Patent Application
20130278757
This disclosed embodiment relates to a space survey system for monitoring Near-Earth space from the ground to detect objects present within this space, determine their precise trajectories and monitor these trajectories.
Such a system makes it possible to track the changes in the objects' trajectories and to catalog these objects and their trajectories.
Near-Earth space is defined as the portion of space located up to several hundred thousand Km from Earth. The detection therefore concerns objects that are primarily—but not only—in orbit around Earth.
The context of this disclosed embodiment is the increase seen in the number of objects in orbit around Earth.
These objects will be called debris in the rest of the text, bearing in mind that this notion of debris includes actual debris, operational satellites and even meteorites.
The disclosed embodiment is more specifically concerned with debris in Low Earth Orbit (“LEO”) from 200 km to 2,000 km, whose numbers lead to an increasing risk of collisions that could, over the long term, bring about a worsening of the situation, and, above all, risks relating to operational aerospace means, irrespective of whether they are military, scientific or commercial.
In order to control these risks, it is essential to catalog all of the potentially hazardous debris and to associate valid orbital parameters to them, which makes it possible to describe their trajectories.
Observed from a fixed point on Earth, objects in Low Earth orbit have the property of traveling quickly through the sky. In addition, at every moment multiple objects are crossing the sky in several places.
Depending on its orbital parameters, each object crosses the local sky at more or less regular time intervals ranging from several tens of minutes to several hours.
These orbits are affected by various effects such as the tide, atmospheric drag, radiation pressure and irregularities in the Earth's gravitational field.
This prevents a precise long-term description of these orbits with an invariant set of orbital elements.
In addition, the distribution of debris sizes varies from a characteristic radius of several millimeters, e.g. propulsion or paint residue, to meteorites with several tens of meters, satellites or artificial orbital systems in particular, whether they are operational or not.
Monitoring the Low Earth orbit requires the following functions to be realized:
In addition, it is necessary to reacquire the same objects and to regularly refresh the measurements of their orbital parameters, so that their precision remains suitable for their intended utilization, e.g. to implement the processes of identifying and consolidating collision risks.
Lastly, the system must be able to refine the precision of a given object's known orbital parameters upon request, so as to be able to accurately predict its position in the near future, typically several days, in order, for example, to consolidate a collision risk and to plan possible avoidance maneuvers.
The first three functions are traditionally grouped together under the space survey topic, which is the main subject of this disclosed embodiment, while the fourth belongs to the space tracking topic.
Realizing these survey functions requires having:
The orbital parameters are estimated on the basis of a time series of measurements of the position/velocity vectors of the objects, acquired during their transit in the field of view.
Patent U.S. Pat. No. 7,319,556 concerns a wide-field telescope, suitable for a system performing these functions.
The main techniques currently proposed and implemented to realize low-orbit monitoring are based on ground-based radars:
Even though they offer many advantages (wide field of view making it possible to capture areas of 180° in azimuth over several tens of degrees in elevation, simplified access to velocity information thanks to Doppler measurements, lack of sensitivity to the weather and the day/night cycle, etc.), the radar solutions have many drawbacks, residing mainly in their development, maintenance and operational costs, as well as in their ecological impact:
As an alternative, optical systems have already been considered to realize space surveys. Purely passive, their principle is based on detecting the sunlight reflected by natural or man-made objects in orbit around the Earth or beyond, e.g. asteroids and planetoids. Such systems provide access to time series of measurements of the objects' angular positions, e.g. their azimuth and elevation.
Various methods are used to measure these positions; the most advantageous of these relies on measuring the position of the detected objects at each instant in relation to the stars present in the field of view, stars whose position is known with very high precision.
The major benefits of optical systems over radar systems are their low development, production, operating and maintenance costs, their reliability and their simplicity of implementation.
In addition, since they are purely passive, they require little in the way of infrastructure, energy, buildings and means of transport.
Optical systems are normally used to monitor the GEO (geostationary orbit) and even, more recently, the MEO (intermediate orbit between LEO and GEO), because objects in these orbits have the particularity of traveling very little in the sky; this facilitates the long observation times required to detect objects that are small and/or have very low light intensity.
