Effective Observation Time Research Articles

Machine learning-based prediction approach for ranging resolution enhancement of FMCW LiDAR system with LSTM networks

Ranging resolution is one of the most important factors of frequency-modulated continuous wave (FMCW) light detection and ranging (LiDAR) systems, which is determined by the effective sweep bandwidth and observation time of the ranging signal used for Fourier transform. Conventional methods for resolution enhancement either require expensive devices or complicated operation with human intervention, which is high-cost and inefficient. For the first time, a machine learning-based prediction approach is proposed for resolution improvement. A long short-term memory (LSTM) network with a special architecture, which is driven by a theoretical model of FMCW ranging signals, is established for signal prediction. A resampling-based preprocessing block is then designed to denoise the original ranging data and enhance its feature. Finally, multi-step prediction of the input time-series signal is performed using the well-trained LSTM network to increase effective data length so as to enhance the resolution. The effectiveness of the proposed method is verified through several experiments. The results show that the proposed method not only has the advantages of low-cost and high-efficiency, but also enables the FMCW LiDAR system to possess larger equivalent bandwidth, better ranging resolution, and higher signal-to-noise ratio (SNR), enlightening the development of machine learning in high-precision ranging systems.

The First GECAM Observation Results on Terrestrial Gamma‐Ray Flashes and Terrestrial Electron Beams

AbstractGravitational‐wave high‐energy Electromagnetic Counterpart All‐sky Monitor (GECAM) is a space‐borne instrument dedicated to monitoring high‐energy transients, including Terrestrial Gamma‐ray Flashes (TGFs) and Terrestrial Electron Beams (TEBs). We implemented a TGF/TEB search algorithm for GECAM, with which 147 bright TGFs, 2 typical TEBs and 2 special TEB‐like events are identified during an effective observation time of ∼9 months. We show that, with gamma‐ray and charged particle detectors, GECAM can effectively identify and distinguish TGFs and TEBs, and measure their temporal and spectral properties in detail. A very high TGF‐lightning association rate of ∼80% is obtained between GECAM and GLD360 in east Asia region.

Research on Wind Tower Position Optimization Method of Radio Telescope Site Based on Numerical Simulation

Wind field is a critical issue for radio telescope antenna structure design and observation. As the telescope performance requirements are getting higher and higher, the antenna structure design becomes more and more complicated, and the influence of wind load on antenna pointing in the observation is also serious. How to ensure the stiffness and strength of the structure during the design and to improve the effective observation time during the observation all need accurate wind field data. Due to the traditional wind tower setup method cannot quantitatively evaluate the reliability of the setting point position, so a method based on numerical simulation to optimize the position of the wind tower is proposed. The boundary conditions of numerical simulation are set based on specification parameters, the overall trend is consistent with the measured data, and therefore the simulation accuracy could meet the requirement. Four wind tower positions were set at the test site. According to the simulation results, the root-mean-square error (RMSE) of wind speed between point P2 and the antenna position is the smallest. So, the measured data in P2 could better represent the wind field characteristics of the antenna area.

A study on the association between eye movements and regular mouthing movements (RMMs) in normal fetuses between 24 to 39 weeks of gestation.

Regular Mouthing Movements (RMMs) are movements in which lips and lower jaw movements occur regularly and can be observed in the fetus using transabdominal ultrasonic tomography. In near term infants, it is known that RMMs form clusters during the quiet sleep period. The notation of RMMs is not uniform, and is described as spontaneous sucking movement or non-nutritive sucking in newborns. Non-nutritive sucking is used to evaluate neurological function after birth, but there are no fetal indicators. The purpose of this study was to clarify the changes in the RMM clusters in fetuses at 24-39 weeks of gestation, and to investigate the relationship with the non-eye movement (NEM) period, which corresponds to the quiet sleep period after birth. Subjects included 83 normal single pregnancy cases. Fetal RMMs and eye movement (EM) were observed for 60 minutes using ultrasonic tomography and recorded as moving image files. We created time series data of eye movements and mouth movements from video recordings, and calculated RMM clusters per minute within effective observation time, RMM clusters per minute in EM period, RMM clusters per minute in NEM period, mouthing movements per cluster and ratio of number of RMM clusters per minute between NEM and EM periods and analyzed using linear regression analysis. As a result, critical points were detected in at two time points, at 32-33 weeks and 36-37 weeks of gestation, in RMM clusters per minute within the effective observation time and RMM clusters per minute in NEM period, respectively. RMM clusters in human fetuses increased from 32-33 to 36-37 weeks. This change is thought to represent fetal sleep development and central nervous system development.

