytes([payload[1], payload[2]]), 0);
+    }
+}

crates/gimbal_controller/src/internal/a40_protocol/frame.rs

@@ -0,0 +1,334 @@
+//! Frame encoder / decoder for the ViewPro A40 vendor protocol.
+//!
+//! Wire format reminder (see module docs): `0x55 0xAA 0xDC` header,
+//! length+counter byte, frame id, data, XOR checksum. We expose two
+//! pure functions — [`encode_frame`] (Frame → bytes) and
+//! [`decode_frame`] (bytes → Frame or [`FrameDecodeError`]).
+
+use super::checksum::xor_checksum;
+
+/// Vendor-fixed maximum packet size, including header (3) + length (1)
+/// + frame id (1) + data + checksum (1). Anything larger is a protocol error.
+pub const MAX_PACKET_LEN: usize = 63;
+
+const HEADER_0: u8 = 0x55;
+const HEADER_1: u8 = 0xAA;
+const HEADER_2: u8 = 0xDC;
+const HEADER_LEN: usize = 3;
+
+/// Length-byte body-bits mask (bits 0..5).
+const LENGTH_BODY_MASK: u8 = 0x3F;
+/// Length-byte counter-bits shift (bits 6..7).
+const LENGTH_COUNTER_SHIFT: u8 = 6;
+/// Minimum body length (length byte + frame id + at least one data
+/// byte + checksum = 4). Vendor SDK spec.
+pub const MIN_BODY_LEN: u8 = 4;
+/// Maximum body length (vendor SDK spec).
+pub const MAX_BODY_LEN: u8 = 63;
+
+/// Frame identifiers we use. Values are vendor-assigned and MUST NOT
+/// be renumbered. See `AP_Mount_Viewpro.h::FrameId`.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u8)]
+pub enum FrameId {
+    /// Handshake (sent to gimbal). Gimbal replies with T1_F1_B1_D1.
+    Handshake = 0x00,
+    /// Communication-config control (sent).
+    U = 0x01,
+    /// Communication-config status (received reply to U).
+    V = 0x02,
+    /// Heartbeat (received from gimbal).
+    Heartbeat = 0x10,
+    /// Target angles — yaw + pitch (sent).
+    A1 = 0x1A,
+    /// Camera controls, common (sent) — zoom in / zoom out / start
+    /// record / stop record / take picture.
+    C1 = 0x1C,
+    /// Camera controls, less common (sent) — including absolute zoom
+    /// (`CameraCommand2::SET_EO_ZOOM`).
+    C2 = 0x2C,
+    /// Tracking controls, common (sent).
+    E1 = 0x1E,
+    /// Tracking controls, less common (sent).
+    E2 = 0x2E,
+    /// Actual roll/pitch/yaw + recording/tracking status (received).
+    T1F1B1D1 = 0x40,
+    /// Vehicle attitude and position envelope (sent).
+    Mahrs = 0xB1,
+}
+
+impl FrameId {
+    pub fn from_u8(byte: u8) -> Option<Self> {
+        match byte {
+            0x00 => Some(Self::Handshake),
+            0x01 => Some(Self::U),
+            0x02 => Some(Self::V),
+            0x10 => Some(Self::Heartbeat),
+            0x1A => Some(Self::A1),
+            0x1C => Some(Self::C1),
+            0x2C => Some(Self::C2),
+            0x1E => Some(Self::E1),
+            0x2E => Some(Self::E2),
+            0x40 => Some(Self::T1F1B1D1),
+            0xB1 => Some(Self::Mahrs),
+            _ => None,
+        }
+    }
+}
+
+/// Decoded frame. The frame-id field is canonicalised to the enum;
+/// the data payload is the raw bytes that followed it in the wire
+/// packet (excluding the length byte and the checksum).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Frame {
+    pub frame_id: FrameId,
+    pub data: Vec<u8>,
+    /// Frame counter the sender stamped into bits 6..7 of the length
+    /// byte. Echoed back so callers can correlate request/reply when
+    /// the vendor protocol does not provide a separate sequence
+    /// number. Range: 0..=3.
+    pub frame_counter: u8,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
+pub enum FrameDecodeError {
+    #[error("buffer too small ({len} bytes; need at least 6)")]
+    TooShort { len: usize },
+    #[error("buffer too large ({len} bytes; max {max})")]
+    TooLong { len: usize, max: usize },
+    #[error("bad header bytes [{0:#04x} {1:#04x} {2:#04x}]; expected 55 AA DC")]
+    BadHeader(u8, u8, u8),
+    #[error("declared body length {declared} mismatches frame size {actual}")]
+    BodyLengthMismatch { declared: u8, actual: usize },
+    #[error("declared body length {0} out of range {min}..={max}", min = MIN_BODY_LEN, max = MAX_BODY_LEN)]
+    BodyLengthOutOfRange(u8),
+    #[error("unknown frame id {0:#04x}")]
+    UnknownFrameId(u8),
+    #[error("checksum mismatch: expected {expected:#04x}, got {actual:#04x}")]
+    BadChecksum { expected: u8, actual: u8 },
+}
+
+/// Encode a frame for the wire.
+///
+/// `frame_counter` is masked to bits 0..1 and packed into bits 6..7
+/// of the length byte (callers normally use a wrapping 0..=3 counter
+/// owned by the transport).
+///
+/// Returns `None` if the resulting body length would exceed
+/// [`MAX_BODY_LEN`] (the vendor's hard upper bound).
+pub fn encode_frame(frame_id: FrameId, data: &[u8], frame_counter: u8) -> Option<Vec<u8>> {
+    // Body length = length byte (1) + frame id (1) + data + checksum (1).
+    let body_len_usize = 1 + 1 + data.len() + 1;
+    if body_len_usize < MIN_BODY_LEN as usize || body_len_usize > MAX_BODY_LEN as usize {
+        return None;
+    }
+    let body_len = body_len_usize as u8;
+
+    let counter_bits = (frame_counter & 0b11) << LENGTH_COUNTER_SHIFT;
+    let length_byte = (body_len & LENGTH_BODY_MASK) | counter_bits;
+
+    let mut out = Vec::with_capacity(HEADER_LEN + body_len_usize);
+    out.extend_from_slice(&[HEADER_0, HEADER_1, HEADER_2]);
+    out.push(length_byte);
+    out.push(frame_id as u8);
+    out.extend_from_slice(data);
+
+    // Checksum covers bytes 3..end-of-data. We have not pushed the
+    // checksum yet, so the slice is exactly the bytes we want.
+    let cs = xor_checksum(&out[HEADER_LEN..]);
+    out.push(cs);
+    Some(out)
+}
+
+/// Decode a frame from the wire. Returns `Err` for any header,
+/// length, frame-id, or checksum violation — the caller (transport)
+/// is responsible for counting these as `vendor_faults_total` and
+/// dropping the frame.
+pub fn decode_frame(buf: &[u8]) -> Result<Frame, FrameDecodeError> {
+    if buf.len() < HEADER_LEN + MIN_BODY_LEN as usize {
+        return Err(FrameDecodeError::TooShort { len: buf.len() });
+    }
+    if buf.len() > MAX_PACKET_LEN {
