Precise Point Positioning (PPP) and Real-Time Kinematic (RTK) techniques provide a position estimate with an accuracy that is within the centimeter-level in open sky. Their basic principle is to take advantage of the high accuracy of the phase measurements made by the receiver Phase Lock Loops, compared to the delay estimate made by the receiver Delay Lock Loops. PPP and RTK techniques are not common for mobile users travelling in difficult environment, like urban area, for several reasons: frequent carrier-phase cycle slips and loss of tracking, reduced number of visible satellites, strong multipath on carrier phase and code pseudorange measurements. Indeed, in a constrained environment, several GNSS signals can be blocked, and the received GNSS signals are usually a combination of several multipath components. The aim of the proposed paper is to present a new precise positioning algorithm, based on single-differenced (between stations) measurement that applies to constrained environments. This technique enables a decreased convergence time of RTK in these environments, and provides a better code-only position if the rover carrier phase measurements are not available. Indeed, the contribution of the reference receiver noise to the initial position estimation is drastically reduced by using a non ambiguous carrier phase combination instead of code measurements. The new model is based on a careful management of code-phase hardware biases that keeps the rover widelane ambiguity as an integer and makes use of the newly and freely available widelane satellite biases originally intended for PPP with ambiguity resolution. The advantages of using single-differenced observations instead of double-differenced measurements are also underlined in this paper.