diff --git a/docker-compose.yaml b/docker-compose.yaml
index 7fd8ce0..e0432a9 100644
--- a/docker-compose.yaml
+++ b/docker-compose.yaml
@@ -1,6 +1,7 @@
 services:
   maptool:
     build: .
+    image: maptool:latest
     restart: unless-stopped
     ports:
       - "4567:4567"
@@ -26,3 +27,19 @@ services:
       # docs/development.adoc for how the deploy job treats this
       # directory).
       - ./data:/app/data
+
+  # Fakes a GNSS fix (a loop around Bratislava, see
+  # bin/gnss_simulator.rb/docs/development.adoc) since there's no real
+  # receiver attached here — the "GNSS" control would otherwise just show
+  # "no fix" always. Remove this service (and nothing else) whenever a real
+  # receiver/gpsd setup replaces it. network_mode ties it to maptool's own
+  # network namespace rather than a bridge network, since GpsdClient
+  # connects to a hardcoded 127.0.0.1:2947 — same as gpsd itself normally
+  # being local to the machine it serves.
+  gnss-simulator:
+    image: maptool:latest
+    depends_on:
+      - maptool
+    restart: unless-stopped
+    network_mode: "service:maptool"
+    command: ["ruby", "bin/gnss_simulator.rb"]
diff --git a/docs/deployment.adoc b/docs/deployment.adoc
index 0c3ab33..ae5c6e0 100644
--- a/docs/deployment.adoc
+++ b/docs/deployment.adoc
@@ -107,6 +107,18 @@ id/secret to store as the two `OAUTH_CLIENT_*` secrets above.
 usernames, e.g. `randy,friend1,friend2` — not their emails, the
 `login` field from Gitea's `/api/v1/user`.
 
+== GNSS simulator
+
+`docker-compose.yaml`'s `gnss-simulator` service (`bin/gnss_simulator.rb`,
+also documented for local dev in `docs/development.adoc`) runs
+alongside `maptool` in production too, sharing its network namespace
+(`network_mode: "service:maptool"`) so it's reachable at the
+hardcoded `127.0.0.1:2947` `Maptool::GpsdClient` expects — there's no
+real GNSS receiver attached to the server, so without this the "GNSS"
+control would just always show "no fix". It drives a loop around
+Bratislava; remove the service block (nothing else needs to change)
+whenever a real receiver/gpsd setup replaces it.
+
 == Garmin LiveTrack session webhook
 
 `POST /api/livetrack/session` (`session_id`, `session_token`, `secret` form