MULTILATERATION UNDER FLIP AMBIGUITY FOR UAV POSITIONING USING ULTRAWIDE-BAND

Ultrawide-band (UWB) ranging technology and multilateration techniques have recently been emerging solutions for positioning unmanned aerial vehicles (UAVs) in GNSS-denied environments. This solution offers cm-level ranging accuracy and considerable robustness to multipath receptions. UWB modules are commonly used in an anchor-based configuration; i.e., one UWB tag is mounted on the UAV, and several UWB anchors are installed on the ground. In real-world operational conditions, anchors can form a planar or a near-planar surface. This causes a geometric ambiguity, called flip ambiguity, in position estimation. Flip ambiguity can lead to considerable errors in the estimated position by multilateration. In this paper, we present a multilateration approach, which automatically resolves the flip ambiguity for UAV-positioning using UWB ranging. The proposed multilateration method first computes an algebraic solution through recursive least squares. If the initially estimated position is found to be flipped, then it is corrected by a symmetric reflection with respect to the anchor plane. Finally, the estimated position is refined by non-linear optimization. Extensive experiments in a real environment show that the proposed algorithm can effectively tackle the issue of flip ambiguity in multilateration, leading to a significant improvement in positioning accuracy.

An analytical model for asymmetric Mach reflection configuration in steady flows

An analytical model is presented for the configuration of Mach reflection (MR) due to the interaction of two-dimensional steady supersonic flow over asymmetric wedges. The present asymmetric MR model is an extension of an earlier model for the symmetric MR configuration. The overall Mach reflection (oMR) in the asymmetric wedge configuration is analysed as a combination of upper and lower half-domains of symmetric reflection configurations. Suitable assumptions are made to close the combined set of equations. The subsonic pocket downstream of the Mach stem is taken to be oriented along an average inclination, based on the streamline deflections by the Mach stem at the triple points. This assumption is found to give satisfactory results for all types of oMR configurations. The oMR configuration is predicted for all types of reflections such as direct Mach reflection (DiMR), stationary Mach reflection (StMR) and inverse Mach reflection (InMR). The reflection configuration and Mach stem shape given by the model for various sets of wedge angles, especially those giving rise to InMR, have been predicted and validated with the available numerical and experimental data. The von Neumann criterion for oMR is accurately predicted by this model. The asymmetric Mach stem profile is well captured. The variation of Mach stem height with wedge angle is also analysed and it is found that simplification of an asymmetric MR to a symmetric MR leads to over-prediction of the Mach stem height and hence the extent of the subsonic region.

Hybrid multiple diffraction in semipolar wurtzite materials: (\bf 01\overline{1}2)-oriented ZnMgO/ZnO heterostructures as an illustration

X-ray diffraction has been widely used to characterize the structural properties (strain and structural quality) of semiconductor heterostructures. This work employs hybrid multiple diffraction to analyzer-oriented Zn1−xMgxO layers grown by molecular beam epitaxy on ZnO substrates. In such a low-symmetry material system, additional features appear in symmetric reflection scans, which are described as arising from hybrid multiple diffraction. First, the Bragg conditions necessary for these high-order processes to occur are introduced and applied to explain all the observed satellite reflections, identify the planes that contribute and computea priorithe angles at which they are observed. Furthermore, thanks to this hybrid multiple-diffraction technique, it is possible to determine the layer lattice parameters (in-plane and out-of-plane) in an easy and accurate way by using one single measurement in standard symmetric conditions. The achieved precision is at least as high as that obtained from the combination of symmetric and asymmetric reciprocal space map measurements.

Hybrid reciprocal lattice: application to layer stress determination in GaAlN/GaN(0001) systems with patterned substrates

Epitaxy of semiconductors is a process of tremendous importance in applied science and in the optoelectronics industry. The control of defects introduced during epitaxial growth is a key point in manufacturing devices of high efficiency and durability. In this work, it is demonstrated how useful hybrid reflections are for the study of epitaxial structures with anisotropic strain gradients due to patterned substrates. High accuracy in detecting and distinguishing elastic and plastic relaxations is one of the greatest advantages of measuring this type of reflection, as well as the fact that the method can be exploited in a symmetric reflection geometry on a commercial high-resolution diffractometer.

High-resolution X-ray diffraction analysis of InN films grown by metalorganic vapor phase epitaxy

The growth temperature dependence of the InN film’s crystalline quality is reported. InN films are grown on sapphire substrates from 570 to 650 °C with low-temperature GaN buffers by metalorganic vapor phase epitaxy (MOVPE). The X-ray rocking curves and reciprocal space mappings of the symmetric reflection (0 0 0 2) and asymmetric reflection (1 0 1 2) are measured with high resolution X-ray diffraction. The results indicate that the crystallinity is sensitive to the growth temperature for MOVPE InN. At growth temperature 580 °C, highly crystalline InN film has been obtained, for which the full-width-at-half-maxima of (0 0 0 2) and (1 0 1 2) rocking curves are 24 and 28 arcmin, respectively. The crystalline quality deteriorates drastically when the growth temperature exceeds 600 °C. Combined with the carrier concentration and mobility, the approach to improve the quality of InN film by MOVPE is discussed.

Analytic determination of the three-dimensional distribution of dislocations using synchrotron X-ray topography

A technique, using a symmetric reflectionviaazimuthal rotation of a sample, is presented for characterization of the three-dimensional distribution of dislocations in single crystals. An analytic formula is derived to transform the three-dimensional geometry of a straight dislocation into its two-dimensional projection onto the detector plane. By fitting topographs to the formula, the orientations and locations of dislocations are quantitatively determined. The dislocations in a thermally stressed Si wafer are examined as an example.

Synchrotron X-ray reticulographic measurement of lattice deformations associated with energetic ion implantation in diamond

A natural diamond plate of thickness 0.55 mm, polished parallel to (110), contained a dislocation-free and unusually distortion-free area into which a beam of fluorine ions of 17 MeV energy had been injected. The beam, the diameter of which was pinhole-limited to ∼3/4 mm, produced a radiation-damaged layer at ∼5 µm depth and uplifted the crystal above to form an optically detectable mesa. Conventional X-ray topography registered lattice distortion strongly at the mesa periphery, but did not disclose the deformation of near-surface lattice layers at the mesa centre. In contrast, reticulography showed directly that the central surface lattice layers were slightly domed outwards. (In reticulography, a fine-scale X-ray absorbing mesh placed between a Laue-reflecting crystal and the topograph-recording plate splits the diffracted beam into individually trackable microbeams, the direction differences of which are measured by recording them at different mesh-to-plate distances.) Reticulographs were recorded in back-reflection, 2θB= 135°, using both symmetric reflection from the (110) surface, reflection 440 dominant, with λ = 0.116 nm, and reflection 331¯, with λ = 0.151 nm. The metrological procedures and geometrical analysis applied are described. The central surface curvature was derived from ten reticulographs recorded under different conditions, which tested the reproducibility of the method. Deviations of individual radius values from the mean were ∼4% for both curvature in (001) and orthogonally in (1¯10), the overall mean being 5 m. Two-beam optical interferometry failed to measure this curvature.