This tutorial paper presents a range-aided Extended Kalman Filter (EKF) for relative navigation, combining IMU data with range observations from onboard sensors that measure the distance between nodes in the network (i.e. a drone swarm). This method, typically called range-only EKF navigation, mitigates the growth of position, navigation, and timing (PNT) errors during periods of GPS signal loss. 

Click on this link to see the paper: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10720587