High-precision Distance Measurement Research Articles

TRENDS OF THE MICROWAVE PHOTONIC RADARS

Today, the world's leading countries are intensively working on the development of new generation radars - microwave photonic radars. Microwave photonic radars make it possible to significantly reduce the mass and size characteristics of radar stations, to increase the information capability and range of target detection due to the reduction of losses in long communication lines when using optical fiber, to ensure high immunity due to the significantly lower sensitivity of optical-electronic equipment and fiber-optic lines of communication connection to external electromagnetic influences. Microwave photonics provides wide bandwidth, flat response, low loss transmission, multi-dimensional multiplexing, ultra-fast analog signal processing and immunity to electromagnetic interference. Radar implementation in the optical domain can provide better resolution, coverage, and speed performance, which would be difficult to implement with traditional electronics. The review article examines the state of development and system architectures of such photonic radars as optoelectronic hybrid radars, all-optical radars, multifunctional microwave photonic radar systems, distributed microwave photonic radars, software-defined radars, and cognitive radars. New technologies in this field and possible future directions of research are discussed. As an example, a broadband microwave photon radar reproduced on the basis of a microcircuit is considered. The broadband signal generator and receiver are built into the silicon crystal on the insulator. A high-precision distance measurement with a resolution of 2.7 cm and an error of less than 2.75 mm was obtained. Visualization of multiple targets with complex profiles has been implemented. But the performance of most integrated microwave photonic chips is not yet satisfactory for practical radar applications. Monolithic integration of key microwave photonic subsystems is also not mature enough for practical applications, so hybrid integration of devices fabricated on their optimal integration platforms is of practical interest. At present, indium phosphide, silicon nitride and silicon on insulator are the three leading platforms for photonic integration

Symmetrical dual-sideband oppositely chirped differential FMCW LiDAR.

A differential FMCW LiDAR for high-precision distance measurements of remote non-stationary targets is proposed and demonstrated experimentally. The required positive and negative symmetrically oppositely chirped laser beams are generated synchronously through a fixed-frequency laser by employing externally unified broadband optical phase modulation and symmetrical dual-sideband optical filtering. After coaxial transmission and reception, orthogonally polarized optical beat signals containing target distance and vector velocity data are de-chirped separately by optical in-phase and quadrature demodulations and then synchronously received by four-channel photoelectric balance detectors. After differential processing of the received beat signals and a fast Fourier transform, it is possible to implement real-time simultaneous range and vector velocity measurements. The inherent symmetrically oppositely chirped optical frequency make it possible to measure the target distance immune to the internal random phase noise introduced by the spectral linewidth of the frequency-swept laser and the external random phase noise introduced by atmospheric turbulence, speckle, and vibration. Meanwhile, the measurement of the target velocity is immune to the nonlinearity of the frequency-swept laser. These results encourage an approach to overcome the barriers of coherence length, nonlinearity, and external noise, and implement simultaneous real-time ranging and velocimetry of long-range, rapid-moving targets.

Supercontinuum-based hyperspectral LiDAR for precision laser scanning.

Hyperspectral LiDAR enables non-contact mapping of the 3D surface geometry of an object along with its spectral reflectance signature and has proved to be effective for automated point cloud segmentation in various remote sensing applications. The established hyperspectral LiDAR methods offer a range precision of a few mm to a few cm for distances exceeding several meters. We propose a novel approach to hyperspectral LiDAR scanning based on a supercontinuum (SC) coherently broadened from a 780 nm frequency comb. It provides high precision distance measurements along with target reflectance over the 570-970 nm range of the SC output. The distance measurements are carried out by monitoring the differential phase delay of intermode beat notes generated by direct photodetection, while the backscattered light spectrum is acquired using a commercial CCD spectrometer with 0.16 nm resolution across the 400 nm bandwidth of the SC output. We demonstrate a measurement precision below 0.1 mm for a stand-off range up to 50 m on a diffuse target with around 89% reflectance. The measured relative accuracy as compared to a reference interferometer is on the order of 10-5 for distances up to 50 m. Initial results also indicate spectrum-based material classification within a 3D point cloud using a linear support vector machine. The results highlight the potential of this approach for joint high-precision laser scanning and automated material classification.

Micrometer-precision absolute distance measurement with a repetition-rate-locked soliton microcomb.

