URE and URA for Predicted LEO satellites Orbits at different altitudes

In recent years, low Earth orbit (LEO) satellites have been frequently discussed for their benefits in positioning and navigation services as an augmentation to the global navigation satellite systems (GNSSs). Similar to the positioning concept based on ranging to GNSS satellites, precise positioning of single-receiver users needs high-accuracy orbits and clocks of LEO satellites as a pre-condition. For real-time users, high prediction accuracies of these orbits at different latencies are needed. Unlike the satellite clocks, the GNSS orbits can be typically predicted for hours with high accuracy. LEO satellites, however, face more complicated perturbing dynamic terms due to their low altitudes. Therefore, the prediction accuracy and integrity of their orbits need to be addressed. In this study, using real data of three test LEO satellites GRACE C, Sentinel-1A and Sentinel-3B of different altitudes, various reduced-dynamic prediction strategies are assessed, with the appropriate methods selected for different prediction times up to 6 h. The global-averaged orbital user range errors (OUREs) are shown to be altitude-related. For the 700–800 km Sentinel satellites and 500 km GRACE satellite, the RMS of the OUREs is at sub-dm and dm-level for the prediction time of 1 h, respectively, and around 0.2 m and 0.6 m at the prediction time of 6 h, respectively. For integrity purposes, the worst-location OURE are calculated for the predicted orbits using a proposed algorithm considering the Earth as an Ellipsoid, not a sphere as usually done for the GNSS satellites. The orbital user range accuracy (OURA) is then evaluated for different prediction periods, having a time-dependent model proposed to compute the overbounding OURA at any prediction time within 6 h. With an integrity risk of 10 5, using hourly quadratic polynomials as the time-dependent model, the overbounding OURA is around 0.1 m at the prediction of 1 h, and at the sub-meter level for the prediction of 6 h for the Sentinel satellites. 2022 COSPAR. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org

Patient-specific simulation of stent-graft deployment in type B aortic dissection: model development and validation

Thoracic endovascular aortic repair (TEVAR) has been accepted as the mainstream treatment for type B aortic dissection, but post-TEVAR biomechanical-related complications are still a major drawback. Unfortunately, the stent-graft (SG) configuration after implantation and biomechanical interactions between the SG and local aorta are usually unknown prior to a TEVAR procedure. The ability to obtain such information via personalized computational simulation would greatly assist clinicians in pre-surgical planning. In this study, a virtual SG deployment simulation framework was developed for the treatment for a complicated aortic dissection case. It incorporates patient-specific anatomical information based on pre-TEVAR CT angiographic images, details of the SG design, and the mechanical properties of the stent wire, graft and dissected aorta. Hyperelastic material parameters for the aortic wall were determined based on uniaxial tensile testing performed on aortic tissue samples taken from type B aortic dissection patients. Pre-stress conditions of the aortic wall and the action of blood pressure were also accounted for. The simulated post-TEVAR configuration was compared with follow-up CT scans, demonstrating good agreement with mean deviations of 5.8% in local open area and 4.6 mm in stent strut position. Deployment of the SG increased the maximum principal stress by 24.30 KPa in the narrowed true lumen but reduced the stress by 31.38 KPa in the entry tear region where there was an aneurysmal expansion. Comparisons of simulation results with different levels of model complexity suggested that pre-stress of the aortic wall and blood pressure inside the stent-graft should be included in order to accurately predict the deformation of the deployed S

Strategies for the production of maize-derived pharmaceuticals using cytoplasmic male sterile lines: In vitro tissue culture/transformation and field breeding approaches

