WO2025229347A1

WO2025229347A1 - Radar reflector and associated systems and methods

Info

Publication number: WO2025229347A1
Application number: PCT/GB2025/050948
Authority: WO
Inventors: Timothy Constandinou; Alan Bannon
Original assignee: Imperial College Innovations Ltd
Current assignee: Ip2ipo Innovations Ltd
Priority date: 2024-05-03
Filing date: 2025-05-02
Publication date: 2025-11-06
Anticipated expiration: 2026-11-03
Also published as: GB202406275D0

Abstract

The present disclosure relates to frequency-selective radar reflector for reflecting radar transmissions transmitted by a radar system, the radar reflector comprising: an active frequency-selective surface, having an adjustable radar absorbency; and a radar reflecting surface; wherein the radar reflector is operable to modulate, in the time domain, the radar absorbency of the frequency-selective surface to control the reflection of radar signals in one or more frequency bands from the radar reflecting surface, to thereby signal information to the radar system.

Description

_{RADAR REFLECTOR AND ASSOCIATED SYSTEMS AND METHODS} Field of the Invention _{The invention relates to radar apparatus, systems and methods. Merely by way of} example, the invention relates to active frequency-selective surfaces for _{synchronisation, communication and identification. Associated apparatus and} _{methods are provided.} Background to the Invention _{Radar systems are often considered as pure observation platforms, gathering data} _{regarding unaware or non-cooperative targets. The options for a target wishing to} _{transmit information back to an observing radar sensor are typically complex and} _{often require clock synchronisation.} Long-term, unobtrusive monitoring of vulnerable or unwell individuals remains a persistent challenge in healthcare. With the global population ageing at a rapid pace, an ever-increasing number of individuals require healthcare monitoring for care, study, and treatment. A radar-based remote sensing system can be used monitor patients in their homes, and provide clinically relevant data to patients, their carers, _{and healthcare providers to inform clinical decisions, or simply as a system to} _{provide parties reassurance that their health status is stable. People without} diagnosed conditions (i.e. healthy individuals) also benefit from health monitoring to provide ongoing input on their health status and also to detect changes in features _{early. These systems may be referred to as biomedical radar systems. For} biomedical radar systems, the targets to monitor using the radar are generally _{individual people. Should any additional information be required, such as} _{timestamps or other information the target wishes to transmit to the radar system,} _{this is either sent using a separate channel or actively transmitted, requiring at} _{minimum a level of clock-synchronization between the active transmitter at the} _{target and the receiving radar.} Numerous challenges remain in biomedical radar systems, particularly concerning the synchronisation of networked sensors and the robust identification of individuals. _{Given the highly individual nature of biomedical data, it is important to distinguish} between individuals when multiple individuals are within the radar system’s view. Currently, most focus lies in utilising biometric information derived from radar data for this purpose. However, there are scenarios where only a subset of individuals may be of interest; for instance, in situations involving a caregiver or clinician interacting with a patient only the patient’s data is of interest. In such cases, it is _{important to ensure the exclusion of non-patient data from storage and analysis.} The increasing complexity of biomedical radar systems often necessitates the networking of sensors to provide coverage over larger areas and to exploit data fusion across multiple overlapping sensor fields-of-view. However, maintaining sufficient clock synchronicity between multiple radar sensors, particularly when channel-separation may be enforced, is technically challenging. This synchronicity is vital to allow successful data fusion of time-varying signals, such as heart-beat. Additionally, in complex deployment environments with more than one sensor, knowledge of sensor positions may be important for downstream data fusion. This is typically solved with manual measurements which carries a significant time and capital cost during deployment. The present disclosure provides methods and apparatus for addressing, or at least partially ameliorating, one or more of the above needs. Summary of the Invention Aspects of the present invention are set out in the appended independent claims, while details of certain embodiments are set out in the appended dependent claims. _{In a first aspect the invention provides a frequency-selective radar reflector for} reflecting radar transmissions transmitted by a radar system, the radar reflector _{comprising: an active frequency-selective surface, having an adjustable radar} absorbency; and a radar reflecting surface; wherein the radar reflector is operable _{to modulate, in the time domain, the radar absorbency of the frequency-selective} surface to control the reflection of radar signals in one or more frequency bands from the radar reflecting surface, to thereby signal information to the radar system. _{The radar reflector may be for biomedical monitoring or human sensing.} The radar reflector may be for an automotive or aerospace application. The radar reflector may be for a geospatial monitoring or geospatial communication application. The radar reflector may be configured to modulate the radar absorbency of the frequency-selective surface using on-off-keying and/or amplitude shift keying to signal the information. The radar reflector may be operable to modulate the radar absorbency to indicate, to the radar system, an identity associated with the radar reflector. The radar reflector may be operable to modulate the radar absorbency to provide a _{timing signal for synchronisation of the radar system.} _{The frequency-selective surface may comprise a set of first portions of the surface} _{and a second portion of the surface; and modulating the radar absorbency may} _{comprise modulating a voltage bias of diodes arranged around each first portion of} the surface, that connect each of the first portions of the surface to the second portion of the surface. _{Each of the first portions of the surface may be circular. More generally, each of the} first portions may have any geometric form, including but not limited to circles, ovals, _{polygons, and custom contours. The first portions may be of repeating form.} The circular first portions may each have a width that is approximately one quarter, _{one eighth, or one sixteenth of the width of the centre wavelength used in the radar} system. Four diodes may be arranged around each of the first portions of the surface. _{The frequency-selective surface may comprise a repeating pattern of the first} portions. The radar reflector may have a substantially tetrahedral shape. The radar reflecting surface may be planar. The radar reflector may be operable to modulate the absorbency of the frequency- _{selective surface at a frequency of between 1 Hz and 1 MHz, for example 1 kHz.} _{The radar reflector may be operable to modulate the radar absorbency of the} _{frequency-selective surface to control the radar cross-section of the radar reflector} in one or more frequency bands, to thereby signal information to the radar system. In a second aspect the invention provides a system comprising one or more of the radar reflectors according to the first aspect. _{The system may comprise a plurality of the radar reflectors, wherein each of the} radar reflectors is configured to modulate the radar absorbency of its respective _{frequency-selective surface using a different modulation pattern in the time domain.} _{The system may comprise one or more radar sensors, each radar sensor} _{comprising: at least one radar transmitter for transmitting radar signals to illuminate} _{the one or more of the radar reflectors, and at least one radar receiver for receiving} _{corresponding radar return signals reflected from the one or more radar reflectors.} _{The system may comprise a plurality of the radar sensors; and the system may be} _{configured to modulate the radar absorbency of the frequency-selective surface of} _{the one or more radar reflectors to provide a reference signal for the radar sensors.} _{The reference signal may be for time synchronization between the radar sensors.} _{The system may comprise a plurality of the radar sensors; wherein the system is} configured to modulate the radar absorbency of the frequency-selective surface of the one or more radar reflectors to provide a signal for spatial calibration of the radar _{sensors; and wherein the system is configured to determine the relative position of} the radar sensors using the signal for spatial calibration. _{The radar system may be for biomedical monitoring or human sensing.