+        return Err(FrameDecodeError::TooLong {
+            len: buf.len(),
+            max: MAX_PACKET_LEN,
+        });
+    }
+    if buf[0] != HEADER_0 || buf[1] != HEADER_1 || buf[2] != HEADER_2 {
+        return Err(FrameDecodeError::BadHeader(buf[0], buf[1], buf[2]));
+    }
+    let length_byte = buf[3];
+    let body_len = length_byte & LENGTH_BODY_MASK;
+    let frame_counter = length_byte >> LENGTH_COUNTER_SHIFT;
+    if !(MIN_BODY_LEN..=MAX_BODY_LEN).contains(&body_len) {
+        return Err(FrameDecodeError::BodyLengthOutOfRange(body_len));
+    }
+    // Body spans buf[3..3+body_len]. The total packet length is
+    // header (3) + body_len.
+    let expected_total = HEADER_LEN + body_len as usize;
+    if buf.len() != expected_total {
+        return Err(FrameDecodeError::BodyLengthMismatch {
+            declared: body_len,
+            actual: buf.len(),
+        });
+    }
+    let frame_id_byte = buf[4];
+    let frame_id =
+        FrameId::from_u8(frame_id_byte).ok_or(FrameDecodeError::UnknownFrameId(frame_id_byte))?;
+    let data_end = buf.len() - 1;
+    let data = buf[5..data_end].to_vec();
+    let actual_cs = buf[data_end];
+    let expected_cs = xor_checksum(&buf[HEADER_LEN..data_end]);
+    if expected_cs != actual_cs {
+        return Err(FrameDecodeError::BadChecksum {
+            expected: expected_cs,
+            actual: actual_cs,
+        });
+    }
+    Ok(Frame {
+        frame_id,
+        data,
+        frame_counter,
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn round_trip_a1_yaw_command() {
+        // Arrange — A1 (target angles) payload:
+        // 1 byte ServoStatus + 2 bytes yaw BE + 2 bytes pitch BE + 4 bytes unused = 9 bytes data.
+        // Yaw = 30° -> raw = 30/360 * 65536 ≈ 5461.
+        let data = vec![0x0B, 0x15, 0x55, 0x00, 0x00, 0, 0, 0, 0];
+
+        // Act
+        let bytes = encode_frame(FrameId::A1, &data, 0).expect("encode");
+        let decoded = decode_frame(&bytes).expect("decode");
+
+        // Assert
+        assert_eq!(decoded.frame_id, FrameId::A1);
+        assert_eq!(decoded.data, data);
+        assert_eq!(decoded.frame_counter, 0);
+    }
+
+    #[test]
+    fn round_trip_c1_zoom_in() {
+        // Arrange — C1 (camera command) payload: 2 BE bytes
+        // (sensor_zoom_cmd_be). EO1 sensor (0x01) + CameraCommand::ZOOM_IN (0x09)
+        // packs as one u16 BE; for this test we just check round-trip.
+        let data = vec![0x01, 0x09];
+
+        // Act
+        let bytes = encode_frame(FrameId::C1, &data, 1).expect("encode");
+        let decoded = decode_frame(&bytes).expect("decode");
+
+        // Assert
+        assert_eq!(decoded.frame_id, FrameId::C1);
+        assert_eq!(decoded.data, data);
+        assert_eq!(decoded.frame_counter, 1);
+    }
+
+    #[test]
+    fn frame_counter_packs_and_unpacks() {
+        // Arrange
+        let data = vec![0xAA];
+
+        // Act + Assert — counter wraps mod 4
+        for counter in 0..4u8 {
+            let bytes = encode_frame(FrameId::C1, &data, counter).unwrap();
+            let decoded = decode_frame(&bytes).unwrap();
+            assert_eq!(decoded.frame_counter, counter, "counter={counter}");
+        }
+        // High bits of the counter argument are masked off
+        let bytes = encode_frame(FrameId::C1, &data, 0xFF).unwrap();
+        let decoded = decode_frame(&bytes).unwrap();
+        assert_eq!(decoded.frame_counter, 0b11);
+    }
+
+    #[test]
+    fn corrupted_checksum_is_detected() {
+        // Arrange
+        let data = vec![0x01, 0x09];
+        let mut bytes = encode_frame(FrameId::C1, &data, 0).unwrap();
+        let last = bytes.len() - 1;
+        bytes[last] ^= 0x01; // flip one bit
+
+        // Act
+        let err = decode_frame(&bytes).unwrap_err();
+
+        // Assert
+        assert!(matches!(err, FrameDecodeError::BadChecksum { .. }));
+    }
+
+    #[test]
+    fn bad_header_rejected() {
+        // Arrange — replace the magic header with 00 00 00
+        let mut bytes = encode_frame(FrameId::C1, &[0x01, 0x09], 0).unwrap();
+        bytes[0] = 0x00;
+        bytes[1] = 0x00;
+        bytes[2] = 0x00;
+
+        // Act
+        let err = decode_frame(&bytes).unwrap_err();
+
+        // Assert
+        assert!(matches!(err, FrameDecodeError::BadHeader(0, 0, 0)));
+    }
+
+    #[test]
+    fn truncated_frame_rejected() {
+        // Arrange
+        let bytes = encode_frame(FrameId::C1, &[0x01, 0x09], 0).unwrap();
+        let truncated = &bytes[..bytes.len() - 1];
+
+        // Act
+        let err = decode_frame(truncated).unwrap_err();
+
+        // Assert
+        assert!(matches!(err, FrameDecodeError::BodyLengthMismatch { .. }));
+    }
+
+    #[test]
+    fn empty_data_falls_under_min_body_len() {
+        // Arrange — empty data would mean body_len = 3 (length + frame id + checksum)
+        // which is below MIN_BODY_LEN (4). encode_frame rejects.
+
+        // Act
+        let result = encode_frame(FrameId::C1, &[], 0);
+
+        // Assert
+        assert!(result.is_none());
+    }
+
+    #[test]
+    fn oversize_data_rejected_by_encoder() {
+        // Arrange — data large enough to overflow MAX_BODY_LEN
+        let data = vec![0; MAX_BODY_LEN as usize];
+
+        // Act
+        let result = encode_frame(FrameId::C1, &data, 0);
+
+        // Assert
+        assert!(result.is_none());
+    }
+
+    #[test]
+    fn unknown_frame_id_rejected() {
+        // Arrange — manually craft a frame with frame_id = 0x99
+        let data = vec![0x01, 0x09];
+        let bytes_ok = encode_frame(FrameId::C1, &data, 0).unwrap();
+        let mut bytes = bytes_ok.clone();
+        bytes[4] = 0x99; // overwrite frame id
+                         // Recompute checksum so the decoder gets to the frame-id check
+        let cs_idx = bytes.len() - 1;
+        bytes[cs_idx] = xor_checksum(&bytes[3..cs_idx]);
+
+        // Act
+        let err = decode_frame(&bytes).unwrap_err();
+
+        // Assert
+        assert!(matches!(err, FrameDecodeError::UnknownFrameId(0x99)));
+    }
+}

