Time of Arrival

We propose an asynchronous, low cost, and accurate mobility estimation scheme for wireless mobile networks. This scheme considers the round-trip time (RTT) of the signal from the mobile station to the base station as observation and estimates position and speed of the mobile user in two dimensions. Our scheme uses an earlier proposed autoregressive mobility model,

This paper describes the characterisation of a set of 3 He proportional counters differing in gas composition and pressure. Each step of the signal generation in the detector is analysed. Optimal working conditions are defined in order to choose the high voltage to be applied. Dead time with short shaping time is measured in such optimal conditions. In addition, the overall neutron efficiency and the gamma sensitivity are reported. Such characterisation is required for a correct choice of the associated counting electronics and the applications of the counters.

Two plastic scintillator arrays with fast read-out electronics have been constructed for proton detection in an electron scattering environment. The detectors, one consisting of 47 and the other of 53 scintillators, each subtend a 39 msr solid angle and cover dynamic ranges of 25-158 and 37-198 MeV, respectively. The segmentation of the detector allows detection at forward scattering angles (> 50°) at a luminosity of 8 X 10 36 nucleons s-1 CM-Z (at about 1 MHz per element). The charge and time of arrival of each photomultiplier signal are digitized by two Flash ADCs. The read-out of background events is suppressed efficiently by a proper coincidence requirement in the first-level trigger. An unambiguous identification of protons and pions is obtained. The energy resolution amounts to 3.3 MeV for 87 MeV protons, while the intrinsic timing resolution is estimated at 0.6 ns .

Observations of radio emissions from thunderstorms were made during the summer of 1996 using two arrays of sensors located in northern New Mexico. The first array consisted of three fast electric field change meters separated by distances of 30 to 230 km. The second array consisted of three broadband (3 to 30 MHz) HF data acquisition systems separated by distances of 6 to 13 km. Differences in signal times of arrival at multiple stations were used to locate the sources of received signals. Relative times of arrival of signal reflections from the ionosphere and Earth were used to determine source heights. A distinct class of short-duration electric field change emissions was identified and characterized. The emissions have previously been termed narrow positive bipolar pulses (NPBPs). NPBPs were emitted from singular intracloud discharges that occurred in the most active regions of three thunderstorms located in New Mexico and west Texas. The discharges occurred at altitudes between 8 and 11 km above mean sea level. NEXRAD radar images show that the NPBP sources were located in close proximity to high reflectivity storm cores where reflectivity values were in excess of 40 dBZ. NPBP electric field change waveforms were isolated, bipolar, initially positive pulses with peak amplitudes comparable to those of return stroke field change waveforms. The mean FWHM (full width at half maximum) of initial NPBP field change pulses was 4.7 gs. The HF emissions associated with NPBPs were broadband noise-like radiation bursts with a mean duration of 2.8 gs and amplitudes 10 times larger than emissions from typical intracloud and cloud-to-ground lightning processes. Calculations indicate that the events represent a distinct class of singular, isolated lightning discharges that have limited spatial extents of 300 to 1000 m and occur in high electric field regions. The unique radio emissions produced by these discharges, in combination with their unprecedented physical characteristics, clearly distinguish the events from other types of previously observed thunderstorm electrical processes. that different lightning processes produce characteristically different emissions. The emissions may differ in their amplitudes, spectra, temporal evolution, or other respects. Three previously reported classes of events that have been characterized by their distinct radio emissions are narrow positive bipolar pulses [Le Vine, 1980; Willett et al., 1989; Medelius et al., 1991], transionospheric pulse pairs [Holden et al., 1995; Massey and Holden, 1995; Massey et al., 1998], and subionospheric pulse pairs [Smith, 1995; Smith and Holden, 1996]. A comparison presented in this paper provides evidence that all three types of emissions are produced by the same sources within thunderstorms. First evidence of this association was presented

The loose part monitoring system (LPMS) in the primary circuit of the WWER-440 / WWER-1000 as in any other PWR or BWR type of NPP is one of the fundamental and essential to safety diagnostic tools. The purpose of the system is to detect, localize and analyze detached or loose metal pieces. When the metal impact occurs it causes the mechanical vibration of the structures and sensors detect a burst signal. The aim of this work is to show the potential of using modem signal processing methods in enhancing the LPMS performance by solving the tasks of noise cancellation, time of arrival detection, discrimination between real and faulty alarms, and loose metal piece mass estimation. KEYWORDS artificial neural networks; computational intelligence; loose part monitoring systrm; singular value decomposition; wavelet transform