The soliton microcomb has sparked interest in high-precision distance measurement, owing to its ultrahigh repetition rate and chip-integrated scale. We report absolute distance measurements based on synthetic wavelength interferometry with a soliton microcomb. We stabilized the repetition rate of 48.98 GHz through injection locking, with fluctuations below 0.25 Hz. Distance measurements up to 64 mm were demonstrated, presenting residuals below 2.7 μm compared with a referenced laser interferometer. Long-term distance measurements were made at two fixed positions of approximately 0.2 m and 1.4 m, resulting in a minimum Allan deviation as low as 56.2 nm at an average time of 0.05 s. The dynamic demonstration illustrated that the proposed system could track round-trip motion of 3 mm at speeds up to 100 mm/s. The proposed distance measurement system is, to our knowledge, the first microcomb-based synthetic wavelength interferometer and achieves a ranging precision of tens of nanometers, with potential applications in the fields of satellite formation flying, high-end manufacturing, and micro-nano processing.

A super-zoom algorithm for envelope peak position resolution using discrete Fourier transform with multiple-warped time-sampling point calculations

We propose a new method for super-zooming the position resolution of envelope peak signals using a discrete Fourier transform (DFT) with multiple-warped time-sampling point calculations. Position resolution of warped DFT (WDFT)-based envelope demodulation is nonlinear but can be externally controlled. Compared with previous studies of WDFT-based envelope demodulation, the proposed method further warps the placement of sampling points along the time axis, allowing us to evaluate the envelope with a denser collection of position sampling points. The proposed approach is introduced within the framework of optical frequency comb-based Michelson interferometry. Our experimental results demonstrated the superiority of the proposed algorithm over the conventional WDFT-based method for envelope position resolution. To the best of the authors’ knowledge, this is the first article to provide a definition of multiple-warped DFT and its application to envelope demodulation. The present technique is expected to be useful for high-precision distance measurements for scientific and industrial purposes.

Method of high-precision free-space distance measurement for noncooperative targets.

Aimed at high-precision distance measurement for noncooperative targets in free space, a spatial distance measurement method is proposed. Based on the concept of optical carrier-based microwave interferometry, this method extracts distance information from the radiofrequency domain. The interference model of broadband light beams is established, and the optical interference can be eliminated by using a broadband light source. A spatial optical system with a Cassegrain telescope as the main body is designed to effectively receive the backscattered signal without cooperative targets. A free-space distance measurement system is built to verify the feasibility of the proposed method, and the results agree well with the set distances. Long-distance measurements with a resolution of 0.033µm can be achieved, and the errors of the ranging experiments are within 0.1µm. The proposed method has the advantages of fast processing speed, high measurement accuracy, and high immunity to disturbances as well as the potential for measurement of other physical quantities.

The MPU RX-4 Project: Design, Electronics, and Software Development of a Geofence Protection System for a Fixed-Wing VTOL UAV

Unmanned aerial vehicles (UAVs) are at the forefront of this century’s technological shift, becoming ubiquitous in research and market areas. However, as a UAV navigates autonomously, there are unanticipated occasions, e.g., collisions with dynamic obstacles or loss of data provided by the global navigation satellite system (GNSS), where the aircraft has to change its mission plans. In particular, to be protected from possible accidents, the platform’s geofence protection system should adjust the trajectory appropriately when obstacles are detected and select the proper ground surface when emergency landing is prompted. As these processes require fast reaction times, utilizing low latency sensors and algorithms is necessary. This article proposes a complete and low-complexity geofence protection system for recognizing moving objects and assessing the ground surface’s suitability using onboard sensing and processing modules. The proposed system is implemented on a novel fixed-wing UAV, designated as MPU RX-4, which features an unconventional flying wing layout and vertical take-off and landing (VTOL) capabilities. Our system is based on a forward-facing laser imaging, detection, and ranging (LIDAR) sensor and three downward-facing laser rangefinders. We take advantage of the high-precision distance measurements and operational speed to identify moving obstacles using the LIDAR module, while the ground’s slope and the existence of any obstacle therein are computed through the rangefinders. First, the article describes the UAV design procedure and its aerodynamic performance characteristics, which allowed us to evaluate our approach on a testbed aircraft. Then, the evaluation protocol shows that our system can perform robustly and under real-time constraints reaching an overall latency of only 165.5 ms, sufficient for reliable detection and avoidance of moving obstacles.

Современные методы, методики и технические средства мониторинга движений земной коры

This article gives an overview of available methods and techniques for geodynamic monitoring, such as the high-precision geometric re-leveling, high-precision linear-angular constructions, high-precision distance measurements, laser scanning, global navigation satellite systems, laser interferometers and strainmeters, radar satellite interferometry. The experience of applying these technologies in different physiographic regions is also described. The tools used in measurements, which provide the maximum achievable accuracy of methods in modern conditions, are demonstrated. A table is made with the summary characteristic of instrumental methods to measure the parameters of current movements of the Earth crust. On the basis of the performed comparative analysis of modern methods, techniques and technical means for execution of geodynamic monitoring, the most effective ones have been chosen from the methods listed above. Such methods include methods of high-precision geometric levelling of I and II classes, as well as the global navigation satellite systems. Complex use of these methods will allow to obtain reliable data about the geodynamic situation of the investigated area.

