Добірка наукової літератури з теми "Drift-orbit geometrics"

This study proposes real-time orbit/clock determination of Korean Navigation Satellite System (KNSS), which employs the kinematic precise point positioning (PPP) solutions of multiple Global Navigation Satellite System (multi-GNSS) to compensate for receiver clock offset. Global visibility of KNSS satellites in terms of geometric coverage is first analyzed for the purpose of selecting optimal locations of KNSS monitoring stations among International GNSS Service (IGS) and Multi-GNSS Experiment (MGEX) network. While the receiver clock offset is obtained from multi-GNSS PPP clock solutions of real observation data, KNSS measurements are simulated from the dynamically propagated KNSS reference orbit and the receiver clock offset. The offset and drift of satellite clock are also generated based on two-state clock model considering atomic clock noise. Real-time orbit determination results are compared with an artificially generated true or bit, wihch show 0.4m and 0.5m of 3-dimensional root-mean-square (RMS) position errors for geostationary (GEO) and ellitically-inclined-geosynchronous-orbit (EIGSO) satellites, respectively. The overall results show that the real-time precise orbit determination of KNSS should be achievable in meter level by installing KNSS-compatible multi-GNSS receivers on the IGS and/or MGEX network. The overall process can be also used to verify integrity of KNSS monitoring stations.

The Earth’s revolution is modified by changes in inclination of its rotation axis. Its trajectory is not closed and the equinoxes drift. Changes in polar motion and revolution are coupled through the Liouville–Euler equations. Milanković (1920) argued that the shortest precession period of solstices is 20,700 years: the summer solstice in one hemisphere takes place alternately every 11,000 year at perihelion and at aphelion. Milanković assumed that the planetary distances to the Sun and the solar ephemerids are constant. There are now observations that allow one to drop these assumptions. We have submitted the time series for the Earth’s pole of rotation, global mean surface temperature and ephemeris to iterative Singular Spectrum Analysis. iSSA extracts from each a trend a 1 year and a 60 year component. Both the apparent drift of solstices of Earth around the Sun and the global mean temperature exhibit a strong 60 year oscillation. We monitor the precession of the Earth’s elliptical orbit using the positions of the solstices as a function of Sun–Earth distance. The “fixed dates” of solstices actually drift. Comparing the time evolution of the winter and summer solstices positions of the rotation pole and the first iSSA component (trend) of the temperature allows one to recognize some common features. A basic equation from Milankovic links the derivative of heat received at a given location on Earth to solar insolation, known functions of the location coordinates, solar declination and hour angle, with an inverse square dependence on the Sun–Earth distance. We have translated the drift of solstices as a function of distance to the Sun into the geometrical insolation theory of Milanković. Shifting the inverse square of the 60 year iSSA drift of solstices by 15 years with respect to the first derivative of the 60 year iSSA trend of temperature, that is exactly a quadrature in time, puts the two curves in quasi-exact superimposition. The probability of a chance coincidence appears very low. Correlation does not imply causality when there is no accompanying model. Here, Milankovic’s equation can be considered as a model that is widely accepted. This paper identifies a case of agreement between observations and a mathematical formulation, a case in which an element of global surface temperature could be caused by changes in the Earth’s rotation axis. It extends the range of Milankovic cycles and resulting global temperature variations to shorter periods (1–100 year range), with a major role for the 60-year oscillation).