Various methods are used to locate cloud-to-ground lightning flash. The accuracy of the methods depends on the number of sensor stations, station topology, mathematical model adopted for the analyses, and type of measuring equipment. For a single station with the configuration of short baseline in a measuring system, the accuracy analysis is mostly based on the chosen mathematical equations (and all relative assumptions) which can be solved in either linear or nonlinear mode. This paper concentrates on the simulation of a lightning flash based on Time of Arrival (TOA) method by utilizing three broadband antennas. The accuracy of the TOA method is also evaluated in this work. A Matlab based simulation of the measuring system is implemented to construct a randomly located lightning flash with its corresponding electromagnetic radiation. Then, suitable mathematical models used as measurement systems based on the TOA method to determine the azimuth and elevation angles.

Various methods are used to locate cloud-toground lightning flash. The accuracy of the methods depends on the number of sensor stations, station topology, mathematical model adopted for the analyses, and the type of measuring equipment. For a single station with the configuration of short baseline in a measuring system, the accuracy of analysis is mostly based on the chosen mathematical equations (and assumptions) which can be solved in either linear or nonlinear mode. This paper concentrates on the simulation of a lightning flash and its detection based on the Time of Arrival (TOA) method by utilizing four broadband antennas. The accuracy of the TOA method is also evaluated in this work. A Matlab based simulation of the measuring system is implemented to construct a randomly located lightning flash with its corresponding electromagnetic radiation. Then, suitable mathematical models used as measurement systems based on the 3-antenna TOA method to determine the azimuth and elevation angles, which in turn can be used for the calculation of flash coordinates in term of (X,Y,Z) with the help of the fourth antenna.

The time-of-arrival (TOA) estimation ofpulse signals is widely used in radar, sonar and other sensor systems for geo-locating targets. This paper presents a new estimator for the TOA and width (W) of a pulse. Three auto-convolutions of a full and partial pulse determine the location ofthree convolution peaks. These peak locations are given by linear equations that contain as unknowns TOA and W. Solving these equations yields estimates of the pulse parameters. Simulation results show the performance of the estimator in noise and are compared with results obtained using an existing estimator.

This paper presents a comparative analysis of three nonlinear filters for estimation of the location and velocity of a moving emitter, using time difference of arrival (TDOA) measurements received by two unmanned aerial vehicles (UAVs) as they traverse the surveillance region. The TDOA measurements are generated over time by comparing and subtracting leading edge time of arrivals (TOAs) of signals. The Cramér Rao lower bound (CRLB) of estimation errors is derived and used as the benchmark in performance analysis. The three nonlinear filters considered in the comparison are: a Gaussian mixture measurement integrated track splitting filter (GMM-ITSF), a multiple model filter with unscented Kalman filters (UKFs) and a multiple-model filter with extended Kalman filters (EKFs).

The paper addresses the general problem of estimating the position of an underwater target carrying an acoustic emitter by measuring the times of arrival (TOAs) of the acoustic signals at a set of surface buoys equipped with submerged hydrophones and GPS receivers. Examples of underwater targets of interest include AUVs (Autonomous Underwater Vehicles) and ROVs (Remotely Operated Vehicles) as well as manned submersibles, divers, and even marine animals. When compared with classical systems, the class of underwater acoustic positioning systems considered in this paper is far more versatile and portable and the costs of operation are greatly reduced. This justifies the increasing interest that such systems have received over the past few years, both from a theoretical and practical standpoint. Research and development in this area have progressed to the point where a commercial product made its appearance in the market: the so-called GIB (GPS Intelligent Buoys). However, much work remains to be done towards the development of operational systems capable of yielding adequate performance in the presence of multi-path effects and acoustic outliers. The paper gives a brief overview of this area of research and discusses theoretical and practical issues that arise in the development and operation of acoustic positioning systems at sea.

