Multi-GNSS-Weighted Interpolated Tropospheric Delay to Improve Long-Baseline RTK Positioning.

Until now, RTK (real-time kinematic) and NRTK (Network-based RTK) have been the most popular cm-level accurate positioning approaches based on Global Navigation Satellite System (GNSS) signals in real-time. The tropospheric delay is a major source of RTK errors, especially for medium and long baselines. This source of error is difficult to quantify due to its reliance on highly variable atmospheric humidity. In this paper, we use the NRTK approach to estimate double-differenced zenith tropospheric delays alongside ambiguities and positions based on a complete set of multi-GNSS data in a sample 6-station network in Europe. The ZTD files published by IGS were used to validate the estimated ZTDs. The results confirmed a good agreement, with an average Root Mean Squares Error (RMSE) of about 12 mm. Although multiplying the unknowns makes the mathematical model less reliable in correctly fixing integer ambiguities, adding a priori interpolated ZTD as quasi-observations can improve positioning accuracy and Integer Ambiguity Resolution (IAR) performance. In this work, weighted least-squares (WLS) were performed using the interpolation of ZTD values of near reference stations of the IGS network. When using a well-known Kriging interpolation, the weights depend on the semivariogram, and a higher network density is required to obtain the correct covariance function. Hence, we used a simple interpolation strategy, which minimized the impact of altitude variability within the network. Compared to standard RTK where ZTD is assumed to be unknown, this technique improves the positioning accuracy by about 50%. It also increased the success rate for IAR by nearly 1.

Real-Time PPP-RTK Performance Analysis Using Ionospheric Corrections from Multi-Scale Network Configurations.

The long convergence time required to achieve high-precision position solutions with integer ambiguity resolution-enabled precise point positioning (PPP-RTK) is driven by the presence of ionospheric delays. When precise real-time ionospheric information is available and properly applied, it can strengthen the underlying model and substantially reduce the time required to achieve centimeter-level accuracy. In this study, we present and analyze the real-time PPP-RTK user performance using ionospheric corrections from multi-scale regional networks during a day with medium ionospheric disturbance. It is the goal of this contribution to measure the impact the network dimension has on the ambiguity-resolved user position through the predicted ionospheric corrections. The user-specific undifferenced ionospheric corrections are computed at the network side, along with the satellite phase biases needed for single-receiver ambiguity resolution, using the best linear unbiased predictor. Such corrections necessitate the parameterization of an estimable user receiver code bias, on which emphasis is given in this study. To this end, we process GPS dual-frequency data from four four-station evenly distributed CORS networks in the United States with varying station spacings in order to evaluate if and to what extent the ionospheric corrections from multi-scale networks can improve the user convergence times. Based on a large number of samples, our experimental results showed that sub-10 cm horizontal accuracy can be achieved almost instantaneously in the ionosphere-weighted partially-ambiguity-fixed kinematic PPP-RTK solutions based on corrections from a network with 68 km spacing. Most of the solutions (90%) were shown to require less than 6.0 min, compared to the ionosphere-float PPP solutions that needed 68.5 min. In case of sparser networks with 115, 174 and 237 km spacing, 50% of the horizontal positioning errors are shown to become less than one decimeter after 1.5, 4.0 and 7.0 min, respectively, while 90% of them require 10.5, 16.5 and 20.0 min. We also numerically demonstrated that the user's convergence times bear a linear relationship with the network density and get shorter as the density increases, for both full and partial ambiguity resolution.

Precise Point Positioning on the Reliable Detection of Tropospheric Model Errors.