The Landsat 8 (L8) spacecraft and its two instruments, the operational land imager (OLI) and thermal infrared sensor (TIRS), have been consistently characterized and calibrated since its launch in February 2013. These performance metrics and calibration updates are determined through the U.S. Geological Survey (USGS) Landsat image assessment system (IAS), which has been performing this function since its launch. The TIRS on-orbit geometric calibration procedures include TIRS-to-OLI alignment, TIRS sensor chip assembly (SCA) alignment, and TIRS band alignment. In December 2014, the TIRS instrument experienced an anomalous condition related to the instrument’s ability to accurately measure the location of the scene select mechanism (SSM). The SSM is a rotating mirror that allows the instrument’s field of view to be pointed at the Earth, for normal imaging, or at either deep space or an onboard black body, for radiometric calibration purposes. This anomalous condition in the SSM’s position sensor made it necessary to implement a new mode of operation for this mirror, termed mode-0. Mode-0 involves operating the mirror in an open-loop control state during normal mission operations when acquiring Earth data. Closed-loop mode-4 is needed for directing the mirror towards the radiometric calibration targets and is used approximately once every two weeks to collect radiometric calibration data. Mode-0 is used for most operational imaging because it does not require SSM encoder data, thereby allowing the SSM encoder electronics to remain unpowered most of the time, reducing its use throughout the lifetime of the TIRS instrument, thus helping to preserve its nominal behavior during it use. This paper discusses the geometric calibration of the SSM mirror during its current normal mode-0 set of image operations, as its open-loop control allows the mirror to drift over time in its uncontrolled state and its effects on products. The results shown in this paper demonstrate that the ability to have ongoing updates to the modeling of the TIRS SSM mirror model, in both an automated fashion and with a set of more manual operations, allows accuracy that approaches mode-4 results within days from the start of a mode-0 event.

Abstract. The Advanced Microwave Sounding Unit-B (AMSU-B) and Microwave Humidity Sounder (MHS) are total power microwave radiometers operating at frequencies near the water vapor absorption line at 183 GHz. The measurements of these instruments are crucial for deriving a variety of climate and hydrological products such as water vapor, precipitation, and ice cloud parameters. However, these measurements are subject to several errors that can be classified into radiometric and geometric errors. The aim of this study is to quantify and correct the radiometric errors in these observations through intercalibration. Since the bias in the calibration of microwave instruments changes with scene temperature, a two-point intercalibration correction scheme was developed based on averages of measurements over the tropical oceans and nighttime polar regions. The intercalibration coefficients were calculated on a monthly basis using measurements averaged over each specified region and each orbit, then interpolated to estimate the daily coefficients. Since AMSU-B and MHS channels operate at different frequencies and polarizations, the measurements from the two instruments were not intercalibrated. Because of the negligible diurnal cycle of both temperature and humidity fields over the tropical oceans, the satellites with the most stable time series of brightness temperatures over the tropical oceans (NOAA-17 for AMSU-B and NOAA-18 for MHS) were selected as the reference satellites and other similar instruments were intercalibrated with respect to the reference instrument. The results show that channels 1, 3, 4, and 5 of AMSU-B on board NOAA-16 and channels 1 and 4 of AMSU-B on board NOAA-15 show a large drift over the period of operation. The MHS measurements from instruments on board NOAA-18, NOAA-19, and MetOp-A are generally consistent with each other. Because of the lack of reference measurements, radiometric correction of microwave instruments remain a challenge, as the intercalibration of these instruments largely depends on the stability of the reference instrument.

Abstract. The quality of the calibrated radiances of the medium-resolution Imaging Infrared Radiometer (IIR) on-board the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite is quantitatively evaluated from the beginning of the mission in June 2006. Two complementary relative and stand-alone approaches are used, which are related to comparisons of measured brightness temperatures and to model-to-observations comparisons, respectively. In both cases, IIR channels 1 (8.65 µm), 2 (10.6 µm), and 3 (12.05 µm) are paired with the Moderate Resolution Imaging Spectroradiometer (MODIS)/Aqua Collection 5 companion channels 29, 31, and 32, respectively, as well as with the Spinning Enhanced Visible and Infrared Imager (SEVIRI)/Meteosat companion channels IR8.7, IR10.8, and IR12, respectively. These pairs were selected before launch to meet radiometric, geometric, and space-time constraints. The prelaunch studies were based on simulations and sensitivity studies using the 4A/OP radiative transfer model and the more than 2300 atmospheres of the climatological Thermodynamic Initial Guess Retrieval (TIGR) input dataset further sorted into five air mass types. Using data from over 9.5 years of on-orbit operation, and following the relative approach technique, collocated measurements of IIR and of its companion channels have been compared at all latitudes over ocean, during day and night, and for all types of scenes in a wide range of brightness temperatures. The relative approach shows an excellent stability of IIR2–MODIS31 and IIR3–MODIS32 brightness temperature differences (BTDs) since launch. A slight trend within the IIR1–MODIS29 BTD, that equals −0.02 K yr−1 on average over 9.5 years, is detected when using the relative approach at all latitudes and all scene temperatures. For very cold scene temperatures (190–200 K) in the tropics, each IIR channel is warmer than its MODIS companion channel by 1.6 K on average. For the stand-alone approach, clear sky measurements only are considered, which are directly compared with simulations using 4A/OP and collocated ERA-Interim (ERA-I) reanalyses. The clear sky mask is derived from collocated observations from IIR and the CALIPSO lidar. Simulations for clear sky pixels in the tropics reproduce the differences between IIR1 and MODIS29 within 0.02 K and between IIR2 and MODIS31 within 0.04 K, whereas IIR3–MODIS32 is larger than simulated by 0.26 K. The stand-alone approach indicates that the trend identified from the relative approach originates from MODIS29, whereas no trend (less than ±0.004 K yr−1) is identified for any of the IIR channels. Finally, using the relative approach, a year-by-year seasonal bias between nighttime and daytime IIR–MODIS BTD was found at mid-latitude in the Northern Hemisphere. It is due to a nighttime IIR bias as determined by the stand-alone approach, which originates from a calibration drift during day-to-night transitions. The largest bias is in June and July when IIR2 and IIR3 are warmer by 0.4 K on average, and IIR1 is warmer by 0.2 K.