This paper describes a novel approach of a robust and efficient 3D location estimation technique for noncooperative, active or passive RF emitters in a noisy environment with two or more arbitrary shaped arrays. Each array can provide precise frequency, direction of arrival (DOA) estimation and time of -arrival (TOA) estimation for the far field RF emitters. For an M-element array, the DOA estimator can adaptively estimate and track up to 2/3M independent emitters. We then apply the TDOA estimation and adaptive tracking algorithms to adaptive position finding and tracking involving one or two plane waves impinging on two synchronized uniform circular arrays, in the presence of uncorrelated receiver noise. Matlab simulations for mobile RF emitters location estimation are presented and discussed. The algorithms have been implemented to utilize an 8element uniform circular array that operates at 5.8GHz. Two identical arrays have been used for locating and tracking two fixed or/and mobile RF emitters.. Test results show that the proposed system can achieve accuracy within 6 meters and resolution about 25 meters with the 8 -element circular array at a distance of 10 km. 1,2

The high time resolution of ultra-wideband (UWB) signals facilitates very precise position estimation in many scenarios, which makes a variety applications possible. This paper reviews the problem of position estimation in UWB systems, beginning with an overview of the basic structure of UWB signals and their positioning applications. This overview is followed by a discussion of various position estimation techniques, with an emphasis on time-based approaches, which are particularly suitable for UWB positioning systems. Practical issues arising in UWB signal design and hardware implementation are also discussed.

Acoustic emission (AE) source location provides a powerful tool for the engineer. Current methods, time of arrival (TOA) location and single sensor modal analysis location (SSMAL), are based on assumptions derived from a simplistic structure. In complex geometric structures these assumptions are incorrect. This paper describes a novel solution for AE source location in complicated geometric structures. ''Delta T'' source location utilises an artificial source; recording differences in times of arrival information from a number of locations, to improve source location. This method does not require knowledge of the sensor location or wavespeed. A 5-step description of the process is provided and practical results from an initial trial are presented. Results from the initial trial demonstrated a considerable improvement over the conventional TOA source location method, with a reduction in location error from 4.81% using the conventional TOA technique, to 1.77% using ''Delta T'' location. r

A b s t ra c t The berth scheduling problem deals with the assignment of vessels to berths in a marine terminal, with the objective to maximize the ocean carriers' satisfaction (minimize delays) and/or minimize the terminal operator's costs. In the existing literature, two main assumptions are made regarding the status of a vessel: (a) either all vessels to be served are already in the port before the planning period starts, or (b) they are scheduled to arrive after the planning period starts. The latter case assumes an expected time of arrival for each vessel, which is a function of the departure time of the vessel from the previous port, the average operating speed and the distance between the two ports. Recent increases in fuel prices have forced ocean carriers to reduce current operating speeds, while stressing to terminal operators the need to maintain the integrity of their schedule. In addition, several collaborative efforts between industry and government agencies have been proposed, aiming to reduce emissions from marine vessels and port operations. In light of these issues, this article presents a berth-scheduling policy to minimize vessel delayed departures and indirectly reduce fuel consumption and emissions produced by the vessels while in idle mode. Vessel arrival times are considered as a variable and are optimized to accommodate the objectives of the proposed policy while providing ocean carriers with an optimized vessel speed. Example problems using real data show that the proposed policy reduces the amount of emissions produced by vessels at the port in idle mode, optimizes fuel consumption and waiting time at the port by reducing vessel operating speeds to optimal levels and minimizes the effects of late arrivals to the ocean carriers' schedule.

Temporal variation in the diet and chick growth of rhinoceros auklets (Cerorhinca monocerata), on Teuri Island, Sea of Japan, was studied to understand how local marine environmental changes aect the reproduction of this piscivorous seabird. The food delivered by parents to chicks was sampled every 1±2 weeks from late May to July, 1994±1998. Overall, the diet of nestling rhinoceros auklets consisted of (by mass) 61% Japanese anchovy (Engraulis japonicus), 18% Japanese sand lance (Ammodytes personatus), 18% Japan Sea greenling (Pleurogrammus azonus), 2% other ®sh and 1% squid. Among years, the contribution of anchovy ranged from 16% to 93%. Once anchovy occurred in the diet, it dominated (80% on average) thereafter. Accordingly, when anchovy appeared in the diet early in the chick-rearing season (1994, 1998), the contribution of anchovy overall was large. The ®rst appearance of anchovy in the diet of auklets late in the summer of 1997 was possibly related to negatively anomalous sea-surface temperature. Food loads composed of anchovy (34.0 g) were heavier than those of sand lance (22.5 g) and greenling (28.5 g). The energy density of anchovies also was higher: 6.3 kJ g ±1 wet mass compared to 0+ greenling (4.78 kJ g ±1 ) and 0+ sand lance (3.78 kJ g ±1 ). Thus, a high proportion of anchovy in the diet resulted in high food load mass, high daily growth rates of chicks and high¯edging success. This study highlighted the importance of the time of arrival of migratory high-lipid prey, which is in¯uenced by local oceanographic conditions, to the reproductive performance of a piscivorous seabird.