The US Air Force GEODSS is an operational example of such systems. It comprises mainly telescopes with an aperture of one meter or more with a narrow field of view, of the order of one degree.
Long integration (exposure) times ranging from 1 to several seconds can be used for these GEO and MEO applications, which allows the signal-to-noise ratio to be increased so as to detect small objects with a characteristic diameter of several tens of cm.
An example of multi-sensor realization is described in document US 2009/0147238.
Some studies have also been initiated to define solutions able to monitor the LEO.
For example the French experimental system SPOC (“Système Probatoire pour l'Observation du Ciel” [Sky Observation Test System]) included 4 small telescopes with an aperture of the order of 10 cm pointed towards the 4 cardinal points at an elevation of several tens of degrees, each offering a field of view of the order of 10°.
Other concepts propose sensitive catadioptric systems with an aperture of one meter or greater, called “wide field”, of the order of 5°, dedicated to LEO monitoring, such as for example the system that is the subject of the aforementioned patent U.S. Pat. No. 7,319,556.
The solutions mentioned above and currently proposed do not however allow the fundamental difficulties and constraints linked to LEO monitoring to be resolved, i.e.:
Because of these constraints, LEO monitoring also requires specific optical systems with very good sensitivity, excellent resolution and a wide field of view.
In effect, existing telescopes usually have high sensitivity, wide apertures and/or long integration times and high resolution, which are detrimental to wide fields of view, because they are designed for conventional astronomy applications or for surveying minor planets and asteroids: they are therefore not compatible with LEO surveying.
In addition, the very principle of surveying does not provide for tracking objects. As a result, during LEO observation, the long integration periods do not improve the detectability of an object, which is evaluated in relation to the signal-to-noise ratio of each illuminated pixel, because, in the case of conventional integration (one second) the object traverses several pixels of the sensor (CCD sensor) over the integration period; this is disadvantageous not only for determining the position and date-stamping same, and it also includes noise, consequently degrading the signal-to-noise ratio once the pixel has been traversed.
From another point of view, the known solutions are not suited to the conditions of LEO detection and are therefore unable to provide observations of all the observable objects with a suitable revisit time.
Lastly, wide-field telescopes remain limited, as known in particular from document US 2009/009897 or document EP 1 772 761.
Other examples of telescopes are given in documents U.S. Pat. No. 7,045,774, US 2007/0188610 and US 2009/0015914.
In the light of this situation, the disclosed embodiment consists of realizing a ground-based LEO survey system that utilizes optical means distributed over the Earth's surface to detect these objects present in low orbit, <2,000 km, without knowing them beforehand and to provide an initial estimate of their orbital parameters.
The disclosed embodiment therefore aims to define a ground-based LEO survey system, based on purely passive optical solutions that, at a competitive cost compared to radar solutions (a factor of 2 to 10), provide comparable performance levels, as follows:
The disclosed embodiment thus relates to LEO tracking using optical technology and solutions for implementing such tracking.
To achieve this, the disclosed embodiment proposes a space survey system comprising networked optical surveying systems designed to scan areas of earth orbit, in particular the LEO layer, arranged in a grid on the surface of a planet characterized in that it comprises means designed to acquire images of objects traversing the areas scanned by the optical systems and calculation means designed to realize a measurement of the time series of positions of objects crossing the scanned areas by short integration over a fixed field; the optical systems being designed to scan the observation area 4 to 6 times faster than the transit velocity of the objects to be detected. The calculator is advantageously designed to realize the measurement returning, relative to the stars within the field of view, a minimum of 3 measurement points for each object crossing the scanned area.
The space survey system according to the disclosed embodiment comprises a computer means for processing the images coming from the optical systems to extract the date-stamped positions of the objects going across the field.
Advantageously, the optical systems comprise telescopes arranged on the surface of the planet according to a configuration designed to provide an effective daily cycle close to 24 hours and a selected revisit rate for the monitored LEO area.
Preferably, each optical system is positioned and controlled according to specific observation conditions based on its geographical location designed to ensure optimum illumination of the objects to be detected.
Preferably, the optical systems are wide-field optical systems designed to scan an area of sky at least 10 to 40° by 10 to 60°.