At low Reynolds numbers, axisymmetric ellipsoidal particles immersed in a shear flow undergo periodic tumbling motions known as Jeffery orbits, with the orbit determined by the initial orientation. Understanding this motion is important for predicting the overall dynamics of a suspension. While slender fibres may follow Jeffery orbits, many such particles in nature are neither straight nor rigid. Recent work exploring the dynamics of curved or elastic fibres have found Jeffery-like behaviour along with chaotic orbits, decaying orbital constants and cross-streamline drift. Most work focuses on particles with reflectional symmetry; we instead consider the behaviour of a composite asymmetric slender body made of two straight rods, suspended in a two-dimensional shear flow, to understand the effects of the shape on the dynamics. We find that for certain geometries the particle does not rotate and undergoes persistent drift across streamlines, the magnitude of which is consistent with other previously identified forms of cross-streamline drift. For this class of particles, such geometry-driven cross-streamline motion may be important in giving rise to dispersion in channel flows, thereby potentially enhancing mixing.

At present, multi-satellite geolocation systems based on the TDOA are actively used to localization of radio emission sources in satellite communication systems operating via relay satellites without on-board processing. In General, information about the location of the radio emission sources is contained in the difference of the inclined range from the multiple fixed points with known coordinates. Such points of space in the classical geolocation system are two or more relay satellites in geostationary orbit. It is not always possible to have two or more satellites retransmitting the same signal. Therefore, it is necessary to develop a mathematical model for geolocation using a single relay satellite. Single-satellite geolocation is based on the use of Doppler, TDOA, or phase direction finding methods. With this approach, it is desirable that a single satellite has the ability to move in a controlled manner, either in altitude or at different speeds relative to its standing point. Moving the satellite along the equator in position and along the meridian in height allows you to calculate several orthogonal bases of estimates of the inclined range to the radio source. In this case, the determination of coordinates is based on the increment of the distance of the object's signal runs between the end points of each base. This provides the construction of position lines (hyperballs), the intersection of which is the source location. If the movement of the satellite along the equator and the meridian is performed with a change in speed, then geolocation is based on measurements of several orthogonal components of the Doppler frequency shift of the radio source signals. The base will be called two, four or more pairwise taken orbital positions of the satellite at points with fixed coordinates; S x y z1 1 1 1( , , ) S x y z2 2 2 2( , , ); S x y z2 2 2 2( , , ) S x y z3 3 3 3( , , ); etc. in all possible combinations. An arbitrary inclined base formed in the spacecraft orbit has an extension of Бп (x2  x1)2  (y2  y1)2  (z2  z1)2 . Differential range Дд = Дн2 – Дн1. To geolocate the M-object, you must: 1. Measure the difranges between M over two or more different shifted Дн bases at multiple satellite drift positions – Дд1, Дд2, ..., Дд4, etc. 2. Calculate the parameters al, bl, cl of each l-th hyperbolic surface of the section of the conic equations of the geometric location of the points of position M with the measured Дд1, Дд2, ..., Дд4 and the known Дн. Construct a common point of intersection of several such hyperbolic surfaces of the cross-section of the conic equations of the geometric location of the points of the position of the object M(x, y, z). The resulting vector of linear coordinates M(x, y, z) of the object must be converted from geocentric to geographical coordinates of the spherical coordinate system of the object M (longitude, latitude, Position-vector).