Precise point positioning (PPP) is one of the well-known applications of Global Navigation Satellite System (GNSS) and provides precise positioning solutions using accurate satellite orbit and clock products. The tropospheric delay due to the neutral atmosphere for microwave signals is one of the main sources of measurement error in PPP. As one component of this delay, the hydrostatic delay is usually compensated by using an empirical correction model. However, how to eliminate the effects of the wet delay during a weather event is a challenge because current troposphere models are not capable of considering the complex atmosphere around the receiver during situations such as typhoons, storms, heavy rainfall, et cetera. Thus, how positioning results can be improved if the residual wet delays are taken into account needs to be investigated . In this contribution, a real-time procedure of recursive detection, identification and adaptation (DIA) is applied to detect the model errors which have the same effects on both phase and code observables; e.g., the model error caused by the tropospheric delay. Once the model errors are identified, additional parameters are added to the functional model to account for the measurement residuals. This approach is evaluated with Global Positioning System (GPS) data during two rainfall events in Darwin, Australia, proving the usefulness of compensated residual slant wet delay for positioning results. Comparisons with the standard approach show that the precision of the up component is improved significantly during the periods of the weather events; for the two case studies, 72.46 % and 64.41 % improvements of root mean squared error (RMS) resulted, and the precision of the horizontal component obtained by the proposed approach is also improved more than 30 % compared to the standard approach. The results also show that the identified model errors are concentrated at the beginning of both heavy rainfall processes when the front causes significant spatial and temporal gradients of the integrated water vapor above the receiver.

An Efficient Implementation of Fixed Failure-Rate Ratio Test for GNSS Ambiguity Resolution.

Ambiguity Resolution (AR) plays a vital role in precise GNSS positioning. Correctly-fixed integer ambiguities can significantly improve the positioning solution, while incorrectly-fixed integer ambiguities can bring large positioning errors and, therefore, should be avoided. The ratio test is an extensively used test to validate the fixed integer ambiguities. To choose proper critical values of the ratio test, the Fixed Failure-rate Ratio Test (FFRT) has been proposed, which generates critical values according to user-defined tolerable failure rates. This contribution provides easy-to-implement fitting functions to calculate the critical values. With a massive Monte Carlo simulation, the functions for many different tolerable failure rates are provided, which enriches the choices of critical values for users. Moreover, the fitting functions for the fix rate are also provided, which for the first time allows users to evaluate the conditional success rate, i.e., the success rate once the integer candidates are accepted by FFRT. The superiority of FFRT over the traditional ratio test regarding controlling the failure rate and preventing unnecessary false alarms is shown by a simulation and a real data experiment. In the real data experiment with a baseline of 182.7 km, FFRT achieved much higher fix rates (up to 30% higher) and the same level of positioning accuracy from fixed solutions as compared to the traditional critical value.

Secretion of adipocytokines by perivascular adipose tissue near stenotic and non-stenotic coronary artery segments in patients undergoing CABG.

OBJECTIVE: Perivascular adipose tissue (pvAT) may induce a local pro-inflammatory environment, possibly contributing to coronary atherosclerosis. We investigated whether there is a difference in adipocytokine production by pvAT near stenotic and non-stenotic coronary artery segments in patients with coronary artery disease (CAD). METHODS: In patients undergoing CABG with or without valve replacement (n = 38) pvAT near stenotic and near non-stenotic coronary segments was harvested. PvAT was incubated ex vivo for 24h. Concentrations of 23 adipocytokines were measured in the supernatants with a Multiplex assay. The number of macrophages (CD68, CD11c, CD206) and lymphocytes (CD45) in pvAT was determined. Differences between stenosis and control pvAT were tested with Wilcoxon signed rank test corrected for multiple comparisons. RESULTS: Production of IL-5, IL-1α, IL-17, IL-18 and IL-23 was higher in control than stenosis pvAT samples (p < 0.0021). Macrophages were more abundant in stenosis than in control pvAT (median n/400× field: 2.3 IQR: 0.3-4.5 versus 1.2 IQR: 0.1-2.5). There was a predominance of M2 macrophages in both stenosis and control pvAT (median n/400× field: macrophages stenosis: M1: 0.0; M2: 1.0 p = 0.004; control: M1: 0.0; M2: 0.6 p = 0.013). The relation between adipocytokine production and macrophage infiltration was not different in stenosis and control pvAT. CONCLUSION: In patients with CAD, multiple adipocytokines were secreted at higher levels by pvAT near non-stenotic than near stenotic coronary artery segments. Furthermore, pvAT macrophages are associated with stenosis of the adjacent vessel. M2 macrophages were more abundant than M1 macrophages in pvAT.

C-reactive protein and incident diabetes in patients with arterial disease.

