504 results about "Satellite observation" patented technology

A computer-implemented method for crop type identification using satellite observation and weather data. The method includes extracting current and historical data from pixels of satellite images of a target region, generating temporal sequences of vegetation indices, based on the weather data, converting each timestamp of the temporal sequences into a modified temporal variable correlating with actual crop growth, training a classifier using a set of historical temporal sequences of vegetation indices with respect to the modified temporal variable as training features and corresponding historically known crop types as training labels, identifying a crop type for each pixel location within the satellite images using the trained classifier and the historical temporal sequences of vegetation indices with respect to the modified temporal variable for a current crop season, and estimating a crop acreage value by aggregating identified pixels associated with the crop type.

The invention discloses a BDS / GPS high-accuracy positioning method, relating to the field of satellite navigation. The method includes establishing a virtual reference station, acquiring the common-view satellite ephemeris and the satellite observation data received by a reference station Bi and a roving station M, and acquiring the geometry distance between the satellite and the reference station Bi; acquiring the pseudo range correction number of the reference station Bi; interpolating the pseudo range correction number of the virtual reference station by means of an algorithm of inverse distance to a power; interpolating the pseudo range correction number of the roving station by the roving station by means of an algorithm of inverse distance to a power; correcting the satellite pseudo range observation data received by the roving station by means of the pseudo range correction number of the roving station; and establishing a roving station satellite pseudo range observation equation to obtain the accurate coordinate of the roving station to complete the positioning. According to the invention, the safety hidden troubles of information leakage of the reference station because a multi-reference difference positioning method in the prior art must use the accurate coordinate of the reference station can prevented, and high-accuracy positioning is obtained.

The invention discloses a target clustering method of imaging satellite observation scheduling. In the method, an optimal target clustering scheme is obtained by obtaining target clustering conditions and qualities of satellite observation, a maximum gain value of the target clustering scheme of a satellite in the orbit circle and an optimal target clustering scheme and finishing a target clustering step, and a set of targets covered by various observation activities of the satellite is further obtained. The target clustering method of the imaging satellite observation scheduling can satisfy using rules and performance characteristics of various operations of the satellite with small computation cost, ensure that the satellite can finish more observation tasks within the given time and improve the using efficiency and the benefits of satellite resources, and has small imaging distortion.

The invention discloses a multi-satellite cooperative observation business scheduling method and relates to satellite observation business planning and scheduling technology in a satellite operation control field. The multi-satellite cooperative observation business scheduling method is based on a satellite operation control system, builds a star land resource management model, a satellite orbit calculation model and a satellite access information calculation model, adopts a satellite observation business dynamic planning algorithm to regulate satellite observation business rapidly and dynamically, adopts an automatic processing mechanism to make a satellite load control plan and a data receiving plan immediately, adopts a satellite actual load control instruction compilation business trigger mechanism to compile satellite actual load control instructions in real time, and schedules an electronic signal detection satellite and an imaging observation satellite to complete cooperative observation business. The multi-satellite cooperative observation business scheduling method has the advantages of being rapid in response speed, accurate in calculation method, high in automation and the like and is particularly suitable for a marine moving target multi-satellite cooperative observation field.

The invention discloses a double-frequency GPS receiver which is arranged in a user application system of a CORS system. The CORS system comprises a data processing center and a plurality of reference stations. The double-frequency GPS receiver comprises a GPS module, a central processing module, a communication module and an output module, wherein the GPS module receives GPS satellite observation data sent by a GPS satellite to generate receiver positioning data, and transmits the receiver positioning data to the central processing module; the central processing module sends the receiver positioning data to the data processing center through the communication module; the data processing center generates difference information and feeds back the difference information to the communication module; the communication module sends the difference information to the GPS module through the central processing module; the GPS module calculates real-time positioning information according to the difference information and the GPS satellite observation data and sends the real-time positioning information to the central processing module; and the central processing module outputs the real-time positioning information to a control holding book through the output module. The data of the data processing center of the CORS system and the receiver can be transmitted more reliably and more quickly.

