The electric field sensing technology based on the Rydberg atom has attracted widespread attention due to its powerful detection ability for the amplitude, phase, and polarization of the electric field. In this work, based on the electromagnetically induced transparency of the Rydberg atom and combined with a heterodyne Rydberg atom-based mixer, we measured the binary array composed of the Rydberg Field Probes. The measured phase difference of the electric field at two different Rydberg Field Probes is related to the direction of incidence. We digitally transform the received signal at 2.63 GHz and conduct digital beamforming processing to obtain its array pattern and beam direction. The measurement accuracy and reliability are greatly improved compared with the phase comparison method, and the angle-of-arrival variance is less than 1.5°. The effectiveness of digital beamforming based on the Rydberg atomic sensor is verified, which lays the foundation for the subsequent linear and planar array reception of the Rydberg atomic sensor. In addition, the occlusion based on Rydberg Field Probes is also measured, and its weak occlusion characteristics are confirmed. This unique feature provides feasible support for the three-dimensional receiving array based on the Rydberg atomic sensor.

In this study, we propose a circularly polarized directional resonator (CPDR) to enhance the reception capability of circularly polarized (CP) waves by the Rydberg atomic sensor and suppress unwanted directional waves to improve interference resistance. By utilizing the ac Stark effect, the CPDR provides a 14.1 dB enhancement for left-hand CP electric fields at 2.388 GHz. In the atomic heterodyne scenario, the CPDR enables a sensing sensitivity of 148.8 mu V/cm$\sqrt {\text{Hz}} $, with an axial ratio reduced to 2.4 dB. Furthermore, we experimentally investigated the reception pattern of the CPDR, which aligns with the results of the characteristic-mode simulation. The measured pattern indicates that the proposed CPDR exhibits excellent directional reception performance with a 3 dB beamwidth of 39 degrees. This CPDR expands the range of enhancement resonators for Rydberg atomic sensing and shows significant potential, particularly for satellite signal reception.

Highly sensitive detection of very high frequency (VHF) band fields poses a challenge for Rydberg atomic sensing since resonant detection requires exciting atoms to extremely high energy states. We demonstrate a PCB-based electrically tunable resonator (PETR) for enhancing the sensing sensitivity of the Rydberg atomic sensor to VHF band fields. In this approach, the sub-wavelength PETR is made of PCB, which offers improved mechanical strength and miniaturization. During the measurements, a parallel-plate waveguide (PPWG) is utilized as the radio frequency (RF) field launcher to eliminate unintentional scattering from the surroundings. By adjusting the bias voltage applied to the tuning varactor of the PETR, we can continuously tune the resonant frequencies from 187 to 293.6 MHz, corresponding to a relative bandwidth of 44%. There is a fluctuation in the enhancement factor of the PETR, ranging from 17.6 to 60.1, which is primarily caused by misalignment between the laser beam and the resonant area. Furthermore, employing the off-resonant Rydberg atomic heterodyne technique, we investigated the sensitivity of the system. In the absence of the PETR, the sensitivity for an RF signal at 250.5 MHz was found to be 97.44 mu V/cm root Hz. However, with the presence of our PETR, the sensitivity significantly improved to 1.73 mu V/cm root Hz, indicating a remarkable enhancement capacity of 56.23 times or 35 dB. This enhancement resonator holds great potential for improving Rydberg atomic sensing, particularly in the VHF band.

Quantum microwave measurement based on Rydberg atoms is a novel microwave measurement technique. To improve its measurement sensitivity over a broadband, this letter presents a mechanically tunable electric field enhancement device, loaded on the the vapor cell. By adjusting the length of the split ring resonator, an electric field enhancement capability of more than 30 dB over a bandwidth of 0.52 GHz to 1.54 GHz is realized in simulation. The experimental results show that its resonance frequency covers from 0.52 GHz to 1.86 GHz with a relative bandwidth of 113%, exhibiting excellent broadband performance. This broadband electric field enhancement device is expected to further improve the performance of quantum microwave measurements.

Rydberg atom-based superheterodyne with additional local oscillator (LO) signal is a novel approach to detect electric field with high measured sensitivity. However, the LO signal is often supplied to the atomic vapor cell by free-space illumination, which lacks mobility and integration for practical applications. Here, we present a LO port integrated split-ring resonator for realizing high sensitivity-enhanced electric field measurements. The LO signal is sent directly to the resonator through a parallel-plate waveguide, which is shown to achieve a sensitivity enhancement of 32 dB. The integrated resonator has an electrical size of 0.088λ and the feed port S11 reaches −38.2 dB.