Interferometric evaluation of positioning error and position repeatability on CNCs

The interferometer is widely used for various applications concerning high precision distance and position measurements, especially for the calibration of numerically controlled machines. While the setup procedures of the interferometer may be provided by the manufacturer, some calibration procedures are well defined by the international standards. Since the operation of interferometers requires qualified personnel, some helpful hints for the novice users are to be presented in this paper. Even if the interferometer is highly recommended by international calibration standards, some factors are prone to intervene into the quality of results. This paper presents the experimental results for the evaluation of positioning accuracy and repeatability on Y axis of two vertical machining centres. The results show the high importance of environmental conditions compensation, the impact of time pauses during the measurement procedure and the implications of the increment of measuring target positions.

Advanced Measuring System

The proposed measuring system is a practical method of measuring linear objects, straight or angular, of variable length, and with great precision. The system uses a digital measuring system, consisting of a motor connected to a stopper that can be moved to the required cutting position, a mechanical movement system consisting of a screw and ball nut and a locking device with a rotary offset. The electronic system consists of a Human Machine Interface (HMI) data input, an electronic measuring controller, and a servo motor with an encoder for high precision distance measurement. The whole system is coordinated by software which allows for several measurement options: in inches, illustrated with either decimals or fractions, or in millimeters. The user input and stopper position are displayed in real time. The device facilitates the adjustment of speed and acceleration parameters. Additionally, the offset dimensions can vary according to user-defined lengths. The graphical interface is very user-friendly and consists of 5 pages for various settings and display of information.

Distance Assessment by Object Detection—For Visually Impaired Assistive Mechatronic System

Techniques for the detection and recognition of objects have experienced continuous development over recent years, as their application and benefits are so very obvious. Whether they are involved in driving a car, environment surveillance and security, or assistive living for people with different disabilities, not to mention advanced robotic surgery, these techniques are almost indispensable. This article presents the research results of a distance assessment using object detection and recognition techniques. The first is a new technique based on low-cost photo cameras and special sign detection. The second is a classic approach based on a LIDAR sensor and an HQ photo camera. Its novelty, in this case, consists of the concept and prototype of the hardware subsystem for high-precision distance measurement, as well as fast and accurate object recognition. The experimentally obtained results are used for the motion control strategy (directional inverse kinematics) of the robotic arm (virtual prototype) component in special assistive devices designed for visually impaired persons. The advantages of the original technical solution, experimentally validated by a prototype system with modern equipment, are the precision and the short time required for the identification and recognition of objects at relatively short distances. The research results obtained, in both the real and virtual experiments, stand as a basis for the further development of the visually impaired mechatronic system prototype using additional ultrasonic sensors, stereoscopic or multiple cameras, and the implementation of machine-learning models for safety-critical tasks.

Statistical Evaluation and Synthetic Generation of Ultra-Wideband Distance Measurements for Indoor Positioning Systems

Wireless positioning systems are a key enabler for a variety of applications. In indoor environments, Global Navigation Satellite Systems are not available, hence technological alternatives are required. Ultra-Wide Band (UWB) radios provide high-precision distance measurements, however are still prone to ranging errors due to the environment. For localization system design and testing, this leads to cost-intense field trials in which localization methods are validated. As an alternative, publicly available UWB datasets exist, which however are also limited to only certain environmental conditions. In order to provide a more general evaluation basis, we present and discuss a procedure which allows the synthetic generation of UWB distance measurements with respect to theoretical error sources and parameterization based on a conducted measurement campaign. This contribution builds an understanding of commonly present errors for wireless distance measurements and their effects on ranging residual distributions in the distance domain. In addition, we discuss the influences of non-line-of-sight and multipath conditions. Based on this work, distance measurements with respect to the multipath richness of the propagation environment can be generated, helping to flexibly evaluate positioning methods for various emerging applications for telematics systems in the consumer, industrial, or transportation sector. These application fields are ranging from unmanned aerial vehicles, in-house parking systems, and intelligent freight wagons to connected aircraft cabins.