} The radar system may be for an automotive or aerospace application. The radar system may be for a geospatial monitoring or geospatial communication application. The one or more radar sensors may be ultra-wideband (UWB) radar sensors. In a third aspect the invention provides a method performed by a frequency- selective radar reflector for reflecting radar transmissions transmitted by a radar system to signal information to the radar system, wherein the radar reflector _{comprises an active frequency-selective surface having an adjustable radar} _{absorbency, and a radar reflecting surface, and wherein the method comprises:} _{modulating, in the time domain, the radar absorbency of the frequency-selective} surface to control the reflection of radar signals in one or more frequency bands from the radar reflecting surface, to thereby signal the information to the radar system. _{In a fourth aspect the invention provides a frequency-selective radar reflector for} reflecting radar transmissions transmitted by a radar system, the radar reflector _{comprising: a first active frequency-selective surface, having an adjustable radar} _{absorbency; a radar reflecting surface; and a first cavity or first region of dielectric} material arranged between the first active frequency-selective surface and the radar _{reflecting surface; wherein the first active frequency-selective surface is provided at} _{a first predetermined distance away from the radar reflecting surface; and wherein} the radar reflector is operable to modulate, in the time domain, the radar absorbency of the first frequency-selective surface to control the reflection of radar signals in one or more frequency bands from the radar reflecting surface, to thereby signal information to the radar system by modulating the phase of radar signals reflected from the frequency-selective radar reflector. _{The radar reflector may be configured to modulate the radar absorbency of the first} frequency-selective surface using on-off-keying and/or amplitude shift keying to signal the information to the radar system. _{The first predetermined distance may be configured for resonance with a frequency} or range of frequencies of one or more transmissions of the radar system. _{The first predetermined distance may be N·λ or λ/N, wherein N is a positive integer} and λ is a centre wavelength transmitted by the radar system. The first region of dielectric material may comprise FR4, polytetrafluoroethylene or a ceramic material. The first region of dielectric material may comprise a dielectric material having a _{controllable permittivity; and the radar reflector may be configured for control of the} permittivity of the dielectric material to modulate the phase of the radar signals reflected from the frequency-selective radar reflector. _{The radar reflector may further comprise: a second active frequency-selective} _{surface, having an adjustable radar absorbency, and a second cavity or second} region of dielectric material arranged between the second active frequency- selective surface and the first active frequency-selective surface; wherein the first active frequency-selective surface is arranged between the second active _{frequency-selective surface and the radar reflecting surface; wherein the second} active frequency-selective surface is provided at a second predetermined distance _{away from the first active frequency-selective surface; and wherein the radar} reflector is operable to modulate, in the time domain, the radar absorbency of the first frequency-selective surface and the second frequency-selective surface to signal information to the radar system by modulating the phase of radar signals reflected from the frequency-selective radar reflector. _{The first predetermined distance and the second predetermined distance may be} configured for resonance with a frequency or range of frequencies of one or more transmissions of the radar system. _{The radar reflector may be configured for control of the radar absorbency of the first} active frequency-selective surface independently of control of the radar absorbency of the second active frequency-selective surface. _{The radar reflector may be configured to modulate the radar absorbency of the first} frequency-selective surface and the second frequency-selective surface using on- off-keying and/or amplitude shift keying. _{A first dielectric material having a controllable permittivity may be arranged between} the first active frequency-selective surface and the radar reflecting surface, and a _{second dielectric material having a controllable permittivity may be arranged} between the second active frequency-selective surface and the first active _{frequency-selective surface; and the radar reflector may be configured for control of} the permittivity of the first dielectric material and the permittivity of the second dielectric material to modulate the phase of the radar signals reflected from the frequency-selective radar reflector. _{The radar reflector may be operable to signal information to the radar system by} modulating the phase of radar signals reflected from the frequency-selective radar reflector for angles of incidence of between 0° and 90° for a radar transmission incident on the frequency-selective radar reflector. _{In a fifth aspect the invention provides a system comprising one or more of the radar} reflectors according to the fourth aspect. The system may comprise a plurality of the radar reflectors, and wherein each of the radar reflectors is configured to signal information to the radar system by modulating the phase of radar signals reflected from the radar reflector using a different modulation pattern in the time domain. _{In a sixth aspect the invention provides a method performed by a frequency-} selective radar reflector for reflecting radar transmissions transmitted by a radar system to signal information to the radar system, wherein the radar reflector comprises at least one active frequency-selective surface having an adjustable radar absorbency, and a radar reflecting surface, and wherein the method _{comprises: modulating, in the time domain, the radar absorbency of the frequency-} selective surface to control the reflection of radar signals in one or more frequency bands from the radar reflecting surface, to thereby signal information to the radar system by modulating the phase of radar signals reflected from the frequency- selective radar reflector. Brief Description of the Drawings Embodiments of the invention will now be described by way of example only with reference to the attached figures in which: _{Figure 1 schematically illustrates examples of a structure of a radar reflector;} _{Figure 2 shows a further schematic illustration of an example of a structure of a} radar reflector; Figure 3 shows an example of a tetrahedral radar reflector; Figure 4 shows an example of a planar radar reflector; _{Figure 5 shows a further schematic illustration of an example of a structure of a} radar reflector; _{Figure 6 shows a further schematic illustration of an example of a structure of a} radar reflector; _{Figure 7 shows a plot of an azimuth amplitude response of a radar reflector;} _{Figure 8 illustrates frame header detections for a radar reflector;} _{Figure 9 illustrates a count of number of detections of the radar reflector beat} frequency; _{Figure 10 schematically illustrates an example of an environment in which a radar} _{system of the present disclosure may be deployed;} _{Figure 11 schematically illustrates an example of a radar system;} _{Figure 12 schematically illustrates an example of a radar sensor comprising on-} device data flow; _{Figure 13 schematically illustrates a further example of a radar reflector;} _{Figure 14 shows plots of the radio frequency (RF) signal and frequency spectrum} _{for ‘on’ and ‘off’ configurations of the frequency-selective surface;} _{Figure 15 shows plots of the phase spectrum and amplitude-scaled phase for ‘on’} _{and ‘off’ configurations of the frequency-selective surface;} _{Figure 16 schematically illustrates a further example of a radar reflector; and} _{Figure 17 shows a plot of a representative azimuth response of a radar reflector.} Detailed Description of Preferred Embodiments The present embodiments represent the best ways known to the Applicant of putting the invention into practice. However, they are not the only ways in which this can be achieved. _{The disclosure provides a radar reflector 100 that can advantageously signal} _{information to a radar system. Encoding and communications schemes for signalling} _{information to the radar system using the radar reflector 100 are also disclosed.} _{This present disclosure provides active frequency-selective surfaces (AFSS). The} AFSS may also be referred to as a ‘tunable’ frequency-selective surface. _{Advantageously, the AFSS can be used to provide beacons (which are also referred} _{to as ‘radar reflectors’) configured to modulate their reflectivity in a controllable} manner. Using this modulation with an on-off-keying (OOK) scheme, and/or _{amplitude shift keying with multiple amplitude characters (M-ary ASK) and a} _{communications protocol using reversals and frame header codes, the beacons can} be located by a radar, and information may be encoded and transmitted from beacon to radar without the need for clock synchronization. The apparatus and _{methods enable individuals to be identified, enable clock synchronization between} channel-separated radars, and enable the position of radar sensors to be determined (e.g. calibrated). _{The AFSS are operable to modulate their reflectivity to appear as a controllable} _{vacillating target to an observing radar system. Furthermore, using a customised} _{communication scheme, the beacons can effectively modulate data onto the} received radar range profiles. Whilst examples will be described in which the AFSS is used for biomedical radar _{sensing, the apparatus and methods are applicable to many potential applications.} Temporal Calibration (Synchronisation) _{By modulating the reflectivity of the AFSS for radar waves transmitted in a radar} _{system, the AFSS can be used as a common timing signal/clock for the radar} system, enabling the radar system can be synchronised precisely, for example to _{the nearest microsecond. In other words, the AFSS is operable to provide a} _{reference signal to provide time synchronization between the radar sensors.} Spatial Calibration _{By arranging a plurality of spatially separated AFSS devices in an area in which a} networked radar system has been installed, uniquely modulated signals from each _{AFSS can be used to reconstruct the positions of each radar node of the radar} _{system within the room, without the need for manual error-prone measurement. This} _{advantageously enables networked radar systems to be installed rapidly. By} combining range profiles using a single beacon, this can be used to identify proximity to different radars. If two beacons are used, then the 2D position of radars _{can be determined; if three beacons are used then 3D position can be determined} (e.g. using back projection to establish radar sensor locations). Identification _{The AFSS is configurable for use as an identification beacon. For example, when} _{worn as a badge, the reflectivity of the AFSS for reflecting one or more frequencies} _{or frequency ranges can be modulated using on-off-keying (OOK), to provide a} _{binary-coded ID string using the reflections from an individual. This enables,} _{example, a person wearing the identification beacon to be uniquely identified by the} radar system. Vehicle-to-vehicle communication An AFSS beacon can be installed on a ground vehicle in such a way that it is visible and illuminated by the radar sensors mounted on other vehicles and ground _{infrastructure. Modulating the AFSS beacon can indicate information the first vehicle} may wish to make known to others, such as destination, intended manoeuvres, or other information about the vehicle. Ground-to-space communication _{An AFSS beacon can be placed on the ground, building, ship, or other place in such} _{a way that it is visible and illuminated by satellites in orbit. Modulating the AFSS} beacon can indicate information the controller of the beacon may wish to make known to radar sensing satellites. This information may be information to be collected and returned to another ground station, or could be of immediate importance such as indication of distress or emergency. Air-vehicle communication An AFSS beacon can be installed on an aerial vehicle in such a way that it is visible and illuminated by radar sensors of other aerial vehicles or other ground-based radars such as air-traffic control radars. Modulating the AFSS can indicate _{information the aerial vehicle may wish to make known to other aerial vehicles and} _{ground stations, such as identity, altitude, speed, destination, or other information.