This paper describes a monolithic integrated transmitter intended for impulse radio (IR) Ultra-wide band (UWB) applications including indoor communication and indoor localization. The transmitter operates in the higher UWB band centered at 7.68 GHz and it is optimized for a pulse bandwidth of about 1.5 GHz. The transmitter generates single pulses with a repetition rate of 60 MHz and utilizes pulse position modulation (8-PPM) for data communication. A dedicated time-of-arrival (TOA) measurement extension supports precise indoor localization in conjunction with an appropriate UWB receiver. The demonstrated spatial ranging resolution is about 3.9 centimeter under line-of-sight conditions.

The fractional Fourier transform (FRFT) is a potent tool to analyze the chirp signal. However, it fails in locating the fractional Fourier domain (FRFD)-frequency contents which is required in some applications. The short-time fractional Fourier transform (STFRFT) is proposed to solve this problem. It displays the time and FRFD-frequency information jointly in the short-time fractional Fourier domain (STFRFD). Two aspects of its performance are considered: the 2-D resolution and the STFRFD support. The time-FRFD-bandwidth product (TFBP) is defined to measure the resolvable area and the STFRFD support. The optimal STFRFT is obtained with the criteria that maximize the 2-D resolution and minimize the STFRFD support. Its inverse transform, properties and computational complexity are presented. Two applications are discussed: the estimations of the time-of-arrival (TOA) and pulsewidth (PW) of chirp signals, and the STFRFD filtering. Simulations verify the validity of the proposed algorithms.

1] The arrival times at Earth of 166 flare-related shocks identified exiting the Sun (using metric radio drift data) during the maximum phase of Solar Cycle 23, were forecast in near-real time using the Shock Time of Arrival Model (STOA), the Interplanetary Shock Propagation Model (ISPM) and the Hakamada-Akasofu-Fry Model (version 2, HAFv.2). These predictions are compared with the arrival at L1 of shocks recorded in plasma and magnetic data aboard the ACE spacecraft. The resulting correspondences are graded following standard statistical methods. Among other parameters, a representative reference metric defined by {(''hits'' + ''correct nulls'') Â 100}/(total number of predictions) is used to describe the success rates of the predictions relative to the measurements. Resulting values for STOA, ISPM, and HAFv.2 were 50%, 57%, and 51%, respectively, for a hit window of ±24 hours. On increasing the statistical sample by 173 events recorded during the rise phase of the same cycle, corresponding success rates of 54%, 60%, and 52%, respectively, were obtained. A 2 test shows these results to be statistically significant at better than the 0.05 level. The effect of decreasing/increasing the size of the hit window is explored and the practical utility of shock predictions considered. Circumstances under which the models perform well/poorly are investigated through the formation, and statistical analysis, of various event subsets, within which the constituent shocks display common characteristics. The results thereby obtained are discussed in detail in the context of the limitations that affect some aspects of the data utilized in the models.

The near-continuous record of the New Zealand terrestrial flora since the landmass broke away from Gondwana provides an excellent opportunity to assess the history of the pteridophyte flora in the region. A database of all published records of fossil ferns from the late Cretaceous onwards is presented, along with references to original reports. This compilation provides evidence for the time of arrival for fern lineages, continuity of their presence, and times of radiation, thereby greatly assisting the reconstruction of the history and biogeography of ferns in the region. Additionally, it provides a valuable source of lineage age estimates, which are required to calibrate the molecular clock for much needed molecular studies on pteridophytes.

The signal emitted by an airborne acoustic source arrives at a stationary sensor located above a flat ground via a direct path and a ground-reflected path. The difference in the times of arrival of the direct path and ground-reflected path signal components, referred to as the multipath delay, provides an instantaneous estimate of the elevation angle of the source. A model is developed to predict the variation with time of the multipath delay for a jet aircraft or other broadband acoustic source in level flight with constant velocity over a hard ground. Based on this model, two methods are formulated to estimate the speed and altitude of the aircraft. Both methods require the estimation of the multipath delay as a function of time. The methods differ only in the way the multipath delay is estimated; the first method uses the autocorrelation function, and the second uses the cepstrum, of the sensor output over a short time interval. The performances of both methods are evaluated and compared using real acoustic data. The second method provides the most precise aircraft speed and altitude estimates as compared with the first and two other existing methods.