Integrated method for measuring distance and time difference between small satellites

The advancement of small satellites is promoting the development of distributed satellite systems, and for the latter, it is essential to coordinate the spatial and temporal relations between mutually visible satellites. By now, dual one-way ranging (DOWR) and two-way time transfer (TWTT) are generally integrated in the same software and hardware system to meet the limitations of small satellites in terms of size, weight and power (SWaP) consumption. However, studies show that pseudo-noise regenerative ranging (PNRR) performs better than DOWR if some advanced implementation technologies are employed. Besides, PNRR has no requirement on time synchronization. To apply PNRR to small satellites, and meanwhile, meet the demand for time difference measurement, we propose the round-way time difference measurement, which can be combined with PNRR to form a new integrated system without exceeding the limits of SWaP. The new integrated system can provide distributed small satellite systems with on-orbit high-accuracy and high-precision distance measurement and time difference measurement in real time. Experimental results show that the precision of ranging is about 1.94 cm, and that of time difference measurement is about 78.4 ps, at the signal to noise ratio of 80 dBHz.

Long span ranging system based on aircraft vision

At present, the transmission line in the field has a large potential safety hazard, and professionals often encounter difficulties in patrol inspection and distance measurement. In view of the current situation of safe distance between equipment in power grid construction, this paper proposes a long-span distance measurement method based on aircraft vision, which can automatically achieve high-precision distance measurement between equipment, and ensure personal safety, avoid equipment failure and other problems. Then the long-span ranging technology based on aircraft vision is analyzed, and the research and application of the given technology are discussed.

Transferred-Rotational-Echo Double Resonance.

Internuclear distance determination is the foundation for NMR-based structure calculation. However, high-precision distance measurement is a laborious process requiring lengthy data acquisitions due to the large set of multidimensional spectra needed at different mixing times. This prevents application to large or challenging molecular systems. Here, we present a new approach, transferred-rotational-echo double resonance (TREDOR), a heteronuclear transfer method in which we simultaneously detect both starting and transferred signals in a single spectrum. This co-acquisition is used to compensate for coherence decay, resulting in accurate and precise distance determination by a single parameter fit using a single spectrum recorded at an ideal mixing time. We showcase TREDOR with the microcrystalline SH3 protein using 3D spectra to resolve resonances. By combining the measured N–C and H–C distances, we calculate the structure of SH3, which converges to the correct fold, with a root-mean-square deviation of 2.1 Å compared to a reference X-ray structure. The TREDOR data used in the structure calculation were acquired in only 4 days on a 600 MHz instrument. This is achieved due to the more than 2-fold time saving afforded by co-acquisition of additional information and demonstrates TREDOR as a fast and straightforward method for determining structures via magic-angle spinning NMR.

High-precision distance measurements with classical pulsating stars

Classical Cepheid and RR Lyrae variables are radially pulsating stars that trace young and old-age stellar populations, respectively. These classical pulsating stars are the most sensitive probes for the precision stellar astrophysics and the extragalactic distance measurements. Despite their extensive use as standard candles thanks to their well-defined Period-Luminosity relations, distance measurements based on these objects suffer from their absolute primary calibrations, metallicity effects, and other systematic uncertainties. Here, I present a review of classical Cepheid, RR Lyrae, and Type II Cepheid variables starting with a historical introduction and describing their basic evolutionary and pulsational properties. I will focus on recent theoretical and observational efforts to establish absolute scale for these standard candles at multiple wavelengths. The application of these classical pulsating stars to high-precision cosmic distance scale will be discussed along with observational systematics. I will summarize with an outlook for further improvements in our understanding of these classical pulsators in the upcoming era of extremely large telescopes.

In situ observation of conformational dynamics and protein ligand-substrate interactions in outer-membrane proteins with DEER/PELDOR spectroscopy.

Observing structure and conformational dynamics of membrane proteins at high-resolution in their native environments is challenging because of the lack of suitable techniques. We have developed an approach for high-precision distance measurements in the nanometer range for outer membrane proteins (OMPs) in intact E. coli and native membranes. OMPs in Gram-negative bacteria rarely have reactive cysteines. This enables in-situ labeling of engineered cysteines with a methanethiosulfonate functionalized nitroxide spin label (MTSL) with minimal background signals. Following overexpression of the target protein, spin labeling is performed with E. coli or isolated outer membranes (OM) under selective conditions. The interspin distances are measured in-situ using pulsed electron-electron double resonance spectroscopy (PELDOR or DEER). The residual background signals, which are problematic for in-situ structural biology, contributes specifically to the intermolecular part of the signal and can be selectively removed to extract the desired interspin distance distribution. The initial cloning stage can take 5–7 d and the subsequent protein expression, OM isolation, spin labeling, PELDOR experiment, and the data analysis typically take 4–5 d. The described protocol provides a general strategy for observing protein-ligand/substrate interactions, oligomerization, and conformational dynamics of OMPs in the native OM and intact E. coli.

