Vertical Position Measurements Research Articles

Non-inductive plasma vertical position measurement for the 1056s discharge on EAST.

Vertical position stability plays a crucial role in maintaining safe and reliable plasma operation for long-pulse fusion devices. In general, the vertical position is measured by using inductive magnetic coils installed inside the vacuum vessel; however, the integration drift effects are inherent for steady-state or long-pulse plasma operation. Developing a non-magnetic approach provides a fusion reactor-relevant steady-state solution that avoids the negative impact of integration drift. In this paper, we compare the non-inductively determined vertical position achieved by line-integrated interferometer and polarimeter measurements to that employing an inductive flux loop for a 1056s discharge recently achieved on EAST (Experimental Advanced Superconducting Tokamak). Experimental results show that the non-inductive measurement is more robust than flux loops after 300s if the integrator is not reset to suppress integrator drift. Real-time vertical position control using the non-inductive system is proposed for the next EAST experimental campaign.

Measuring Global-Mean Sea-Level Rise With Surface Drifting Buoys

AbstractWe propose to establish a new ocean observing system for monitoring global and regional mean sea-level changes. This system will consist of a global array of thousands of water-following drifting buoys tracked by a global navigation satellite system—such as the Global Positioning System (GPS)—which will continuously provide the geographical positions and the height of the sea surface along the buoys' trajectories. The sea-level height data collected in this way, averaged over regional basins and the global ocean, will provide daily measures of regional and global mean sea levels. An essential climate variable, mean sea level is an intrinsic measure of climate change, integrating the thermal expansion of the ocean's waters and additions to the ocean's mass from melting terrestrial ice. The realization of this new system requires that standardized vertical position measurements with controlled accuracy be acquired and regularly transmitted from relatively small and expendable drifting buoys, which constitutes a technological challenge, yet one with a clear path for being met. The development and implementation of this ocean shot concept will ultimately provide an independent, resilient, sustainable, and economical observational system to quantify natural and anthropogenic sea-level changes, augmenting the existing satellites and tide gauge observing systems.

Measuring Global Mean Sea Level Changes With Surface Drifting Buoys

AbstractCombining ocean model data and in situ Lagrangian data, I show that an array of surface drifting buoys tracked by a Global Navigation Satellite System (GNSS), such as the Global Drifter Program, could provide estimates of global mean sea level (GMSL) and its changes, including linear decadal trends. For a sustained array of 1,250 globally distributed buoys with a standardized design, I demonstrate that GMSL decadal linear trend estimates with an uncertainty less than 0.3 mm yr−1 could be achieved with GNSS daily random error of 1.6 m or less in the vertical direction. This demonstration assumes that controlled vertical position measurements could be acquired from drifting buoys, which is yet to be demonstrated. Development and implementation of such measurements could ultimately provide an independent and resilient observational system to infer natural and anthropogenic sea level changes, augmenting the ongoing tide gauge and satellites records.

Improved sensing of vertical velocity for vertical position control using loop voltage signals on EAST

Vertical position control of shaped plasma is essential for all elongated tokamaks to achieve safe and stable plasma discharge. In EAST, the in-vessel coil (IC) is employed for fast control of plasma vertical motion which power supply capabilities limits the controllability of vertical instability. Besides, adequate signal-to-noise ratio of vertical position measurement can also realize an improvement in vertical control. In EAST, plasma vertical position Zp is produced nearly by all integrated magnetics, fast Z error signal is obtained by filtering from Zp. Then this fast Z error and its derivative term (dz/dt) are controlled by IC with a separate PD controller. However, it has a low signal-to-noise ratio which limits the gains that can be used successfully in the PD control loops. To provide less noisy dz/dt signal, the successful use of multiple up/down loop voltage pairs instead of derivative of z derived from magnetic probe signals will be discussed. Experiment show that even a single pair of loop voltage signal has better signal-to-noise ratio than differentiation of Zp. In addition, flux loop pickup signals from the IC current are also measured with series of positive and negative IC current pulses.

Non-inductive vertical position measurements by Faraday-effect polarimetry on EAST tokamak.

Vertical instability control in an elongated plasma is highly desirable for a tokamak reactor. A multi-channel 694 GHz far-infrared laser-based polarimeter-interferometer system has been used to provide a non-inductive vertical position measurement in the long-pulse EAST tokamak. A detailed comparison of vertical position measurements by polarimetry and external inductive flux loops has been used to validate Faraday-effect polarimetry as an accurate high-time response vertical position sensor.

A virtual vertical reference concept for aided inertial navigation at the sea surface

When trying to capture the heaving motion of a marine vessel in waves, conventional position references such as global navigation satellite systems fall short in many cases. Because of design and satellite geometry, the vertical position measurement is typically inferior in both accuracy and precision compared to its horizontal counterparts. For aiding an inertial navigation system (INS), using conventional vertical position references may therefore be suboptimal. This article presents an alternative algorithm utilizing a virtual vertical reference (VVR) concept, based on the mean sea level, to aid the INS. The novelty of the algorithm arises where the VVR concept is augmented with an error model, used to improve the heave estimation performance. The motion of the vessel is estimated using an INS, employing a feedback-interconnected observer framework, based on nonlinear theory. The INS, using low-cost micro-electro-mechanical-system-based inertial measurement units, provides position, velocity and attitude in a dead-reckoning fashion, and is aided by a horizontal position reference, the VVR, and a compass to prevent the estimates from drifting. The estimation performance obtained with the observer structure is evaluated using Monte Carlo simulations and compared to preceding results. The algorithm is also validated experimentally and compared to an industry standard vertical reference unit using data collected on board an offshore vessel.