} Active Frequency-Selective Surfaces An AFSS is a type of meta-material that can change its reflectivity/permittivity for _{particular frequency bands. Advantageously, the AFSS enables the provision of} _{beacons for synchronisation, communication, and identification. The reflectivity of} _{the AFSS can be modulated in a manner that makes the modulations visible to a} radar system. These modulations appear as rapid changes in reflectivity in reflected _{radar return signals. Advantageously, the reflectivity can be actively modulated, and} _{the AFSS do not rely on resonance.} As will be described in more detail later, the AFSS may comprise, for example, _{circular elements that have a width that is one quarter of the width (or approximately} _{one quarter of the width) of the centre wavelength used in a radar system that} illuminates the AFSS. The modulation effect is driven by modulating the voltage bias _{of diodes (e.g. PIN diodes) arranged around each circular element. Using the radar} _{transmissions that are reflected from the AFSS as common timing signal (or clock),} _{the networked radar system can be precisely synchronised, for example to the} nearest microsecond. The AFSS can also be configured to provide an indication of an identity. For example, when worn as a badge, the AFSS can be modulated using _{on-off-keying (OOK) to signal a binary-coded ID string using the reflected radar} transmissions, for example, allowing people to be distinguished indoors. Moreover, by placing multiple AFSS devices in a grid on the floor of a room in which a networked radar system has been installed, it becomes possible to use the uniquely modulated signals from each AFSS to reconstruct the positions of each radar node _{of the radar system within the room, without the need for manual error-prone} _{measurement. This enables the networked radar system to be installed rapidly and} _{sensor nodes to be mounted where convenient. For example, a single 1x1-meter} _{grid of AFSS devices could be used to locate and calibrate the radar system (e.g.} automatically). Frequency-Selective Surfaces (FSS) exhibit diverse responses (absorption, _{reflection, transmission) to radio frequency (RF) energy across a spectrum of} _{frequencies. AFSS provide, in addition, the capability to change some or all of these} _{responses and/or the frequency at which they occur. From a radar sensing} _{perspective, an AFSS capable of dynamically changing reflectivity introduces a} _{fluctuating target to a radar sensor. Consequently, changes in reflectivity will be the} _{most visible to an observing radar sensor, regardless of other characteristics.} _{In one example, the present disclosure provides a tunable (or ‘active’) frequency-} _{selective surface based on planar circular elements and diodes (e.g. PIN diodes).} _{A simplified schematic illustration of a radar reflector 100 comprising an AFSS is} _{illustrated in Fig. 1. In this example, the radar reflector 100 comprises a planar} _{reflector 101, bias routing 102, an electrically-insulating (e.g. FR4, PTFE, or} _{ceramic) dielectric substrate 103, and a frequency-selective layer 104 comprising} _{circular elements and a ground plane. As will be described later, the elements of the} _{frequency-selective layer 104 need not necessarily be circular. Advantageously, the} _{additional planar reflector layer 101 maximises visibility of the radar reflector 100 to} _{the radar system. Whilst a simple planar reflector can been used, a polyhedral} reflector could alternatively be used. _{The radar system used to illuminate the radar reflector may be, for example, a} _{networked radar system. The system may comprise pulsed sensors that use swept-} _{threshold sampling (STS) to directly capture RF reflections. Due to the} _{implementation of STS with a chaotically dithered sampler, channel separation may} _{be enforced, making it difficult to synchronise radar clocks or samplers between} _{different nodes of the radar system. This limitation means that many of the} _{contemporary methods of synchronization or registration of networked radar} _{systems are not possible to implement; forcing the system to operate in a multi-} _{static-like mode of independent-but-networked sensors. Advantageously, the radar} reflectors 100 of the present disclosure, comprising the AFSS, enable meaningful _{information to be conveyed between the radar reflector and the radar sensors, by} _{using a communications and encoding scheme based on using on-off-keying (OOK)} to modulate the reflectivity of the radar reflectors. _{Figure 2 schematically illustrates a diode-based AFSS design for the frequency-} selective layer 104 of the radar reflector 100. In the present examples the diodes are PIN diodes, although other types of diodes may alternatively be used. In this _{example, the radar reflector 100 comprises sets four diodes 202 connecting each} _{respective circular element 201 to a surrounding ground plane 204. A ground pin} _{and bias pin 203 are also provided, for suppling bias to the diodes and to the centre} _{205 of each circular element 201 via traces on the reverse side of the reflector 100} _{(not shown in the figure). To operate the AFSS and modulate the absorbency of the} _{surface, the diode bias voltage 203 is increased to place the diodes into a forward-} biased conducting state. The size of each circular element 201 will depend on the _{particular implementation and on the centre frequency of the radar system (for} _{example, 8GHz). The width of the circular elements may be, for example, one} _{quarter, one eighth, or one sixteenth of a wavelength used in the radar system. The} _{circular elements 201, bias routing 102, and ground plane 204 may be formed, for} _{example, of copper, gold, or any other suitable conductive material, and may be} _{between 10 microns and 100 microns in thickness.} _{Further examples of configurations that could be used for the frequency-selective} _{layer 104 of the radar reflector 100 are illustrated in Figures 3 to 6. For simplicity of} illustration, the bias voltage supply 203 and the diodes 202 connecting each _{respective reflective element to the ground plane are not illustrated in all of Figures} _{3 to 6, but are nevertheless provided. Figure 3 illustrates an example in which the} _{radar reflector 100 comprises circular elements 302 arranged on the surface of a} tetrahedron 301. Figure 4 shows a further example of a planar radar reflector 100 comprising circular elements 401. Figure 5 shows an example of a radar reflector _{100 in which crosses 501 are provided to form the tunable reflective elements.} _{Figure 6 shows a further example of a radar reflector 100 in which an arrangement} _{of triangles 601 is provided to form the tunable reflective elements. The part of the} _{surface of the radar reflector around which the diodes are provided (e.g. the circular} _{elements) may be referred to as the ‘first portion’ of the surface, and the ground} _{plane may be referred to as the ‘second portion’ of the surface.} Synchronisation The radar reflector 100 can be used to provide an amplitude modulation (AM) system, where the carrier is the radar cross-section (RCS) of the backing reflector 101. The modulation is achieved by varying the absorbency of the frequency- selective layer 104, which results in an observable change in returned power to the interrogating radar system. The interrogating radar illuminates the radar reflector so that the backscattered energy from the radar reflector is detectable. As a result of the carrier being the RCS, other objects illuminated by the radar transmissions can result in noise on the communication channel. This includes, for example, objects or people wearing or carrying the radar reflector 100. A human may present a large RCS relative to that of the radar reflector 100, posing a challenge to the detection of the reflector 100. _{To solve this problem, frame synchronisation may be achieved by inserting a binary} sequence into the transmission with reversals and a sequence of finite binary values with ideal autocorrelation properties and high peak to sidelobe ratio as a frame header which can be detected with matched filters by a receiver. Following the frame _{header, which enables frame synchronisation, a fixed-width data payload may} _{follow. For example, 16, 32, or 64 bits of payload may be used. The total length of} each transmitted frame will depend on the frame header sequence length and data payload. By way of example, following reversals, the frame header may employ a Legendre sequence of length 5 followed by a 64-bit payload. The accurate detection and demodulation of the modulated signal requires that range profiles are available at, at minimum, the Nyquist rate for the chosen baud rate. For example, range profiles may be available at a frequency of 500 Hz for a maximum baud rate of 250 symbols/second. The detectability of the signal is improved by decreasing the baud rate; however, the radar reflector 100 may be mobile within the scene. Range migration will increase the number of confuser targets (in this context, sources of noise power) within the down-range section occupied by the beacon and decrease SNR. Characterisation of Radar Reflectors Azimuth modulation amplitude Measurements of the effective azimuth of the radar reflector 100 were performed _{using a radar system to illuminate the AFSS beacons, which were turned in azimuth} _{between measurements. A square-wave was used to bias the AFSS, and the} _{difference in reflected amplitude between forward-bias and reverse-bias states was} _{measured. The results of these measurements are illustrated in Figure 7. As} _{illustrated, the azimuth amplitude response is narrow. Advantageously, to mitigate} _{the narrow effective azimuth of the AFSS, a repeating pattern of the surface} _{elements (e.g. a tessellated design) such as those illustrated in Figures 4 to 6) can} _{be used to reduce the incident azimuth for as many possible sensor location as} possible. Element size modulation amplitude _{AFSS beacons were fabricated with element sizes of} ^ ^ ^, ^ and ^^ _{λ of a centre} _{frequency of 8Ghz of the radar system. Each AFSS was placed 750mm away from} _{a radar transceiver. A square-wave was used to bias the AFSS, and the difference} _{in reflected amplitude between forward-bias and reverse-bias states was measured.