The optical systems are advantageously designed to scan the observation area 4 to 6 times faster than the transit velocity of the objects to be detected.
According to a particular embodiment, the space survey system according to the disclosed embodiment comprises, associated to each optical system a dedicated tracking device with a conventional field designed to acquire, on the basis of the designation realized by the survey system, more numerous and precise position measurements of objects designed to achieve the required precision on the determination of the orbital parameters of said objects in order to transform the survey system into a system designed to define and track a precise trajectory of previously detected objects.
Advantageously, the basic optical systems of the network are designed to traverse the area to be observed at a frequency 4 times greater than the minimum transit time of the objects in the targeted population in the scanned area of the sky.
The disclosed embodiment also relates to a space survey and tracking system comprising a space survey system as described above, characterized in that it comprises, in addition to the optical survey systems, dedicated tracking systems, means of connecting said optical survey systems and said dedicated tracking systems designed to transmit designations of objects coming from the optical survey systems to the dedicated tracking systems; the orbits of said objects then being determined precisely by the dedicated tracking systems using said designations as input data.
Other features and advantages will become apparent in reading the following description of a non-limiting example of realization of the disclosed embodiment with reference to the drawings, which show:
In the context of the disclosed embodiment, in order to realize the grid a system configuration is defined, using computer-based means of simulating the performance and positions of optical systems, which system configuration consists of a suitable networking ground-based optical systems along said grid or an approximation of said grid over the surface of the Globe or surface of the planet, to provide an effective daily cycle of the system close to 24 hours, i.e. continuous coverage of the planet's entire environment.
By calculating the phase angle (in astronomy, the phase angle is the angle made by the sun, the object observed and the observer or, more generally, the angle made by the incident light ray and the reflected ray) and the sun's position and by simulation, specific observation conditions are defined for each optical system, depending on its geographical situation, to provide each optical system with optimum illumination of the objects to be detected.
For the optical systems, detectors are used that have a wide field, greater than 5° or preferably greater than 10°, and are designed to detect the objects in the scanned area.
For example, 70 cm reflecting telescopes able to detect objects of 10 cm at a distance of 1,000 km are chosen.
The conditions for scanning areas of the sky by optical systems are then optimized, so as to traverse the observation area 4 to 6 times faster than the transit speed of the objects to be detected.
As an example of realization, the grid is realized by simulation according to either one of the methods below or to a combination of these methods.
The steps common to both methods are:
Starting with these common steps, method no. 1 comprises:
So they can be cataloged, the objects must be seen at least once by meeting the detection conditions within the allotted period and the minimum search can then be done using a conventional minimum search algorithm.
Method no. 2 comprises a simulation of the visibility episodes for the entire reference population for a preferred subset of sites selected by criteria (e.g. according to criteria of ease of access, specific properties of the site, etc.) and an assessment of the coverage rate, i.e. the percentage of the reference population visible at least once meeting the detection conditions, and also the convergence period, i.e. the simulated period of time required to achieve this level of coverage.
This simulation is completed by iteration, modifying the subset of preferred sites by adding or removing sites until the required performance is achieved, e.g. 98% coverage of the reference population, and a convergence period of, for example, 1 month.
For detecting objects, the disclosed embodiment provides a measurement of the position time series of the objects crossing the scanned areas by short integration over a fixed field, returning, in relation to the stars in the field of view, a minimum of 3 measurement points for each object crossing the scanned area.
The use of short integration times is one of the innovative elements of the disclosed embodiment.
In survey mode, objects cannot be tracked because their presence is not known beforehand. Images are therefore captured at a fixed pointing position for the exposure time (or integration period).
Thereafter, the sensitivity (ability to see a star or an orbital object relative to background noise) is defined by the signal-to-noise ratio in each pixel, defined by the simplified formula:
Signal=PhotonsFlux×TimeExposure
ΣNoises2=PhotonNoise2+SkyBackgroundNoises2+ΣElectronicNoises2
PhotonNoise=√{square root over (Signal)}
SkyNoise=Skyphotonflux×TimeExposure
The observation of distant stars is generally performed by compensating for the Earth's rotation so as to maintain a fixed sky in the field of view.
