---
type: plan
status: active
updated: 2026-09-04
---
# Initial plan

## Order

1. MQTT client, subscribing to the retained vehicle topics. Text readout only —
   prove the contract before drawing anything.
2. Gauges.
3. Moving map: MapLibre against the PMTiles basemap served by the Orin, with the
   position from `yukart/gnss/fix`. No routing involved.
4. Guidance from `yukart/nav/guidance`, once [svc-nav](gitea:yukart/svc-nav) publishes it.

A moving map with position, heading and trace is most of the value in a car and
needs no routing engine at all. Routing is the expensive last fraction.

## Open

- **Transport.** Over USB the tablet has an IP link via RNDIS, so it can reach the
  broker directly. Whether it should, or should go through `svc-gw`, is decided in
  that repo — see its initial plan.
- **CJK labels.** MapLibre cannot ship Japanese glyph sets as SDF ranges. Local
  font rendering must be configured, or labels will be slow or absent.
- **Sunlight.** A typical tablet is unreadable through a windscreen in daylight.
  This is a hardware problem, but the UI should not make it worse.

## The boundary, from this side

Two channels with different guarantees, not one socket:

| | Down (state) | Up (commands) |
|---|---|---|
| Transport | push stream | request/response |
| Loss | tolerated; next sample supersedes | **not** tolerated; retry until acknowledged |
| On reconnect | expect current state immediately | resend anything pending |

This client always initiates and owns reconnection — the Orin never dials out.
Assume the link drops constantly: cable knocked, screen off, WiFi lost.

## Session marking is worth building early

A button that stamps an event (`indicator on`, `cold start`, `hard braking`) into
the recording, on the same clock as the CAN frames, turns the Phase 1 manual event
log from a notebook-and-wristwatch exercise into structured data. It is what makes
a raw capture decodable later, and it is a Phase 1 capability that happens to live
in this repo.

## Mock data must look like mock data

`svc-gw` can synthesise vehicle state without any real readings, which is how this
app is developed with no Jetson present. The stream carries a flag when it does.

Rendering synthetic readings indistinguishable from real ones, on a screen in a
moving car, is a safety problem. The indication belongs on the display surface
itself, not in a settings page or a startup log.

## This app is the audio sink

The tablet owns the car's aux input and is the music player, so the Orin's speech
has to arrive here to be heard.

- TTS arrives as **discrete utterances** over the reliable channel, not as a
  real-time stream. Receive the whole thing, then play it — no jitter buffer.
- Duck other audio with `AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK` for the duration of
  the utterance, then release. That is what makes Spotify drop under the voice
  without either side coordinating.
- The microphone stays on the Orin. This app never captures audio; ASR input never
  crosses the boundary.