An analysis method for data taken by Imaging Air Cherenkov Telescopes at very high energies under the presence of clouds

The effective observation time of Imaging Air Cherenkov Telescopes (IACTs) plays an important role in the detection of γ-ray sources, especially when the expected flux is low. This time is strongly limited by the atmospheric conditions. Significant extinction of Cherenkov light caused by the presence of clouds reduces the photon detection rate and also complicates or even makes impossible proper data analysis. However, for clouds with relatively high atmospheric transmission, high energy showers can still produce enough Cherenkov photons to allow their detection by IACTs. In this paper, we study the degradation of the detection capability of an array of small-sized telescopes for different cloud transmissions. We show the expected changes of the energy bias, energy and angular resolution and the effective collection area caused by absorption layers located at 2.5 and 4.5 km above the observation level. We demonstrate simple correction methods for reconstructed energy and effective collection area. As a result, the source flux that is observed during the presence of clouds is determined with a systematic error of ≲ 20%. Finally, we show that the proposed correction method can be used for clouds at altitudes higher than 5 km a.s.l. As a result, the analysis of data taken under certain cloudy conditions will not require additional time-consuming Monte Carlo simulations.

Commissioning progress of the FAST

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) was completed with its main structure installed on September 25, 2016, after which it entered the commissioning phase. This paper aims to introduce the commissioning progress of the FAST over the past two years. To improve its operational reliability and ensure effective observation time, FAST has been equipped with a real-time information system for the active reflector system and hierarchical commissioning scheme for the feed support system, which ultimately achieves safe operation of the two systems. For meeting the high-performance indices, a high-precision measurement system was set up based on the effective control methods that were implemented for the active reflector system and feed support system. Since the commissioning of the FAST, a low-frequency ultra-wideband receiver and 19-beam 1.05-1.45 GHz receiver have been mainly used. Telescope efficiency, pointing accuracy, and system noise temperature were completely tested and ultimately achieved the acceptance indices of the telescope. The FAST has been in the process of national acceptance preparations and has begun to search for pulsars. In the future, it will still strive to improve its capabilities and expand its application prospects.

Enhanced Brownian motion of the group self-propellant Janus particles based on dynamic light scattering technique

Janus particles collectively refer to the non-uniform particles with different properties on two sides. Due to their asymmetry, asymmetric physical or chemical reactions shall be performed by the functionalized Janus particles by using “fuel” in the environment, thereby forming the gradient fields of different physical quantities, such as concentration, electric field or light intensity on both sides of the particles, so the autonomous motion shall be carried on by using the asymmetrical momentum distribution formed by the gradient. At present, the main methods to study the self-propulsion of Janus particles include single particle tracking (SPT) and dynamic light scattering (DLS). In this paper, the movement behavior of Pt-SiO2 Janus particles with a nominal diameter of 1 μm in pure water and in H2O2 solutions was analyzed, whose results were compared with that of the SPT. The experimental results show that the movement behavior of group Janus particles in H2O2 solution showed the enhanced Brownian motion. With the increase of the concentration of H2O2 solution, the effective diffusion coefficient of group Janus particles was increased by using the DLS method, which was less than the effective diffusion coefficient of single Janus particles measured by using the SPT method. In addition, in this paper, according to the inverse calculation of the self-correlation function measured by DLS, the mean square displacement of group Janus particles in the extremely short time interval was obtained. The results showed that at the same observation time interval, the mean square displacement shall be increased with the density enhancement of the H2O2 solutions, while in the H2O2 solutions with the same density, the mean square displacement shall be increased with the increase of the observation time interval. Finally, the characteristics and differences of SPT and DLS methods to study the Janus particle motion behavior were analyzed. It is shown that the observation interval is the key factor among the measurement of DLS and SPT. By SPT method, the movement characteristics of a single Janus particle at different stages can be obtained by recording the trajectory of Janus particle, while it is difficult for SPT to record the trajectory of Janus particle for small particles ( d In this case, SPT is not suitable for the study of Janus particles, while DLS is an effectively method to compensate this shortcoming of the SPT method. By DLS, the motion characteristics of Janus particles can be obtained by analyzing the light intensity fluctuations caused by particles motion, and the applicable range is 2 nm −3 μm, but he results of DLS are limited to the longest effective observation time (the time for the autocorrelation coefficient decreasing from 1 to 0). When the longest effective observation time is less than the characteristic time, the motion characteristics of the “directional driven diffusion” stage can be obtained, while when the longest effective observation time is greater than the characteristic time, the motion characteristics of the “Brownian-like motion” stage can be obtained. Due to the longest effective observation time, only the Janus particles’ movement characteristics at a certain stage in the above two phases can be obtain by DLS. DLS and SPT are two important methods to study the self-propelling movement of Janus particles. Although their mechanisms are different, the movement behavior of Janus particle can be studied more thoroughly by these two methods with supplementation mutually.

