where it retransmits them without any modification. This way the adversary can avoid detection if cryptography is employed, while it can “present” a victim with GNSS sig- nals that are not normally visible at the victim’s location. In this paper, we abstract away the placement of adversar- ial nodes, and we characterize the replay attack by two fea- tures: (i) the adversarial node capability to receive, record and replay GNSS signals, and (ii) the delay t$_{replay}$ between reception and re-transmission of a signal. The GNSS signal reception and replay can be done at the message or symbol level, or it can be done by recording the entire frequency band and replaying it with- out de-spreading signals. The latter, more involved and thus costly, would enable the attacker to mount an at- tack against the delayed-disclosure secret spreading code approach, as pointed out in [7], not only for long replay- ing delays but also for very short ones. Clearly, such an instantiation of the replaying attack implies a more sophis- ticated adversary than one replaying symbols or messages. For example, the adversary would need to infer, possibly by possessing a legitimate receiver, the start of NAV messages to replay signals accordingly The t$_{replay}$ delay between reception and re-transmission depends on the attack configuration (e.g., the distance be- tween the receiving and re-transmitting adversarial radios, the physics of the signal propagation, and, when applica- ble, the delay for the adversary to decode the GNSS signal). We capture such factors by considering t min replay > 0, a min- imum delay that the adversary cannot avoid. Beyond this, the attacker can choose some additional delay τ ≥ 0, such that it replays the signal after t$_{replay}$ = t min replay + τ . We illustrate a replay attack in Fig. 1: The recording of the NAV message starts after its beginning is detected, due to the preamble 10001011, with length of eight chips, and the decoding of the NAV message first bit. This corresponds to t min replay = 20 ms : the transmission rate of 50 bit/s implies that 20ms are needed for the first bit to be received by an adversarial radio. The adversary can choose different t$_{replay}$ values for sig- nals from different satellites, even though “blind” replaying of all NAV signals with the same delay can be effective. The selection of which signals (from which satellites) to relay of- fer flexibility. But even the “blind” replaying of all NAV signals (the entire band) can be effective: t$_{replay}$ controls the “shift” in the PVT solution. Essentially, t$_{replay}$ con- trols the “shift” in the PVT solution the adversary induces to the victim node(s). Fig. 2 shows the impact of a replay attack as a function of the spoofing stage of the attack: (i) the location offset or error, i.e., the distance between the attack-induced and the actual victim receiver position, and (ii) the time offset or error, that is, the time difference between the attack- induced clock value and the actual time. We consider for this example t$_{relay}$ = 20ms, as the first bit decoding de- lay dwarfs the preamble detection and propagation delays. This is indeed a very subtle attack we refer to [9] for a range of t$_{replay}$ values, which shows that the larger the t$_{replay}$ , as We investigate three defense mechanisms that rely on a common underlying three-step idea. First, the receiver col- lects data for a given parameter during periods of time it deems it is not under attack; we term this the normal mode . 4 Defense mechanisms 4 Figure 2: Impact of the replay attack, as a function of the spoofing attack duration. (a) Location offset or er- ror: Distance between the attack-induced and the actual victim receiver position. (b) Time offset or error: Time difference between the attack-induced clock value and the actual time. the adversary tunes its τ value, the higher the location and time offsets. Even for a very low t$_{replay}$ , while the mobile node re- ceiver is still locked on the attacker-transmitted signals, the location error increases, with the victim receiver “dragged” away from its actual position. Each millisecond of t$_{relay}$ translates approximately into 300m of location offset for each pseudorange (as the speed of light, c , is taken into account), with the actual “displacement” of the victim de- pending on the geometry (e.g., position of the satellite whose signals were replayed). As for the time offset, which can be viewed as a side- effect of the attack: it is in the order of less than one mil- lisecond per second, and it can very well go easily unnoticed by the user. With a given t$_{relay}$ , every time the victim re- ceiver re-synchronizes, typically at the end of a NAV mes- sage that lasts 30 sec, t$_{replay}$ will emerge as t$_{V}$ from the PVT solution and thus will be accumulated as part of the time offset shown in Fig. 2. 0 50 100 150 200 250 300 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Attack duration [s] Distance offset [m] (a) 0 50 100 150 200 250 300 50 100 150 200 250 300 350 Attack duration [s] Time offset [ms] (b)