This thesis is concerned with neoclassical transport in the H-1NF heliac, and contains an examination of drift-orbit geometries, a description of a neoclassical Monte Carlo transport code, and a description of a method to use that code to self-consistently calculate ambipolar radial electric fields. We set out to study the contributions to neoclassical transport in H-1NF, by first describing the topology and the abundance of collisionless, trapped particle orbits in the presence of radial electric fields. We give an overview of the trapped orbit geometries in H-1NF, and develop a method to numerically classify the trapped particle orbits. On average, the trapped particle fraction in H-1NF is 40%, with approximately 5%, 15%, and 20% of the orbits in the deeply trapped, helically trapped, and toroidally trapped states, respectively. A condensed version of this component of the thesis has been submitted to Nuclear Fusion. The orbit studies provide a background for the development of a neoclassical Monte Carlo transport code, MCMuPPeT (for Monte Carlo, Multi Processing Plasma Transport). Using the code, we compare several Monte Carlo transport diagnostics, taken from the literature. Confinement times and diffusion coefficients are calculated for plasma conditions which will be achievable in H-1NF after the National Facility upgrade. Since the electric field can dominate in the determination of the transport, we develop an iterative method to self-consistently calculate the ambipolar radial electric field, using the Monte Carlo code. The method is applied to the Argon plasma conditions observed in H-1NF, in the experimentally observed Improved Conhnement Mode (ICM). To help interpret the results, the ambipolar electric fields were calculated in the same conditions using a well-known analytic model which was geometrically-fitted to H-1NF for our purposes. Qualitative agreement was found between both of the neoclassical models and the experimental results; the electric fields predicted in the ICM conditions are typically twice as large as those predicted in the conditions before the transition. The two models were also used to look for the neoclassically predicted transition from negative to positive radial electric field. Positive radial electric fields were observed, at long mean free path, in Hydrogen plasma conditions which will be achievable in H-1NF after the National Facility upgrade. We have also developed methods to optimise the Monte Carlo code for both parallel and vector computing environments. Two Message Passing algorithms that we use to parallelise the MC code are presented in the appendix.

Traditional analysis of running gait utilizes averaged biomechanical data from several strides to generate a mean curve. This curve is then used to define the average picture of a runners gait. However, such measures are frequently accompanied by time normalization, which results in a loss of temporal variations in the gait patterns. An examination of stability requires analysis of both time and position, therefore loss of such information makes stability analysis difficult. On the contrary, the use of a dynamical systems approach for gait analysis allows for a better understanding of how variations in gait pattern change over time. In the current study runners ran on a treadmill, with both a flat and uneven surface, at a self selected speed. Three-dimensional position data was captured for 11 different anatomical locations at a frequency of 120 Hz using a Qualysis motion capture system. The data was first shifted to a lumbar coordinate system to account for low frequency drift attributed to the subjects’ drift on the treadmill. Since all of the markers were rigidly connected, via the subject, the movements and variations of certain components of the 33-dimensional measurements were not independent. As a result, it was possible to reduce the dimensionality of the transformed data using singular value decomposition techniques. The primary components were then analyzed using the method of delay embeddings to extract geometric information, revealing the natural structure found in the data as a result of the periodicity of each running stride. A nearest neighbor mean stride orbit was then computed to create a reference orbit, so that deviations from the mean stride orbit can be measured. The expectation was that a more stable running configuration would lead to smaller deviations from the mean stride orbit. On-going work that will be reported includes: (i) analysis of running stability related to the reference stride comparator, (ii) compensation of lumbar centroid dynamics, (iii) reconstructions using one dimension from the lumbar centroid transformed data, and (iv) consideration of transients, fatigue, adaptation, etc.