**2.1. Alternatives for UAV localization systems**

In this section, we review the most important works proposed to deal with UAV localization problem. First, this section presents an analysis of the environment representation that these works used as a map, their advantages and disadvantages. Second, what these works use to compare the sensor readings with different parts of the map. Then, **Table 1** in this section introduces other qualitative comparisons between these works, such as the single or multi MAV pose estimation and indoor or outdoor localization.