crates/gimbal_controller/src/internal/a40_protocol/mod.rs

@@ -0,0 +1,31 @@
+//! ViewPro A40 vendor UDP protocol.
+//!
+//! Frame layout (per the ViewPro A40 Pro SDK / `AP_Mount_Viewpro.h` in
+//! ArduPilot, which is the canonical open-source reference for this
+//! camera family):
+//!
+//! ```text
+//! Field     Index  Bytes  Description
+//! Header    0..2   3      0x55 0xAA 0xDC
+//! Length    3      1      bits 0..5 = body length (n = bytes 3..checksum, min 4 max 63)
+//!                         bits 6..7 = frame counter (increments per send, wraps mod 4)
+//! Frame Id  4      1      see FrameId enum
+//! Data      5..    n      first byte is command id; remainder is per-frame payload
+//! Checksum  n+2    1      XOR of bytes 3..n+1 (inclusive)
+//! ```
+//!
+//! IMPORTANT — spec correction: AZ-653's task spec lists "CRC16
+//! (vendor polynomial)". The actual ViewPro vendor protocol uses an
+//! 8-bit XOR checksum, NOT CRC16. We implement the real vendor
+//! protocol (the airframe will accept nothing else); the spec
+//! deviation is documented in the batch report.
+
+pub mod checksum;
+pub mod commands;
+pub mod frame;
+
+pub use checksum::xor_checksum;
+pub use commands::{
+    build_a1_angles, build_c1_camera, build_c2_set_zoom, CameraCommand, ImageSensor, ServoStatus,
+};
+pub use frame::{decode_frame, encode_frame, Frame, FrameDecodeError, FrameId, MAX_PACKET_LEN};

crates/gimbal_controller/src/internal/mod.rs

@@ -0,0 +1,4 @@
+//! Internal modules for `gimbal_controller`. Not part of the public API.
+
+pub mod a40_protocol;
+pub mod transport;

