268 research outputs found
Effectiveness of observation-domain sidereal filtering for GPS precise point positioning
Sidereal filtering is a technique used to reduce errors caused by multipath in the positioning of static receivers via the Global Positioning System (GPS). It relies upon the receiver and its surrounding environment remaining static from one day to the next and takes advantage of the approximately sidereal repeat time of the GPS constellation geometry. The repeating multipath error can thus be identified, usually in the position domain, and largely removed from the following day. We describe an observation-domain sidereal filter algorithm that operates on undifferenced ionospheric-free GPS carrier phase measurements to reduce errors caused by multipath. It is applied in the context of high-rate (1 Hz) precise point positioning of a static receiver. An observation-domain sidereal filter (ODSF) is able to account for the slightly different repeat times of each GPS satellite, unlike a position-domain sidereal filter (PDSF), and can hence be more effective at reducing high-frequency multipath error. Using 8-h long datasets of GPS measurements from two different receivers with different antenna types and contrasting environments, the ODSF algorithm is shown overall to yield a position time series 5–10 % more stable, in terms of Allan deviation, than a PDSF over nearly all time intervals below about 200 s in length. This may be particularly useful for earthquake and tsunami early warning systems where the accurate measurement of small displacements of the ground over the period of just a few minutes is crucial. However, the sidereal filters are also applied to a third dataset during which two short episodes of particularly high-frequency multipath error were identified. These two periods are analyzed in detail and illustrate the limitations of using sidereal filters with important implications for other methods of correcting for multipath at the observation level
A shadow function model based on perspective projection and atmospheric effect for satellites in eclipse
Accurate Solar Radiation Pressure (SRP) modelling is critical for correctly describing the dynamics of satellites. A shadow function is a unitless quantity varying between 0 and 1 to scale the solar radiation flux at a satellite’s location during eclipses. Errors in modelling shadow function lead to inaccuracy in SRP that degrades the orbit quality. Shadow function modelling requires solutions to a geometrical problem (Earth’s oblateness) and a physical problem (atmospheric effects). This study presents a new shadow function model (PPM_atm) which uses a perspective projection based approach to solve the geometrical problem rigorously and a linear function to describe the reduction of solar radiation flux due to atmospheric effects. GRACE (Gravity Recovery And Climate Experiment) satellites carry accelerometers that record variations of non-conservative forces, which reveal the variations of shadow function during eclipses. In this study, the PPM_atm is validated using accelerometer observations of the GRACE-A satellite. Test results show that the PPM_atm is closer to the variations in accelerometer observations than the widely used SECM (Spherical Earth Conical Model). Taking the accelerometer observations derived shadow function as the “truth”, the relative error in PPM_atm is −0.79% while the SECM 11.07%. The influence of the PPM_atm is also shown in orbit prediction for Galileo satellites. Compared with the SECM, the PPM_atm can reduce the radial orbit error RMS by 5.6 cm over a 7-day prediction. The impacts of the errors in shadow function modelling on the orbit remain to be systematic and should be mitigated in long-term orbit prediction
Softstar: Heuristic-guided probabilistic inference
Recent machine learning methods for sequential behavior prediction estimate the motives of behavior rather than the behavior itself. This higher-level abstraction improves generalization in different prediction settings, but computing predictions often becomes intractable in large decision spaces. We propose the Softstar algorithm, a softened heuristic-guided search technique for the maximum entropy inverse optimal control model of sequential behavior. This approach supports probabilistic search with bounded approximation error at a significantly reduced computational cost when compared to sampling based methods. We present the algorithm, analyze approximation guarantees, and compare performance with simulation-based inference on two distinct complex decision tasks
Probabilistic movement modeling for intention inference in human-robot interaction.
Intention inference can be an essential step toward efficient humanrobot interaction. For this purpose, we propose the Intention-Driven Dynamics Model (IDDM) to probabilistically model the generative process of movements that are directed by the intention. The IDDM allows to infer the intention from observed movements using Bayes ’ theorem. The IDDM simultaneously finds a latent state representation of noisy and highdimensional observations, and models the intention-driven dynamics in the latent states. As most robotics applications are subject to real-time constraints, we develop an efficient online algorithm that allows for real-time intention inference. Two human-robot interaction scenarios, i.e., target prediction for robot table tennis and action recognition for interactive humanoid robots, are used to evaluate the performance of our inference algorithm. In both intention inference tasks, the proposed algorithm achieves substantial improvements over support vector machines and Gaussian processes.