The star in question then illuminates a set of fixed pixels. In those circumstances, so as to increase the SNR (signal-to-noise ratio), it is sufficient to increase the exposure time so that for an object with a given brightness, the signal dominates all the other noises, in particular the sky background noise.
The same situation prevails for optical monitoring of the GEO: the objects therein are practically still relative to the axis of observation on the ground, and here also, the axis of observation is fixed in relation to the local reference and the increase in sensitivity is obtained by increasing the exposure time.
In the context of low orbit surveying, the object is not still in relation to the axis of observation. Consequently, its image travels across the CCD. Each of the COD's pixels is only illuminated by the signal during the time the object's image is traveling over that pixel. In contrast, each pixel of the CCD is illuminated by the background noise over the entire exposure time.
Thus, according to this disclosed embodiment, in order to maximize the SNR, an exposure time is set that is close to the object's transit time in the field of the pixel. However in LEO, depending on the axis of observation (apparent angular velocity lower at low elevation than at zenith), the instrument's configuration (field of view, size of each pixel) and the object's orbit (angular velocity less at high altitude), this transit time is of the order of several milliseconds to several hundred milliseconds.
Consequently, exposure times (or integration times) are selected that make it possible to obtain the required signal-to-noise ratio.
The ideal exposure time is chosen by calculating the photometric link budget, taking into account the favored orbits, the different observation configurations (elevation, phase angle, exposure time), the quality of the sky background, the effect of the atmosphere (signal attenuation and dilution by turbulence), the instrument's configuration (telescope and focal plane) and the characteristics of the targeted objects (minimum size and minimum albedo).
Basically, this consists of analyzing sensitivity to various parameters, making it possible to define the most suitable instrument configuration and observation configuration.
As the brightness of an object is directly related to its size and its ability to reflect the sunlight (albedo), the system's great sensitivity makes it possible to see small objects.
The images captured during the transit of objects are processed taking into account the positioning of the optical system by utilizing an image processing computer system to extract the date-stamped positions of objects crossing the field.
In addition, by adding a dedicated tracking system to each optical survey system, realized for example with a conventional field telescope system motorized and controlled by a computerized tracking system connected to the optical survey system's computer system, more numerous and accurate position measurements are acquired, based on the designation realized by the survey system, which make it possible to obtain the required precision for the determination of the objects' orbital parameters.
This makes it possible to change the survey system into a tracking system, i.e. a system able to define and track an accurate trajectory of previously detected objects.
The ground-based optical survey systems are networked over the surface of the Globe or of the planet by keeping to the following rules:
The local implementation conditions of the observation are such that each optical system installed at each node of the grid scans only areas of sky some 10° to 40°, preferably 20° to 40°, in azimuth above 35° and of 10° to 60° in elevation, preferably 20° to 60°, around azimuths varying according to the time, season and latitude, which corresponds to a fixed right ascension belt, depending on the required performance, i.e. the population of LEO objects to be covered, the objective coverage rate and the precision to be maintained for the catalog.
To achieve this, basic optical survey systems comprise an image capture device motorized and controlled by a computerized aiming and image acquisition system.
The basic optical survey systems of the network and their means of control are designed to traverse the area to be observed at a frequency 4 times greater than the minimum transit time of the objects in the targeted population in the scanned area of the sky.
In addition, a version wherein the optical systems comprise a telescope with a 5°×5° field that is made to sweep an area of space can be envisaged within the context of the disclosed embodiment.
To determine the orbits of the objects, an initial detection of objects in LEO is realized by measuring the position time series of objects traversing the scanned areas.
This is achieved by short integration on a fixed field, as discussed above, with a minimum of 3 measurement points for each object traversing the scanned area, and a determination of the object's position in relation to the stars within the field of view is performed, these stars being referenced at the local aiming computer system or at an additional computer system possibly located remotely at the command center for all the network's tracking systems and comprising a map of the sky.
Possibly, the orbits are then determined precisely by utilizing a dedicated tracking system, such as described above, that uses as input data the designations of the basic optical system previously described.
In this context, an algorithm inspired by “startrackers” is used, making it possible to determine the position of the orbital object in each image of the object captured by the telescope, either in right ascension and declination, or in azimuth and elevation, by a relative measurement of its position in the image compared to the position of the stars, known absolutely and very precisely in the system, which includes a catalog (such as the Hiparcos catalog, for example).