The invention discloses a task planning common platform based on a satellite-ground resource model. Through the configuration of the satellite-ground resource model, the platform is suitable for the type, the characteristics, the working mode and the use constraints of each satellite; for a specific business rule and a satellite use constraint, a special plugin is expanded through the personalized customization of a contract plugin interface; and through the integration of a universal plugin and the special plugin as well as the custom expansion of a personalized application, various types of task planning application software which meet each application type and each satellite use requirement are created, functions that multi-satellite joint planning is carried out, each satellite observation task is subjected to optimal distribution and observation mode optimization, each receiving resource task is subjected to optimal distribution and the like are realized, intersatellite task conflicts, satellite sensor task conflicts and data receiving conflicts are eliminated, a task planning scheme is generated, and visual deduction display is carried out. By use of the platform, a "platform+plugin" technology implementation way is adopted to realize task planning generality basic functions and frame functions, and exhibits high universality, adaptation and expandability.

The invention provides an MODIS satellite high-precision monitoring method for chlorophyll-a in a eutrophic lake water body. The method comprises the following steps: screening a chlorophyll-a evaluation index (BNDBI) which is sensitive to the concentration change of chlorophyll-a and not affected by highly suspended matters, thin cloud and solar flares; acquiring the quantitative relationship between BNDBI and the concentration of chlorophyll-a on the basis of biological optical model simulation; acquiring a chlorophyll-a inversion algorithm based on ground measured spectrum (Rrs) and BNDBI by referring to water body spectral information and corresponding water body chlorophyll-a concentration of 2013-2014 lake field monitoring; acquiring the quantitative relationship between the ground monitoring and remote sensing reflectance ratio (Rrs) and Rrc after simulated Rayleigh scattering correction by simulating different aerosol types and thicknesses and different solar elevation angles, satellite observation angles and azimuth angles; and extending the chlorophyll-a inversion algorithm based on ground measured spectrum data to satellite image data after Rayleigh scattering correction. Based on the method, inter-annual and inter-monthly change rules and spatial distribution of chlorophyll-a in a eutrophic lake can be acquired accurately.

Systems and methods are disclosed herein to use what is referred to as adaptive continuous tracking (ACT) to reduce the power consumption of GNSS receivers. In GNSS receivers, performance as measured by position accuracy is a function of the observation time of the satellites. A longer observation time translates into more reliable range measurements and demodulated data, and ultimately into better positioning accuracy of the receivers. However, a longer observation time also means more power consumption. ACT allows satellite observation time to be tuned to the desired positioning performance by dynamically adjusting the on time period of the receivers while maintaining a minimum performance metric. The performance metric may be formed from a combination of the estimated position error, the horizontal dilution of precision (HDOP), the data collection state, and the receiver operating environment as characterized by the carrier to noise ratio (CN0). ACT cyclically switches on / off the radio frequency (RF) front-end and also cyclically enables / disables the baseband hardware of the receivers to reduce power consumption while allowing the receivers to meet the minimum performance metric and ensuring continuous satellite tracking, continuous positioning fix operation, multiplexed GNSS operation, and continuous data collection.

The invention discloses a multiple-fault detecting device and detecting method for a tightly-integrated inertial satellite navigation system, and belongs to the technical field of satellite positioning and navigation. The multiple-fault detecting device comprises an observed quantity measurement unit, an inertial navigation aided unit, a satellite fault evaluation unit, a measurement information filtering unit and a Kalman filter, which can detect and identify the faults (of multiple satellites) occurring simultaneously. The fault detecting method transfers the inertial navigation information to an observed quantity matrix; according to a corrected detection and observation equation of the observed quantity matrix and the quality evaluation for the satellite observed quantity, the satellite fault evaluation unit reconstructs the observation system and the observation equation, and therefore the precision of the tightly-integrated navigation system under fault condition is improved.

A time-attitude based method for decomposition and synthesis of imaging satellite observation task is disclosed. The method is characterized by comprising the following steps: (1) determining the time-orientation vector of a target according to the weft height of the target and the position of a satellite; (2) establishing an external characteristic rectangle, with a long edge parallel to a star line and including all target points, as a target region, determining a pitch angle, and calculating the characteristic vector of a region target; (3) according to the width of the satellite, dividing bands; and (4) according to the region target borders, cutting the bands to a right length. The method can intuitively and efficiently conduct target decomposition and synthesis, and more easily generate element task information input to a task planning and scheduling model. The method puts forward a concept of time-attitude, which is not only easy to understand and calculate, but also provides a unified description method for target decomposition and synthesis, and has certain operability, generality and scalability.