Evaluating performance of MEMS barometric sensors in differential altimetry systems

Micro-electro-mechanical-systems (MEMS) barometric sensors are extensively employed in positioning systems for the acquisition of altitude data. Confidence in their general approval on position location applications is due to the following reasons: A. the relatively high accuracy of vertical position measurements, which is constantly improved in the most recent commercial sensor devices; B. the inability of the allied—and dominant in outdoor environment— GPS and GNSS products to operate within buildings. Indicative examples of barometric altitude readings in consumer, medical, and aerospace applications [1] are as follows. Detecting floor [2] and mode of transition (i.e., elevators, escalators or stairs) [3] inside large buildings can be addressed for indoor navigation; Identifying human physical activities (e.g. sit-to-stand) [4] and falls [5] is able to provide telehealth solutions; Fusing information from barometric altimeters with GPS may improve performance of GPS-based landing systems [6], while the employment of two barometers in a system can be used for measuring the airplane position angles (e.g. pitch and roll) [7].

Constraints on snow accumulation and firn density in Greenland using GPS receivers

AbstractData from three continuously operating GPS sites located in the interior of the Greenland ice sheet are analyzed. Traditionally these kinds of GPS installations (where the GPS antenna is placed on a pole deployed into the firn) are used to estimate the local horizontal speed and direction of the ice sheet. However, these data are also sensitive to the vertical displacement of the pole as it moves through the firn layer. A new method developed to measure snow depth variations with reflected GPS signals is applied to these GPS data from Greenland. This method provides a constraint on the vertical distance between the GPS antenna and the surface snow layer. The vertical positions and snow surface heights are then used to assess output from surface accumulation and firn densification models, showing agreement better than 10% at the sites with the longest records. Comparisons between the GPS reflection method and in situ snow sensors at the Dye-2 site show good agreement, capturing the dramatic changes observed in Greenland during the 2012 summer melt season. The geocentric elevation of the snow surface can be inferred by subtracting the snow surface height estimates from the vertical position measurements. It should be possible to use those surface elevation estimates to help validate elevation results obtained from satellite altimetry.

Canopy temperature for peach tree at various soil water contents

Canopy temperature measurements with infrared thermometry have been extensively studied as a means of assessing plant water status for field and row crops. Achieving high quality peach fruit depends on the ability to maintain mild to moderate levels of water stress in the crop during the growing season. The paper examined the spatial distribution of tree canopy temperature (Tc) using thermal images in a peach orchard for irrigation scheduling. The variation of Tc was investigated in three irrigation regime treatments (factor A) that produced various soil moisture content (SMC) values, three cardinal points (factor B): South, North and East-West aspects combined, and five up-down vertical position measurements (factor C: upper, middle upper, middle, middle lower and lower) across the tree canopy thermal images. It was found that Tc was significantly influenced by the irrigation regime. Cardinal point showed a significant Tc difference between South on the one hand and the other aspects. The vertical position within canopy image did not significantly influence Tc.

Using APCS for Plasma Vertical Control at TCV

Early tokamaks with circular cross-section plasmas were vertically stable in nature and no problems were reported on the control of the vertical position. However, the concept of vertically elongated plasma cross-section, with benefits to the energy confinement time, led to vertical instabilities. To overcome this problem, complex closed feedback loop control systems with a vertical position measurement, signal processing, control algorithm, power supplies and active actuating coils are used. The Tokamak a Configuration Variable (TCV) permits the stabilization of highly elongated plasmas, with a powerful vertical position control system and fast power supplies capable of response times under 0.1 ms. The introduction of the new Advanced Plasma Control System (APCS) in TCV has required the development of a new vertical position observer built to optimally exploit the capabilities of the new system and correct for the changes that were detected by the introduction of signal digitizing in the feedback loop. This paper discusses the need for a new measurement of the plasma vertical position, the method to build the observer and the validation of the measured position by comparing with previous methods and observers. A summary of the ongoing work is provided. We also discuss its aim and the goals that are expected to be achieved by exploring the new digital non-linear control algorithm capabilities of APCS.

Experimental vertical stability studies for ITER performance and design guidance

Operating experimental devices have provided key inputs to the design process for ITER axisymmetric control. In particular, experiments have quantified controllability and robustness requirements in the presence of realistic noise and disturbance environments, which are difficult or impossible to characterize with modelling and simulation alone. This kind of information is particularly critical for ITER vertical control, which poses the highest demands on poloidal field system performance, since the consequences of loss of vertical control can be severe. This work describes results of multi-machine studies performed under a joint ITPA experiment (MDC-13) on fundamental vertical control performance and controllability limits. We present experimental results from Alcator C-Mod, DIII-D, NSTX, TCV and JET, along with analysis of these data to provide vertical control performance guidance to ITER. Useful metrics to quantify this control performance include the stability margin and maximum controllable vertical displacement. Theoretical analysis of the maximum controllable vertical displacement suggests effective approaches to improving performance in terms of this metric, with implications for ITER design modifications. Typical levels of noise in the vertical position measurement and several common disturbances which can challenge the vertical control loop are assessed and analysed.

