Low power levels of global positioning system (GPS) signals make them susceptible to radio frequency interference (RFI) from intentional or unintentional sources. Reliance of safety critical and modern applications on GPS has made GPS critical infrastructure which must be protected. Time-difference-of-arrival (TDOA) is a common approach for locating emitters. The TDOAs are estimated by correlating signals received at different sensor nodes. However, RFI detection can be hampered by the in-band GPS signals which create undesired correlation peaks that can be mistaken for weak RFI. A sub-space projection-based approach for removing the GPS signals and leaving the RFI is proposed, and its performance is evaluated using signals from two synchronised Spirent GPS signal generators captured using a NordNav multi-front-end receiver. The results show that the proposed approach can remove the GPS signals, resulting in better RFI detection and localisation performance under various scenarios.