INTRODUCTION: Systemic low-grade inflammation, as measured by high-sensitive C-reactive protein (hsCRP), may contribute to the risk of type 2 diabetes in patients with manifest arterial disease. METHODS: Cohort study in 4072 patients with manifest arterial disease without diabetes. The relation between quartiles of hsCRP and type 2 diabetes was assessed with Cox regression analyses, taking age, smoking and blood pressure-lowering medication and lipid-lowering medication into account. Insulin resistance was estimated with homeostasis model of insulin resistance (HOMA-IR). In exploratory models, adjustments were performed for body mass index (BMI) and visceral and subcutaneous adipose tissue thickness. RESULTS: During a median follow-up of 5·0 (IQR 2·5-8·2) years, 288 subjects developed diabetes. High hsCRP was independently associated with incident diabetes (Q4 vs. Q1 males: HR 1·62; 95% CI 1·06-2·48; females: HR 3·12; 95% CI 1·57-6·21). HOMA-IR at baseline is related to hsCRP plasma levels (Q4 vs. Q1: males: ß 0·27; 95% CI 0·19-0·36; females: ß 0·35; 95% CI 0·22-0·48). The risk of diabetes associated with hsCRP was abolished in males (Q4 vs. 1 HR 1·23; 95% CI 0·80-1·88) and attenuated in females (Q4 vs. 1 HR 2·32; 95% CI 1·14-4·75) after adding BMI to the model, but not modified by statin use (P for interaction: 0·61). CONCLUSIONS: Patients with manifest arterial disease with high hsCRP plasma levels are at increased risk to develop type 2 diabetes and are more insulin resistant as compared to those with low hsCRP levels. This increase in risk is more pronounced in females than in males and is not modified by statin use.

Relationship between myocardial bridges and reduced coronary atherosclerosis in patients with angina pectoris.

BACKGROUND: A myocardial bridge (MB) is a band of myocardium covering a coronary artery segment, typically located in the left anterior descending coronary artery (LAD). Bridged segments of the coronary artery are isolated from the influence of perivascular adipose tissue. The aims of this study were to investigate the relationship between MBs and atherosclerosis in bridged LAD segments and to evaluate whether perivascular adipose tissue is involved in this relationship. METHODS: MBs were identified in the coronary arteries of patients referred for diagnostic cardiac CT. The calcium score of MBs of the LAD or, in patients without LAD-MBs, of a corresponding LAD segment at the same distance from its origin and over the same length was measured. RESULTS: Of 128 patients, 56 (44%) had in total 73 MBs. The mean MB length was 22 ± 14 mm and the median MB thickness was 0.8mm (interquartile range 0.3-2.1mm). MBs in the LAD were present in 40 patients (31%). The calcium score was 0 in 95% of the LAD segments with MBs compared with 52% of the corresponding LAD segments without MBs. The association between LAD-MBs and calcium score (OR 0.06, 95% CI 0.01-0.25) was not influenced by age and gender, but was attenuated by local perivascular adipose tissue thickness (OR 0.35, 95% CI 0.04-2.70). CONCLUSIONS: Coronary artery segments covered with an MB have a lower calcium score than segments without an MB. The association between MBs and calcium scores was influenced by local perivascular adipose tissue thickness.

Coronary perivascular adipose tissue characteristics are related to atherosclerotic plaque size and composition. A post-mortem study.

BACKGROUND: Perivascular adipose tissue (pvAT) may influence atherosclerotic plaque formation. We aim to determine the association between the local amount and inflammatory properties of pvAT and the size and composition of atherosclerotic plaque in the left anterior descending artery (LAD). METHODS: Post-mortem, a total of 139 cross sections of the LAD were obtained from 16 patients. PvAT quantity was measured within an area of 3 mm around the LAD (pvAT-area (%)). Furthermore, inflammatory properties of pvAT were measured (macrophages/400× field and adipocyte size). From plaque area (mm(2)), plaque/media-ratio was calculated and morphologic characteristics were scored (presence of a lipid core and calcification; collagen and smooth muscle cell content; macrophage (<10, 10-50, >50/400× field) and lymphocyte infiltration (<10, 10-25, >25/400× field)). RESULTS: Plaque/media-ratio increased with increasing pvAT-area (ß0.02; 95%CI 0.01-0.03) and pvAT-macrophages (ß 0.10; 95%CI 0.05-0.16), but not with adipocyte area (ß -0.00; 95%CI -0.07-0.06). PvAT-area was related to the presence of a lipid core (OR 1.05; 95%CI 1.03-1.08) and with macrophage and lymphocyte infiltration of atherosclerotic plaque (per increase in category OR 1.05; 95%CI 1.02-1.07 and OR 1.04; 95%CI 1.01-1.07 respectively). PvAT macrophage infiltration was correlated with adventitia and plaque macrophages. CONCLUSION: PvAT quantity and macrophage infiltration are highly related to atherosclerotic plaque size and composition in patients with coronary atherosclerosis. These results indicate potential involvement of pvAT in coronary atherosclerotic plaque development, although the causality of the relation has yet to be determined.