Compact, robust, and repetition-rate-locked all-polarization-maintaining femtosecond fiber laser system

We demonstrate a 200-MHz all polarization-maintaining, repetition-rate-locked femtosecond fiber laser system with a total electrical power consumption of 11 W. The center wavelength, spectral width, pulse width, and average output power of the laser are 1558.8 nm, 34 nm, 139 fs, and 77.6 mW, respectively. The proposed laser system that integrates all optical components and locking electronics has a volume less than 1.5 L, a mass of 1.3 kg, and a fast locking time of 3 s (from the free running state to the repetition-rate-locked state). Using a hydrogen maser as the frequency reference, after locking, the Allan deviation is 2.8 mHz at a gate time of 1 s. Further, we place the repetition-rate-locked fiber laser system on a homemade shaker table with peak and rms accelerations of 1.97 and 0.7 g, respectively; the experimental results show that the locking state can be maintained robustly with Allan deviation of 2.0 mHz. The highly integrated, robust fiber laser system has potential applications in the areas of ultralow-noise microwave generation and high-precision distance measurement in outdoor environments.

The X-Ray Variable Sky as Seen by MAXI: The Future of Dust-echo Tomography with Bright Galactic X-Ray Bursts

Abstract Bright, short duration X-ray flares from accreting compact objects produce thin, dust scattering rings that enable dust-echo tomography: high-precision distance measurements and mapping of the line-of-sight distribution of dust. This work looks to the past activity of X-ray transient outbursts in order to predict the number of sight lines available for dust-echo tomography. We search for and measure the properties of 3σ significant flares in the 2–4 keV light curves of all objects available in the public MAXI archive. We derive a fluence sensitivity limit of 10−3 erg cm−2 for the techniques used to analyze the light curves. This limits the study mainly to flares from Galactic X-ray sources. We obtain the number density of flares and estimate the total fluence of the corresponding dust echoes. However, the sharpness of a dust-echo ring depends on the duration of a flare relative to quiescence. We select flares that are shorter than their corresponding quiescent period to calculate a number density distribution for dust-echo rings as a function of fluence. The results are fit with a power law of slope −2.3 ± 0.1. Extrapolating this to dimmer flares, we estimate that the next generation of X-ray telescopes will be 30 times more sensitive than current observatories, resulting in 10–30 dust ring echoes per year. The new telescopes will also be 10–100 times more sensitive than Chandra to dust ring echoes from the intergalactic medium.

Long-range, high-precision absolute distance measurement technology based on alternately oscillating optoelectronic oscillator

Absolute distance measurement plays an important role in many areas, such as aerospace and scientific research. Traditional measurement methods generally cannot meet requirements for long-range and high-precision at the same time. In this paper, an absolute distance measurement method based on alternately oscillating optoelectronic oscillator is proposed. This method places the distance to be measured in the loop of optoelectronic oscillator and takes advantage of accumulative magnification effect to achieve high accuracy. The measurement and the reference optoelectronic oscillators are established and selected by an optical switch, and a microwave switch is used to choose the high-order or low-order oscillating frequency. The high-order frequency and low-order oscillating frequency of the measurement and reference optoelectronic oscillators are measured in turn by frequency counter to calculate the loop lengths of two optoelectronic oscillators. The low-order frequencies are used to measure the fundamental frequency roughly and the high-order frequencies are used to calculate loop length precisely. Although the mode hopping occurs in the measurement process, it does not affect the loop length calculation by substituting the corresponding oscillating mode number. Note that the loop length measurement moments of two optoelectronic oscillators are different due to the switching order of optical switch and microwave switch. In order to calculate the absolute distance, which is the length difference between two optoelectronic oscillators at the same moment, the measured loop lengths should be averaged.In this way, systematic error accumulation caused by slow drift of environment can be eliminated, and this method does not need to control the length of reference optoelectronic oscillator. Meanwhile, the measurement system is simple. In the experiment, 1 km, 5 km and 8 km fibers are placed in a common part of the measurement and reference optoelectronic oscillators to simulate different long-range distances in space. A high-resolution optical delay line is placed in the measurement optoelectronic oscillator to verify the performance of the measurement system. The experimental results show that the measurement error is 3.5 μm with a 3.5 μm maximum standard deviation of each measurement distance at an emulated round trip distance of 6 km. The relative measurement accuracy reaches 5.8 × 10<sup>－10</sup>. This method provides a feasible idea for solving the technical problems of long-range and high-precision absolute distance measurement.