The security and reliability of GPS receivers are critical to Phasor Measurement Units (PMUs). These devices provide synchronized voltage and current phasor measurements in power systems based on GPS timing. However, due to the unencrypted nature and low received signal to noise ratio of civil GPS signals, jamming and spoofing attacks on GPS receivers can pose a risk to both the position and timing solutions. In this work, we aim for robust GPS time transfer for PMUs. We achieve this goal by taking advantage of the static nature of PMUs. We propose a position-information- Aided vector tracking loop to improve tracking capabilities and ensure continued operation through various attacks. To evaluate the effectiveness of the algorithm presented in this paper, we further conduct field experiments. Our test shows that the proposed position-information-aided vector tracking approach 1) improves the robustness of GPS receivers used in PMUs against jamming; 2) is able to successfully detect record-and-reply attacks; and 3) improves the accuracy of timing solutions.