Constraints on particle acceleration in SS433/W50 from MAGIC and H.E.S.S. observations

Context. The large jet kinetic power and non-thermal processes occurring in the microquasar SS 433 make this source a good candidate for a very high-energy (VHE) gamma-ray emitter. Gamma-ray fluxes above the sensitivity limits of current Cherenkov telescopes have been predicted for both the central X-ray binary system and the interaction regions of SS 433 jets with the surrounding W50 nebula. Non-thermal emission at lower energies has been previously reported, indicating that efficient particle acceleration is taking place in the system.Aim. We explore the capability of SS 433 to emit VHE gamma rays during periods in which the expected flux attenuation due to periodic eclipses (Porb~ 13.1 days) and precession of the circumstellar disk (Ppre ~ 162 days) periodically covering the central binary system is expected to be at its minimum. The eastern and western SS 433/W50 interaction regions are also examined using the whole data set available. We aim to constrain some theoretical models previously developed for this system with our observations.Methods. We made use of dedicated observations from the Major Atmospheric Gamma Imaging Cherenkov telescopes (MAGIC) and High Energy Spectroscopic System (H.E.S.S.) of SS 433 taken from 2006 to 2011. These observation were combined for the first time and accounted for a total effective observation time of 16.5 h, which were scheduled considering the expected phases of minimum absorption of the putative VHE emission. Gamma-ray attenuation does not affect the jet/medium interaction regions. In this case, the analysis of a larger data set amounting to ~40–80 h, depending on the region, was employed.Results. No evidence of VHE gamma-ray emission either from the central binary system or from the eastern/western interaction regions was found. Upper limits were computed for the combined data set. Differential fluxes from the central system are found to be ≲ 10−12–10−13 TeV−1 cm−2 s−1 in an energy interval ranging from ~few × 100 GeV to ~few TeV. Integral flux limits down to ~ 10−12–10−13 ph cm−2 s−1 and ~ 10−13–10−14 ph cm−2 s−1 are obtainedat 300 and 800 GeV, respectively. Our results are used to place constraints on the particle acceleration fraction at the inner jetregions and on the physics of the jet/medium interactions.Conclusions. Our findings suggest that the fraction of the jet kinetic power that is transferred to relativistic protons must be relatively small in SS 433, qp ≤ 2.5 × 10−5, to explain the lack of TeV and neutrino emission from the central system. At the SS 433/W50 interface, the presence of magnetic fields ≳10 μG is derived assuming a synchrotron origin for the observed X-ray emission. This also implies the presence of high-energy electrons with Ee− up to 50 TeV, preventing an efficient production of gamma-ray fluxes in these interaction regions.

Dimensionality of Spatio-Temporal Broadband Signals Observed Over Finite Spatial and Temporal Windows

Dimensionality of spatio-temporal signals is fundamental to the performance limits of any wireless communications system with multiple antennas. In this paper, we use the spherical harmonic analysis of wave propagation to derive a closed-form expression for the dimensionality of band-limited wireless signals observed over a finite region of space within a finite time window. Specifically, we decompose spatio-temporal signals into independent and orthogonal spatial modes (waveforms) and apply Shannon’s classical signal dimensionality result, namely, time-frequency bandwidth product to each spatial mode. Our analysis shows that: 1) the effective observation time of all the spatial modes is the sum of the finite time window and the time it takes a wave front to propagate across the entire spatial region and 2) though the effective bandwidth at lower spatial modes is equal to the bandwidth of the underlying transmitted signal, the effective bandwidth decreases as the mode index increases depending on the acceptable signal-to-noise ratio at each spatial mode. Thus, only a finite number of spatial modes (independent channels) carry information with its own time-frequency bandwidth product. These findings show that the classical signal dimensionality result of time-bandwidth product does not extend directly to the product of spatial degrees of freedom and time-bandwidth product. Simulation result indicates that increasing the radius of the region leads to a sub-quadratic growth in the derived signal dimensionality irrespective of the bandwidth or the observation time.

