Standard single-point positioning (SPP) still plays an essential role for some applications with high-level requirements for navigation robustness. However, the accuracy of SPP is usually meter level and may not be sufficient for dynamic applications such as hydrology and vehicle surface activities in the airport. In order to solve this problem, an instantaneous sub-meter level navigation solution can be achieved by incorporating a priori altitude information into SPP. In this paper, an altitude constrained SPP algorithm is proposed, by applying the least squares method with quadratic equality constraints. The real GPS data was used for algorithm validation. The results demonstrate that the altitude constrained SPP’s positioning accuracy is improved effectively compared to the conventional SPP method and it is robust due to not depending on the carrier phase measurements.