Algorithm 1. The pseudocode of proposed adaptive cooperative localization method for heterogeneous air-to-ground robots based on relative distance constraints

1.  Initialization      Cooperative network time synchronization and state initialization ${\mathit{X}}_0,{\mathit{P}}_0$ 2.  Set adaptive fusion threshold $\lambda$ based on dynamic change in number of known-position UAVs in dynamic cluster cooperative network 3.  State recursive equation of UGV based on Equation (1) 4.  Construct position observation equation of UGV INS based on the different number of known-position UAVs in cluster cooperative network      If ($\lambda ={\lambda }_1$)      Construct position observation equation of UGV INS at time $t_{\mathrm{k}}$ based on Equation (10)      Else if ($\lambda ={\lambda }_2$)      Construct position observation equation of UGV INS at time $t_{\mathrm{k}+1}$ based on Equation (15)      Else if ($\lambda ={\lambda }_3$)      Construct position observation equation of UGV INS at time $t_{\mathrm{k}+2}$ based on Equation (20) 5.  State optimal fusion estimation based on AEKF 6.  Optimal state estimation value of UGV