Plant-made pharmaceuticals (PMPs) offer promise as efficient and cost-effective products for the treatment of human and animal diseases. An advantage of producing pharmaceuticals in maize is the large storage capacity and stability for proteins and starch in seed, allowing for manufacturing recombinant proteins such as antigens and antibodies. Other advantages of the maize system include safety, high yields, and scalability of production and processing. However, the benefits of this technology must be balanced against potential health and environmental risks that may be associated with its use. Because PMPs presently have no provision for regulatory tolerance, their inadvertent occurrence in foods and feeds remains an important economic consideration, even when the health and environmental risks are low. Pollen drift is considered a source of potential contamination of maizemade pharmaceuticals in the food chain. In addition to physical and temporal isolation requirements, open field pharmaceutical maize production also calls for controlled pollen release. Here, we describe two strategies to address the issue of transgenic pollen drift. First, we describe the development and genetic transformation of a tissue culture-amenable male-sterile line using biolistic- or Agrobacterium-mediated transformation methods. Secondly, we describe the introgression of a transgene from male-fertile transgenic maize to male-sterile germplasm by conventional breeding. After six seasons of breeding, this second strategy allows us to obtain 100% transgenic seeds from an open-field production using a non-transgenic line as the pollinator

Van der Waals epitaxy of Bi2Se3 on Si(111) vicinal surface: An approach to prepare high-quality thin films of topological insulator

Epitaxial growth of topological insulator Bi2Se3 thin films on nominally flat and vicinal Si(111) substrates is studied. In order to achieve planner growth front and better quality epifilms, a two-step growth method is adopted for the van der Waal epitaxy of Bi2Se3 to proceed. By employing vicinal Si(111) substrate surfaces, the in-pane growth rate anisotropy of Bi2Se3 is explored to achieve single crystalline Bi2Se3 epifilms, in which threading defects and twins are effectively suppressed. Optimization of the growth parameters has resulted in vicinal Bi2Se3 films showing a carrier mobility of ~ 2000 cm2V-1s-1 and the background doping of ~ 3 x 1018 cm-3 of the as-grown layers. Such samples not only show relatively high magnetoresistance but also a linear dependence on magnetic field.Comment: 18 pages, 4 figure

Integrity Monitoring of PPP-RTK Positioning; Part II: LEO Augmentation

Low Earth orbit (LEO) satellites benefit future ground-based positioning with their high number, strong signal strength and high speed. The rapid geometry change with the LEO augmentation offers acceleration of the convergence of the precision point positioning (PPP) solution. This contribution discusses the influences of the LEO augmentation on the precise point positioning—real-time kinematic (PPP-RTK) positioning and its integrity monitoring. Using 1 Hz simulated data around Beijing for global positioning system (GPS)/Galileo/Beidou navigation satellite system (BDS)-3 and the tested LEO constellation with 150 satellites on L1/L5, it was found that the convergence of the formal horizontal precision can be significantly shortened in the ambiguity-float case, especially for the single-constellation scenarios with low precision of the interpolated ionospheric delays. The LEO augmentation also improves the efficiency of the user ambiguity resolution and the formal horizontal precision with the ambiguities fixed. Using the integrity monitoring (IM) procedure introduced in the first part of this series of papers, the ambiguity-float horizontal protection levels (HPLs) are sharply reduced in various tested scenarios, with an improvement of more than 60% from 5 to 30 min after the processing start. The ambiguity-fixed HPLs can generally be improved by 10% to 60% with the LEO augmentation, depending on the global navigation satellite system (GNSS) constellations used and the precision of the ionospheric interpolation

Performance improvement of MXene-based perovskite solar cells upon property transition from metallic to semiconductive by oxidation of Ti₃C₂Tₓ in air

The unique properties of MXenes that arise from terminating functional groups and oxidation of MXenes make them attractive for application in photovoltaic devices like perovskite solar cells (PSCs). Here, oxidation of Ti3C2Tx hydrocolloid was carried out to tune its properties desirable for an electron transport layer (ETL) in low-temperature processed PSCs. The calculations of the energy levels were carried out using the Vienna ab initio simulation package (VASP) code based on density functional theory (DFT). Oxidation of Ti_{3}C_{2}T_{x} can generate Ti–O bonds and effectively reduce the macroscopic defects of the film fabricated by spin-coating, while a transition from metallic material to semiconductor occurred after heavy oxidation. A better matching of energy levels between perovskite and ETL layer in the case of a hybrid of oxidized and pristine Ti_{3}C_{2}T_{x} renders a champion power conversion efficiency (PCE) of 18.29%. The improvement in PCE can be attributed to the increased electron mobility in the ETL, which promotes electron transport and reduces the electron–hole recombination. Hence, by presenting a simple method for high performance in PSCs by MXene-derived materials, this work demonstrates the great potential of these materials for applications in low-temperature processed PSCs and other photovoltaic technologies