crates/gimbal_controller/src/internal/transport.rs

@@ -0,0 +1,330 @@
+//! UDP transport for the ViewPro A40.
+//!
+//! Owns the [`UdpSocket`], the rolling frame counter, the bounded
+//! retry policy, and the vendor-fault counters that feed the
+//! component's health surface. Inbound frames are checksum-validated
+//! by [`super::a40_protocol::decode_frame`]; mismatches are counted
+//! as `vendor_faults_total{kind="crc"}` and dropped.
+//!
+//! The transport is **command/response** keyed by `(FrameId, frame_counter)`:
+//! each `send_with_response` issues a frame, awaits the next
+//! matching inbound frame within a per-command deadline, and retries
+//! up to `max_retries` on timeout. Unmatched inbound frames (e.g.
+//! the gimbal's HEARTBEAT) are still surfaced through the
+//! broadcast stream so a future telemetry pump can consume them.
+
+use std::net::SocketAddr;
+use std::sync::Arc;
+use std::time::Duration;
+
+use tokio::net::UdpSocket;
+use tokio::sync::{broadcast, Mutex};
+use tokio::task::JoinHandle;
+use tokio::time::{timeout, Instant};
+
+use super::a40_protocol::frame::{decode_frame, encode_frame, Frame, FrameDecodeError, FrameId};
+
+/// Default per-command response deadline. The NFR is ≤200 ms on a
+/// healthy link; 150 ms leaves headroom for the bounded-retry budget.
+pub const DEFAULT_COMMAND_DEADLINE: Duration = Duration::from_millis(150);
+
+/// Default retry budget for `send_with_response`. Vendor link is
+/// best-effort UDP; bounded retries match the AZ-651 ladder pattern.
+pub const DEFAULT_MAX_RETRIES: u8 = 3;
+
+/// Broadcast channel capacity for inbound frames. Slow consumers
+/// see `Lagged`; the transport itself is unaffected.
+pub const INBOUND_CHANNEL_CAPACITY: usize = 64;
+
+/// Counters surfaced through `health()`. Tracked atomically by the
+/// transport; readers see a coherent snapshot via the public
+/// getters.
+#[derive(Debug, Default)]
+pub struct VendorFaults {
+    /// Inbound frames that failed checksum / framing validation.
+    pub crc: std::sync::atomic::AtomicU64,
+    /// Outbound commands that exhausted their retry budget without a
+    /// matching response.
+    pub timeout: std::sync::atomic::AtomicU64,
+    /// Inbound frames whose `FrameId` could not be decoded.
+    pub unknown_frame_id: std::sync::atomic::AtomicU64,
+}
+
+impl VendorFaults {
+    fn inc_crc(&self) {
+        self.crc.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+    }
+    fn inc_timeout(&self) {
+        self.timeout
+            .fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+    }
+    fn inc_unknown_frame_id(&self) {
+        self.unknown_frame_id
+            .fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+    }
+    pub fn snapshot(&self) -> VendorFaultsSnapshot {
+        VendorFaultsSnapshot {
+            crc: self.crc.load(std::sync::atomic::Ordering::Relaxed),
+            timeout: self.timeout.load(std::sync::atomic::Ordering::Relaxed),
+            unknown_frame_id: self
+                .unknown_frame_id
+                .load(std::sync::atomic::Ordering::Relaxed),
+        }
+    }
+}
+
+/// Read-side snapshot of [`VendorFaults`].
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
+pub struct VendorFaultsSnapshot {
+    pub crc: u64,
+    pub timeout: u64,
+    pub unknown_frame_id: u64,
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum A40Error {
+    #[error("frame too large for vendor protocol (max body 63 bytes)")]
+    FrameTooLarge,
+    #[error("max retries exceeded ({attempts} attempts) waiting for {expected:?}")]
+    MaxRetriesExceeded { attempts: u8, expected: FrameId },
+    #[error("UDP I/O: {0}")]
+    Io(#[from] std::io::Error),
+    #[error("inbound broadcast channel closed")]
+    InboundChannelClosed,
+}
+
+/// UDP transport for the A40. Cheap to clone — both the socket and
+/// the inbound broadcast sender are wrapped in `Arc`.
+#[derive(Clone)]
+pub struct A40Transport {
+    socket: Arc<UdpSocket>,
+    peer: SocketAddr,
+    inbound_tx: broadcast::Sender<Frame>,
+    faults: Arc<VendorFaults>,
+    frame_counter: Arc<Mutex<u8>>,
+    command_deadline: Duration,
+    max_retries: u8,
+}
+
+impl A40Transport {
+    /// Build a transport bound to a local UDP port and pre-connected
+    /// to `peer`. The receive task is spawned and returned alongside
+    /// the transport so the caller owns the join handle.
+    pub async fn bind(
+        local: SocketAddr,
+        peer: SocketAddr,
+    ) -> Result<(Self, JoinHandle<()>), A40Error> {
+        let socket = UdpSocket::bind(local).await?;
+        socket.connect(peer).await?;
+        Self::from_socket(Arc::new(socket), peer)
+    }
+
+    /// Construct a transport directly from a pre-bound socket. Used
+    /// by tests that need to control both endpoints.
+    pub fn from_socket(
+        socket: Arc<UdpSocket>,
+        peer: SocketAddr,
+    ) -> Result<(Self, JoinHandle<()>), A40Error> {
+        let (inbound_tx, _rx) = broadcast::channel::<Frame>(INBOUND_CHANNEL_CAPACITY);
+        let faults = Arc::new(VendorFaults::default());
+        let transport = Self {
+            socket: socket.clone(),
+            peer,
+            inbound_tx: inbound_tx.clone(),
+            faults: faults.clone(),
+            frame_counter: Arc::new(Mutex::new(0)),
+            command_deadline: DEFAULT_COMMAND_DEADLINE,
+            max_retries: DEFAULT_MAX_RETRIES,
+        };
+        let recv_task = tokio::spawn(receive_loop(socket, inbound_tx, faults));
+        Ok((transport, recv_task))
+    }
+
+    pub fn with_command_deadline(mut self, deadline: Duration) -> Self {
+        self.command_deadline = deadline;
+        self
+    }
+
+    pub fn with_max_retries(mut self, retries: u8) -> Self {
+        self.max_retries = retries;
+        self
+    }
+
+    /// Subscribe to inbound frames. Receivers that lag past the
+    /// channel capacity see `RecvError::Lagged` and are responsible
+    /// for resyncing.
+    pub fn subscribe_inbound(&self) -> broadcast::Receiver<Frame> {
+        self.inbound_tx.subscribe()
+    }
+
+    pub fn faults(&self) -> VendorFaultsSnapshot {
+        self.faults.snapshot()
+    }
+
+    /// Send a fire-and-forget frame; no response is awaited and no
+    /// retry is performed. Use for outbound packets the vendor does
+    /// not acknowledge (e.g. `M_AHRS` attitude pushes).
+    pub async fn send_oneway(&self, frame_id: FrameId, data: &[u8]) -> Result<(), A40Error> {
+        let counter = self.next_counter().await;
+        let bytes = encode_frame(frame_id, data, counter).ok_or(A40Error::FrameTooLarge)?;
+        self.socket.send(&bytes).await?;
+        Ok(())
+    }
+
+    /// Send a frame and await the first inbound frame whose
+    /// `FrameId` matches `expected_reply` within the per-command
+    /// deadline. Retries up to `max_retries` times on timeout;
+    /// returns `Err(MaxRetriesExceeded)` on cap exhaustion.
+    ///
+    /// Inbound frames with non-matching ids are still broadcast to
+    /// subscribers; they just don't satisfy *this* call.
+    pub async fn send_with_response(
+        &self,
+        frame_id: FrameId,
+        data: &[u8],
+        expected_reply: FrameId,
+    ) -> Result<Frame, A40Error> {
+        let bytes_template = {
+            // Re-encode per attempt because the counter increments;
+            // do one bounds check up-front so we never enter the
+            // retry loop with a doomed frame.
+            let probe_counter = 0u8;
+            encode_frame(frame_id, data, probe_counter).ok_or(A40Error::FrameTooLarge)?
+        };
+        // Use `bytes_template` purely as a size validator above; the
+        // counter we actually use is fresh per attempt.
+        drop(bytes_template);
+
+        let mut inbound_rx = self.inbound_tx.subscribe();
+        let deadline = self.command_deadline;
+        let max_retries = self.max_retries.max(1);
+
+        let mut attempts: u8 = 0;
+        while attempts < max_retries {
+            attempts += 1;
+            let counter = self.next_counter().await;
+            let bytes = encode_frame(frame_id, data, counter).ok_or(A40Error::FrameTooLarge)?;
+            self.socket.send(&bytes).await?;
+
+            // Await the next matching inbound frame within the
+            // deadline. We re-loop on non-matching frames so the
+            // gimbal's HEARTBEAT etc. doesn't cancel our wait.
+            let started = Instant::now();
+            loop {
+                let remaining = deadline.saturating_sub(started.elapsed());
+                if remaining.is_zero() {
+                    break;
+                }
+                match timeout(remaining, inbound_rx.recv()).await {
+                    Ok(Ok(frame)) if frame.frame_id == expected_reply => {
+                        return Ok(frame);
+                    }
+                    Ok(Ok(_other)) => continue,
+                    Ok(Err(broadcast::error::RecvError::Lagged(_))) => {
+                        // We may have missed the reply; treat as
+                        // timeout for this attempt rather than
+                        // hanging.
+                        break;
+                    }
+                    Ok(Err(broadcast::error::RecvError::Closed)) => {
+                        return Err(A40Error::InboundChannelClosed);
+                    }
+                    Err(_elapsed) => break, // timed out
+                }
+            }
+            self.faults.inc_timeout();
+            tracing::warn!(
+                attempts,
+                max_retries,
+                ?frame_id,
+                ?expected_reply,
+                "A40 command timeout; retrying"
+            );
+        }
+        Err(A40Error::MaxRetriesExceeded {
+            attempts,
+            expected: expected_reply,
+        })
+    }
+
+    pub fn peer(&self) -> SocketAddr {
+        self.peer
+    }
+
+    async fn next_counter(&self) -> u8 {
+        let mut c = self.frame_counter.lock().await;
+        let v = *c;
+        *c = (*c).wrapping_add(1) & 0b11;
+        v
+    }
+}
+
+async fn receive_loop(
+    socket: Arc<UdpSocket>,
+    inbound_tx: broadcast::Sender<Frame>,
+    faults: Arc<VendorFaults>,
+) {
+    // Vendor packet ceiling is 63 bytes; round up to 128 for safety.
+    let mut buf = [0u8; 128];
+    loop {
+        match socket.recv(&mut buf).await {
+            Ok(len) => match decode_frame(&buf[..len]) {
+                Ok(frame) => {
+                    let _ = inbound_tx.send(frame);
+                }
+                Err(FrameDecodeError::BadChecksum { .. }) => {
+                    faults.inc_crc();
+                    tracing::debug!("A40 inbound checksum mismatch; dropping frame");
+                }
+                Err(FrameDecodeError::UnknownFrameId(_)) => {
+                    faults.inc_unknown_frame_id();
+                }
+                Err(e) => {
+                    // Other framing errors share the crc counter
+                    // (they are all "frame envelope invalid" faults
+                    // from the operator's perspective).
+                    faults.inc_crc();
+                    tracing::debug!(error=?e, "A40 inbound frame rejected");
+                }
+            },
+            Err(e) => {
+                tracing::error!(error=%e, "A40 transport recv error; shutting down receive loop");
+                return;
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn faults_default_zero() {
+        // Arrange + Act
+        let f = VendorFaults::default();
+
+        // Assert
+        let s = f.snapshot();
+        assert_eq!(s.crc, 0);
+        assert_eq!(s.timeout, 0);
+        assert_eq!(s.unknown_frame_id, 0);
+    }
+
+    #[test]
+    fn faults_counters_increment_independently() {
+        // Arrange
+        let f = VendorFaults::default();
+
+        // Act
+        f.inc_crc();
+        f.inc_crc();
+        f.inc_timeout();
+
+        // Assert
+        let s = f.snapshot();
+        assert_eq!(s.crc, 2);
+        assert_eq!(s.timeout, 1);
+        assert_eq!(s.unknown_frame_id, 0);
+    }
+}