Rydberg atomic electric field sensors have attracted the attention of international researchers due to their pow erful electric field measurement capabilities. Especially in the shortwave (SW) frequency band (3-30 MHz), using only millimeter-level sensors to receive SW signals is undoubtedly attractive. However, due to the extremely difficult excitation of the Rydberg atoms that resonate with the SW frequency, the sensitivity of the Rydberg atoms SW measurement is low. Here, we report that a method to improve the SW sensitivity by using an SW resonator, which is extremely small in electrical size (0.0043 lambda x 0.0035 lambda x 0.007 lambda), can achieve continuous resonant frequency coverage from 9.365 to 10.223 MHz and can increase the sensitivity of electric field measurement by a factor of 8633. In the state of far off-resonant, we have achieved a detectable field strength as low as 2.41 mu V/m with a sensitivity of 1.71 mu V/mHz(1/2) using the heterodyne Rydberg atom-based mixer method, which realizes ultrahigh-sensitivity measurement of SWs. In addition, for the first time outside the laboratory, we directly received an amplitude-modulated broadcast signal with 880 km (the straight- line distance between Changsha and Baoji) at 9.81 MHz in the form of a subminiature resonator combined with a Rydberg field probe (RFP), which verified the powerful ability of this sensing scheme.

Rydberg atomic sensors (RASs) have garnered significant attention as a novel type of electric field sensor with high sensitivity. While the integration of electric field enhancement structure (EFES) and RASs has been proven to improve the sensitivity and the minimal detectable electric field of RAS, there are concerns regarding their omnidirectional and polarization performance. However, most existing RASs or RASs loaded with EFES operate with linear polarization, which reduces the sensitivity to circularly polarized (CP) microwave reception. It is a challenging method to change the property of the lasers to modify the RAS's CP property, while it is simpler to use EFES to change RASs' polarization indirectly. Meanwhile, current EFES research mainly focuses on the response of electric field enhancement at a fixed incident angle, limiting extension to 3-D CP patterns and lacking clear physical insights to guide. To address this, we propose an RAS with CP EFES to achieve CP reception capability. The EFES converts spatial CP electric field into local linearly polarized (LP) electric field, adjusting the RAS characteristics accordingly. We use characteristics mode (CM) methods, dividing the problem into modal current, governing the modal patterns, and external CP excitation sources, determining the coefficients of each modal pattern. This method reduces the complexity of the pattern analysis and improves the efficiency of optimization design. To validate the proposed method's efficiency, we fabricate a prototype to measure reception pattern and further study the CP performance using an atomic heterodyne approach. The results indicate that the RAS with EFES achieves the right-hand CP (RHCP) omnidirectional reception capability, and the sensitivity is improved by 19.2 dB at 2.195 GHz. The cross-polarized level can be further improved to 24.5 dB using an atomic heterodyne approach.