An imager for time-resolved optical sensing was fabricated in CMOS technology. The sensor comprises an array of 128 2 128 single-photon pixels, a bank of 32 time-to-digital-converters, and a 7.68 Gbps readout system. Thanks to the outstanding timing precision of single-photon avalanche diodes and the optimized measurement circuitry, a typical resolution of 97 ps was achieved within a range of 100 ns. To the best of our knowledge, this imager is the first fully integrated system for photon time-of-arrival evaluation. Applications include 3-D imaging, optical rangefinding, fast fluorescence lifetime imaging, imaging of extremely fast phenomena, and, more generally, imaging based on time-correlated single photon counting. When operated as an optical rangefinder, this design has enabled us to reconstruct 3-D scenes with milimetric precisions in extremely low signal exposure. A laser source was used to illuminate the scene up to 3.75 m with an average power of 1 mW, a field-of-view of 5 and under 150 lux of constant background light. Accurate distance measurements were repeatedly achieved based on a short integration time of 50 ms even when signal photon count rates as low as a few hundred photons per second were available. Index Terms-Depth sensor, FCS, flash laser camera, fluorescence lifetime imaging microscopy (FLIM), Förster Resonance Energy Transfer (FRET), Geiger-mode avalanche photodiode, rangefinder, range imaging, single-photon avalanche diode, single-photon detector, solid-state 3-D imaging, SPAD, TCSPC, time-correlated single-photon counting, time-of-flight, 3-D image sensor.

Scheduling aircraft landings has been a complex and challenging problem in air traffic control for long time. In this paper, we propose to solve the aircraft landing scheduling problem (ALSP) using multi-objective evolutionary algorithms (MOEAs). Specifically, we consider simultaneously minimizing the total scheduled time of arrival and the total cost, and formulate the ALSP as a 2-objective optimization problem. A MOEA named Multi-Objective Neighborhood Search Differential Evolution (MONSDE) is applied to solve the 2-objective ALSP. Besides, a ranking scheme named non-dominated average ranking is also proposed to determine the optimal landing sequence. Advantages of our approaches are demonstrated on two example scenarios.

Cardiac surgery (PCS) has a low error tolerance, is dependent upon sophisticated organisational structures and demands high levels of cognitive and technical performance. The aim of the study was to assess the role of intraoperative non-routine events (NREs) and team performance on paediatric cardiac surgery outcomes. The current paper focuses on improving methods for studying teamwork; a companion paper will report on the empirical results. The authors trained human factors observers to observe and code the NRE's and teamwork from time of arrival of the patient into the operating room (OR) to the patient handover in the intensive care unit. The observers underwent immersive training in which each observer attended 10 operations, learnt in detail about the technical procedures and clinical tasks and received practice in coding teamwork. Two observers were used interchangeably to observe OR teamwork. The authors instigated a rigorous training and assessment protocol, with indepen...

This paper investigates the impact of realistic antennas on joint anchor-less localization and indoor characterization based on Impulse Radio (IR) Ultra Wideband (UWB) communications. In this frame, the Maximum Averaged Likelihood (MAL) algorithm and its extended version are considered, both relying on a tree approach consisting in two stages. The first part of the process, which is common to both algorithms, exploits the cross-correlation between received and locally predicted paths. The second stage calculates the ...

Positioning algorithms and their implementation in mobile networks are being investigated in the literature due to their importance in location services. Nowadays, the need for superior accuracy has cast attention to hybrid positioning techniques. In this paper, we introduce a novel algorithm for the identification of NLOS propagation using both angle and time estimates, which leads to enhanced versions of the Time of Arrivals and Angle of Arrivals positioning methods. Furthermore, a novel GSM procedure for the implementation of the latter techniques is proposed. In contrast to specified network-based GSM solutions (U-TDOA), the proposed requires minimum modifications in the GSM Phase 2+ infrastructure and protocol stack, and therefore increases the upgrade flexibility and minimizes the implementation cost. The proposed GSM positioning procedure has been experimentally validated using a GSM emulator and the modified signalling messages given by a measurement tool of the emulator are exhibited. Finally, the enhanced cost functions are experimentally evaluated using several GSM-like, high-capacity simulation environments and the results have shown significant reduction of the location error compared to the conventional techniques.