crates/gimbal_controller/src/lib.rs

@@ -1,7 +1,12 @@
 //! `gimbal_controller` — ViewPro A40 UDP control + smooth-pan primitive.
 //!
-//! Real implementation lands in:
-//! - AZ-653 `gimbal_a40_transport`
+//! AZ-653 lands:
+//! - The vendor frame codec ([`internal::a40_protocol`])
+//! - The UDP transport with bounded retry + vendor-fault counters
+//!   ([`internal::transport`])
+//! - The real `set_pose` / `zoom` paths on [`GimbalControllerHandle`]
+//!
+//! Subsequent gimbal tasks layer onto the same transport:
 //! - AZ-654 `gimbal_zoom_out_sweep`
 //! - AZ-655 `gimbal_smooth_pan_plan`
 //! - AZ-656 `gimbal_centre_on_target`
@@ -13,27 +18,63 @@ use shared::error::{AutopilotError, Result};
 use shared::health::ComponentHealth;
 use shared::models::gimbal::GimbalState;
 
+mod internal;
+
+pub use internal::a40_protocol::{
+    build_a1_angles, build_c1_camera, build_c2_set_zoom, decode_frame, encode_frame, xor_checksum,
+    CameraCommand, Frame, FrameDecodeError, FrameId, ImageSensor, ServoStatus, MAX_PACKET_LEN,
+};
+pub use internal::transport::{
+    A40Error, A40Transport, VendorFaults, VendorFaultsSnapshot, DEFAULT_COMMAND_DEADLINE,
+    DEFAULT_MAX_RETRIES, INBOUND_CHANNEL_CAPACITY,
+};
+
 const NAME: &str = "gimbal_controller";
 
+/// Caller-supplied target pose. Yaw + pitch are absolute angles in
+/// degrees (vendor convention: yaw 0° = airframe nose, pitch 0° =
+/// horizon, pitch +90° = straight up).
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize)]
 pub struct GimbalCommand {
     pub yaw_deg: f32,
     pub pitch_deg: f32,
 }
 
+/// Owns the state publisher and (optionally) the A40 transport. When
+/// constructed without a transport (`GimbalController::new`), the
+/// controller is in **disabled** mode — `set_pose` and `zoom` return
+/// `AutopilotError::NotImplemented`. This matches the AZ-651 /
+/// AZ-652 pattern where transports are wired by the composition root
+/// in `autopilot/runtime.rs`.
 pub struct GimbalController {
     state_tx: watch::Sender<GimbalState>,
+    transport: Option<A40Transport>,
 }
 
 impl GimbalController {
     pub fn new(initial: GimbalState) -> Self {
         let (state_tx, _rx) = watch::channel(initial);
-        Self { state_tx }
+        Self {
+            state_tx,
+            transport: None,
+        }
+    }
+
+    /// Construct a controller already wired to the A40 transport.
+    /// The composition root uses this overload after binding the
+    /// vendor UDP socket.
+    pub fn with_transport(initial: GimbalState, transport: A40Transport) -> Self {
+        let (state_tx, _rx) = watch::channel(initial);
+        Self {
+            state_tx,
+            transport: Some(transport),
+        }
     }
 
     pub fn handle(&self) -> GimbalControllerHandle {
         GimbalControllerHandle {
             state_tx: self.state_tx.clone(),
+            transport: self.transport.clone(),
         }
     }
 }
@@ -41,19 +82,54 @@ impl GimbalController {
 #[derive(Clone)]
 pub struct GimbalControllerHandle {
     state_tx: watch::Sender<GimbalState>,
+    transport: Option<A40Transport>,
 }
 
 impl GimbalControllerHandle {
-    pub async fn set_pose(&self, _command: GimbalCommand) -> Result<()> {
-        Err(AutopilotError::NotImplemented(
-            "gimbal_controller::set_pose (AZ-653)",
-        ))
+    /// Issue an absolute-angle target to the A40. Returns once the
+    /// vendor has acknowledged via a T1_F1_B1_D1 reply (its standard
+    /// angle-feedback frame) or the bounded retry budget exhausts.
+    pub async fn set_pose(&self, command: GimbalCommand) -> Result<()> {
+        let transport = self.transport.as_ref().ok_or(AutopilotError::NotImplemented(
+            "gimbal_controller::set_pose: no transport wired",
+        ))?;
+        let data = build_a1_angles(command.yaw_deg, command.pitch_deg);
+        let _reply = transport
+            .send_with_response(FrameId::A1, &data, FrameId::T1F1B1D1)
+            .await
+            .map_err(map_a40_error)?;
+        // `send_replace` updates the watched value regardless of
+        // subscriber count; using plain `send` would silently fail
+        // when no consumer is listening yet (the composition root
+        // wires consumers after construction in some test flows).
+        let mut state = *self.state_tx.borrow();
+        state.yaw = command.yaw_deg;
+        state.pitch = command.pitch_deg;
+        state.ts_monotonic_ns = monotonic_ns();
+        self.state_tx.send_replace(state);
+        Ok(())
     }
 
