Algorithm 1. Estimation of Absolute heading angle using Sun sensor and Inclinometer (EASI algorithm)

Get Sun ray vector from sensor output angles. $S^s={\left[\begin{array}{ccc}{\mathrm{S}}_x& {\mathrm{S}}_y& {\mathrm{S}}_z\end{array}\right]}^T$ Transform SS to rover frame. $S^b=\left[\begin{array}{c}{\mathrm{S}}_x^b\\ {\mathrm{S}}_y^b\\ {\mathrm{S}}_z^b\end{array}\right]=C_s^b\left[\begin{array}{c}{\mathrm{S}}_x\\ {\mathrm{S}}_y\\ {\mathrm{S}}_z\end{array}\right]\therefore C_s^b=\left[\begin{array}{ccc}1& 0& 0\\ 0& -1& 0\\ 0& 0& -1\end{array}\right]$ Transform Sb to local-flat frame. $S^n=\left[\begin{array}{c}{\mathrm{S}}_x^n\\ {\mathrm{S}}_y^n\\ {\mathrm{S}}_z^n\end{array}\right]=C_b^n\left[\begin{array}{c}{\mathrm{S}}_x^b\\ {\mathrm{S}}_y^b\\ {\mathrm{S}}_z^b\end{array}\right]\therefore C_b^n=\left[\begin{array}{ccc}\cos \theta & \sin \varphi \sin \theta & \cos \varphi \sin \theta \\ 0& \cos \varphi & -\sin \theta \\ -\sin \theta & \sin \varphi \cos \theta & \cos \varphi \cos \theta \end{array}\right]$ Get Sun’s azimuth (αSS) and elevation angles (${\varsigma }^{ss}$) from $S^n$ vector as: $\begin{array}{l}{\alpha }^{ss}=a\tan 2({\mathrm{S}}_y^{n^{\prime }},{\mathrm{S}}_x^{n^{\prime }})\\ {\varsigma }^{ss}=-{\sin }^{-1}({\mathrm{S}}_z^{n^{\prime }})\end{array}$ Using local date, time and location (latitude, longitude), get Sun’s azimuth (${\alpha }^{Ephe.}$) and elevation (${\varsigma }^{Ephe.}$) angles from Ephemeris models. Determine rover’s absolute heading by comparing the Sun’s position in steps 4 and 5. ${\psi }_{ss}=\{\begin{array}{l}{\alpha }^{Ephe}-{\alpha }^{SS}if({\alpha }^{Ephe}>{\alpha }^{SS})\\ {\alpha }^{SS}-{\alpha }^{Ephe}otherwise\end{array}$