The impact of clouds on image parameters in IACT at very high energies

The effective observation time with the Cherenkov telescopes arrays is limited to clear sky conditions due to considerable absorption of Cherenkov light by the possible presence of clouds. However below the cloud altitude the primary particles with high energies can still produce enough Cherenkov photons to allow the detection by the large telescopes. In this paper, using the standard CORSIKA code, we investigate the changes of image parameters due to the absorption of Cherenkov radiation by the cloud (for γ-ray and proton showers with various energies – from 2 TeV to 100 TeV and from 10 TeV to 200 TeV, respectively). We consider the clouds with different transmissions located at various altitudes above the ground level (between 8 km and 3 km). We show that, for both simulated primary particles at fixed energy, the WIDTH and the DIST distributions are shifted towards larger values in the presence of clouds in comparison to the clear sky simulations. This shift decreases with the cloud altitude. The LENGTH distributions are shifted towards smaller values for images of primary γ-rays, while for primary protons this shift is not observed. We conclude that the large Cherenkov telescopes with large camera FOV could be used for observation of γ-ray showers with high energies in the presence of clouds.

Atmospheric monitoring in MAGIC and data corrections

A method for analyzing returns of a custom-made “micro”-LIDAR system, operated alongside the two MAGIC telescopes is presented. This method allows for calculating the transmission through the atmospheric boundary layer as well as thin cloud layers. This is achieved by applying exponential fits to regions of the back-scattering signal that are dominated by Rayleigh scattering. Making this real-time transmission information available for the MAGIC data stream allows to apply atmospheric corrections later on in the analysis. Such corrections allow for extending the effective observation time of MAGIC by including data taken under adverse atmospheric conditions. In the future they will help reducing the systematic uncertainties of energy and flux.

A new method of reconstructing VHEγ-ray spectra: the Template Background Spectrum

Very-high-energy (VHE, E>0.1 TeV) gamma-ray emission regions with angular extents comparable to the field-of-view of current imaging air-Cherenkov telescopes (IACT) require additional observations of source-free regions to estimate the background contribution to the energy spectrum. This reduces the effective observation time and deteriorates the sensitivity. A new method of reconstructing spectra from IACT data without the need of additional observations of source-free regions is developed. Its application is not restricted to any specific IACT or data format. On the basis of the template background method, which defines the background in air-shower parameter space, a new spectral reconstruction method from IACT data is developed and studied, the Template Background Spectrum (TBS); TBS is tested on published H.E.S.S. data and H.E.S.S. results. Good agreement is found between VHE gamma-ray spectra reported by the H.E.S.S. collaboration and those re-analysed with TBS. This includes analyses of point-like sources, sources in crowded regions, and of very extended sources down to sources with fluxes of a few percent of the Crab Nebula flux and excess-to-background ratios around 0.1. However, the TBS background normalisation introduces new statistical and systematic errors which are accounted for, but may constitute a limiting case for very faint extended sources. The TBS method enables the spectral reconstruction of data when other methods are hampered or even fail. It does not need dedicated observations of VHE gamma-ray-free regions (e.g. as the On/Off background does) and circumvents known geometrical limitations to which other methods (e.g. the reflected-region background) for reconstructing spectral information of VHE gamma-ray emission regions are prone to; TBS would be, in specific cases, the only feasible way to reconstruct energy spectra.

Sighting frequency and relative abundance of bottlenose dolphins (<i>Tursiops truncatus</i>) along the northeast coast of Margarita Island and Los Frailes Archipelago, Venezuela