-    pub async fn zoom(&self, _level: f32) -> Result<()> {
-        Err(AutopilotError::NotImplemented(
-            "gimbal_controller::zoom (AZ-654)",
-        ))
+    /// Issue an absolute optical-zoom factor (e.g. `4.0` for 4×).
+    /// Routed through the C2 SET_EO_ZOOM command per the vendor
+    /// protocol. The continuous-rate C1 ZOOM_IN / ZOOM_OUT pair is
+    /// reserved for AZ-654's sweep primitive.
+    pub async fn zoom(&self, level: f32) -> Result<()> {
+        let transport = self.transport.as_ref().ok_or(AutopilotError::NotImplemented(
+            "gimbal_controller::zoom: no transport wired",
+        ))?;
+        let data = build_c2_set_zoom(level);
+        // C2 SET_EO_ZOOM ack arrives as a T1_F1_B1_D1 (the vendor's
+        // generic angle/status feedback frame).
+        let _reply = transport
+            .send_with_response(FrameId::C2, &data, FrameId::T1F1B1D1)
+            .await
+            .map_err(map_a40_error)?;
+        let mut state = *self.state_tx.borrow();
+        state.zoom = level;
+        state.ts_monotonic_ns = monotonic_ns();
+        self.state_tx.send_replace(state);
+        Ok(())
     }
 
     pub fn state(&self) -> GimbalState {
@@ -64,26 +140,105 @@ impl GimbalControllerHandle {
         self.state_tx.subscribe()
     }
 
-    pub fn health(&self) -> ComponentHealth {
-        ComponentHealth::disabled(NAME)
+    /// Direct vendor-fault counter snapshot. The composition root
+    /// uses this to populate the health surface; unit tests use it
+    /// to assert AC-2 ("CRC mismatch counted") and AC-3 / AC-4
+    /// (`vendor_faults_total{kind="timeout"}` increments).
+    pub fn faults(&self) -> Option<VendorFaultsSnapshot> {
+        self.transport.as_ref().map(|t| t.faults())
     }
+
+    pub fn health(&self) -> ComponentHealth {
+        let Some(transport) = self.transport.as_ref() else {
+            return ComponentHealth::disabled(NAME);
+        };
+        let f = transport.faults();
+        // Any timeout fault flips to yellow; ≥ 5 to red. The exact
+        // thresholds are conservative starting points — the
+        // operator-surface team will refine once flight data exists.
+        if f.timeout >= 5 {
+            ComponentHealth::red(NAME, format!("timeout faults={}", f.timeout))
+        } else if f.timeout > 0 || f.crc > 0 {
+            ComponentHealth::yellow(
+                NAME,
+                format!("vendor faults: crc={} timeout={}", f.crc, f.timeout),
+            )
+        } else {
+            ComponentHealth::green(NAME)
+        }
+    }
+
+    /// Direct transport handle for the AZ-654/655/656 primitives
+    /// that need to issue ZOOM_IN/ZOOM_OUT rate commands rather than
+    /// going through `set_pose` / `zoom`.
+    #[doc(hidden)]
+    pub fn transport(&self) -> Option<&A40Transport> {
+        self.transport.as_ref()
+    }
+}
+
+fn map_a40_error(e: A40Error) -> AutopilotError {
+    match e {
+        A40Error::FrameTooLarge => {
+            AutopilotError::Internal("A40 frame exceeds vendor 63-byte max".into())
+        }
+        A40Error::MaxRetriesExceeded { attempts, expected } => AutopilotError::Internal(format!(
+            "A40 max retries exceeded ({attempts} attempts) waiting for {expected:?}"
+        )),
+        A40Error::Io(io) => AutopilotError::Internal(format!("A40 UDP I/O: {io}")),
+        A40Error::InboundChannelClosed => {
+            AutopilotError::Internal("A40 inbound broadcast channel closed".into())
+        }
+    }
+}
+
+fn monotonic_ns() -> u64 {
+    use std::time::{SystemTime, UNIX_EPOCH};
+    SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .map(|d| d.as_nanos() as u64)
+        .unwrap_or(0)
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
-    #[test]
-    fn it_compiles() {
-        let initial = GimbalState {
+    fn initial_state() -> GimbalState {
+        GimbalState {
             yaw: 0.0,
             pitch: 0.0,
             zoom: 1.0,
             ts_monotonic_ns: 0,
             command_in_flight: false,
-        };
-        let h = GimbalController::new(initial).handle();
+        }
+    }
+
+    #[test]
+    fn disabled_controller_has_disabled_health() {
+        // Arrange + Act
+        let h = GimbalController::new(initial_state()).handle();
+
+        // Assert
         assert_eq!(h.state().zoom, 1.0);
         assert_eq!(h.health().level, shared::health::HealthLevel::Disabled);
+        assert!(h.faults().is_none());
+    }
+
+    #[tokio::test]
+    async fn disabled_controller_rejects_set_pose() {
+        // Arrange
+        let h = GimbalController::new(initial_state()).handle();
+
+        // Act
+        let res = h
+            .set_pose(GimbalCommand {
+                yaw_deg: 10.0,
+                pitch_deg: 0.0,
+            })
+            .await;
+
+        // Assert
+        assert!(matches!(res, Err(AutopilotError::NotImplemented(_))));
     }
 }

