Urban Canyons: Why GPS Fails Among Tall Buildings

An urban canyon is a street flanked by tall buildings, so that the sky is visible only as a narrow strip overhead. GPS suffers there for two reasons: buildings block many satellites completely, and they bounce signals so that they arrive late. Both effects can move your estimated position by tens of metres.

Less sky means fewer satellites

A receiver needs a good view of satellites spread across the sky. In a canyon, satellites low on the horizon are hidden behind buildings, and only those almost overhead can be heard. Those few satellites are clustered together, so the geometry is poor. Poor geometry makes the same small timing errors turn into larger position errors.

Reflections and multipath

Glass, concrete and metal reflect radio waves. A signal can arrive directly, and a second copy can arrive after bouncing off a building. The receiver locks onto a signal and assumes it travelled in a straight line, so a reflected path looks like a satellite that is further away than it really is. When several satellites are affected differently, the calculated position shifts in an unpredictable direction.

In the worst case, the direct signal is blocked entirely and the receiver sees only reflected ones. The result may look confident while still being wrong.

What you will notice

• The blue dot appears on the wrong side of a street or inside a building.
• The route line jumps when you walk or drive.
• The accuracy circle grows when you enter a dense district.
• Position improves suddenly when you reach a square or wide road.

How devices cope

Phones combine several techniques:

• Listening to more than one satellite system, as described in what GNSS is, increases the number of usable signals.
• Using nearby Wi-Fi networks and cell towers, covered in how phones use Wi-Fi and cell towers, gives an independent estimate.
• Using motion sensors and map data to smooth or snap the track to roads.

These methods reduce the problem, but they cannot remove it.

What you can do

When you need a precise point in a built-up area, do not rely only on the dot. Zoom in on the map and place the marker on the entrance or the exact corner. Choosing a point on the map is what a location-code system relies on: in UPCS, the code you receive corresponds to the 3 m by 3 m cell around the point you select, whatever your phone first thought.

For general causes of poor readings, see why phone GPS is inaccurate.