Alkali atom vapor cells are the essential component of quantum microwave measurement equipment. By means of laser pumping technology,alkali atoms can be easily excited from the ground to Rydberg states. Alkali atoms is very sensitive to electric filed because of their very large polarizability,huge electric dipole and low ionization threshold field and so on. Recently,by virtue of the strong interaction of microwave field and Rydberg atoms, alkali atom vapor cells have been widely applied to detect the amplitude, frequency,phase and polarization of electric field,especially the microwave electric field. Quantum microwave measurement technology has shown significant advantages, such as the break of probe size independent of wavelength, extremely high sensitivity and accuracy, very broad spectrum measurement,and very large dynamic range. In the past decade,the technology has shown great potential for application in the monitoring of ultra wide band electromagnetic spectrum,the metering of microwave electric field,microwave imaging and communication,etc. Thereinto,the state population of Rydberg atoms in vapor cell is one of the decisive factors affecting measurement capability. Up to now,certain properties of vapor cell can be obtained by optical or other measurement technologies, such as the thickness,refractive index and transmittance of glass envelope,atomic ratio and density in vapor cells. However,all of them can not directly reflect the Rydberg atomic states in vapor cell,which could give rise to difficulties to the performance optimization of quantum measuring equipment based on atom vapor cells. In this paper,a theoretical calculation model of Rydberg atomic state population excited by two-photon resonance has been established by using the ideal gas state equation,and the Rydberg blockade effect and gas atomic distribution are comprehensively analyzed. Meanwhile,an estimation method of Rydberg atomic state population,which is acomplished by aid of optimal Electromagnetically Induced Transparency(EIT) signal of Rydberg atoms,has been proposed and demonstrated experimentally under shading condition and room temperature,by means of the cylindrical(similar to 1.0 cm in diametre and length)and cuboid(similar to 1.0 cm in width and height,and similar to 2.0 cm in length)vapor cell,respectively. The vapor cells are filled with saturated cesium (Cs-133) atoms at 300 K,and the intensity of pressure is similar to 6 666.1 Pa. In order to obtain the theoretical calculated and experimental estimates of the Rydberg state population excited by two-photon resonance,the EIT experimental setup has been put up by similar to 852 nm and similar to 1 020 nm semiconductor laser. Both of them are produced by TOPTICA Photonics. The typical spectral linewidths (5 mu s integration time) of two semiconductor lasers are similar to 100 kHz. The similar to 852 nm laser,which is stabilized on the saturated absorption spectral signal of 133Cs D2-line,is employed as a probe laser to irradiate into the vapor cells to excite the 133Cs atoms from the ground state( 6S(1/2))to intermediate state( 6P(3/2)). Simultaneously,after frequency-doubling of similar to 1 020 nm laser, the coupling laser in the wavelength of similar to 510 nm counterpropagates through the vapour cells to excite the atoms from the intermediate state to the Rydberg state ( 42D(5/2)). Thereinto,the probe and coupling laser are collimated and linearly polarized. Experimentally, the wavelengths of probe and coupling laser also can be certified by wavelength meter(Bristol 771A VIS). And then,by scanning the frequency of coupling laser and adjusting the laser powers,the optimal EIT spectrum can be displayed on the oscilloscope, which is connected into the photodetector (Thorlabs PDA36A2). The gain(50 O)of photodetector is similar to 7.5x10(6) V/A,and the responsivity at the wavelength of similar to 852 nm is similar to 0.55 A/ W. The laser parameters of optimal EIT spectrum can be measured by laser beam quality analyzer(Ophir SP920 s)and power meter(Thorlabs PM160). For the cylindrical vapor cell,the power and 1/e(2) beam diameter of probe laser are similar to 13.0 mu W and similar to 886.0 mu m,and the ones of coupling laser are similar to 22.0 mW and similar to 1 425.0 mu m. For the cuboid vapor cell,above-mentioned laser parameters are similar to 16.2 mu W,similar to 901.0 mu m,similar to 23.5 mW and similar to 1 437.0 mu m,respectively. Comparing the natural linewidth of Rydberg state ( 42D(5/2)) and two lasers,the Rydberg blockade radius r B of 42D(5/2) is calculated to be similar to 6.3 mu m. Because of the two-photon resonance,the Rydberg atoms only can be excited in the overlapping region of lasers. According to the ideal gas state equation and dense packing model,a theoretical calculation model of Rydberg atomic state population can be established. So the Rydberg atomic state populations in the cylindrical and cuboid vapor cell can be obtained,3.06x10(6) and 6.33x10(6),respectively. Otherwise,in the optimal EIT spectrum,the energy difference between EIT peak and ground noise can be regarded as the energy of similar to 852 nm enhanced transmission laser. Furthermore,a photon represents a Rydberg atom at 42D(5/2) level. Hence, the Rydberg atomic state populations in two vapor cells can be estimated to be2.32x10(6) and 5.86x10(6). The experimental results are in good agreement with the values calculated by aforementioned theoretical model. The theoretical model and measurement method is benefit to characterization and optimization of vapor cell properties,which can promote the rapid development of quantum microwave measurement technology based on Rydberg atoms in future.

Atomic sensing and electric field measurements based on the use of the all-optical electromagnetic induction transparency (EIT) spectrum of Rydberg atoms have attracted great interest due to the advantages of self-calibration, Le Système International d'unités traceability, and a broadband spectrum. At the same time, this methodology also promotes the development of atomic sensor technology. We present a novel high-efficiency Rydberg-atom radio-frequency (RF) electric (E) field integrated probe (Rydberg Field Probe or RFP). The high-efficiency RFP consists of a 10 mm cubic 133Cs vapor cell, a dichroic mirror, three collimating lenses, and three fibers. This system can be used to independently control two counterpropagating lasers passing through a vapor cell, easing the design effort. In addition, the overall system efficiency can reach a value of 40.4%, reducing the EIT power broadening doe conventional RFPs. Based on the designed high-efficiency RFP, near-field electric field measurement and imaging of a C-band horn antenna at 4.48 GHz are performed by using the resonant RF field of the 133Cs atomic Rydberg state at room temperature with a spatial resolution of λ/6. These results promote the development of atomic sensing technology and show great application potential for the measurement of measuring high-resolution radio frequency electric fields and arrays of RFPs.

