I. Introduction
Global navigation satellite systems (GNSS) have been widely used in civil and military fields over the past decade. Nowadays, there are many scenarios in which multiple receivers work in a certain region [1]. For instance, a platoon of military men carries out the task together, a crew does rescue work in a building, people locate their positions in a shopping center, etc. However, in challenging environments such as urban canyons and forests, the occlusions and other issues would severely affect the positioning performance, such as availability and accuracy [2]. Up to now, there are no good positioning solutions in these scenarios for conventional GNSS standalone receivers.