The general principle of the image processing realized by image processing software is as follows:
For this determination, 5 known stars are used even though, in theory, 3 stars are enough; but in this way the precision and reliability of the calculations are improved.
Lastly, the measured position is date-stamped with the date of the image capture.
The grid and nodes where the optical systems will be located need to be defined so as to position the optical systems.
The analysis of the visibility conditions of LEO objects from the ground defines preferred latitudes depending on the type of orbit.
These latitudes are typically for a belt in right ascension, moving by 1° per day to compensate for the Earth's rotation around the Sun, centered on a right ascension providing the smallest possible illumination phase angle, depending on the objects' altitude.
In the context of positioning the optical systems in latitude, the following facts in particular are taken into account:
As regards the visibility of objects in longitude, the disclosed embodiment also consists of monitoring the areas of the sky where there will be the highest probability of detecting objects.
These areas meet the following criteria:
These criteria make it possible to define a mean direction around which the objects on an orbit family have a phase angle less than 45°. The phase angle depending on the position of the sun relative to Earth, of the object in the local sky and on the local time, this direction changes with the rotation of the Earth, and consequently with the local time.
It should also be noted that, the apparent angular velocity of satellites in LEO being less at lower elevations than at zenith, sensitivity is lower at zenith (excluding the effect of the atmosphere) for a given exposure time.
In this case, the visibility areas are in relation to the solar flux; area 4 for site 1 and orbit a, area 5 for site 1 and orbit b, areas 6 and 7 for site 3, these areas being separated by the area of the Earth's shadow, areas 6 and 7 covering the orbits a, b and c, areas 8 and 9 for site 2, zone 8 making it possible to detect the objects on orbit b and area 9 the objects on orbit a.
It can be seen that the visibility areas for sites 1 and 2 are offset from the local zeniths 10 and 11, whereas for site 3, the local zenith 12 is in the visibility area 7.
The analysis of revisit times at every point on the globe, which takes into account these conditions of visibility for the entire LEO population to be detected, demonstrates that a longitudinal grid provides at every moment a longitudinal belt wherein these visibility conditions are met;
These two analyses, in longitude and latitude, make it possible to define a grid of the globe providing an effective daily cycle of the system close to 24 hours, for each type of orbit, providing both the required coverage and revisit period.
Taking the seasonal weather statistics into account makes it possible to define a necessary redundancy rate: the analysis of weather configurations reveals a strong decorrelation between local cloud cover conditions for points separated by a few hundred km on the globe.
In this way, the addition to the longitudinal grid of nodes provides a rate of redundancy as to cover that makes it possible overall to ignore local weather conditions.
This leads to the specific case below of the implementation of a network that comprises 15 sites spread over various latitudes:
Sites in continental Europe (southern Spain), Central Asia (on the sites of existing astronomical observatories), in Japan and Canada (southern part) that make it possible to focus on most objects with an inclination greater than 45°, while detecting objects in a non-SSO polar orbit.
Sites in the Pacific (Tahiti, the Marquesas Islands), Chile (ESO sites), in East Africa (Malindi), on Diego Garcia in the Indian Ocean and on the north coast of Australia make it possible to cover equatorial orbits, as well as all the other orbits (even though providing shorter observation periods for these than at higher latitudes).
Lastly, sites located at latitudes above 60° (North or South) make it possible to detect more specifically objects on SSO and polar orbits: Alaska (Poker Flat tracking station, contributing to the ESA tracking network), northern Canada, Iceland, Kiruna, Kerguelen Islands and southern Argentina.
The choice of sites in both the northern and southern hemispheres makes it possible to partly erase the seasonal effect that limits the observation possibilities.
Within the context of realizing a survey and tracking system, each station is equipped with an optical survey system and a tracking telescope.
At the survey stations, for each optical system, predefined areas of the local sky are scanned, depending on the time of day or night, corresponding to a fixed right ascension band changing by 1° per day providing optimum illumination conditions (standard phase angle<45°) depending on the latitude.