The study of local cetaceans in Venezuela has a very recent history, and few efforts have been made in the assessment of coastal populations based on field research. The occurrence of whales and dolphins along the northeast coast of Venezuela has been documented through sightings and stranding records. Given the underwater topographical features and the influence of upwelling processes, this area is considered a very productive coastal ecosystem. Our objective was to establish the sighting frequency and relative abundance of bottlenose dolphins in the area. Sighting records were gathered on bottlenose dolphins and other cetacean species occurring along the northeast coast of Margarita Island and Los Frailes Archipelago through direct observation during land-based (6 surveys, 48 hours of observation) and boat-based surveys (24 surveys, 121 hours of observation, 1295 km covered). A sighting frequency was calculated using two methodologies and then compared, considering: 1) a mean effective observation time (4.27 hours), and 2) distance covered with cetacean sightings (1108 kin). A third method is proposed relating a mean effective distance covered with cetacean sightings and expressed as a percentage. The abundance index was calculated using the mean effective observation time. The sighting frequency of Tursiops truncattus in the study area was 3 - 4 sightings per day of 4.27 observation hours, or by 185 kilometers covered. The relative abundance was calculated as 35 dolphins in the study area, so a total population of less than 60 dolphins could inhabit the proposed range. Tursiops truncatus is the dominant species in the northeast coast of Margarita Island and Los Frailes Archipelago with 70% of all the sightings, so this locality could be termed as the distribution range of a possible local population of bottlenose dolphins.

Tunka-Rex: a Radio Extension of the Tunka Experiment

Tunka-Rex, the Tunka radio extension, is an array of about 20 antennas currently under construction, which covers an area of 1 km2. Tunka-Rex measures the radio emission of cosmic-ray air showers above 1016 eV. It is triggered by the photomultipliers of the Tunka-133 experiment which simultaneously measure the Cherenkov light emitted by the same air showers. The radio-Cherenkov-hybrid measurements thus offer a unique opportunity for a cross-calibration of both detection methods. The main goal of Tunka-Rex is to determine the precision of the radio reconstruction for the energy and the atmospheric depth of the shower maximum, Xmax, and thus to experimentally test theoretical predictions that the radio precision can be similar to the precision of air-Cherenkov and fluorescence measurements. At the same time, Tunka-Rex can demonstrate that radio measurements can be performed on a large area for a relatively cheap price, since the antennas will be connected to the already existing Tunka DAQ. Finally, radio-antenna arrays have the perspective to increase the effective observation time compared to air-Cherenkov and fluorescence detectors by an order of magnitude, since radio measurements are possible under almost any atmospheric and light conditions.

In-Orbit Performance of the MAXI/SSC onboard the ISS

We report here the in-orbit performance of the CCD camera (MAXI/SSC) onboard the International Space Station (ISS). It was commissioned in 2009 August. This is the first all-sky survey mission employing X-ray CCDs. It consists of 32 CCDs, each of which is 1 inch square. It is a slit camera with a field of view of 1$^\circ\!\!\!.$5 $\times$ 90$^\circ$, and scans the sky as the rotation of the ISS. The CCD on the SSC is cooled down to the working temperature around $-$60$^\circ$C by the combination of a peltier cooler, a loop heat pipe and a radiator. The standard observation mode of the CCD is in a parallel sum mode (64-binning). The CCD functions properly, although it suffers an edge glow when the Sun is near the field of view (FOV), which reduces the observation efficiency of the SSC down to about 30%. The performance of the CCD is continuously monitored both by the Mn-K X-rays and by the Cu-K X-rays. There are many sources detected, not only point sources, but extended sources. Due to the lack of an effective observation time, we need more observation time to obtain an extended emission analysis extraction process.

NMR q-space imaging of macroscopic pores using singlet spin states

NMR q-space imaging is a powerful non-invasive technique used to determine structural characteristics of pores in applications ranging from medical to material science. To date, the application of q-space imaging has primarily been limited to microscopic pores in part because of limitations of the effective observation time due to relaxation. Here we report on the use of singlet spin states for NMR q-space imaging, which allow significantly greater observation times. This opens the way for studying larger pores in materials such as biological tissue, emulsions, and rocks.

Scanning sonar of rolling porpoises during prey capture dives

Dolphins and porpoises have excellent biosonar ability, which they use for navigation, ranging and foraging. However, the role of biosonar in free-ranging small cetaceans has not been fully investigated. The biosonar behaviour and body movements of 15 free-ranging finless porpoises (Neophocaena phocaenoides) were observed using electronic tags attached to the animals. The porpoises often rotated their bodies more than 60 deg., on average, around the body axis in a dive bout. This behaviour occupied 31% of the dive duration during 186 h of effective observation time. Rolling dives were associated with extensive searching effort, and 23% of the rolling dive time was phonated, almost twice the phonation ratio of upright dives. Porpoises used short inter-click interval sonar 4.3 times more frequently during rolling dives than during upright dives. Sudden speed drops, which indicated that an individual turned around, occurred 4.5 times more frequently during rolling dives than during upright dives. Together, these data suggest that the porpoises searched extensively for targets and rolled their bodies to enlarge the search area by changing the narrow beam axis of the biosonar. Once a possible target was detected, porpoises frequently produced short-range sonar sounds. Continuous searching for prey and frequent capture trials appeared to occur during rolling dives of finless porpoises. In contrast, head movements ranging +/-2 cm, which can also change the beam axis, were regularly observed during both dives. Head movements might assist in instant assessment of the arbitrary direction by changing the beam axis rather than prey searching and pursuit.