crates/gimbal_controller/tests/a40_transport.rs

@@ -0,0 +1,358 @@
+//! AZ-653 integration tests for the ViewPro A40 transport.
+//!
+//! Strategy: bring up a fake A40 endpoint on a second `UdpSocket` in
+//! the same process; pair it with the transport under test via a
+//! pre-bound `peer` address; drive scenarios by scripting the fake's
+//! reply behaviour (echo, drop, corrupt CRC).
+
+use std::net::{Ipv4Addr, SocketAddr};
+use std::sync::atomic::{AtomicU8, Ordering};
+use std::sync::Arc;
+use std::time::Duration;
+
+use tokio::net::UdpSocket;
+use tokio::sync::Mutex;
+
+use gimbal_controller::{
+    build_a1_angles, decode_frame, encode_frame, A40Transport, CameraCommand, FrameId,
+    GimbalCommand, GimbalController, ImageSensor,
+};
+use shared::models::gimbal::GimbalState;
+
+const LOCALHOST: Ipv4Addr = Ipv4Addr::new(127, 0, 0, 1);
+
+fn loopback(port: u16) -> SocketAddr {
+    SocketAddr::new(LOCALHOST.into(), port)
+}
+
+fn initial_state() -> GimbalState {
+    GimbalState {
+        yaw: 0.0,
+        pitch: 0.0,
+        zoom: 1.0,
+        ts_monotonic_ns: 0,
+        command_in_flight: false,
+    }
+}
+
+/// Bind a UDP socket on an OS-chosen ephemeral port and return both
+/// the socket and the bound address.
+async fn bind_ephemeral() -> (Arc<UdpSocket>, SocketAddr) {
+    let s = UdpSocket::bind(loopback(0)).await.expect("bind ephemeral");
+    let addr = s.local_addr().expect("local_addr");
+    (Arc::new(s), addr)
+}
+
+/// Helper — minimal fake A40 endpoint. Behaviour is supplied as a
+/// closure invoked for every inbound frame.
+struct FakeA40 {
+    socket: Arc<UdpSocket>,
+    addr: SocketAddr,
+}
+
+impl FakeA40 {
+    async fn bind() -> Self {
+        let (socket, addr) = bind_ephemeral().await;
+        Self { socket, addr }
+    }
+}
+
+#[tokio::test]
+async fn ac1_crc_round_trip_no_faults() {
+    // Arrange — bring up the fake; build a yaw-30 A1 frame; spawn a
+    // task that echoes the (well-formed) command back as a
+    // T1_F1_B1_D1 reply (the vendor's angle-feedback frame).
+    let fake = FakeA40::bind().await;
+    let (test_socket, test_addr) = bind_ephemeral().await;
+    test_socket.connect(fake.addr).await.expect("connect");
+
+    let fake_socket = fake.socket.clone();
+    let echo_task = tokio::spawn(async move {
+        let mut buf = [0u8; 128];
+        let (n, from) = fake_socket
+            .recv_from(&mut buf)
+            .await
+            .expect("fake recv_from");
+        // Validate the incoming A1 frame parses cleanly.
+        let inbound = decode_frame(&buf[..n]).expect("inbound decode");
+        assert_eq!(inbound.frame_id, FrameId::A1);
+        // Reply with T1_F1_B1_D1 (12 bytes of arbitrary feedback
+        // payload — content unchecked by the transport).
+        let reply = encode_frame(FrameId::T1F1B1D1, &[0; 12], 0).expect("encode reply");
+        fake_socket
+            .send_to(&reply, from)
+            .await
+            .expect("fake send_to");
+    });
+
+    let (transport, _recv_task) =
+        A40Transport::from_socket(test_socket.clone(), fake.addr).expect("from_socket");
+    let _ = test_addr;
+
+    let payload = build_a1_angles(30.0, 0.0);
+
+    // Act
+    let reply = transport
+        .send_with_response(FrameId::A1, &payload, FrameId::T1F1B1D1)
+        .await
+        .expect("send_with_response");
+
+    // Assert
+    assert_eq!(reply.frame_id, FrameId::T1F1B1D1);
+    assert_eq!(transport.faults().crc, 0);
+    assert_eq!(transport.faults().timeout, 0);
+
+    echo_task.await.expect("echo task");
+}
+
+#[tokio::test]
+async fn ac2_crc_mismatch_counted_and_dropped() {
+    // Arrange — fake echoes a frame whose checksum is one bit off.
+    let fake = FakeA40::bind().await;
+    let (test_socket, _) = bind_ephemeral().await;
+    test_socket.connect(fake.addr).await.expect("connect");
+
+    let fake_socket = fake.socket.clone();
+    tokio::spawn(async move {
+        let mut buf = [0u8; 128];
+        let (_n, from) = fake_socket
+            .recv_from(&mut buf)
+            .await
+            .expect("fake recv_from");
+        // Craft a corrupt frame (flip the checksum).
+        let mut reply = encode_frame(FrameId::T1F1B1D1, &[0; 12], 0).expect("encode reply");
+        let last = reply.len() - 1;
+        reply[last] ^= 0x01;
+        fake_socket
+            .send_to(&reply, from)
+            .await
+            .expect("fake send_to");
+    });
+
+    let (transport, _recv_task) =
+        A40Transport::from_socket(test_socket, fake.addr).expect("from_socket");
+    let transport = transport
+        .with_command_deadline(Duration::from_millis(80))
+        .with_max_retries(1);
+
+    let payload = build_a1_angles(30.0, 0.0);
+
+    // Act — must fail (the corrupt frame is dropped; no valid reply
+    // arrives within the deadline).
+    let result = transport
+        .send_with_response(FrameId::A1, &payload, FrameId::T1F1B1D1)
+        .await;
+
+    // Assert — CRC counter incremented; timeout counter incremented
+    // because no valid reply arrived.
+    assert!(
+        result.is_err(),
+        "expected MaxRetriesExceeded; got {result:?}"
+    );
+    // The receive loop is asynchronous; give it a tick to record.
+    tokio::time::sleep(Duration::from_millis(20)).await;
+    let faults = transport.faults();
+    assert!(faults.crc >= 1, "expected ≥1 CRC fault, got {}", faults.crc);
+    assert!(
+        faults.timeout >= 1,
+        "expected ≥1 timeout fault, got {}",
+        faults.timeout
+    );
+}
+
+#[tokio::test]
+async fn ac3_command_timeout_retries_then_succeeds() {
+    // Arrange — fake drops the FIRST inbound frame silently; replies
+    // to every subsequent one.
+    let fake = FakeA40::bind().await;
+    let (test_socket, _) = bind_ephemeral().await;
+    test_socket.connect(fake.addr).await.expect("connect");
+
+    let drop_count = Arc::new(AtomicU8::new(0));
+    let fake_socket = fake.socket.clone();
+    let drop_count_for_task = drop_count.clone();
+    tokio::spawn(async move {
+        loop {
+            let mut buf = [0u8; 128];
+            let Ok((_n, from)) = fake_socket.recv_from(&mut buf).await else {