The method relates to a method for increasing the positioning precision of a PDT terminal. The method is divided into a base station assisting RTD broadcast mode and a network RTK mode according to the different functions of the satellite positioning modules of the PDT terminal. The base station assisting RTD broadcast mode is characterized in that the PDT terminal is provided with the satellite positioning module supporting the RTD technology, a PDT core network periodically acquires satellite positioning pseudo-range differential information in the coverage of base stations in a local area from a CORS data center and broadcasts the information to the terminal, and the satellite positioning module of the terminal calculates precise positions. The network RTK mode is characterized in that the PDT terminal is provided with the satellite positioning module supporting the RTK technology, grouping data connection between the PDT terminal and PDT base stations is built to transmit self-satellite observation information to the CORS data center through the PDT core network, and the CORS data center calculates the precise positions and transmits the same to the terminal. The positioning precision of the PDT terminal using the base station assisting RTD broadcast mode can be increased to a sub-meter level, and the positioning precision of the PDT terminal using the network RTK mode can be increased a centimeter level.

The invention provides an MODIS remote sensing monitoring method for the vertical distribution pattern of eutrophic lake water algae. The method includes: acquiring the vertical distribution pattern of the algae by means of field monitoring; on the basis of field measured water surface spectral information and environmental information, creating a ground measured spectral data (Rrs) based remote sensing monitoring method for vertical algae distribution; acquiring the quantitative relationship between ground monitoring and remote sensing reflectance Rrs and simulated Rrc after Rayleigh scattering correction by simulating different aerosol types and thicknesses and different solar altitudes, satellite observation angles and azimuthal angles; further promoting the ground measured spectral data based monitoring method for vertical algae distribution to MODIS satellite image data subjected to Rayleigh scattering correction. On the basis of the method, interannual and inter-monthly change rules and spatial distribution of the vertical distribution pattern of the eutrophic lake water algae can be acquired accurately, and scientific support is provided for scientific decision-making on water resource management and water environment protection water conservancy and environmental protection departments.

The invention discloses a satellite-borne AIS (Automatic Identification System) real-time information guidance based on-satellite independent imaging method, which comprises the following steps of (1), capturing and confirming a ship target needing to be observed; (2), determining satellite observation moment and observation area; (3), calculating an observation attitude of a satellite to a vessel; and (4), adjusting the attitude of the satellite, and determining that an SAR (Synthetic Aperture Radar) load can be used for carrying out imaging work. According to the method, the matching screening is carried out on an interesting target in real time by utilizing satellite-borne AIS information; an area to which the target is located is determined; the real-time resolving of data of a time, the attitude and an imaging parameter and the like needed by the imaging of an SAR load is realized; a satellite attitude adjustment instruction and an SAR load startup imaging instruction are automatically generated; and imaging tasks of a selected area and a selected target are completed. Through the comprehensive utilization of AIS information and SAR imaging information, the identification andconfirmation efficiency of a seaborne ship target is improved; the imaging accurate rate of the ship target is improved; and the observation efficiency of a system is improved.

The invention discloses a satellite dynamic strip region resolution method described by static mesh. The satellite dynamic strip region resolution method comprises the following steps: firstly, dynamically generating an observation strip through satellite parameter characteristics according to a satellite track; secondly, dividing the observation strip by utilizing a static mesh, so as to calculate observation earnings of the mesh and an observation region. A region observation task is dynamically divided into mask tasks according to characteristics of a satellite, and the capability characteristics and observation earnings of the satellite can be sufficiently considered, so that the satellite dynamic strip region resolution method is especially suitable for optimizing the satellite task in a joint observation process of the plurality of satellites. The observation region is divided into a plurality of sub-regions by utilizing the static mesh; the meta tasks are uniformly described and processed; difference between an imaging target and imaging region coverage in the aspect of layout processing is avoided; under the condition that the resolution calculation amount of the strip is remarkably increased, the satellite observation strip is resolved dynamically and rapidly, and uniform processing of the tasks is convenient to realize when the satellite accesses a calculation plan.

