This chapter presents the ERTK: extra-wide-lane real-time kinematic positioning. It dedicates to fully exploit the RTK capability of virtual extra-wide-lane (EWL) signals over long baselines, which is referred to as ERTK. Ionosphere-ignored and ionosphere-float models which are two ERTK models are formulated. And then the ionosphere-smoothed ERTK model is introduced. In addition, the ERTK equivalence of using any two EWL observations is proven, and the condition of selecting either ionosphere-ignored or ionosphere-float model is discussed. Through the experiment and analysis, we find some useful remarks. The ERTK is promising and can be applied in many applications.