Global Navigation Satellite Systems — GPS, GLONASS, Galileo, BeiDou — work by a elegant geometric trick. Your phone's receiver measures the time delay of radio signals from at least four satellites, converts each delay into a pseudorange (an estimated distance to that satellite), and solves a system of equations for your three-dimensional position plus a clock bias correction. Outdoors under open sky, horizontal accuracy is often 3–8 meters at the 95th percentile. That is good enough to find a restaurant. It is not good enough to find your car on level B3 of a concrete parking garage — and the failure is not a software bug you can patch with a better map tile. It is physics: signal attenuation, multipath reflection, and geometry collapse.

How signals die underground

GPS signals broadcast at L1 band around 1575 MHz — microwave radio that behaves roughly like light at architectural scales. Reinforced concrete walls attenuate these signals by 20–30 dB or more per pass. By the time you are two levels underground, the receiver may see zero usable satellites, or worse, it may see reflections from a partial sky view at the ramp entrance. A receiver with fewer than four healthy satellite signals cannot solve for position reliably. Some phones fall back to stale dead-reckoning estimates or Wi-Fi positioning, which drifts or jumps unpredictably. The blue dot on the map keeps moving not because the phone knows where you are, but because it is guessing with degraded inputs and updating the guess every second.

Multipath: the silent position killer

Even when signals penetrate — near ramp mouths, open-air upper decks, or garages with skylights — multipath dominates the error budget. Multipath occurs when the direct signal from a satellite arrives at your antenna along with a delayed copy reflected off a concrete wall, metal roof, or another car. The receiver's correlation engine tracks the strongest peak, which may be a composite of direct and reflected paths equivalent to an extra path length Δd. That extra path translates directly into a pseudorange error, which translates into a position error on the order of Δd itself. Standing still in a garage, you can watch your GPS dot orbit 10–30 meters as satellite geometry changes and different reflections dominate. This is not incompetent map software. It is the receiver honestly reporting the best estimate from ambiguous measurements.

Vertical error and the floor problem

Horizontal GPS error gets the complaints, but vertical error is typically 1.5–3 times worse than horizontal even in good conditions. Satellites are mostly above you, so geometry poorly constrains altitude — the constellation is flat from your perspective. This means GPS cannot reliably answer "which floor am I on?" even when latitude and longitude look reasonable at the garage entrance. Wheria's barometer floor hints exist precisely because GPS altitude is useless underground; see our barometer parking math post for the hydrostatic approach. Other failure modes compound the problem: urban canyons between tall buildings create canyon effects similar to underground multipath; cheap RF jammers (illegal but real) flood the band with noise; faulty electronics emit broadband interference. Each mode produces a different signature, but the user experience is the same — the pin is not where you are.

The afternoon Wheria was born

Landmark 81, Ho Chi Minh City, level B3. I had parked, taken a photo of the pillar number, walked to the elevator. GPS showed my car floating in the Saigon River, four hundred meters east of reality. I walked two full laps of the floor before I found it, comparing the map dot against concrete reality and losing every time. That evening the decision was simple: stop showing a lying blue dot underground. Show the path I actually walked, reconstructed from step detection and heading fusion, with honest uncertainty bands. The indoor pipeline — IMU dead reckoning, barometer floor hints, adaptive compass fusion — is documented in our indoor navigation post. Step odometry details live in the IMU post. GPS still works fine when you park outdoors. We just stopped pretending it works where physics says it cannot.