> ## Documentation Index
> Fetch the complete documentation index at: https://docs.metar.ws/llms.txt
> Use this file to discover all available pages before exploring further.

# Channels Overview

> Three data namespaces, one per-airport addressing scheme, and a replay mechanism that gets you current state instantly.

Every channel is keyed by the lowercase ICAO code of an airport. There are three namespaces; the two observation namespaces share one payload shape, while forecasts carry a different one.

## Namespaces

| Channel pattern               | Contents                                 | Minimum plan |
| ----------------------------- | ---------------------------------------- | ------------ |
| `metar.obs.<icao>`            | Official METAR observations              | Starter      |
| `metar.atis.<icao>`           | D-ATIS fast lane for US hubs             | Pro          |
| `metar.forecast.basic.<icao>` | Global model forecasts (GFS, ICON)       | Sandbox      |
| `metar.forecast.hires.<icao>` | High-resolution regional model forecasts | Starter      |
| `sandbox.demo`                | Fixed demo observation                   | Sandbox      |

See [METAR Observations](/channels/metar-observations), [D-ATIS Fast Lane](/channels/atis-fast-lane) and [Model Forecasts](/channels/model-forecasts) for the full field reference of each namespace.

<Tip>
  The station list grows over time. Subscribing to a channel that doesn't exist yet simply fails with a `channel_format` error, so it is safe to probe for a station before you know it is live.
</Tip>

## Last-value replay

Immediately after a successful subscribe, the server replays the most recently cached message for that channel, so a new subscriber gets current state instantly instead of waiting for the next update.

* Observations are cached for 45 minutes, since new reports typically arrive every 20 to 30 minutes.
* Forecasts are cached for 12 hours, since a model run is only published every 1 to 6 hours.

A replayed frame carries `"cached": true`. Never use a cached frame to measure delivery latency.

<Card title="Next: METAR Observations" icon="cloud" href="/channels/metar-observations">
  See the full field reference for the observation channel.
</Card>