write arrticle base on : With the advent of autonomous cyber-physical systems, many safety and security critical applications rely on radars for enhanced real-time situational awareness. More specif- ically, mmWave frequency modulated continuous wave (FMCW) radars are used in many systems today as they provide robust object detection even under adverse weather such as fog and low-light scenarios, where other light- based sensors such as cameras and Lidars fail [1]–[4]. For example, they already play a critical role in advanced driver assistance systems (ADAS) [5] of modern automobiles for pedestrian and blind spot detection, adaptive cruise control, and emergency braking [6], [7]. Due to the precision and robustness of mmWave radars, they are also used to en- hance the autonomy of unmanned aerial vehicles [8], [9], multilane trafﬁc monitoring systems [10] and protect critical infrastructure, e.g., through robust detection of unauthorized drones [11], [12]. Given the nature of applications, it is imperative to investigate the security guarantees of mmWave FMCW radars against adversarial threats.
FMCW radars continuously transmit known chirp sig- nal that bounces off objects and get reﬂected back. The radar receiver processes the received signal and detects the surrounding objects that caused the reﬂection. There are broadly two categories of attacks on a radar system: Jam- ming and Spooﬁng [13], [14]. Radar jamming occurs when the attacker radiates a noise signal at high power in the same frequency band as the radar [15]–[17]. This jamming signal saturates the radar making it blind and unable to operate normally. Since jamming causes the radar to malfunction and fail, it is easily detected [18]–[20]. In contrast, spooﬁng attacks are hard to detect; they try to manipulate the radar measurements by forcing it to detect “ghost” objects (which are not present in the environment) or miss actual objects in the environment. For instance, an attacker driving ahead of a radar-equipped victim’s vehicle can manipulate the radar signals such that the victim radar measures a false distance to the attacker vehicle, as shown in Figure 1(a). Upon falsely detecting the attacker’s ghost as a real vehicle, the radar may trigger the vehicle to apply a sudden brake, risking the
 
passenger’s life and causing accidents.
Several studies have shown that FMCW radars are vul- nerable to spooﬁng attacks [21]–[27]. In these works, the attacker uses an active transmitter to generate a spooﬁng signal, which is received by the radar and results in a spoof- ing attack (as shown in Figure 1(b)). However, these studies have several limitations. Firstly, they require the attacker to have complete knowledge of the radar signal parameters. Secondly, they assume or achieve perfect synchronization between the victim’s radar and the attacker’s hardware using wires. Finally, these methods are not robust to radar security measures [28]. For example, the radar can introduce short pauses in transmission to detect the spooﬁng signal or dynamically alter the transmit signal characteristics, such as the frequency sweep slope and the time period of individual chirps, to prevent successful spooﬁng.
We present mmSpoof, a robust and ﬂexible spooﬁng mechanism using a millimeter-wave (mmWave) reﬂect array. Our reﬂect array captures the incident radar signal and reﬂects it towards the radar so that it is indistinguishable from the original radar signal but can still cause malicious detection that appears as a ghost object. In contrast to the active transmitter style, mmSpoof is based on the reﬂection of the radar’s signal with a modulation that perturbs the properties of measured distance and velocity by the victim’s radar as shown in Figure 1(c). The reﬂection-based spooﬁng model allows mmSpoof to eliminate the synchronization re- quirement between the victim radar and the attacker. More- over, the security encoding of radar, if any, are preserved in a reﬂection-based attack, e.g., when the radar is paused, the reﬂections are paused too, thus making the attack robust to several security features and countermeasures of radar. mmSpoof’s spooﬁng is also ﬂexible to spoof any arbitrary distance and velocity at the radar without requiring synchro- nization between the victim and the attacker. We further develop a mechanism to learn important radar parameters required for spooﬁng in the background without assuming they are known to the attacker. In this way, mmSpoof is a robust and ﬂexible spooﬁng mechanism.
