A GNSS receiver needs a clear line of sight to the satellites it is tracking. If the line of sight to a satellite is blocked by objects such as buildings, trees, bridges, etc., the receiver cannot receive signals from that satellite. In locations that have a lot of obstructions, such as the urban canyons found in downtown areas of a large city, the obstructions can block so many satellites that the receiver cannot calculate its position or time.
One solution to signal blockage is for the receiver to track more than one constellation. By tracking more than one constellation, there will be more satellites available and a better chance of finding enough satellites to determine a position and time. The use of multiple navigation sensors, such as IMUs, helps not only in bridging outages such as those due to signal blockage, but also in the reacquisition of the GNSS signals after the outage.