Based on the Rydberg cascade electromagnetically induced transparency, we propose a simultaneous dual-wavelength locking method for Rydberg atomic sensing at room temperature. The simplified frequency-locking configuration uses only one signal generator and one electro-optic modulator, realizing real-time feedback for both lasers. We studied the effect of the different probe and coupling laser powers on the error signal. In addition, the Allan variance and a 10 kHz amplitude-modulated signal are introduced to evaluate the performance of the laser frequency stabilization. In principle, the laser frequency stabilization method presented here can be extended to any cascade Rydberg atomic system.

Objective Rydberg atoms became increasingly crucial in the last decade because of their fascinating characteristics that distinguish them from conventional radio frequency (RF) sensors. First, the Rydberg atoms are self-calibrating thanks to the invariance of the atomic parameters, and their response is linked to Plank's constant. Second, atomic sensing systems break a key assumption behind the Chu limit of traditional electronic sensors by allowing a small vapor cell to operate over multiple octaves of frequencies from DC to THz. Third, instead of demodulated circuitry, Rydberg atoms can naturally extract the baseband signals from the carrier frequency. Fourth, Rydberg atoms may avoid internal thermal (Johnson) noise, even at room temperature. In recent years, the amazing introduction of the local oscillator (LO) RF field has assisted us in controlling ensembles of Rydberg atoms. However, most current reports on Rydberg atomic heterodyne sensors focus on measurements in the resonant region, which can only achieve highly sensitive detection at discrete frequencies due to the quantum nature of the atomic energy level. In this work, by extending the Rydberg atomic heterodyne technique from the resonant region to the off-resonant region, we experimentally validated the continuous broadband and high sensing sensitivity of Rydberg atoms. Methods When a strong LO field and a weak signal (SIG) field with frequency detuning on the order of kHz are irradiated to the atoms, the energy level will be modulated by the intermediate frequency (IF) in the resonant and off-resonant regions, which can be directly detected by optical electromagnetically induced transparency (EIT). At room temperature, a probe laser of 852 nm and a coupling laser of 509 nm propagate in opposite directions and overlap inside a 2 cm-long vapor cell containing cesium atoms, exciting the atoms to the Rydberg state for atomic sensing. In the resonant region, the LO frequency is set to 2. 63 GHz, and the SIG frequency is set to 2. 63 GHz+ 10 kHz. Both fields are illuminated into the vapor cell by a horn antenna 7 cm away from the optical path, and the polarization of the two RF fields is the same as that of the probe and coupling beams and propagates in a vertical direction to the laser beams. While in the off-resonant region, the frequencies of the LO and SIG fields are tuned to 300 MHz and 300 MHz+ 10 kHz, respectively. An aluminum parallel-plate waveguide serves as the microwave transmitter in the off-resonant region. The reflection coefficient ( S11) of the input port is below - 20 dB from DC to 850 MHz (Fig. 2), indicating the excellent port matching performance of the parallel-plate waveguide. Results and Discussions In the resonant region, we calibrated the electric (E) field strength of the RF field using the Autler-Townes (AT) splitting effect. By adjusting the output power of the signal generator to satisfy the linear relationship between AT-splitting and RF field amplitude, we obtained the relationship between the square root of the signal generator output power and the E-field intensity calculated by AT- splitting (Fig. 3). The results show excellent linearity, and the weak RF E- field strength can be inferred from the fit line. Then, a spectrum analyzer was used to measure the intensity of the beat-note signal under Rydberg atomic heterodyne conditions. We measured a series of data points of the beat-note signal strength versus the applied SIG power (Fig. 4). The intensity of the received beat- note signal is approximately proportional to the strength of the applied SIG field with a linear dynamic range of over 45 dB. The minimum SIG output power is - 85 dBm, which is limited by the background noise of the spectrum analyzer. By leveraging the gradient of the fit line, we can obtain the minimal detectable E-field of 220. 94 nV/ cm, with the corresponding sensing sensitivity of - 131. 9 (dBm/ cm(2))/ Hz. Similarly, in the off-resonant region, through the relationship between the power injected in the parallel-plate waveguide and the E-field strength, we measured the minimum E-field strength of 19 mu V/cm in the offresonant region at 300 MHz, with a sensitivity of - 93. 2 (dBm/ cm(2))/ Hz. Besides, we also measured the instantaneous bandwidth of the system in the off-resonant region (Fig. 5). By taking into account the negative detuning of the SIG and LO fields, the instantaneous bandwidth of 3 dB of the system reaches 90 kHz. Conclusions In the present study, two typical frequency points in the resonant and off-resonant regions were selected to experimentally verify the broadband and high sensitivity detection capability of Rydberg atomic sensors. During the measurement, a horn antenna and a parallel-plate waveguide were used as microwave transmitters in the resonant and offresonant regions, respectively. Using the Rydberg atomic heterodyne technique, we successfully measured a minimum Efield strength of 220. 94 nV/ cm with a sensitivity of - 131. 9 ( dBm/cm(2))/Hz in the resonant region at 2. 63 GHz and a minimum E- field strength of 19 mu V/cm with a sensitivity of - 93. 2 (dBm/cm(2))/Hz in the off-resonant region at 300 MHz, respectively. In principle, by adjusting the laser frequency to excite the alkali metal atoms to various Rydberg states and incorporating the distinct responses of Rydberg atoms to E-fields in the resonant and off-resonant regions, highly sensitive sensing of microwave E-fields can be achieved in the broadband continuous spectral range.