} _{There is a significant positive correlation between element size and modulation} amplitude response. Synchronization of networked radar systems _{In many radar systems, channel separation is enforced using a chaotic dither in the} _{sampler analogue to digital converter ADC. While this simplifies operating multiple} _{radars in the same space, it also makes conventional techniques for synchronization} _{impossible. However, advantageously, an AFSS beacon visible to multiple sensors} _{will present a common event that can be used to determine the slow-time clock} _{offsets between multiple sensors. In some scenarios, such as the design and} _{validation of networked radar systems, this information can be used as a test to} _{ensure the existing clock synchronization method - for example Network Time} _{Protocol (NTP) - is providing sufficient synchronicity. In other conditions, a} _{consistent shared slow-time clock could be important to ensure synchronization is} _{maintained during analysis of high-frequency period signals, such as individual} _{section of a heart-beat rhythm.} _{By way of example, two networked radar sensors can be arranged to illuminate an} _{AFSS beacon. Each of the networked sensors may use NTP from a local network} _{server to set and synchronise their respective internal clocks. The beacon may be} activated with reversal modulations followed by a frame header code. The _{modulation scheme can be repeated periodically until the end of the recording. For} _{each sensor, the beacon can be located in range using the reversal frequency, and} _{the time-offsets between the frame header codes can be established. The output of} _{the frame header code detection stage is illustrated in Figure 8.} Identification of Individuals _{A single radar sensor comprising a radar transceiver can be used, for example, to} _{observe two individuals, A and B, seated at different distances from the sensor.} Individual A may be a person for which data is not to be recorded, while person B _{acted may be a “subject” whose data is to be recorded using the radar system.} _{Classically, without additional information or some previous recognition training} _{dataset, identifying either individual from such limited data would be extremely} _{challenging. However, advantageously, by providing individual A with the AFSS} _{beacon 100 (e.g. attached to their shirt pocket) configured with a modulation} _{scheme for the reflectivity (e.g. comprising an oscillating reflectivity, optionally} followed by a frame header code), individual A can be recognised (identified) by the system, and distinguished from individual B. Figure 9 illustrates an example in which searching for the expected reversal _{frequency resulted in a strong detection at range bin 16 - the expected location for} _{individual A. Searching for the frame header code with a matched filter resulted in a} _{high-confidence detection, indicating that the beacon was highly likely present in bin} _{16. Individuals can be distinguished even when the beacon is directly attached to} _{clothing (and thus may be moving with the wearer’s chest motion, for example). The} _{data may be high-pass filtered in slow-time to show visually the contributions made} _{by the breathing movements of each individual.} Ultra-Wideband (UWB) Radar System _{An exemplary radar system 110 that could be used with the radar reflectors 100} described above will now be described. However, it will be appreciated that any _{other suitable configuration or type of radar system 110 could alternatively be used.} UWB radar is a radio technology that uses very low energy level for short-range, high-bandwidth communications over a large portion of the radio spectrum. UWB radar sensing is a safe, contactless, regulatory-compliant, privacy-preserving, and scalable method to monitor physiology and movement indoors. The radar system _{may comprise tabletop, wall-mounted, or ceiling mounted sensors in clinics,} _{hospitals, and homes. The sensors could also be embedded within other devices} _{(e.g. a TV or a lamp). The system is arbitrarily scalable - it works with a single device} or dozens, for example. By networking multiple devices (e.g. via Wi-Fi) an entire indoor space may be monitored. The system is operable for the collection of data with high spatiotemporal precision. _{Figure 10 schematically illustrates an example of an environment 1000 in which a} _{radar system 110 may be deployed. Radar sensors 10 are illustrated attached to} _{walls and on other surfaces. As shown in Figure 10, the radar may be deployed in} a home environment comprising various items of furniture such as a table 16, television stand 17, sofa 18, kitchen countertops 14 and doors 12. It will be appreciated that any suitable number of networked radar sensors 10 may be _{provided. Each of the radar sensors 10 is operable to transmit radar signals (e.g. in} _{a pulsed mode, or in a continuous transmission mode) and to receive the reflected} _{return signals. Examples of how the radar sensors 10 may be provided will now be} _{described in more detail with reference to Figures 11 and 12. Whilst the examples} of the present disclosure are described with respect to a radar sensor 10 that both transmits the radar signal and receives the return signal, it will be appreciated that _{the radar transmitter and receiver need not necessarily be co-located, and could} alternatively be provided separately. _{UWB provides low-cost at-scale hardware, and provides a wider bandwidth of} pulses with higher data accuracy. UWB operates below the noise floor, and enables simpler performance. Whereas other radar types perform on a dual signal capture _{(reflection and velocity), the UWB radar only uses reflection. This simplifies the data} capture and calibration. _{The system is arbitrarily scalable. A single sensor 10 could be deployed, or} _{hundreds of sensors 10 networked using Wi-Fi or a cellular network could be} _{deployed. Scenarios for which a plurality of sensors 10 could be deployed include:} _{^ Multiple radars 10 in separate spaces (e.g. rooms) across an environment} _{(e.g. one sensor 10 per room across a home or environment such as a} hospital). ^_{Multiple radars 10 separated in space, observing a single space (e.g. four} _{radars 10 on different faces of a wall in a room). In this case local (within} home) data fusion is highly desirable and enabled by our system. ^_{Multiple radars 10 in a single system observing a single space (e.g. multiple} radars in a single device with minimal spatial separation in a single location) –_{in this case local (within device) data fusion is highly desirable.} _{^ Any of the above scenarios as either an indoor or outdoor monitoring system.} _{^ Any combination of the above scenarios.} _{A radar sensor 10 has two key components, a transmitter and a receiver. The former} transmits low-power radio pulses while the latter samples their echoes. The reflected pulses are changed by the objects they interact with, including humans. _{The radar sensors 10 may be networked together using Wi-Fi networking (although} any other suitable form of networking could alternatively be used). Radar System _{With reference to Figures 11 and 12, the present disclosure provides an exemplary} _{networked system 200 of radar sensors 10 that could be used with one or more} _{radar reflectors 100.} _{As illustrated in Figure 12, each sensor 10 comprises a radar transceiver 124. The} _{radar transceiver 124 is operable to transmit radar signals and to receive the} corresponding reflected return signals. The data obtained using the radar _{transceiver 124 is received at an input 126 for the radar data, and is then passed to} _{a frame packet builder 130. The frame packet builder 130 constructs a frame of} _{data, which is then stored in a corresponding buffer 131 along with an associated} time, for synchronisation, from a clock 133. The frame packets are then stored in a _{corresponding area of memory 132 ready for encoding, or for the direct extraction} _{of features from unencoded (uncompressed) data using the algorithms 135. An} _{encoder then performs compression to generated encoded packets 134 of radar} data, which are then stored in a corresponding area of memory 137. When the radar _{sensor 10 includes environmental sensors (e.g. for sensing light, temperature and/or} _{luminosity), the environmental data is received at a corresponding input 128 of the} _{device 10. Processing can then be performed at the radar sensor 10 using the frame} _{packets 132 and the environmental data, to extract one or more features 136 from} _{the data. The extracted features are then stored in the memory 137. The data stored} _{in the memory 137 is then converted into a format for transmission across the} network (in this example JavaScript Object Notation (JSON), although any other suitable data format could alternatively be used) via a data transfer mechanism 139 _{(e.g. Wi-Fi, a cellular network, Bluetooth, USB connection, or any other suitable} wired or wireless connection). By virtue of the compression and feature extraction performed at the radar sensor 10, the volume of data that need by transmitted over _{the network is greatly reduced. This enables the radar system 110 to be operated} _{at a high frame rate of (for example) 500 Hz, enabling the extraction of granular} motion features. The radar system 200 provides sustained high data rate, on-the-fly _{compression, data storage (e.g. using an SD data card), and data read-back for} over-the-air streaming. As illustrated in Figure 10, a plurality of the sensors 10 may be provided at a particular location 1000. The networked sensors 10 may transmit the data stored in _{the memory 137 of the sensor 10 to a remote cloud 112 via the transfer mechanism} _{139. In this example, the cloud 112 comprises container runtimes 209 comprising a} _{web server 114 and a control and application logic module 118. The web server 114} _{comprises a number of endpoint instances 116 for receiving the data from the radar} _{sensors 10, in this example one endpoint instance 116 per sensor. A remote client} _{122a and a local client 122b are also illustrated. The clients 122 are operable to} communicate with the web server 114, for example to view features or analysis _{extracted from the data obtained at the sensors 10. In this example, the cloud 112} _{also comprises a database server 120 that is connected to the web server 114, and} _{a frontend 206 that is also connected to the web server 114.