Comparison of stationary acoustic monitoring and visual observation of finless porpoises

The detection performance regarding stationary acoustic monitoring of Yangtze finless porpoises Neophocaena phocaenoides asiaeorientalis was compared to visual observations. Three stereo acoustic data loggers (A-tag) were placed at different locations near the confluence of Poyang Lake and the Yangtze River, China. The presence and number of porpoises were determined acoustically and visually during each 1-min time bin. On average, porpoises were acoustically detected 81.7+/-9.7% of the entire effective observation time, while the presence of animals was confirmed visually 12.7+/-11.0% of the entire time. Acoustic monitoring indicated areas of high and low porpoise densities that were consistent with visual observations. The direction of porpoise movement was monitored using stereo beams, which agreed with visual observations at all monitoring locations. Acoustic and visual methods could determine group sizes up to five and ten individuals, respectively. While the acoustic monitoring method had the advantage of high detection probability, it tended to underestimate group size due to the limited resolution of sound source bearing angles. The stationary acoustic monitoring method proved to be a practical and useful alternative to visual observations, especially in areas of low porpoise density for long-term monitoring.

Effects of orbital drift on land surface temperature measured by AVHRR thermal sensors

The NOAA series of meteorological satellites that carry the Advanced Very High Resolution Radiometer (AVHRR) suffer from orbital drift so that during each satellite's duty period the overpass time occurs later in the day. Replacement satellites restore the overpass time temporarily, but then it gradually decays. The goals of this paper are to document the effects of variable observation time owing to orbital drift on brightness temperatures (BT) and land surface temperature (LST) calculated from them in the NOAA/NASA Pathfinder AVHRR Land (PAL) data set and to consider possible corrections for the resulting trends and discontinuities in the PAL BT data. The drift effects were found to be greater for bare ground than for vegetated land cover classes, however, significant effects were found for most vegetated classes. The magnitude of the orbital drift effect for most global cover types was at least as large as the other errors that affect LST measurement. A simple empirical correction for observation time based on solar zenith angle (SZA) was used to correct the PAL BT time series following Gutman [Int. J. Remote Sens. 20 (1999a) 3407]. The correction from this method was compared with that predicted by a physically based model and was found to differ in the early part of each satellite's duty cycle. Finally, the impacts of correction on the effective observation time are analyzed and the simple statistical correction was found to suffer from greater variability than has hitherto been recognized. A modification to the statistical correction to adjust the effective observation time is described.

Multiband Optical Polarimetry of the BL Lacertae Object PKS 2155−304: Intranight and Long‐Term Variability

The polarized and total flux of the BL Lac object PKS 2155-304 were monitored intensively and simultaneously in the optical UBVRI bands with the Turin photopolarimeter at the CASLEO 2.15 m telescope during four campaigns in 1998 June, August, and November and 1999 August. The effective observation time amounted to ~47 hr. PKS 2155-304 showed a linear polarization percentage (P) usually ranging between 3% and 7% and a polarization position angle (P.A.) mainly between 70° and 120°. The highest temporal resolution of our observations, 15 minutes, is unprecedented for polarimetric monitoring of this source and has allowed us to detect amplitude variations of the linear polarization percentage from 6% to 7.5% in timescales of hours. In some nights the polarization percentage seems to increase toward shorter wavelengths; however, the polarized spectrum does not vary significantly with time. The most remarkable variability event occurred on 1998 June 18, when the degree of linear polarization decreased by more than a factor 2 in 1 day in all bands, while the P.A. rotated by 90°. This is consistent with the presence of two emission components, of different polarization degree and position angle. Intranight variability of P and P.A. can be interpreted with small-amplitude physical or geometrical changes within the jet. Measurements of the circular polarization over time intervals of days set upper limits of 0.2%. Simultaneous photometry taken with the Turin Photopolarimeter and with a CCD camera at Cordoba Astronomical Observatory did not show light variations correlated with those of the linearly polarized flux.