The invention discloses a multi-information fusion positioning method. A satellite observation value is read from a satellite signal processor so as to generate a pseudo-range equation; and dynamic information including a direction vector, a speed, an accelerated speed, and a jerk and the like is read from an inertial navigation system and data including a road topology structure, a road direction vector, a road length, and electronic elevation and the like are read from a digital map and an elevation database. Initial matching is completed according to the pseudo range as well as the dynamic information, direction information, the road topology and direction that are outputted by the inertial navigation device; and a positioning model is formed based on the initial matching result, wherein the positioning model includes the pseudo-range equation, the road vector equation and the electronic elevation constraint equation and the like; and position calculation is carried out by using the least square method or the Kalman filtering algorithm. In addition, the invention also provides an apparatus for realizing the method. Therefore, a positioning problem of a satellite signal in a shielded environment can be effectively solved; and the positioning coverage range of the satellite navigation is expanded. And thus the method and the apparatus have the high commercial value.

The invention provides a regional division method for satellite observation. The method comprises the follow steps of: step 1, determining the effective region of a target region; step 2, determining a maximum observation lateral swinging angle and a minimum observation lateral swinging angle corresponding to the effective region; step 3, making a current lateral swinging angle equal to the minimum observation lateral swinging angle, and emptying a result sub region set; step 4, adding the current lateral swinging angle into the result sub region set; step 5, adding a set granularity angle to the current lateral swinging angle; step 6, turning to step 4 if the sum of the current lateral swinging angle and the granularity angle is smaller than the maximum observation lateral swinging angle, and otherwise, turning to step 7; and step 7, adding the current lateral swinging angle into the result sub region set so that the effective region is divided into observation strip belts corresponding to each lateral swinging angle in the result sub region set. The regional division method overcomes the limitation of the Gaussian projection on task longitude differences, the accuracy is higher, and in addition, the regional division method is suitable for the target region observation by a plurality of satellites.

The invention provides a Beidou space-based high-precision real-time positioning method. The method comprises the following steps: firstly, generating difference revised data, then generating a difference broadcast telegraph text, adopting two pseudorandom sequences to generate a short precise code, and conducting spectrum spread and carrier modulation on the difference broadcast telegraph text and the short precise code so as to generate a difference broadcast signal; a user positioning terminal combines difference broadcast data of the difference broadcast signal and Beidou navigation satellite observation data to conduct positioning calculation so as to realize high-precision real-time positioning. The method can effectively realize decimeter-degree high-precision real-time positioning in the coverage area of a geosynchronous orbit satellite, and quickly improves and enhances the positioning service performance of a Beidou satellite navigation system.

The invention provides a high-precision satellite MODIS (Moderate-resolution Imaging Spectroradiometer) monitoring method for chlorophyll a of a eutrophic lake water body. The method comprises the following steps: screening a chlorophyll a evaluation index (NDBI) which is sensitive to the concentration change of the chlorophyll a and is not influenced by high suspended solids; obtaining a quantitative relation between the NDBI and the concentration of the chlorophyll a on the basis of biological optical model simulation; combining spectral information of the water body and the corresponding concentration of the chlorophyll a of the water body monitored in the field in Chaohu Lake in year 2013-2014, so as to obtain a chlorophyll a inversion algorithm based on a ground measured spectrum (Rrs) and the NDBI; simulating different aerosol types and thicknesses, different solar altitudes, satellite observation angles and azimuth angles, so as to obtain a quantitative relation between the ground monitored remote sensing reflectance (Rrs) and simulated Rrc subjected to Rayleigh scattering correction; and further extending the chlorophyll a inversion algorithm based on ground measured spectral data to satellite image data subjected to the Rayleigh scattering correction. According to the method, the inter-annual and inter-monthly change rules and space distribution of the rules of the concentration of the chlorophyll a of a eutrophic lake can be accurately obtained.

The invention relates to a method for precisely measuring the earth gravitational field, in particular to a method for inverting the earth gravitational field by using a variance-covariance diagonal tensor principle. The method comprises the following steps of: establishing a cumulative geoidal surface error model on the basis of satellite gravity gradient variance-covariance diagonal tensor principle; accurately and rapidly inverting the earth gravitational field by using the satellite gravity gradient measurement data of a spaceborne gravity gradiometer; and developing requirement argumentation on a GOCE (Gravity Field and Steady-State Ocean Circulation Explorer)-II satellite gravity gradient system by using the satellite orbit altitude and the precision index of the satellite gravity gradiometer. The method disclosed by the invention is high in inversion precision of the earth gravitational field, high in inversion speed of the satellite gravity gradient, explicit in physical content of a satellite observation equation, capable of easily developing the requirement analysis of the satellite gravity gradient system and low in requirement on computer performances. Therefore, the method for inverting the earth gravitational field by using the variance-covariance diagonal tensor principle is an effective method for resolving the earth gravitational field with high precision and high spatial resolution.