Bioinspired materials for underwater adhesion with pathways to switchability

Strong adherence to underwater or wet surfaces for applications like tissue adhesion and underwater robotics is a significant challenge. This is especially apparent when switchable adhesion is required that demands rapid attachment, high adhesive capacity, and easy release. Nature displays a spectrum of permanent to reversible attachment from organisms ranging from the mussel to the octopus, providing inspiration for underwater adhesion design that has yet to be fully leveraged in synthetic systems. Here, we review the challenges and opportunities for creating underwater adhesives with a pathway to switchability. We discuss key material, geometric, modeling, and design tools necessary to achieve underwater adhesion similar to the adhesion control demonstrated in nature. Through these interdisciplinary efforts, we envision that bioinspired adhesives can rise to or even surpass the extraordinary capabilities found in biological systems

"Narrow" Graphene Nanoribbons Made Easier by Partial Hydrogenation

It is a challenge to synthesize graphene nanoribbons (GNRs) with narrow widths and smooth edges in large scale. Our first principles study on the hydrogenation of GNRs shows that the hydrogenation starts from the edges of GNRs and proceeds gradually toward the middle of the GNRs so as to maximize the number of carbon-carbon $\pi$-$\pi$ bonds. Furthermore, the partially hydrogenated wide GNRs have similar electronic and magnetic properties as those of narrow GNRs. Therefore, it is not necessary to directly produce narrow GNRs for realistic applications because partial hydrogenation could make wide GNRs "narrower"

A method of real-time long-baseline time transfer based on the PPP-RTK

Long-baseline time transfer can nowadays reach rather high frequency stability based on post-processed batch least-squares adjustment using the Precise Point Positioning (PPP) or Integer-PPP (IPPP) methods. For real-time PPP users, time transfer results are degraded due to the filter-based processing mode, and the degraded accuracy of the real-time satellite orbits and clocks compared to the final ones. The Real-Time Kinematic (RTK) time transfer can significantly reduce the satellite-related errors, but has limits on the baseline length similar to the RTK positioning. Also, the delivery of raw observations instead of State-Space Representation (SSR) products could result in pressure on data transfer and difficulties related to latency and prediction. In this study, the PPP-RTK technique, which combines the advantages of the PPP and the RTK methods, is tested for real-time long-baseline time transfer. As an alternative approach to the above two methods, it allows for the time transfer of long baselines, while not relying on external high-sampling and high-precision satellite clocks. By delivering the satellite clocks and satellite phase biases produced within the PPP-RTK regional network, time differences can be estimated between users and the reference network station, with which stable time transfer between users separated by long baselines can be realized. Using dual-frequency GPS and Galileo data, the PPP-RTK time transfer is tested using approximately a thousand-kilometer-scale network in Europe. The time transfer results between two hydrogen masers, i.e., those on the 884 km baseline BRUX-ONSA and the 920 km baseline WTZR-ONSA, are computed. At an averaging time of 105 s, Modified Allan Deviation (MDEV) at the level of sub-10-15 to 10-15 can be reached when processing the user coordinates in the station fixed, static, or kinematic modes. The median clock residuals can converge to 1 ns and 0.3 ns within 2 min and 15 min, respectively, in the kinematic mode, while in the static and fixed modes the convergence times are shorter. With the augmentation of 150 Low Earth Orbit (LEO) satellites having simulated observations, the clock residuals can converge to 1 ns and 0.3 ns within 30 s and 3.5 min, respectively, for all the three estimation modes