Better-resolution measurement of vertical scattering angle in a new ion-optical mode of spectrometer “Grand Raiden”

Demand for near-zero-degree measurements using a magnetic spectrometer is increasing with the growing interest in the study of L=0 nuclear excitation modes. For the precise determination of scattering angles near 0°, both horizontal and vertical scattering angle components have to be measured with good accuracy. It is, however, not easy to realize a good vertical angle resolution using modern high-resolution magnetic spectrometers with small vertical angle magnifications, like “Grand Raiden” at RCNP or “K600” at IUCF. A new ion-optical mode which enables the precise determination of vertical components of scattering angles from vertical position measurements in the focal plane has been developed for these spectrometers. For the wide range of spatial as well as momentum acceptances, vertical angle resolution better than 8 mrad is realized for Grand Raiden at RCNP.

Atmospheric pressure loading effects on Global Positioning System coordinate determinations

Earth deformation signals caused by atmospheric pressure loading are detected in vertical position estimates at Global Positioning System (GPS) stations. Surface displacements due to changes in atmospheric pressure account for up to 24% of the total variance in the GPS height estimates. The detected loading signals are larger at higher latitudes where pressure variations are greatest; the largest effect is observed at Fairbanks, Alaska (latitude 65°), with a signal RMS of 5 mm. Out of 19 continuously operating GPS sites (with a mean of 281 daily solutions per site), 18 show a positive correlation between the GPS vertical estimates and the modeled loading displacements. Accounting for loading reduces the variance of the vertical station positions on 12 of the 19 sites investigated. Removing the modeled pressure loading from GPS determinations of baseline length for baselines longer than 6000 km reduces the variance on 73 of the 117 baselines investigated. The slight increase in variance for some of the sites and baselines is consistent with expected statistical fluctuations. The results from most stations are consistent with ∼65% of the modeled pressure load being found in the GPS vertical position measurements. Removing an annual signal from both the measured heights and the modeled load time series leaves this value unchanged. The source of the remaining discrepancy between the modeled and observed loading signal may be the result of (1) anisotropic effects in the Earth's loading response, (2) errors in GPS estimates of tropospheric delay, (3) errors in the surface pressure data, or (4) annual signals in the time series of loading and station heights. In addition, we find that using site dependent coefficients, determined by fitting local pressure to the modeled radial displacements, reduces the variance of the measured station heights as well as or better than using the global convolution sum.

Precision of vertical position estimates from Very Long Baseline Interferometry

We discuss the prospects for using very long baseline interferometry (VLBI) to measure vertical crustal motions. Our analysis of this problem indicates that the main limitations of such VLBI measurements will probably arise from errors in modeling of the propagation delay through the earth's neutral atmosphere and errors in determining the orientation of a “crust fixed” coordinate system in the VLBI (inertial) reference frame. We examine two techniques for assessing the precision of vertical position measurements which can be made with currently available VLBI systems: (1) the repeatability of baseline length measurements, and (2) the repeatability of the vertical site position of one site in a four‐site network when the positions of the other three sites determine the earth orientation parameters. The repeatability of baseline length measurements implies that the precision of the vertical position estimates is ∼8 cm, averaged over 13 sites (separated by 600–8000 km) and 4.5 years of data. The repeatability of vertical position estimates for the Richmond, Florida, site is ∼7 cm for the 42 observing sessions carried out during an 11‐month period. Both of the techniques we have used to estimate the precision of height determinations indicate that the current precision is ∼8 cm for a single 24‐hour VLBI observing session. The effects of errors in the earth orientation parameters will depend on the distances between the VLBI sites and could induce errors as large as the precision given above.

Relative Vertical Positioning Using Ground-Level Transponders with the ERS-1 Altimeter

The microwave altimeters flown on the GEOS-3 and Seasat satellites have demonstrated the remarkable potential of the instrument for making global measurements of ocean currents and sea state. In this paper a technique is proposed for using the altimeter to be carried on the ESA ERS-1 satellite (launch date 1989) to measure the relative vertical position of land-based transponders and the ocean geoid on 2000-km-long segments of the satellite ground track. The absolute position accuracy will be similar to that of the laser tracking stations, relative accuracy achievable may be of the order of a few centimeters, and measurements of changes in transponder elevation during the three-year satellite life may be at the subcentimeter level. The opportunity for such measurements is provided by exploiting together, very accurate range measurements made to ground-based transponders and the demonstrated constancy of form of satellite, short-arc orbits repeated over an identical ground track. Altimetric vertical position measurements of this accuracy would make possible new techniques for altimeter calibration, sea-state bias measurements, orbit measurement, and seismic/geodetic study.