The invention provides a navigation data processing method and system based on cloud computing. The method comprises the steps that ground tracking data of a medium and high earth orbit GNSS satelliteand GNSS satellite observation data of a low earth orbit satellite are acquired; distributed cloud computing is adopted to preprocess the ground tracking data of the medium and high earth orbit GNSSsatellite and the GNSS satellite observation data of the low earth orbit satellite; a first observation model from a ground station to the medium and high earth orbit GNSS satellite and a second observation model from the medium and high earth orbit GNSS satellite to the low earth orbit satellite are constructed according to the preprocessed data; linearization processing is performed on the firstobservation model and the second observation model according to preset parameter initial values; and a least square method is adopted to calculate the first observation model and the second observation model after linearization, and joint orbit determination parameters of the medium and high earth orbit GNSS satellite and the low earth orbit satellite are obtained. Through the navigation data processing method and system, an orbit of the low earth orbit satellite can be determined with high precision, joint orbit determination and time synchronization of the navigation satellite and the low earth orbit satellite are realized, therefore, real-time high-precision service is provided for a user, and system service performance is improved.

The invention discloses a satellite joint observation task planning method, and relates to a satellite joint observation task planning method. The objective of the invention is to solve the problems of insufficient modeling consideration constraint conditions, low algorithm convergence speed and long algorithm calculation time when a satellite joint observation task planning problem is solved in the prior art. The method comprises the steps of 1, constructing a satellite observation task model; 2, establishing a satellite observation task planning model on the basis of a satellite observationtask model under the conditions that satellite observation tasks and observation resources in a given planning time period are not changed and interstellar links are stable; Step 3, constructing a mathematical model of constraint conditions based on the constraint condition set in the satellite observation task planning model obtained in the step 2; and step 4, designing a genetic algorithm basedon a greedy strategy based on the mathematical model of the constraint condition to complete satellite joint observation task planning. The method is applied to the field of satellite joint observation task planning.

The invention relates to a satellite demand processing system based on negotiation countermeasure conflict resolution. The method includes that a client acquisition module determines a preliminary access track of a satellite and outputs the preliminary access track to users; the users submit access demand lists to an acquisition list archiving module; the acquisition list archiving module feeds product data to the corresponding users, sorts the rest legal access demand lists by priority and stores and archives the same; an acquisition list processing module extracts acquisition list information in priority order, performs redundancy resolution on time information and space information in the acquisition list information, generates an acquisition mission list and sends the acquisition mission list to a satellite mission planning module; the satellite mission planning module performs strip division on an observation area in the acquisition mission list according to satellite imaging capacity, and combines with satellite orbit access features to generate a satellite observation list; a command generation module generates the satellite observation list into a remote control command, and uploads the remote control command to the satellite; an acquisition list feedback module acquires data information corresponding to the satellite observation list from a ground data processing public management platform of the satellite.

The invention belongs to the technical field of atmospheric remote sensing, and discloses a method for estimating a near-surface atmospheric temperature by using thermal infrared data of a geostationary meteorological satellite. The satellite is used for observing brightness temperature, meteorological station and numerical prediction model data to obtain a representative thermal infrared observation brightness temperature and near-surface atmospheric temperature; satellite cloud detection products are used for obtaining matched data sets for the observation brightness temperature, the stationactually measured temperature and auxiliary data under cloudless conditions; based on a stepwise regression method, the relationship between the radiation temperature of satellite observation, atmospheric pressure, relative humidity, a satellite observation angle, Julian daily parameters and the like and near-surface atmospheric temperature is analyzed, and key factors used for estimating the atmospheric temperature are determined; and a inversion model of near-surface temperature estimation is constructed by using a neural network technology. The method can realize the purpose of inversion of the near-surface atmospheric temperature under the clear sky condition of the thermal infrared data of the stationary meteorological satellite.