} _{Similar functionality can be achieved by hosting layers of servers that achieve the} _{same function using different protocols. For example, the web server 114 could use} _{a protocol other than hypertext transfer protocol (HTTP). Additionally, the cloud} could be a locally hosted server. The sensors 10 may be arranged so as to provided overlapping fields-of-view. By networking sensors 10 together and overlapping the sensor fields-of-view, a greater spatial coverage is obtained. _{The system 110 is operable to simultaneously capture information related to} _{physiology or gross motion using the sensors 10, with algorithms 135 selected on} _{the basis of the subject’s state. For example, when the subject is at rest, the system} _{110 is operable to measure vital signs. Furthermore, after extracting this feature-} _{level data, the need to submit raw data (rather than the processed data that is} _{generated at the sensors 10) to the cloud server 112 is avoided, which would} otherwise make home deployment unviable due to high data rates. _{In the present example, a Novelda X4 UWB pulsed System-on-Chip (SoC) radar is} _{used. However, it will be appreciated that any other suitable radar-based sensor 10} _{could alternatively be used. This radar provides a ten metre range that proves} _{sufficient for various indoor sensing environments 100. An ESP32-S3} microcontroller [1] is also used. However, it will be appreciated that any other _{suitable microcontroller could alternatively be used. Impulse radio ultra-wideband} (IR-UWB) radar is well-suited for a home-based sleep monitoring system, for example. Similar to continuous-wave (CW) Doppler radars, IR-UWB radars can achieve sub-millimetre movement tracking by extracting phase information. _{Radar sensor 10 electronics, comprising the radar transceiver 124, microcontroller,} _{power supply, memory 137 (e.g. Secure Digital (SD) card interface), transfer} _{mechanism 139 and programming interface, may be integrated and implemented} on a compact single printed circuit board (PCB). This design approach, driven by _{the minimal component count, enables the PCB size to closely match the outline of} _{the radar sensor, resulting in a compact form factor for space efficiency.} _{In this example the firmware implementation at the radar sensor 10 is built upon} freeRTOS, an open-source real-time operating system that offers reliable and accurate timing, thread-safe queues and task and interrupt priorities to ensure the most time-critical functions, such as retrieving data from the radar transceiver 124 _{via the interface 126 (e.g. Serial Peripheral Interface), are reliably executed. This} _{operating system was selected for the improved control over memory use and} _{thread-safe messaging queues to pass radar data buffers located in a 2 MB external} _{RAM chip to-and-from different threads as pointers. However, it will be appreciated} that any other suitable operating system and firmware could alternatively be used. _{The data flow starting from radar data generation by the sensor 10, through} _{compression (encoding), storage, and transmission (e.g. streaming) over the} _{network, is illustrated in Figure 12. Dedicated worker tasks carry out data retrieval} _{from the transceiver 124, compression, storage and read-back from the memory} _{137, and finally sending the packaged (e.g. JSON-packaged) data over the network} _{(e.g. using Hypertext Transfer Protocol (HTTP)). These tasks may run at different} _{frequencies and priorities. Dynamic allocation of memory is kept to a minimum to} improve robustness and predictability, as well as avoid problems from memory _{fragmentation. Finally, in this example the device 10 uses RTOS event groups and} flags to orchestrate the transition of threads from one state to another, such as when _{starting or stopping recordings. Every 2 ms the radar sensor 10 produces new data.} The microprocessor detects this and fetches the data from the transceiver 124. _{Once fifty such data transfers are done and compiled into a single data ‘blob’, the} microprocessor compresses the data. From there, the data is buffered into the _{memory 137. From the memory 137, the data is retrieved and queued for} _{transmission to one of the radar sensors 10 that operates as a supervisor node (a} _{central orchestrating node) via the transfer mechanism 139.} _{Features extracted from the radar data at the radar sensor 10 are transmitted to the} _{secure cloud service 112 for further processing and visualisation in a dashboard-} _{style application as appropriate, depending on the context and particular use case.} _{This real-time monitoring can then be used to inform care decisions. By monitoring} _{in real-time, detect intra-day fluctuations can advantageously be detected.} _{The ultra-wideband (UWB) radar technology of the radar system 110 is a safe, low-} _{cost, privacy-preserving, and scalable method for monitoring physiology and} _{movement indoors. The end-to-end system 110 (hardware and software/algorithms)} _{is operable to capture real-time data at high spatial and temporal resolution, e.g.} _{using tabletop or wall-mounted sensors 10 in homes 1000. Furthermore, this system} _{110 is arbitrarily scalable — it works with a single radar sensor module 10 or with} _{dozens (or more).} The compression methods and other methods performed at the radar sensor 10 significantly reduce the data requirements. In one example, the received data is data binned into 167 range bins at the radar sensor 10, and then the range amplitudes are quantised. This step is lossy, and dramatically reduces data transmission from _{(for example) 668 kB/s to <200 kB/s. Next, a lossless compression scheme is added} _{for further compression. Optionally, features-of-interest 136 can be extracted at the} _{radar sensor 10, and the method may comprise only transmitting feature-level data} _{to the remote device (e.g. the cloud 112), which dramatically reduces the data} _{transmission rates (for example from approximately GB/day of radar data to a few} _{KB to MB/day of comma separated value (CSV) data).} _{The system 110 may use a local Wi-Fi network for establishing connections between} _{individual radar nodes 10 and the central supervisor node. Each radar sensor 10} may discover the IP address of the supervisor through the implementation of a UDP broadcast mechanism. The supervisor initiates a broadcast message that is disseminated to every device on the network. By listening for and processing this _{broadcast, the radar sensors 10 can locate and establish a connection to the} _{supervisor. Once the radar nodes 10 successfully locate the supervisor and} establish a connection, the next step is to initiate status and data transfers. For this _{application, HTTP may be used as the data transfer protocol. HTTP’s packet-based} nature aligns well with the nature of the compressed, discretely sampled data in this application. To facilitate the transfer of Huffman encoded binary data over HTTP, _{the data is encoded at the radar sensors 10 using an additional base64 encoding} scheme. This encoding process transforms the binary data into a text string format that is free of control characters, ensuring safe and reliable transmission over HTTP. For scalable deployment, the supervisor system may be containerised using Docker. The entire system, including all its components, may be encapsulated within a set of Docker containers and deployed using Docker Compose. This approach greatly simplifies the deployment process, as the system can be easily deployed on a host machine with minimal configuration using a few straightforward commands. Furthermore, containerisation enables the supervisor to be platform-independent, allowing it to run on various operating systems and hardware platforms. Figure 11 _{visually illustrates a system 110 that includes the integration of the sensors 10, and} external clients 112, such as data labelling interfaces and live data analysis scripts, within the overall architecture. _{In this example, the radar transceiver 124 is an XeThru X4, manufactured by} _{Novelda AS. However, any other suitable radar transceiver 12 could alternatively be} _{used. The sensor 10 may operate within the 6 to 8.6 GHz and/or 7.25 to 10.2 GHz} frequency ranges, maintaining emissions below the −41 dB/MHz noise floor for _{global Federal Communications Commission (FCC) compliance. The sensor 10} _{may have an effective range of up to approximately 10 meters and a fine range} resolution of close to 6.27 mm. _{The networked system architecture may use a local Wi-Fi connection. To this end,} _{a local network is hosted by the supervisor node. Each supervisor node may host a} _{Network Time Protocol (NTP) server, permitting each sensor node 10 to pull local} Unix time. As described above, the radar reflectors 100 can advantageously be used _{to test the NTP synchronisation of the radar system 110.