■	Asynchronous spooﬁng challenge: A key challenge for mmSpoof is to ﬂexibly spoof distance and velocity using reﬂect array without synchronizing it with the victim’s radar. The reﬂect array should apply appropriate modulation to the incident signal to spoof the radar’s measurements. For spoof- ing distance, one naive solution is to apply a conﬁgurable delay to the incident signal, which causes an increase in the time-of-ﬂight of the signal as detected by the victim’s radar. However, it only increases the distance between the victim and the attacker. We cannot reduce distance as it requires infusing negative delays, making the system non-causal. Our insight is to spoof shorter distances by leveraging the nature of radar chirp waveform that allows us to create a frequency shift (instead of delays) which manifests as a time shift due to a linear frequency-time relationship. It is easy to visualize that if the frequency shift is positive, then radar observes the attacker’s ghost object closer than it is, and for a negative frequency shift, it appears farther. Therefore, the attacker can spoof an arbitrary distance on
 
the radar by changing the frequency shift. Next, spooﬁng velocity is important to give the victim’s radar a perception that the attack is realistic. For instance, in the automotive example, the attacker may want the spoofed velocity to lie in the range of typical driving speed limits or follow a speciﬁc pattern that mimics a vehicle’s natural acceleration or braking. FMCW radar estimates velocity by measuring the Doppler frequency from the reﬂections received from objects in front of the radar. Therefore, we provide a novel mechanism to alter the Doppler frequency of the reﬂections by applying an appropriate frequency shift to spoof velocity.
■	Decoupling distance and velocity challenge: The next challenge for mmSpoof is to spoof distance and ve- locity independently. As discussed, applying frequency shift on the reﬂected signal creates arbitrary variations in the distance as desired, but it also induces random ve- locity change. i.e., a single knob (frequency shift) at the attacker changes two parameters (distance and velocity) at the victim’s end. We call this issue coupling between distance and velocity spooﬁng and present a novel solution to decouple the two entities and spoof them independently. Our decoupling solution is inspired by two key observa- tions. First, the granularity and ﬁne-tuning of frequency- shift, where a small perturbation in frequency shift allows us to control the velocity spooﬁng without making signif- icant changes in distance spooﬁng. Second, we leverage periodicity in velocity variations with frequency shift, i.e., two different frequency-shift would cause the same spoofed velocity. Therefore, we vary the frequency-shift with ﬁne granularity to spoof velocity and with coarse granularity to spoof distance without affecting velocity. Thus, mmSpoof can ﬂexibly and independently spoof distance and velocity.
■	Realistic power variation challenge: In addition to the distance and velocity, the attacker must also have control over the received signal strength at the victim’s radar, allowing them to manipulate power levels to adhere to path loss regulations with the spoofed distance. For in- stance, spooﬁng shorter distances should have higher signal strength than spooﬁng longer distances. We achieve this by controlling the gain of the reﬂected signal with the spoofed distance. The distance-to-gain relationship is obtained from the known path loss models at the operating frequency. To attack with realistic power variations, We use a commercial phased array with a built-in gain control mechanism suitable for the required variable-gain spooﬁng.
■	Parameter estimation challenge: The ﬁnal challenge for mmSpoof is to estimate the frequency-shift at the reﬂect array that creates ﬂexible distance and velocity spooﬁng while being robust to various countermeasures in the radar. The frequency-shift calculation depends on important radar parameters such as the start frequency, chirp time, and chirp slope. The security features in radar may change these parameters occasionally or periodically to alter any inter- ference or spooﬁng. Therefore, it is necessary to estimate these parameters in real-time at the attacker without giving any indication of the attacker’s presence to the victim. We develop a novel digital signal processing mechanism to ex- tract radar parameters. Our technique is based on analyzing the spectrogram of the received signal and detecting known patterns of chirps found in the standard FMCW radars. We estimate the radar parameters periodically and use them to create the desired frequency shift for spooﬁng the radar.