Do GPS trackers work underground or in car parks, and what happens when satellite signal is lost

GPS trackers depend on line-of-sight contact with satellites, which is unavailable underground and often unreliable in multi-storey car parks. This is a genuine limitation that vehicle owners and fleet operators should understand when evaluating GPS tracking for theft prevention. LockCar explains how the technology behaves in GPS-denied environments and what happens to tracking coverage in these situations.

How GPS works and why car parks are a problem

GPS location is calculated from time-of-flight signals received from multiple satellites in medium earth orbit. This requires a clear line of sight between the receiver and at least three to four satellites. Underground structures, thick concrete ceilings, and deep building interiors all block or severely attenuate the satellite signals needed for GPS position calculation.

Multi-storey car parks present varying levels of GPS challenge depending on construction. Open-sided multi-storey car parks with limited roof coverage allow partial GPS signal in outer bays. Fully enclosed concrete structures provide minimal GPS signal on all floors. Underground car parks provide no GPS signal.

The practical implication for vehicle tracking is that a tracker inside an underground car park cannot report its current GPS location. It stores the last known GPS position from before entry and resumes accurate tracking when the vehicle exits to open sky.

Cellular position assist, the partial solution

Higher-specification 4G tracking systems, including LockCar One Plus, supplement GPS with cellular network position assist. When GPS signals are unavailable, the system uses the signal strengths from nearby 4G base stations to calculate an approximate location. This cellular position is significantly less accurate than GPS, typically within 50 to 500 metres rather than a few metres, but provides a useful indication of general location in GPS-denied environments.

Cellular position assist works reasonably well in urban areas with high cell tower density. It is less effective in rural areas or deep underground environments where cell signal is also weak. For theft scenarios involving underground car parks, cellular positioning may indicate the general area of a car park even when precise floor and bay cannot be determined.

LockCar relay immobiliser in car parks

The relay immobiliser function in LockCar systems operates independently of GPS satellite coverage. The physical relay break in the start circuit is controlled by the encrypted proximity tag signal, which operates on a short-range 2.4GHz channel inside the vehicle’s immediate environment. This function works underground, in car parks, in tunnels, and anywhere else regardless of GPS or cellular coverage.

This is the most important security distinction: GPS tracking coverage has gaps in GPS-denied environments, but LockCar’s relay immobiliser protection is continuous regardless of location. A car park relay attack on a LockCar-protected vehicle will fail regardless of whether the GPS is tracking accurately.

LockCar recommended products

Top recommendation

LockCar One Plus, £259

4G cellular backup, last known position, movement alerts

4G GPS system with cellular network position assist. In GPS-denied environments, cellular tower triangulation provides approximate location. Last known GPS position logged before signal loss.

LockCar IC3ST, £139

Relay immobiliser, works without GPS signal

The relay immobiliser works in car parks and underground regardless of GPS coverage. Pure prevention that GPS tracking alone cannot provide in GPS-denied environments.