Insulin resistance increases the occurrence of new cardiovascular events in patients with manifest arterial disease without known diabetes. the SMART study.

BACKGROUND: Insulin resistance is accompanied by a cluster of metabolic changes, often referred to as metabolic syndrome. Metabolic syndrome is associated with an increased cardiovascular risk in patients with manifest arterial disease. We investigated whether insulin resistance is associated with an increased risk for cardiovascular events in patients with manifest arterial disease without known diabetes and whether this can be explained by the components of the metabolic syndrome or by inflammation. METHODS: Prospective cohort study in 2611 patients with manifest arterial disease without known diabetes. Homeostasis model of insulin resistance (HOMA-IR) was used to quantify insulin resistance. The relation of HOMA-IR with cardiovascular events (vascular death, myocardial infarction or stroke) and all cause mortality was assessed with Cox regression analysis. In additional models adjustments were performed for the single components constituting the metabolic syndrome and for inflammation. RESULTS: HOMA-IR increases with the number of metabolic syndrome components (mean HOMA-IR ± SD in groups with 0, 1, 2, 3, 4 and 5 metabolic syndrome components: 1.4 ± 0.7; 1.8 ± 1.2; 2.4 ± 1.5; 3.1 ± 1.8; 4.0 ± 2.6; and 5.6 ± 3.6 respectively). High HOMA-IR was independently associated with an increased risk of cardiovascular events (tertile 2 vs. 1 HR 1.92; 95%CI 1.20-3.08) (tertile 3 vs.1 HR 1.78; 95%CI 1.10-2.89) and with all cause mortality (tertile 2 vs. 1 HR 1.80; 95%CI 1.04-3.10) (tertile 3 vs.1 HR 1.56; 95%CI 0.88-2.75). These relations were not influenced by the individual components of metabolic syndrome or by inflammation. CONCLUSIONS: In patients with manifest arterial disease without known diabetes, insulin resistance increases with the number of metabolic syndrome components, and elevated insulin resistance increases the risk of new cardiovascular events.

Perivascular adipose tissue as a cause of atherosclerosis.

Perivascular adipose tissue surrounds (coronary) arteries and may be involved in local stimulation of atherosclerotic plaque formation. Epicardial adipose tissue, the adipose tissue within the pericardium, is a frequently used measure of coronary perivascular adipose tissue and can be quantified with echocardiography, computed tomography (CT) and magnetic resonance imaging (MRI). The quantity of (coronary) perivascular adipose tissue is correlated with parameters of the metabolic syndrome, such as increased waist circumference, hypertriglyceridemia and hyperglycemia, and with coronary atherosclerosis. Coronary artery segments covered by myocardium are not exposed to coronary perivascular adipose tissue and interestingly, atherosclerosis is absent in these intra-myocardial segments. Pro-inflammatory cytokines and adipokines are expressed and secreted at a higher level in epicardial adipose tissue of patients with coronary artery disease compared to patients without coronary artery disease. Furthermore, in vitro and ex vivo perivascular adipose tissue induces inflammation of the artery wall by secretion of pro-inflammatory proteins. Atherogenesis in the vascular wall is thus stimulated from 'outside to inside'. Based on the results of clinical, ex vivo and in vitro studies, it can be argued that perivascular adipose tissue may be involved in the process of atherosclerosis.