How to solve GNSS problem in critical environment?

Information about user position is very important due to increasing interest in Location Based Services (LBSs). Nowadays, service providers try to provide ubiquitous LBSs. It leads to solve ubiquitous positioning, i.e. localize mobile user anywhere and anytime. This challenge mainly depends on the actual environment where user position is determined. The providers try to divide all environments to two basic types: outdoor and indoor. Positioning in both types requires different ways to estimate position. This paper is focused on outdoor environment. GNSSs (Global Navigation Satellite Systems) provide acceptable reliability in outdoor, but are not very reliable in dense urban areas. Entrances to buildings can also be considered as critical environment, because of interconnection between outdoor and indoor. Therefore user positioning should be as reliable as possible. Generally, tall buildings cause the biggest problems, because mobile device is in GNSS signal shadow and positioning result is negative affected. This paper analyses these problems and try to propose a solution. The proposal is based on alternative positioning solutions based on WLAN, GSM network and Android Location Provider (ALP). These systems are compared to GPS (Global Positioning System) from accuracy point of view.