This paper presents a study of the threat of malicious interference to GNSS and examines the special case where the jamming device is incrementally more sophisticated than a typical always-on interference source. The concept of a systematic jamming attack is considered, where the interference signal is intentionally synchronized with the GNSS signals, with the intention of causing maximum disruption with the minimum power expenditure. Various attack methodologies are examined for the case of a civilian L1 receiver. It is shown that, depending on the attack strategy, the target signal and the target receiver, data-recovery, navigation and timing can be denied to a user with some tens of decibels less average power than a traditional jamming attack. It is further shown that some attacks may be capable to effectively deny some receiver functionality in a subtle manner such that presence of the malicious interference goes undetected. Key signal and receiver features that expose a vulnerability are identified and some means of improving receiver robustness are provided.