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Urban Canyons

Improving accuracy in places with little or no line of sight to the satellites.

GPS receivers tend to loose position accuracy when the direct paths from GPS satelites to a receiver are obstructed. This happens a lot between tall buildings in cities or inside buildings. Places like streets with high buildings on both sides are called urban canyons, and especially improving car navigation in these places have been addressed.

Making use GPS together with other GNSS, like GPS+Glonass [GPS World article on improving performance in urban canyons]

Placing more satellites where they are likely to be in direct line of sight from the canyons. The Japanese seems to do this.