In effect, for objects in LEO, visibility periods are limited to a few hours after dusk and before dawn (duration varying according to the season, latitude, inclination and altitude of the objects), corresponding to right ascension belts providing the optimum observation conditions (illumination phase angle minimized), without the object being in the Earth's shadow.
These are belts 10 to 60° wide in azimuth, average azimuth; the width of the belt varying according to the latitude, date, time and altitude of the orbit, which are located to the east and to the west,
In addition, most of the objects meeting the visibility conditions traverse this azimuth belt in a strip 10 to 60° high in elevation, above 35° elevation.
By taking into account a minimum elevation>30° to limit the atmospheric absorption and a maximum elevation (to limit the apparent traveling speed), an area of sky to be preferably monitored is defined, as well as its changes depending on the time of day, the seasons and various latitudes.
As there is, at each orbit altitude, a visibility area that is more or less wide at a given time, and each orbit altitude comprises disparate populations (in terms of inclination, ascending node, etc.), it is difficult to determine theoretically the ideal area of sky to be monitored. In order to limit the latter, an area in azimuth is determined by simulation, wherein the density of objects meeting the visibility conditions is respected, depending on the time and season. The simulation principle is simple: for a certain number of latitudes (e.g. 0°, 30° N, 30° S, 45° N, 45° S, 60° N, 60° S), the illumination conditions (phase angle) of each object in a reference catalog are simulated over several days and over the two seasons, by building in the relative motions of the objects and of the Earth/Sun pair. Depending on the time, the area of the local sky (azimuth and elevation) comprising the highest density of objects meeting the required illumination conditions is measured
The ideal area is refined by iterations in regards of the desired grid density to be obtained.
To this end, an initial definition of the area of the sky to be monitored is performed for various latitudes; this is modeled in the simulator used to define the grid.
The accessible performance of a given network is measured; if the performance achieved is insufficient or if the configuration of the network of stations becomes too large, the area of sky to be monitored for each latitude is reevaluated by a new analysis.
This is repeated by iteration until a satisfactory compromise in terms of performance and cost is achieved.
Conventional optical systems do not provide a sufficient field of view to scan the sky within the context of this disclosed embodiment,
The principle adopted in this disclosed embodiment is to have a wide field that may reach 60° by 40°.
Combining fields by adding conventional optical systems requires an unacceptable number of optical systems.
Similarly, scanning the area of sky with a set of optical systems with conventional fields raises complex problems, notably of synchronization of the systems and parasitic motions of these systems relative to the others.
The principle of the disclosed embodiment is to utilize medium-field, high-sensitivity telescopes at the location of each optical system, with a sensor positioned thereon, these telescopes being servo-controlled together and grouped so as to operate simultaneously to provide a wide field.
The optical systems' telescopes are sized for observing small pieces of debris in the LEO layer, e.g. debris of the order of 8 to 10 cm at an altitude of 600 Km, and are thus ideally suited to observing objects with an equivalent magnitude at higher altitudes, in the MEO or GEO belts.
However, the required velocity of the mount is greater for the LEO layer.
The determination of the telescopes' parameters arrives at: a diameter of the order of 80 cm to 100 cm; a focal length of the order of 1.5 m to 2 m, this parameter not being critical; a field ideally ranging from 5° and 20° and, more specifically from 5° to approximately 10°, the preferred value being a field of about 10° and in particular 8° to 12°.
In the example where the field of the telescope is of the order of 10°, for image capture a camera is used of the type with a CCD sensor at the focal plane, with about 4,000×4,000 pixels, depending on the combination of the focal length and the field. For a telescope with a 5° field, the CCD would have 2,000×2,000 pixels.
To summarize, the sensor of the telescope(s) is a CCD sensor with 1000×1000 to 6000×6000 pixels and a CCD read-out time less than or equal to 2 seconds and an exposure time of less than 100 milliseconds.
The spectral domain is visible light and the objects to be monitored range from the LEO layer to the GEO layer with a lower magnitude of up to 13.
The detectors can be of CCD, CMOS, SCMOS or EMCCD type, but the sensor preferred for its good signal-to-noise ratio remains the back-illuminated cooled CCD sensor.