Earth Radiation Pressure Modelling for Beidou IGSO Satellites
This study shows ERP (Earth Radiation Pressure) is significant for precise
orbit determination of BeiDou satellites, systematic error is observable in the
orbit products if it is ignored in the orbital dynamics. Through subdividing
CERES (Cloud and Earth Radiation Energy System) 1° by 1° earth radiation grid
into 6 levels of triangles, a precise and runtime configurable earth radiation model
has been obtained. The results reveal that level 4 subdivision can make the relative
error of both longwave and shortwave radiation less than 1%. ERP acceleration
for BeiDou IGSO satellite(C08) and its effects on orbit have been analyzed
through modelling the earth radiation using a box-wing geometry model. The acceleration
in radial is about 2-4 times larger than the along track and normal with
magnitude 10-10m/s2
. 30-hour orbit prediction shows the 3D RMS error due to ERP
is 0.638 m and its maximum can reach 0.9 m
The Bosma effect revisited - I. HI and stellar disc scaling models
The observed proportionality between the centripetal contribution of the
dynamically insignificant HI gas in the discs of spiral galaxies and the
dominant contribution of DM - the "Bosma effect" - has been repeatedly
mentioned in the literature but largely ignored. We have re-examined the
evidence for the Bosma effect by fitting Bosma effect models for 17 galaxies in
the THINGS data set, either by scaling the contribution of the HI gas alone or
by using both the observed stellar disc and HI gas as proxies. The results are
compared with two models for exotic cold DM: internally consistent cosmological
NFW models with constrained compactness parameters, and URC models using fully
unconstrained Burkert density profiles. The Bosma models that use the stellar
discs as additional proxies are statistically nearly as good as the URC models
and clearly better than the NFW ones. We thus confirm the correlation between
the centripetal effects of DM and that of the interstellar medium of spiral
galaxies. The edificacy of "maximal disc" models is explained as the natural
consequence of "classic" Bosma models which include the stellar disc as a proxy
in regions of reduced atomic gas. The standard explanation - that the effect
reflects a statistical correlation between the visible and exotic DM - seems
highly unlikely, given that the geometric forms and hence centripetal
signatures of spherical halo and disc components are so different. A literal
interpretation of the Bosma effect as being due to the presence of significant
amounts of disc DM requires a median visible baryon to disc DM ratio of about
40%.Comment: Accepted by A&A (Paper I
Insights into the mechanism of coreactant electrochemiluminescence facilitating enhanced bioanalytical performance
Electrochemiluminescence (ECL) is a powerful transduction technique with a leading role in the biosensing field due to its high sensitivity and low background signal. Although the intrinsic analytical strength of ECL depends critically on the overall efficiency of the mechanisms of its generation, studies aimed at enhancing the ECL signal have mostly focused on the investigation of materials, either luminophores or coreactants, while fundamental mechanistic studies are relatively scarce. Here, we discover an unexpected but highly efficient mechanistic path for ECL generation close to the electrode surface (signal enhancement, 128%) using an innovative combination of ECL imaging techniques and electrochemical mapping of radical generation. Our findings, which are also supported by quantum chemical calculations\ua0and spin trapping methods, led to the identification of a family of alternative branched amine coreactants, which raises the analytical strength of ECL well beyond that of present state-of-the-art immunoassays, thus creating potential ECL applications in ultrasensitive bioanalysis
Testing of a new single-frequency GNSS carrier phase attitude determination method: land, ship and aircraft experiments
Global navigation satellite system (GNSS) ambiguity resolution is the process of resolving the unknown cycle ambiguities of the carrier phase data as integers. The sole purpose of ambiguity resolution is to use the integer ambiguity constraints as a means of improving significantly on the precision of the remaining GNSS model parameters. In this contribution, we consider the problem of ambiguity resolution for GNSS attitude determination. We analyse the performance of a new ambiguity resolution method for GNSS attitude determination. As it will be shown, this method provides a numerically efficient, highly reliable and robust solution of the nonlinearly constrained integer least-squares GNSS compass estimators. The analyses have been done by means of a unique set of extensive experimental tests, using simulated as well as actual GNSS data and using receivers of different manufacturers and type as well as different platforms. The executed field tests cover two static land experiments, one in the Netherlands and one in Australia, and two dynamic experiments, a low-dynamics vessel experiment and high-dynamics aircraft experiment. In our analyses, we focus on stand-alone, unaided, single-frequency, single epoch attitude determination, as this is the most challenging case of GNSS compass processing
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