Advances in electronics technology have enabled the creation of malicious RF interference of GNSS signals. For example jamming completely denies the GNSS user of position, navigation, and time (PNT) information. While a serious concern when we expect PNT at all times, current generation GNSS receivers often warn the user when PNT is unavailable. A second threat to GNSS integrity is spoofing, the creation of counterfeit GNSS signals with the potential to confuse the receiver into providing incorrect PNT information. This type of attack is considered more dangerous than a jamming attack since erroneous PNT is often worse than no solution at all. A variety of approaches have been proposed in the literature to recognize spoofing and can vary widely based upon the assumed capabilities and a priori knowledge of the spoofer. One method is to compare the GNSS result to data from a non-GNSS sensor. At the January 2016 ION ITM these authors developed and analyzed a spoof detection algorithm based upon measurements from an active ranging system (distances, but no heading). This paper expands the class of signals viable for this spoofing detection approach to passive ranging; equivalently, to range measurements which depend upon knowledge of precise time (effectively pseudoranges).