The integration of spaceborne accelerometry in the precise orbit determination of low-flying satellites

This dissertation describes the integration of accelerometer measurements of LEO-satellites in GPS-based orbit determination.Astrodynamics and Space MissionsAerospace Engineerin

Calibration of GRACE Accelerometers Using Two Types of Reference Accelerations

Two approaches for the calibration of GRACE (Gravity Recovery And Climate Experiment) accelerometers are revisited. In the first approach, surface forces acting on the satellite are considered to derive the reference acceleration. In the second approach, the total acceleration consisting of a gravitational and a non-gravitational contribution is first determined from the reduced-dynamic orbits. The approximation of discrete satellite positions by a polynomial function allows the total acceleration to be obtained by a twofold derivative w.r.t. time. Calibration parameters (scale factor and bias) and statistical values are estimated for periods with a low and high solar activity. The quality of these two approaches shows dependencies on solar activity and consequent variations in the magnitude of the non-gravitational reference acceleration. Besides, the quality of the presented results is affected by the orientation of the orbital plane w.r.t. the Sun. The second approach is vitiated by a periodic disturbing signal on cross-track axis. This signal has been pointed out in earlier studies (Calabia et al., Aerosp Sci Technol 45, 2015; Calabia and Jin, Aerosp Sci Technol 49, 2016). We apply a moving window median filter to recover the underlying non-gravitational signal for accelerometer calibration. The calibration is accomplished by a direct comparison of reference accelerations and observed accelerometer measurements without introducing any a priori values or constraints. The focus of this work is more sensor oriented than gravity field recovery (GFR) related. Nevertheless, the results can be used as initial values for precise orbit determination (POD) or for pre-processing of accelerometer measurements in a multi step gravity field recovery approach (Klinger and Mayer-Gürr, Adv Space Res 58(9), 2016). The final paper is available at https://doi.org/10.1007/1345_2018_46

Arterial stiffness in patients with type 1 diabetes and its comparison to cardiovascular risk evaluation tools

BACKGROUND: Arterial stiffness is a potential biomarker for cardiovascular disease (CVD) risk in patients with type 1 diabetes (T1D). However, its relation with other CV risk evaluation tools in T1D has not been elucidated yet. This study aimed to evaluate arterial stiffness in T1D patients free from known CVD, and compare it to other CV risk evaluation tools used in T1D. METHODS: Cross-sectional study in adults with a T1D duration of at least 10 years and without established CVD. Patients were categorized in CVD risk groups based on 2019 European Society of Cardiology (ESC) guidelines, and the STENO T1D risk engine was used to estimate 10-year risk for CV events. Arterial stiffness was evaluated with carotid-femoral pulse wave velocity (cf-PWV). Coronary artery calcium (CAC) score was assessed and carotid ultrasound was performed. Ambulatory 24-h blood pressure and central hemodynamic parameters were evaluated. Data on renal function and diabetic kidney disease was retrieved. RESULTS: 54 patients (age: 46 ± 9.5 years; T1D duration: 27 ± 8.8 years) were included. One-fourth of patients showed prematurely increased aortic stiffness based on cf-PWV (24%). Cf-PWV was significantly associated with CAC score, carotid intima-media thickness, central hemodynamic parameters and diabetic kidney disease. Based on STENO, 20 patients (37%) were at low, 20 patients (37%) at moderate, and 14 patients (26%) at high 10-year risk for CV event. Cf-PWV was strongly associated with the STENO score (rs = + 0.81; R2 = 0.566, p  10% 10-year risk for CV events (n = 44/54; 81.5% versus n = 34/54; 63%). CONCLUSIONS: This study demonstrated that a substantial proportion of long-standing T1D patients free from known CVD show premature arterial stiffening. Cf-PWV strongly associates with the STENO risk score for future CV events and with cardiovascular imaging and function outcomes, thereby illustrating the clinical importance of arterial stiffness. The data, however, also show considerable heterogeneity in CV risk and differences in risk categorisation between the STENO tool and ESC criteria.There is a need for refinement of CV risk classification in T1D, and future studies should investigate if evaluation of arterial stiffness should be implemented in T1D clinical practice and which patients benefit the most from its assessment

Intestinal transplantation: from the laboratory to the clinics

Intestinal transplantation (ITx) is a valuable treatment for patients suffering from irreversible short bowel syndrome and debilitating complications of total parenteral nutrition. However, the results of ITx remain inferior to other solid organ transplants due to the profound immunogenicity of the organ and the subsequent demand for high levels of immunosuppression with its associated side effects. Since 2000, 10 consecutive cadaveric intestinal transplantations have been performed in our center. All were treated with the same Leuven Tolerogenic Protocol - an experimentally proven, multifactorial immunomodulatory regimen - which includes the administration of donor specific whole blood, low-dose steroids, low-dose tacrolimus, and limitation of peritransplant intestinal ischemia reperfusion injury. The first patient transplanted in this series was the first successful ITx performed in the Benelux. The 5-year patient and graft survival rate is 90% (follow-up: 11 months – 11 years and 10 months), which compares favorably with the average 50% patient survival rate reported by the Intestinal Transplant Registry. Only one patient (10%) developed an early acute rejection, reversible with immediate steroid treatment. Three patients (30%) faced a later episode of acute rejection (at 4 months, 18 months and 46 months). Wider application of ITx depends upon the development of these immunosuppressive protocols which prohibit intestinal rejection while reducing the need for immunosuppression.status: publishe

Tracking and orbit determination performance of the GRAS instrument on MetOp-A

The global navigation satellite system receiver for atmospheric sounding (GRAS) on MetOp-A is the first European GPS receiver providing dual-frequency navigation and occultation measurements from a spaceborne platform on a routine basis. The receiver is based on ESA’s AGGA-2 correlator chip, which implements a high-quality tracking scheme for semi-codeless P(Y) code tracking on the L1 and L2 frequency. Data collected with the zenith antenna on MetOp-A have been used to perform an in-flight characterization of the GRAS instrument with focus on the tracking and navigation performance. Besides an assessment of the receiver noise and systematic measurement errors, the study addresses the precise orbit determination accuracy achievable with the GRAS receiver. A consistency on the 5 cm level is demonstrated for reduced dynamics orbit solutions computed independently by four different agencies and software packages. With purely kinematic solutions, 10 cm accuracy is obtained. As a part of the analysis, an empirical antenna offset correction and preliminary phase center correction map are derived, which notably reduce the carrier phase residuals and improve the consistency of kinematic orbit determination results.Delft Institute of earth Observation and Space SystemsAerospace Engineerin

Design and optimization of ICs for wearable EEG sensor

In modern clinical practice, scalp electroencephalography (EEG) recording is one of the most important noninvasive procedures to measure the electrical activity of the human brain. EEG has a wide range of applications from brain disorder diagnosis, stroke rehabilitation, brain-computer interface (BCI), and gaming. Conventionally, EEG signal is obtained by placing electrodes on the scalp along with conductive gel to reduce the electrode-tissue contact impedance. The recorded EEG signal between two electrodes is a differential voltage representing the average intensity and spontaneous activities of a group of neurons underlying the skull. In time domain, EEG response with peaks and valleys indicates the power spectrum associate with brain activities. In frequency domain, most of the signal falls within a narrow band of 0.5–100 Hz. Some of the prominent frequency bands are called alpha (7–14 Hz), beta (15–30 Hz), theta (4–7 Hz), and delta (1–4 Hz), each having characteristic neurophysiological traits