} Data flow through the network is configurable. Data can be streamed in real-time to the central supervisor node, or alternatively, data may be stored locally on each _{sensor node 10 and downloaded when required. Data transfer can be achieved, for} _{example, with FastAPI hosting a representational state transfer application} programming interface (REST API). _{Since each sensor 10 makes detections in range, a single sensor 10 cannot be used} _{to locate individuals at a precise location within a two-dimensional (or three-} _{dimensional) space. However, by utilising the networked architecture of the system} 110, it is possible to combine the outputs of multiple spatially diverse sensor nodes _{10 to locate and track individuals within the view of the system 110. This has been} _{accomplished using the system 110 to provide real-time data, with a signal} processing chain implemented in python-docker. Signals from each radar 10 are processed to extract range detections using moving-target-indicator (MTI) filtering, followed by constant false-alarm (CFAR) detections output to a multilateration _{process that determines the location of individuals in two-dimensional space. From} this, multilateration positions are formed into consistent tracks using an active tracking process based on the extended Kalman filter. Tracks are able to be detected and isolated. _{In this example, the X4 radar sensor is embedded with a custom ESP32s3-based} _{microcontroller (MCU) PCB. However, any other suitable microcontroller PCB could} alternatively be used. As illustrated in Figure 12, in this example the radar sensor 10 is provided with a _{real-time clock (RTC) 133, which may be integrated with a battery connector. Once} _{set, a commercial-off-the-shelf coin cell battery could be used to enable the device} _{10 keep time without requiring an internet connection, and hardware mode-selector} _{switches and a user-accessible button allow may allow the sensor 10 to operate and} _{record data completely independently of the network connection (e.g. if the network} connection is temporarily unavailable). _{The radar sensor 10 may also be configured, using hardware mode-selector} switches, to act as an X4 USB carrier and buffer, allowing any device with a USB host an easy way to interface with the X4 radar and receive compressed X4 data. _{The radar sensor 10 may host a PostgreSQL database locally, which enables high} concurrency database operations. This means that multiple programs can _{simultaneously read and write to the database without issue, improving real-time} _{operation, before transmitting feature-level data to a server for further processing.} This functionality could be provided as an intermediary node between the end-user and the radar sensor 10. The end-user would interact with a web application (via the _{frontend 206) that provides useful, actionable metrics.} _{To enable remote telemetry of information pertaining to device 10 health and} _{functionality, Message Queuing Telemetry Transport (MQTT) may be used. MQTT} provides a lightweight publish-subscriber based messaging transports for Transmission Control Protocol/Internet Protocol (TCP/IP) and connectionless _{protocols (e.g. User Datagram Protocol (UDP)). It is designed with low-bandwidth,} high-latency, and unreliable networks in mind. MQTT topics of interest include _{device health parameters including power status, Wi-Fi signal strength, recording} status, and data storage status. Extracted features could also be transmitted. MQTT _{may be implemented over Wi-Fi or Global System for Mobile Communications} (GSM, 4G.) with MQTT over Transport Layer Security (TLS), to mitigate the effects _{of Wi-Fi dropouts in home deployment settings.} _{The radar sensor 10 may be configurable to run a full Linux-based operating system} (OS) which allows more complex programs for real-time processing to be deployed, _{including on-device machine learning models (e.g. compressed TensorFlow} _{models). The memory 137 at the radar sensor 10 may comprise an M.2 solid state} drive (SSD) which is significantly faster, more reliable, and capable of storing larger _{volumes than an SD card, for example. The OS of the radar sensor 10 may be} _{hosted on the SSD, which makes the system more user-friendly for engineers and} researchers alike by permitting a full graphical user interface (GUI), akin to a _{desktop experience. The radar sensor 10 may be configurable to operate without a} _{network connection. For example, a 2TB SSD at the radar sensor 10 is able to} continuously record and save data without intervention from researchers for _{approximately 17 weeks, making the system 110 convenient to deploy for longer-} term studies without regular researcher intervention. Multi-Sensor Network _{As described above, the system 110 may be provided with any suitable number of} radar sensors 10. With an increased number of radar sensors 10, machine learning models trained on the radar data are more generalisable than for single radar systems. For example, if an activity is captured using more than one 'view' (i.e., _{more than one radar sensor 10) simultaneously, then the machine learning is more} generalisable. This is analogous to multi-view detection with cameras (versus single camera configurations which overfit to a single scene). Advantageously, therefore, _{the deployment of multiple radar sensors 10 makes the system 200 more robust in} settings with multiple individuals in a single environment/scene. Phase Modulation A radar reflector 100 comprising a planar reflector layer 101, a bias routing layer 102, an electrically-insulating dielectric substrate layer 103 and a frequency- _{selective layer 104 has been described above with reference to Figure 1. As} described above, the AFSS layer 104 has an adjustable radar absorbency which can be used to control the reflection of radar signals in one or more frequency bands from the radar reflecting surface, to signal information to the radar system. A _{modification of the radar reflector 100 of Figure 1 will now be described in which,} _{advantageously, information can be encoded in the phase of the return signals.} _{Figure 13 shows a simplified schematic diagram of a modified version of the radar} reflector illustrated in Fig.2. In this example, the AFSS 104 is spaced apart from the reflector layer 101 by a region of dielectric material 103, such that the dielectric material 103 is arranged between the AFSS 104 and the reflector layer 101. Alternatively, the AFSS 104 may be spaced apart from the reflector layer 101 by a cavity, such that the cavity is arranged between the AFSS 104 and the reflector layer 101. _{Advantageously, by virtue of spacing the reflector layer 103 a predetermined} _{distance behind the AFSS 104, it is possible to introduce phase encoding to the} _{return reflections by modulating the radar absorbency of the AFSS layer 104. In} other words, it is possible for the radar reflector to signal information to the radar system by modulating the phase of radar signals reflected from the frequency- _{selective radar reflector 1300, by modulating the radar absorbency of the frequency-} _{selective surface 104 in the time domain.} The predetermined distance between the reflector layer 103 and the AFSS 104 can be selected to achieve resonance with transmissions from the radar system 110. The predetermined distance may be selected based on the centre wavelength of the observing radar, and may be defined as N·λ or λ/N, where N is a positive integer _{and λ is the centre wavelength transmitted by the radar system. Extraneous} frequencies will experience increased scattering, leaving the frequency-band of interest as the prominent reflected component. As illustrated in Fig.13, a radar transmission 1302 emitted from a transmitter of the _{radar system 110 is incident on the AFSS layer 104. A portion 1306 of the incident} _{radar signal 1302 will be reflected by the AFSS 104. A further portion 1304} _{(depending on the radar absorbency of the AFSS layer 104) will be transmitted} _{through the AFSS 104 to the reflector layer 101, traversing the cavity or region of} _{dielectric material 103. The reflector layer 101 will reflect light that has traversed the} cavity or region of dielectric material 103 back to the AFSS layer 104 as a reflected signal 1310. The reflected signal 1310 will then pass through the AFSS layer 104 and is emitted out of the radar reflector 1300 as a reflected radar return signal 1308. By modulating the radar absorbency of the AFSS layer 104, modulation of the phase of the radar return signal from the radar reflector 1300 is achieved, enabling information to be encoded in the phase of the return signal. This is because the signal 1304 that traverses the cavity or region of dielectric material will have travelled an increased distance compared to the signal 1306 that is reflected directly _{from the AFSS layer 104. Therefore, the phase component of the light 1304 that} _{traverses the cavity or region of dielectric material 103 will be different from the} _{phase of reflections 1306 directly from the AFSS layer 104, and this phase-change} _{will be evident to an observing radar sensor when the AFSS layer 104 switches} between the “on” (e.g. more radar absorbent) and “off” (e.g. less radar absorbent) _{states (between forward-bias and reverse-bias states of the AFSS layer).