Direct GPS Positioning (DP) is a robust method that estimates the GPS navigation solution directly from the received GPS signal. In contrast, traditional methods, such as scalar tracking and vector tracking, compute intermediate channel range and range residual measurements independently, then use these as inputs into the navigation filter to estimate the navigation solution. However, a brute force implementation of DP is not computationally efficient. Our research contribution is to evaluate the robustness of DP using our more computationally efficient DP implementation. Our novel and effective DP receiver architecture initializes the navigation solution guesses as two groups: guesses varying in position and clock bias, guesses varying in velocity and clock drift. We then perform vectorized calculations to get the expected delay and Doppler between the receiver and each satellite in view. Following that, we perform batch calculations using Fast Fourier Transforms (FFTs) to obtain the vector correlation and vector spectrum. The navigation solution residual derived from the vector correlation and vector spectrum is then used as input into the navigation filter. We implemented our receiver architecture using our research platform - PyGNSS. We then evaluated the robustness of our DP receiver architecture by subjecting it to simulated jamming and meaconing attacks. We demonstrate through our experiments the robustness of our DP receiver architecture.