The signal emitted by an airborne acoustic source arrives at a stationary sensor located above a flat ground via a direct path and a ground-reflected path. The difference in the times of arrival of the direct path and ground-reflected path signal components, referred to as the multipath delay, provides an instantaneous estimate of the elevation angle of the source. A model is developed to predict the variation with time of the multipath delay for a jet aircraft or other broadband acoustic source in level flight with constant velocity over a hard ground. Based on this model, two methods are formulated to estimate the speed and altitude of the aircraft. Both methods require the estimation of the multipath delay as a function of time. The methods differ only in the way the multipath delay is estimated; the first method uses the autocorrelation function, and the second uses the cepstrum, of the sensor output over a short time interval. The performances of both methods are evaluated and compared using real acoustic data. The second method provides the most precise aircraft speed and altitude estimates as compared with the first and two other existing methods.

This paper presents a ranging receiver architecture able to timestamp IEEE 802.11b Wireless LAN signals with sub-100 picosecond precision enabling time-based range measurements. Starting from the signal model, the performance of the proposed architecture is assessed in terms of statistical bounds when perturbed by zero-mean additive white Gaussian noise (AWGN) as well as in case of multipath propagation. Results of the proposed architecture, implemented in a Field Programmable Gate Array-(FPGA-) based prototype, are presented for different environments. For AWGN channels, the prototype system is able to attain an accuracy of 1.2 cm while the ranging accuracy degrades in dynamic multipath scenarios to about 0.6 m for 80% of the measurements due to the limited bandwidth of the signal.

Means for the continuous monitoring of seismic phase velocity to precision finer than one part in 104 have been used to detect in situ periodic velocity variations apparently related to solid earth tides. The magnitude of the velocity change implies that the elastic moduli of crustal rock are much more stress sensitive in situ at shallow depths than in laboratory tests on small rock samples. [1969]. Sensitivities of seismic velocities to hydrostatic pressure of the order of dc/cdp ~ 10 -3 bar -•, where c is velocity and p is pressure, are common in granites at

The paper contains a proposal for an energy and time representation. We construct modes that correspond to fuzzy distributions around discrete values of energy or time. The modes form an orthogonal and complete set in the space of square integrable functions. Energy and time are self adjoint in the space spanned by the modes. The widths of the modes are

Methods of heat detection were compared in the Mid- Crest Area Cattle Evaluation Program (MACEP) heifer development program in the 1998-breeding season. A total of 189 heifers from thirteen consignors entered the program on November 10, 1997. These heifers were condition scored, hip height measured, weighed, disposition scored, booster vaccinated, and treated for parasites at the time of arrival. Determination

Accuracy of ordinary sensor node localization in wireless sensor networks mainly depends on the signal parameter such as time-of-arrival and signal strength estimation errors and the accuracy of the anchor node locations. In this paper a lowcomplexity but efficient algorithm is derived to improve anchor location accuracy in the presence of both anchor-to-anchor distance and AOA estimates and GPS measurements. Also, a Lenvenberg-Marquardt (LM) optimization based algorithm is developed for accuracy improvement when anchor-to-anchor distance estimates and GPS measurements are provided. Further, we derive the Cramer-Rao lower bound (CRLB) to benchmark the anchor position accuracy. To our knowledge, improving anchor node location accuracy and deriving the CRLB in the presence of both GPS and anchor-to-anchor measurements in 3-D scenarios are not reported in the literature. Simulation results demonstrate that the proposed approaches can improve the anchor position accuracy substantially and that the accuracy of the two developed algorithms approaches the corresponding CRLB.

We study the construction of probability densities for time-of-arrival in quantum mechanics. Our treatment is based upon the facts that (i) time appears in quantum theory as an external parameter to the system, and (ii) propositions about the time-of-arrival appear naturally when one considers histories. The definition of time-of-arrival probabilities is straightforward in stochastic processes. The difficulties that arise in quantum theory are due to the fact that the time parameter of Schrödinger's equation does not naturally define a probability density at the continuum limit, but also because the procedure one follows is sensitive on the interpretation of the reduction procedure. We consider the issue in Copenhagen quantum mechanics and in history-based schemes like consistent histories. The benefit of the latter is that it allows a proper passage to the continuous limit-there are however problems related to the quantum Zeno effect and decoherence. We finally employ the histories-based description to construct Positive-Operator-Valued-Measures (POVMs) for the timeof-arrival, which are valid for a general Hamiltonian. These POVMs typically depend on the resolution of the measurement device; for a free particle, however, this dependence cancels in the physically relevant regime and the POVM coincides with that of Kijowski.