} _{Figure 14 shows plots of the RF signal and frequency spectrum for ‘on’ and ‘off’} _{configurations of the frequency-selective surface 104 of the radar reflector 1300 of} Figure 13. In the top half of the figure, the amplitude of the signals reflected from the radar reflector 1300 when the AFSS is in the ‘on’ (solid line) and ‘off’ (dashed line) _{states are plotted. In the bottom half of the figure, the corresponding frequency} spectrum is illustrated for the ‘on’ and ‘off’ states. _{Figure 15 shows plots of the phase spectrum and amplitude-scaled phase for the} _{‘on’ and ‘off’ configurations of the frequency-selective surface 104. In the top half of} Figure 15 the phase spectrum is plotted, and in the bottom half of Figure 15 the amplitude-scaled phase is plotted. As illustrated in Figure 15, there is a clear difference in the phase spectrum (and amplitude-scaled phase spectrum) between the two modes of the AFSS. This difference in the phase spectrum is detectable by a radar sensor 10 of the radar system 110, and therefore by modulating the _{reflectivity of the AFSS layer 104 in the time domain information can be signalled to} the radar system by modulating the phase of the radar return signals. A further example of a radar reflector 1400 will now be described with reference to Figure 16. Figure 16 schematically illustrates a modification of the radar reflector 1300 illustrated in Figure 13. In this example, the radar reflector 1400 comprises a _{first AFSS layer 104a and a second AFSS layer 104b. The first AFSS layer 104a is} spaced a first predetermined distance away from the second AFSS layer 104b, and a first region of dielectric material 103a or a first cavity is arranged between the first _{AFSS layer 104a and the second AFSS layer 104b. A reflector layer 101 is also} provided. The reflector layer 101 is provided a second predetermined distance away _{from the second AFSS layer 104b. The second predetermined distance may be the} same as the first predetermined distance (but may alternatively be different from the _{first predetermined distance). As with the example of Figure 13, in the example of} Figure 16 the first predetermined distance and the second predetermined distance _{may be selected for resonance with a transmission by the radar system 110, such} that extraneous frequencies experience increased scattering, leaving the frequency- band of interest as the prominent reflected component. As illustrated in Figure 16, the signal 1304 that passes through the first AFSS layer 104a will be incident on the second AFSS layer 104b. Light reflected 1310 from the second AFSS layer 104b will be reflected back to the first AFSS layer 104a. Light that is transmitted through the second AFSS layer 104b will be incident on the reflector layer 101, and will then be reflected 1408 back to the second AFSS layer 104b. This reflected light will then pass through the second AFSS layer 104b to the first AFSS layer 104a, for transmission 1404 back to a sensor of the radar system _{110 as a reflected radar return signal 1404.} Advantageously, providing two separated AFSS layers 104a, 104b, before the _{reflector layer 101 enables multi-level signalling to be achieved. With a single AFSS} _{layer 104, binary ‘on’ or ‘off’ modulation is possible for both phase and amplitude of} _{the radar return signals. Beneficially, with multiple separated AFSS layers 104a,} 104b, multi-level signalling can be achieved by activating/biasing different _{combinations of the AFSS layers 104. For example, the first AFSS layer 104a may} be in the ‘off’ state whilst the second AFSS layer 104b is in the ‘on’ state. Alternatively, for example, both the first AFSS layer 104a and the second AFSS layer 104b may be in the ‘off’ state. The state of the AFSS layers 104a, 104b can _{therefore be used to control both the overall amplitude modulation level and the} _{phase modulation degree, by modulating the effective propagation length of the} _{radar signals. In other words, when light is directly reflected 1306 from the first AFSS} layer 104a that light will travel no additional distance. In contrast, the light reflected from the second AFSS layer 104b will travel an additional distance (twice the first predetermined distance), and light reflected from the reflector layer 101 will travel a different additional distance (twice the sum of the first predetermined distance and the second predetermined distance). It will be appreciated that, as with the radar reflector 100 described above with _{reference to Figure 1, the radar reflector of Figures 13 and 16 may be used with the} radar system 110 of Figure 11, or with any other suitable radar system. It will also be appreciated that the frequency selective layer(s) 104 may have any suitable form, _{for example any of the configurations illustrated in Figures 3 to 6. Similarly, the} _{electrically-insulating layer 103 may be formed of any suitable material, for example,} _{FR4 (a glass-reinforced epoxy laminate material), polytetrafluoroethylene (PTFE) or} _{ceramic. Moreover, whilst the bias routing layer 102 is not illustrated in Figures 13} and 14 for clarity, it will be appreciated that the bias routing layer (or layers) may nevertheless be provided at any suitable position in the reflector illustrated in Figures 13 and 16. It will be appreciated that the in the example of Figure 13, the spacing selected for the distance between the AFSS layer 104 and the reflector layer 101 for resonance with a transmission of the radar system will depend on whether a cavity is provided between the layers or if dielectric material 103 is provided between the layers. _{Advantageously, providing the region of dielectric material 103 enables the distance} _{between the AFSS layer 104 and the reflector layer 101 to be reduced whilst} maintaining the same phase-relationship and frequency tuning. Moreover, some _{dielectric materials can be used in combination with the AFSS layer 104 to modulate} the phase of the reflected signals. Active dielectric materials, such as Barium Strontium Titanate, can change their effective permittivity as a result of bias signals/current. This change in permittivity leads to changes in phase of signals _{passing through the dielectric layer, and thus can be used as a further source of} phase-modulation. This combination of effects can be exploited in both the single _{AFSS layer 104 example of Figure 13 and multiple AFSS layer example of Figure} _{16, and enables multiple phase and amplitude signal levels.} Waveguide Behaviour Figure 17 shows a plot of a representative azimuth response of the radar reflector _{1300 illustrated in Figure 13. At normal incidence, the reflector 1300 will behave as} a classical resonant cavity. The plot of Fig.17 illustrates angles between 0° and _{180°, which corresponds to angles of incidence of between 0° and 90° in each} _{direction. However, if the cavity (or region of dielectric material 103) is open (with} _{no barriers or walls at its edges), the behaviour of the reflector 1300 will be different} _{at angles of incidence towards perpendicular. At angles closer to perpendicular, the} _{internal structure of the radar reflector 1300 begins acting as a waveguide, with} _{sides of the waveguide formed from the AFSS layer 104 and the reflector layer 101.} _{In this manner, modulation of the AFSS layer causes some frequencies to be} _{scattered/lost between the ‘on’ and ‘off’ states of the AFSS layer 104. The result of} this scattering/loss is reflected modulations that are still clear to the observing radar. _{Advantageously, the overall result of this waveguide characteristic of the radar} _{reflector 1300 is a very wide angle of effective view, allowing observing radars to} obverse modulations from angles across normal to perpendicular incidence. It will be appreciated that the radar reflectors 1300, 1400 of Figures 13 and 16 can be operated in accordance with any of the methods described above, for example any of the above-described methods for temporal calibration, spatial calibration, identification, vehicle-to-vehicle communication, ground-to-space communication or air-vehicle communication. Modifications and Alternatives Detailed embodiments and some possible alternatives have been described above. As those skilled in the art will appreciate, a number of modifications and further alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. It will therefore be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto. The radar reflectors 100 are not limited to use with a particular type of radar system 110. For example, the radar system 110 used to illuminate the reflector need not _{necessarily be an ultra-wideband radar system.} Whilst some of the above examples have been described with respect to monitoring or identifying a human using the radar system 110 and a radar reflector 100, this need not necessarily be the case. Alternatively, for example, the radar system 110 _{could be used to monitor or identify an animal (e.g. a pet such as a dog), or a non-} _{living object (e.g. a vehicle).} _{Whilst the radar reflector 100 and the associated methods may be used for} _{biomedical radar sensing, the apparatus and methods could also be used in many} different technical fields. For example, an automotive application may comprise _{encoding unique ID’s or manoeuvring information onto reflections from a radar} _{reflector 100 mounted to a vehicle.} Reference _{[1] “ESP32-S3 Wi-Fi & Bluetooth 5 (LE) MCU | Espressif Systems.”}

Claims

1. A frequency-selective radar reflector for reflecting radar transmissions _{transmitted by a radar system, the radar reflector comprising:} _{an active frequency-selective surface, having an adjustable radar} _{absorbency; and} a radar reflecting surface; w_{herein the radar reflector is operable to modulate, in the time domain, the} _{radar absorbency of the frequency-selective surface to control the reflection of radar} _{signals in one or more frequency bands from the radar reflecting surface, to thereby} signal information to the radar system. _{2. The radar reflector according to claim 1, wherein the radar reflector is for} _{biomedical monitoring or human sensing.} _{3. The radar reflector according to claim 1, wherein the radar reflector is for an} _{automotive or aerospace application.} _{4. The radar reflector according to claim 1, wherein the radar reflector is for a} geospatial monitoring or geospatial communication application. _{5. The radar reflector according to any preceding claim, wherein the radar} reflector is configured to modulate the radar absorbency of the frequency-selective surface using on-off-keying and/or amplitude shift keying to signal the information. _{6. The radar reflector according to claim 5, wherein the radar reflector is} _{operable to modulate the radar absorbency to indicate, to the radar system, an} identity associated with the radar reflector. _{7. The radar reflector according to claim 5 or claim 6, wherein the radar reflector} _{is operable to modulate the radar absorbency to provide a timing signal for} _{synchronisation of the radar system.} _{8. The radar reflector according to any preceding claim,} _{wherein the frequency-selective surface comprises a set of first portions of} the surface and a second portion of the surface; and w_{herein modulating the radar absorbency comprises modulating a voltage} _{bias of diodes arranged around each first portion of the surface, that connect each} _{of the first portions of the surface to the second portion of the surface.} _{9. The radar reflector according to claim 8 wherein each of the first portions of} the surface is circular. _{10. The radar reflector according to claim 9, wherein the circular first portions} _{each have a width that is approximately one quarter, one eighth, or one sixteenth of} _{the width of the centre wavelength used in the radar system.} _{11. The radar reflector according to any one of claims 8 to 10, wherein four} diodes are arranged around each of the first portions of the surface. _{12. The radar reflector according to any one of claims 8 to 11, wherein the} _{frequency-selective surface comprises a repeating pattern of the first portions.} _{13. The radar reflector according to any preceding claim, wherein the radar} _{reflector has a substantially tetrahedral shape.} _{14. The radar reflector according to any preceding claim, wherein the radar} reflecting surface is planar. _{15. The radar reflector according to any preceding claim, wherein the radar} _{reflector is operable to modulate the absorbency of the frequency-selective surface} _{at a frequency of between 1 Hz and 1 MHz, for example 1 kHz.}