In this work, we demonstrate a Rydberg atom-based radio frequency (RF) sensor that utilizes an atomic homodyne technique to determine the amplitude-modulation (AM) frequency. By detuning the local oscillator (LO) field frequency to the same carrier frequency of the signal (SIG), the AM frequency can be demodulated in both resonant and off-resonant regions. Here we investigate the effect of the phase difference between the LO and the SIG fields on the sensing capability. Due to the presence of the LO field, the sensitivity of the atomic sensor improves by 14.2 dB and 20.3 dB in the resonant and off-resonant regions, respectively. In addition, the AM response bandwidth and linear dynamic range of the atomic sensor in both resonant and off-resonant regions are also studied. In principle, the proposed atomic sensor has the potential to demodulate the envelope frequency of AM waves in a broadband and continuous frequency range, making it a promising candidate for the next-generation RF sensors.

Rydberg-atom electrometers promise traceable standards for RF electrometry by enabling stable and uniform measurement. In this Letter, we propose an approach to increase the sensitivity of the Rydberg-atom electrometer for far-detuned RF field sensing. The key physical mechanism is the addition of a new ingredient—a local RF field near-resonant with a Rydberg transition—so that the far-detuned field can be detected by the shift of an Autler–Townes (AT) splitting peak, which can be dozens of times larger than the AC Stark shift of the electromagnetic induced transparency (EIT) signal without the near-resonant field. The method enables us to measure far-detuned fields with higher sensitivities, including sub-GHz RF fields (even DC electric fields) which are rarely involved in the existing sensitivity enhancement methods.

Rydberg atoms have extremely large polarizability and transition dipole moments, allowing for non-destructive and traceable precise measurement or communications over ultra-broadband electromagnetic signals by the Autler-Townes splitting effect in the resonance region and the alternating current Stark (AC Stark) shift effect in the off-resonance region. In a cesium vapor cell at room temperature, by varying the coupling laser wavelength, three energy levels (vertical bar 70S(1/2)>, vertical bar 42D(5/2)>, and vertical bar 30D(5/2)>) are selected to measure the spatial electric field strength of electromagnetic signals in the far-off-resonance region (2 GHz) and the resonance region (9. 953 GHz and 29. 54 GHz), respectively. On this basis, the attenuation factors caused by environment scattering and vapor cell perturbations are calculated. Meanwhile, the potential of Rydberg atoms for communication applications in the broadband frequency range is demonstrated by experiments on the variation of the signal-to-noise ratios (SNRs) of electromagnetic signals with different modulation frequencies under amplitude modulation and frequency modulation in the far-off-resonance region (2 GHz), near-off-resonance region (9. 5 GHz), and the resonance region (29. 54 GHz). Furthermore, for the amplitude modulation signal with a modulation frequency of 10 kHz, the demodulated signal SNR is investigated in the ultra-broadband frequency range of 100 kHz-40 GHz. The experimental results reveal that Rydberg atoms can break the operational bandwidth limit of conventional electronic sensors, and have the ability of electric field sensing and communications in the ultra-broadband continuous spectrum range.

