 Residual phase of a tracked east-west interferometer

For an east-west baseline $b$ and phase-centre zenith angle $\theta$ in its vertical plane, path compensation subtracts $b\sin \theta$. An offset $\alpha$ leaves $\delta \ell =b[\sin (\theta +\alpha )-\sin \theta ]\simeq b\alpha \cos \theta$. The complex correlation therefore has phase $2\pi b\alpha \cos \theta /\lambda$, up to receiver-order convention. Equal cable lengths do not remove this sky-dependent path difference.