_{16. The radar reflector according to any preceding claim, wherein the radar} _{reflector is operable to modulate the radar absorbency of the frequency-selective} _{surface to control the radar cross-section of the radar reflector in one or more} frequency bands, to thereby signal information to the radar system. _{17. A system comprising one or more of the radar reflectors according to any} preceding claim. _{18. The system according to claim 17, wherein the system comprises a plurality} of the radar reflectors, wherein each of the radar reflectors is configured to modulate _{the radar absorbency of its respective frequency-selective surface using a different} _{modulation pattern in the time domain.} _{19. The system according to claim 17 or claim 18, wherein the system comprises} one or more radar sensors, each radar sensor comprising: a_{t least one radar transmitter for transmitting radar signals to illuminate the} _{one or more of the radar reflectors, and} at least one radar receiver for receiving corresponding radar return signals _{reflected from the one or more radar reflectors.} _{20. The system according to any one of claims 17 to 19,} wherein the system comprises a plurality of the radar sensors; and wherein the system is configured to modulate the radar absorbency of the _{frequency-selective surface of the one or more radar reflectors to provide a} _{reference signal for the radar sensors.} _{21. The system according to claim 20, wherein the reference signal is for time} _{synchronization between the radar sensors.} _{22. The system according to any one of claims 17 to 19,} wherein the system comprises a plurality of the radar sensors; wherein the system is configured to modulate the radar absorbency of the frequency-selective surface of the one or more radar reflectors to provide a signal for spatial calibration of the radar sensors; and wherein the system is configured to determine the relative position of the radar sensors using the signal for spatial calibration. _{23. The system according to any one of claims 17 to 22, wherein the radar} _{system is for biomedical monitoring or human sensing.} _{24. The system according to any one of claims 17 to 22, wherein the radar} system is for an automotive or aerospace application. T_{he system according to any one of claims 17 to 22, wherein the radar} system is for a geospatial monitoring or geospatial communication application. _{26. The system according to any one of claims 19 to 25, wherein the one or more} radar sensors are ultra-wideband (UWB) radar sensors. _{27. A method performed by a frequency-selective radar reflector for reflecting} _{radar transmissions transmitted by a radar system to signal information to the radar} _{system, wherein the radar reflector comprises an active frequency-selective surface} _{having an adjustable radar absorbency, and a radar reflecting surface, and wherein} the method comprises: m_{odulating, in the time domain, the radar absorbency of the frequency-} _{selective surface to control the reflection of radar signals in one or more frequency} bands from the radar reflecting surface, to thereby signal the information to the radar system. _{28. A frequency-selective radar reflector for reflecting radar transmissions} transmitted by a radar system, the radar reflector comprising: a_{first active frequency-selective surface, having an adjustable radar} absorbency; _{a radar reflecting surface; and} a first cavity or first region of dielectric material arranged between the first _{active frequency-selective surface and the radar reflecting surface;} _{wherein the first active frequency-selective surface is provided at a first} _{predetermined distance away from the radar reflecting surface; and} _{wherein the radar reflector is operable to modulate, in the time domain, the} _{radar absorbency of the first frequency-selective surface to control the reflection of} radar signals in one or more frequency bands from the radar reflecting surface, to _{thereby signal information to the radar system by modulating the phase of radar} signals reflected from the frequency-selective radar reflector. _{29. The radar reflector according to claim 28, wherein the radar reflector is} configured to modulate the radar absorbency of the first frequency-selective surface _{using on-off-keying and/or amplitude shift keying to signal the information to the} radar system. _{30. The radar reflector according to claim 28 or 29, wherein the first} _{predetermined distance is configured for resonance with a frequency or range of} _{frequencies of one or more transmissions of the radar system.} _{31. The radar reflector according to any one of claims 28 to 30, wherein the first} _{predetermined distance is N·λ or λ/N, wherein N is a positive integer and λ is a} centre wavelength transmitted by the radar system. _{32. The radar reflector according to any one of claims 28 to 31, wherein the first} _{region of dielectric material comprises FR4, polytetrafluoroethylene or a ceramic} material. _{33. The radar reflector according to any one of claims 28 to 31,} wherein the first region of dielectric material comprises a dielectric material having a controllable permittivity; and wherein the radar reflector is configured for control of the permittivity of the _{dielectric material to modulate the phase of the radar signals reflected from the} frequency-selective radar reflector. _{34. The radar reflector according to any one of claims 28 to 33,} wherein the radar reflector further comprises: a_{second active frequency-selective surface, having an adjustable radar} absorbency, and a_{second cavity or second region of dielectric material arranged between the} _{second active frequency-selective surface and the first active frequency-selective} surface; w_{herein the first active frequency-selective surface is arranged between the} _{second active frequency-selective surface and the radar reflecting surface;} _{wherein the second active frequency-selective surface is provided at a} second predetermined distance away from the first active frequency-selective surface; and w_{herein the radar reflector is operable to modulate, in the time domain, the} _{radar absorbency of the first frequency-selective surface and the second frequency-} _{selective surface to signal information to the radar system by modulating the phase} of radar signals reflected from the frequency-selective radar reflector. _{35. The radar reflector according to claim 34, wherein the first predetermined} _{distance and the second predetermined distance are configured for resonance with} _{a frequency or range of frequencies of one or more transmissions of the radar} system. _{36. The radar reflector according to claim 34 or 35, wherein the radar reflector is} configured for control of the radar absorbency of the first active frequency-selective surface independently of control of the radar absorbency of the second active _{frequency-selective surface.}

_{37. The radar reflector according to any one of claims 34 to 36, wherein the radar} reflector is configured to modulate the radar absorbency of the first frequency- selective surface and the second frequency-selective surface using on-off-keying and/or amplitude shift keying. _{38. The radar reflector according to any one of claims 34 to 37,} wherein a first dielectric material having a controllable permittivity is arranged _{between the first active frequency-selective surface and the radar reflecting surface,} and a second dielectric material having a controllable permittivity is arranged _{between the second active frequency-selective surface and the first active} _{frequency-selective surface; and} the radar reflector is configured for control of the permittivity of the first dielectric material and the permittivity of the second dielectric material to modulate the phase of the radar signals reflected from the frequency-selective radar reflector. _{39. The radar reflector according to any one of claims 28 to 38, wherein the radar} _{reflector is operable to signal information to the radar system by modulating the} phase of radar signals reflected from the frequency-selective radar reflector for _{angles of incidence of between 0° and 90° for a radar transmission incident on the} frequency-selective radar reflector. _{40. A system comprising one or more of the radar reflectors according to any one} of claims 28 to 39. _{41. The system according to claim 40, wherein the system comprises a plurality} of the radar reflectors, and wherein each of the radar reflectors is configured to _{signal information to the radar system by modulating the phase of radar signals} reflected from the radar reflector using a different modulation pattern in the time domain.

_{42. A method performed by a frequency-selective radar reflector for reflecting} radar transmissions transmitted by a radar system to signal information to the radar _{system, wherein the radar reflector comprises at least one active frequency-} _{selective surface having an adjustable radar absorbency, and a radar reflecting} surface, and wherein the method comprises: m_{odulating, in the time domain, the radar absorbency of the frequency-} _{selective surface to control the reflection of radar signals in one or more frequency} bands from the radar reflecting surface, to thereby signal information to the radar _{system by modulating the phase of radar signals reflected from the frequency-} selective radar reflector.