State of polarization (SoP) of light is one of the fundamental characteristics of light and has great significance to optical communication, imaging, quantum optics and medical facilities. The generation and maintenance of polarized light have always been research concerns in polarization optics. Polarization-maintaining fibers are frequently used to transmit polarized light without changing its polarization in optical systems, but the high cost and coupling efficiency problems hinder their usage in large-scale light paths. Polarization controllers, which operate arbitrary polarization generation and conversion at the expense of utilizing at least two optical elements such as a half-wave plate and quarter-wave plate, are too bulky for some special applications. Meanwhile, they can only generate desired output polarization of light by transcendentally determining the input polarization, which means that the existing polarization controllers cannot respond in real time. Metasurfaces composed of subwavelength nanoscatterers offer fruitful functionalities to manipulate the amplitude, phase and polarization of light. Here, we propose and experimentally demonstrate a real-time polarization controller realized by combining a depolarizer and polarizer into one monolithic metasurface. Arbitrary polarization states can be transferred to the required polarization with no requirement to determine the incident polarization in advance. Through combining with ordinary optical fibers, the proposed metasurface may also replace polarization-maintaining fibers and optical fiber polarizers in some polarization-dependent applications. This versatile concept may settle the problems of arbitrary polarization conversion once and for all.

Based on Autler–Townes splitting and AC Stark shifts, we present a Rydberg atom-based receiver for determining the amplitude modulation (AM) frequency among a wideband carrier range utilizing a cesium atomic vapor cell. To verify this approach, we measured the signal-to-noise ratio and the data capacity with a 10 kHz AM frequency in the carrier range from 2 GHz to 18 GHz. Without changing the lasers, the working band can be easily extended to a higher range by optimizing the feed antenna and experimental configurations.

Signal-to-noise ratio (SNR) is a priori information necessary for many signal processing algorithms or techniques. However, there are many problems exsisting in conventional SNR estimation techniques, such as limited application range of modulation types, narrow effective estimation range of signal-to-noise ratio, and poor ability to accommodate non-zero timing offsets and frequency offsets. In this paper, an SNR estimation technique based on deep learning (DL) is proposed, which is a non-data-aid (NDA) technique. Second and forth moment (M2M4) estimator is used as a benchmark, and experimental results show that the performance and robustness of the proposed method are better, and the applied ranges of modulation types is wider. At the same time, the proposed method is not only applicable to the baseband signal and the incoherent signal, but can also estimate the SNR of the intermediate frequency signal.

Spectrum sensing is one of the technologies that is used to solve the current problem of low utilization of spectrum resources. However, when the signal-to-noise ratio is low, current spectrum sensing methods cannot well-handle a situation in which the prior information of the licensed user signal is lacking. In this paper, a blind spectrum sensing method based on deep learning is proposed that uses three kinds of neural networks together, namely convolutional neural networks, long short-term memory, and fully connected neural networks. Experiments show that the proposed method has better performance than an energy detector, especially when the signal-to-noise ratio is low. At the same time, this paper also analyzes the effect of different long short-term memory layers on detection performance, and explores why the deep-learning-based detector can achieve better performance.

Due to the low cost and straightforward structure, the switch antenna array (SAA) frequency modulated continuous wave (FMCW) radar has been widely applied in many fields. However, the motion-induced phase always leads to inaccurate direction estimation of moving targets. Here, we proposed an elimination method of the motion-induced phase for the SAA FMCW radar. A double-time switching scheme (DTSS) is used for the reception of echo signals. Elimination of motion-induced phase is completed without estimating velocity, which can avoid the ambiguous velocity estimation problem. Additionally, the direction estimation of the moving target can be obtained by directly using a conventional digital beam forming (DBF) algorithm. The validity of the proposed method has been proved by the simulated and experimental results.

Switch antenna array (SAA) has obtained increasing attention due to its low-cost, small volume. In the presence of the target motion, the phase distortion will cause problems in the azimuth focusing. By the use of a reference element, a phase recovery method for SAA FMCW radar is proposed. The performance of the method is demonstrated by the numerical results.