Two Different Things Working Together
Your car’s navigation system uses two separate technologies that work together: GPS (Global Positioning System) to determine where you are, and map data (stored on the navigation SD card) to understand what roads exist and how to route between them. Neither is useful without the other.
How GPS Works
GPS uses signals from a network of satellites orbiting the Earth. Your car has a GPS receiver — usually integrated into the roof or dashboard — that picks up signals from multiple satellites simultaneously. By measuring the tiny time differences in signals arriving from different satellites, the receiver calculates its precise position on Earth to within a few metres. This position update happens several times per second while you’re driving.
GPS works anywhere on Earth with a clear view of the sky. It doesn’t require a mobile signal or internet connection — the satellites transmit continuously and freely to any receiver. This is why your factory sat nav continues to work in remote areas with no phone signal.
What the SD Card Provides
GPS tells the navigation system where you are, but it has no idea what roads exist at that location. That’s what the SD card provides — a detailed database of every road, junction, speed limit, one-way system, and restriction in the mapped area. When the navigation system combines your GPS position with the road data on the SD card, it can display your position on the map, identify which road you’re on, and calculate routes.
How Route Calculation Works
When you enter a destination, the navigation processor uses a routing algorithm to find the best path through the road network stored on the SD card. It considers distance, road type (motorway vs A-road vs residential), speed limits, turn restrictions, and your routing preferences (avoid motorways, avoid tolls, etc.). For a long journey, this calculation happens in seconds — the processor is working through millions of possible route combinations to find the optimal path.
Why Maps Go Out of Date
The SD card contains a snapshot of the road network as it existed when the map data was compiled. Roads that were built, changed, or removed after the data cutoff date won’t be in the database. The GPS position will still be accurate — satellites don’t go out of date — but the navigation will be working from an incomplete road picture. This is why keeping the SD card updated matters: accurate GPS + current map data = reliable navigation.
What Happens in Tunnels and Underground Carparks
GPS signals can’t penetrate buildings, tunnels, or underground structures. In these situations, your navigation uses dead reckoning — it tracks your speed and direction using the car’s speed sensor and compass to estimate where you are between GPS fixes. This is generally accurate enough for tunnel navigation and typically reacquires GPS signal within seconds of emerging.