A TMA or Schmidt type telescope will be chosen, and the preferred type of telescope is a three-mirror anastigmatic (TMA) telescope. Such a telescope is schematized in
As an example of realization, the chosen size is a primary mirror diameter of 80 cm; this allows objects of about 10 cm in LEO to be reached, for objects at a distance of 500 km and objects of the order of 20 cm at a distance of 2,000 km for a very dark object, the dimensions being calculated for an albedo of 0.1.
In absolute terms, it is the transmission budget calculation that makes it possible to optimize the size of each telescope, as well as the entire optical and operational configuration for a required minimum detectable object size depending on altitude.
The telescopes are advantageously defined so to provide a usable field of 10° by 10°, and 6 telescopes should be grouped and servo-controlled to realize a basic survey system.
This yields a reconstituted field of 30° elevation by 20° azimuth.
For 5×5 telescopes, the reconstituted field is smaller and the scans may be larger.
Each telescope is mounted on a programmable rotary mount to scan an overall area of up to 60° elevation by 40° azimuth.
The movement velocity of the telescope is such that each object traversing the swept area is detected three times to obtain at least 3 date-stamped position measurements, ideally distributed over the object's arc of transit in the area of sky to predetermine its orbit.
The images are processed with startracker-type image processing algorithms that make it possible to determine the position of moving objects in the field, relative to the background stars, with an angular precision of the order of IFOV (instantaneous Field Of View): 5.85 seconds of arc.
The transit periods in the area thus defined are of the order of a few minutes, depending on the orbital population to be covered (the period being shorter as the orbit is lower).
The number of image captures required to traverse the area concerned is directly related to the field of view of the wide-field optical system utilized.
The duration of image capture and in particular the integration time, combined with the repositioning performance of the optical system (in particular its speed of movement and its stabilization time) and with the number of image captures affects the minimum travel duration over the area of local sky to be scanned.
A compromise must therefore be found between: this number of image captures; the maximum time taken to traverse the area of sky to be scanned; the characteristics of the optical system's mounts, their relocation speed including stabilization and integration time.
In order to ensure a sampling of the area of sky to be scanned limiting losses, the optical system scans this area at least 4 to 6 times faster than the shortest transit time of the targeted objects.
This constraint is added to the optimum integration time constraint to size the optical systems in terms of field of view and motion performance.
The equatorial mounts used allow a movement in azimuth from one field to the subsequent field in less than one second, including stabilization.
The telescopes scan the sky in a belt of 10 to 60° in elevation and 10 to 40° in azimuth, around 45° in elevation, centered on a right ascension near the side opposite the Sun (depending on the latitude).
The belt scanned is traversed by successive round trips. Its tropocentric coordinates change with the movement of the Sun during a single night (typically 15 arcmin per minute, to the west) and from one night to the next to compensate for the revolution of Earth around the Sun (1° per day).
Objects located at an altitude of 500 km, observed at 45° elevation, traverse the scanned area in over 15 secs.
According to this configuration, the system can ensure that the position of each object at an altitude greater than 500 km will be measured 3 times during its transit.
Detecting objects in LEO orbit is not compatible with tracking the objects because they are not known beforehand and their trajectory is even less known.
Therefore, the optical survey systems are kept still during each integration, observing a particular area of the sky that matches their field of view.
The best date-stamped position measurement precision is achieved by evaluating the position of each object in LEO within the field of view relative to the stars also in the field of view.
As seen above, the survey system can be completed by a tracking system whose objective is to acquire more numerous and precise position measurements, on the basis of the designation realized by the survey system, so as to achieve the required precision of determination of the orbital parameters.
The tracking system is based on conventional telescopes, with high sensitivity and a standard field of view of the order of 1°.
These telescopes are positioned on the same sites as the optical survey systems.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1055658 | Jul 2010 | FR | national |
This application is the National Stage of International Application No. PCT/EP2011/061568 International Filing date, 8 Jul. 2011, which designated the United States of America, and which International Application was published under PCT Article 21 (s) as WO Publication 2012/007360 A1 and which claims priority from, and benefit of, French Application No. 1055658 filed on 12 Jul. 2010, the disclosures of which are incorporated herein by reference in their entireties.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/061568 | 7/8/2011 | WO | 00 | 3/28/2013 |