+                return;
+            };
+            let prior = drop_count_for_task.fetch_add(1, Ordering::Relaxed);
+            if prior == 0 {
+                // Silently drop the first command.
+                continue;
+            }
+            let reply = encode_frame(FrameId::T1F1B1D1, &[0; 12], 0).expect("encode reply");
+            let _ = fake_socket.send_to(&reply, from).await;
+        }
+    });
+
+    let (transport, _recv_task) =
+        A40Transport::from_socket(test_socket, fake.addr).expect("from_socket");
+    let transport = transport
+        .with_command_deadline(Duration::from_millis(80))
+        .with_max_retries(3);
+
+    let payload = build_a1_angles(30.0, 0.0);
+
+    // Act
+    let reply = transport
+        .send_with_response(FrameId::A1, &payload, FrameId::T1F1B1D1)
+        .await
+        .expect("retry should succeed");
+
+    // Assert — exactly one timeout (first attempt dropped); reply
+    // arrived on the second attempt.
+    assert_eq!(reply.frame_id, FrameId::T1F1B1D1);
+    let faults = transport.faults();
+    assert_eq!(
+        faults.timeout, 1,
+        "expected 1 timeout fault, got {}",
+        faults.timeout
+    );
+    assert_eq!(faults.crc, 0);
+    assert!(
+        drop_count.load(Ordering::Relaxed) >= 2,
+        "fake should have seen ≥2 commands"
+    );
+}
+
+#[tokio::test]
+async fn ac4_cap_exhaustion_returns_max_retries_exceeded() {
+    // Arrange — fake never replies. The transport should fail after
+    // exactly `max_retries` attempts with `MaxRetriesExceeded`.
+    let fake = FakeA40::bind().await;
+    let (test_socket, _) = bind_ephemeral().await;
+    test_socket.connect(fake.addr).await.expect("connect");
+
+    let attempts_seen = Arc::new(Mutex::new(0u32));
+    let fake_socket = fake.socket.clone();
+    let attempts_for_task = attempts_seen.clone();
+    tokio::spawn(async move {
+        loop {
+            let mut buf = [0u8; 128];
+            let Ok((_, _from)) = fake_socket.recv_from(&mut buf).await else {
+                return;
+            };
+            *attempts_for_task.lock().await += 1;
+            // Never reply.
+        }
+    });
+
+    let (transport, _recv_task) =
+        A40Transport::from_socket(test_socket, fake.addr).expect("from_socket");
+    let transport = transport
+        .with_command_deadline(Duration::from_millis(60))
+        .with_max_retries(3);
+
+    let payload = build_a1_angles(30.0, 0.0);
+
+    // Act
+    let err = transport
+        .send_with_response(FrameId::A1, &payload, FrameId::T1F1B1D1)
+        .await
+        .expect_err("should hit cap");
+
+    // Assert
+    assert!(
+        matches!(
+            err,
+            gimbal_controller::A40Error::MaxRetriesExceeded { attempts: 3, .. }
+        ),
+        "expected MaxRetriesExceeded(3); got {err:?}"
+    );
+    let faults = transport.faults();
+    assert_eq!(
+        faults.timeout, 3,
+        "expected 3 timeout faults; got {}",
+        faults.timeout
+    );
+    // Give the fake one final beat to record the final attempt.
+    tokio::time::sleep(Duration::from_millis(20)).await;
+    let seen = *attempts_seen.lock().await;
+    assert_eq!(seen, 3, "fake should have seen exactly 3 attempts");
+}
+
+#[tokio::test]
+async fn set_pose_via_transport_updates_state_stream() {
+    // Arrange — full GimbalController + transport wired together;
+    // fake echoes every A1 with a T1_F1_B1_D1 ack.
+    let fake = FakeA40::bind().await;
+    let (test_socket, _) = bind_ephemeral().await;
+    test_socket.connect(fake.addr).await.expect("connect");
+
+    let fake_socket = fake.socket.clone();
+    tokio::spawn(async move {
+        loop {
+            let mut buf = [0u8; 128];
+            let Ok((_, from)) = fake_socket.recv_from(&mut buf).await else {
+                return;
+            };
+            let reply = encode_frame(FrameId::T1F1B1D1, &[0; 12], 0).expect("encode reply");
+            let _ = fake_socket.send_to(&reply, from).await;
+        }
+    });
+
+    let (transport, _recv_task) =
+        A40Transport::from_socket(test_socket, fake.addr).expect("from_socket");
+    let controller = GimbalController::with_transport(initial_state(), transport);
+    let handle = controller.handle();
+    let mut state_rx = handle.state_stream();
+
+    // Act
+    handle
+        .set_pose(GimbalCommand {
+            yaw_deg: 45.0,
+            pitch_deg: -10.0,
+        })
+        .await
+        .expect("set_pose");
+
+    // Assert
+    state_rx.changed().await.expect("state changed");
+    let snapshot = *state_rx.borrow();
+    assert_eq!(snapshot.yaw, 45.0);
+    assert_eq!(snapshot.pitch, -10.0);
+    assert_eq!(handle.faults().expect("transport present").timeout, 0);
+}
+
+#[tokio::test]
+async fn zoom_via_transport_updates_zoom_state() {
+    // Arrange
+    let fake = FakeA40::bind().await;
+    let (test_socket, _) = bind_ephemeral().await;
+    test_socket.connect(fake.addr).await.expect("connect");
+
+    let fake_socket = fake.socket.clone();
+    tokio::spawn(async move {
+        loop {
+            let mut buf = [0u8; 128];
+            let Ok((_, from)) = fake_socket.recv_from(&mut buf).await else {
+                return;
+            };
+            let reply = encode_frame(FrameId::T1F1B1D1, &[0; 12], 0).expect("encode reply");
+            let _ = fake_socket.send_to(&reply, from).await;
+        }
+    });
+
+    let (transport, _recv_task) =
+        A40Transport::from_socket(test_socket, fake.addr).expect("from_socket");
+    let controller = GimbalController::with_transport(initial_state(), transport);
+    let handle = controller.handle();
+
+    // Act
+    handle.zoom(4.0).await.expect("zoom");
+
+    // Assert
+    let snapshot = handle.state();
+    assert_eq!(snapshot.zoom, 4.0);
+}
+
+#[tokio::test]
+async fn build_c1_camera_payload_matches_vendor_layout() {
+    // Arrange + Act
+    let payload = gimbal_controller::build_c1_camera(ImageSensor::Eo1, CameraCommand::ZoomIn);
+
+    // Assert — sanity-check the byte layout the transport will send.
+    assert_eq!(payload, [0x01, 0x09]);
+}