The invention discloses a cooperative localization-based terminal-to-terminal orientation method. The cooperative localization-based terminal-to-terminal orientation method is characterized in that wireless ranging and pseudo range double difference orientation methods are used in combination. The cooperative localization-based terminal-to-terminal orientation method comprises the following steps that: a terminal acquires satellite observation data of a global navigation system so as to obtain satellite pseudo range information; a pseudo range difference model is established through utilizing satellite observation values of the global navigation system and navigation satellite observation values of adjacent terminals; terminal-to-terminal distances which are measured through the pseudo range double difference and wireless ranging methods are combined together, so that a terminal-to-terminal orientation equation is established; and orientation results can be obtained through adopting different orientation methods under the situation that common visible satellites are different in quantity, and terminal-to-terminal orientation can be realized when common visible satellites are fewer than four. The invention also discloses a terminal-to-terminal orientation device. With the cooperative localization-based terminal-to-terminal orientation method and the device adopted, terminal-to-terminal orientation when common visible satellites are fewer than four can be realized, and therefore, the cooperative localization-based terminal-to-terminal orientation method and device of the invention have high practical values.

The invention discloses a space-time continuous PM2.5 inversion method based on foundation and satellite observation. The method comprises the following steps: dividing a monitoring area into a plurality of sub-areas; Establishing a random forest regression model for each sub-region, and carrying out inversion under the optimal model to obtain a PM2.5 concentration estimation value of each sub-region; Calculating a spatial interpolation observed by each station in each sub-region by using a common Kriging interpolation algorithm; Based on the root-mean-square error of the PM2.5 concentration satellite estimation value and the spatial interpolation, calculating the PM2.5 concentration satellite estimation value and the spatial interpolation by using an inverse variance weighting method to obtain a final PM2.5 concentration inversion value; According to the method, multi-scale segmentation and a random forest regression model are comprehensively adopted, the ground observation result isinterpolated, and seamless high-precision calculation of the near-ground PM2.5 concentration is achieved.

The invention discloses an integer ambiguity validity check method in a satellite navigation system. The method includes the following steps that: step 1, an observation equation is established according to satellite observation quantity in the current epoch and a stochastic model, and the observation equation is solved, and a satellite integer ambiguity float point solution is obtained; step 2, the fixed solution of satellite integer ambiguity is searched through using a LAMBDA method and according to the satellite integer ambiguity float point solution obtained in the step 1, so that integer ambiguity smallest residual quadratic form and integer ambiguity second smallest residual quadratic form can be obtained; step 3, correct integer ambiguity check statistic quantity and wrong integer ambiguity check statistic quantity are calculated according to the integer ambiguity smallest residual quadratic form and the integer ambiguity second smallest residual quadratic form which are obtained in the step 2, and the validity of the correct integer ambiguity check statistic quantity and wrong integer ambiguity check statistic quantity are checked, and the correctness o satellite integer ambiguity fixation in the current epoch is judged. According to the method of the invention, deviation between the correct integer ambiguity check statistic quantity and the wrong integer ambiguity check statistic quantity is as high as possible, namely, discrimination degree of wrong integer ambiguity and correct integer ambiguity can be enhanced, and the reliability of the validity check can be improved.

An improvement to a method of acquiring and processing a satellite observation image of the earth through the use of one strip or a matrix of charge coupled device type detectors. As the detectors travel over an observed zone, a plurality of pixels are acquired in a sampling method. The sampling method includes "e" subarrays corresponding to sampling thereby satisfying Shannon's condition. The "e" is an integer or rational number greater that 1. Processing is implemented that enables a noise-reduced image to be interpolated at a resolution corresponding to that of a sampling subarray on the basis of initially acquired image pixels.

The invention relates to a multispectral imaging device for satellite observation by “push-broom” scanning over an observed area, operating in N wavelength bands, respectively centered on a first wavelength (λ1), . . . , an nth wavelength (λN) comprising:a source emitting a light beam in a set of the N wavelength bands;a wide-field optic;a set of N rows of detectors making it possible to acquire images of said observed area;optical filtering means,characterized in that it also comprises:a first dispersion element (R1,R) making it possible to disperse the light beam toward the filtering means;optical filtering means comprising at least one micro-opto-electro-mechanical system (MOEMS) capable of carrying out N filtering functions for the N spectral bands, wavelength-tunable;control means for said micro-opto-electro-mechanical system making it possible to select the filtering function.