Patent Application
20250141572
The invention relates to a device and a method for recording a radiofrequency activity of earth-orbiting artificial satellites, and a computer program for their implementation.
The field of the invention relates to active satellites, in all kinds of orbits.
A service for determining the orbits of the geostationary satellites is known, which uses the declarations provided by the operators on the use of the radiofrequency spectrum made by these satellites; it is referred to as “orbital tracking” system.
The known orbital tracking service uses a technique of correlation of the signal emitted by the satellites and acquired by stations remote from each other, typically by a distance of several hundred kilometers. This technique requires transferring large amounts of data to a computing center. A systematic examination of an orbit, such as for example the Clarke belt (geostationary orbit), by this technique, would be limited by the bandwidth of the long-distance communications towards the computing center and would therefore require an unacceptable measurement time and a high cost of communications.
Also, satellites emitting sporadically may not be detected by this method, because the rate of revisiting an orbital position would be too slow (more than a day). In addition, when the orbital tracking by remote stations is failing, it is not obvious to determine what could be the cause of it: change of plan/frequency by the satellite, cessation of satellite communications or other causes.
One objective of the invention is to obtain a device, a method and a computer program for recording a radiofrequency activity coming from earth-orbiting artificial satellites, which make it possible to automatically discover and identify the radiofrequency emissions derived from satellites without prior third-party information, that is to say without considering the declarations provided by the operators and with a short revisit time.
To this end, a first subject matter of the invention is a method for recording a radiofrequency activity of at least one artificial satellite, which is earth orbiting and which is emitting a radiofrequency signal, characterized in that the method comprises the following steps:
Thus, the invention, by the combination of a small antenna and of the inverse convolution makes it possible to detect a greater number of satellites, because the main lobe of the antenna having the small diameter mentioned above is wider, allows covering a greater number of satellites at each antenna pointing position and allows a shorter revisit time (which is the time elapsed to perform all the planned scanning sequences), thus increasing the possibility of detecting a non-permanent emission from a satellite. This makes it possible to increase the measurement pitch from one pointing position to the next pointing position in the sequence and therefore the speed of acquisition of the refined power spectral density, while reducing the cost of the infrastructure.
Thus, the invention makes it possible to detect all the artificial satellites having a radiofrequency activity, whether or not they have been listed. This system is called “spectral mapping”. An examination using only one station at a time on which the device according to the invention is installed can therefore be envisaged. Thanks to the invention, uses not declared by an operator, changes in frequency plan, in orbital position, new station acquisitions or failures regarding earth-orbiting artificial satellites, are listed. The invention makes it possible to reduce the revisit time in order to detect as many events as possible. The invention makes it possible to establish an accurate mapping with a rapid revisit (a few hours at most), while using one or several antennas of limited size and reduced cost. The spectral mapping can therefore be updated frequently, several times a day and every day in order to be capable of identifying with certainty the use changes made by the operator. The invention can create the mapping in the form of a power spectral density map as a function of the pointing position and of the frequency, making it possible to visualize and interpret the origin of the received radiofrequency spectrum, as well as its evolution over time. It is thus possible to detect celestial objects and the use changes (breakdowns, station acquisitions, change of frequency plan) concerning the satellites. The invention can map the use of the frequency spectrum as a function of the orbital position (longitude or anomaly when an orbit is scanned) of all or part of the orbit(s) considered, on one or several receiving frequency sub-bands. The invention can be used to detect satellites on all types of orbits, which therefore comprises the non-geostationary ones.
The invention makes it possible to put the detected satellites into orbital tracking. This makes it possible to dispense with frequency use declarations of the operators and to track these objects, whether or not they have been declared.
According to one embodiment of the invention, the antenna has a maximum opening diameter, which is less than or equal to 4 meters.
According to one embodiment of the invention, the antenna has a maximum opening diameter, which is less than or equal to 2.50 meters.
According to one embodiment of the invention, the antenna has a maximum opening diameter, which is less than or equal to 2.10 meters.
According to one embodiment of the invention, the reprocessing of the recording of raw power spectral densities comprises:
According to one embodiment of the invention, the inverse convolution is performed for a linear combination of several successive raw power spectral densities for the successive prescribed pointing positions.
According to one embodiment of the invention, the method comprises the pointing of the axis of sight of the antenna following a prescribed scanning trajectory passing through the prescribed pointing positions at consecutive instants.
According to one embodiment of the invention, the prescribed scanning trajectory tracks each prescribed pointing position for a non-zero duration.
According to one embodiment of the invention, the prescribed scanning trajectory connects the prescribed successive pointing positions with a regular speed.
According to one embodiment of the invention, the antenna control device interrupts the displacement of the axis of sight when prescribed pointing positions are located below a predefined minimum elevation.
According to one embodiment of the invention, the successive prescribed pointing positions have the same orbital period.
According to one embodiment of the invention, the prescribed pointing positions of at least one of the sequences of prescribed pointing position move in a determined orbit.
According to one embodiment of the invention, the determined orbit is the Clarke belt of the geostationary satellites.
According to one embodiment of the invention, the axes of sight of the successive prescribed pointing positions are spaced apart by an angle smaller than a width of a main radiation lobe of the antenna, in the time interval where the antenna moves from the prescribed pointing position to the next prescribed pointing position in the sequence.
According to one embodiment of the invention, the calculator reads the raw power spectral density at each prescribed pointing position from the digital signal.
According to one embodiment of the invention, the recording of raw power spectral densities is made by an acquisition of the radiofrequency signal, which is collected by the antenna and which is digitized by the radiofrequency chain according to separate windows of a predefined minimum duration.
According to one embodiment of the invention, to measure the recording of raw power spectral densities at each prescribed pointing position, the calculator for each duration window of the digital signal calculates the Fourier transform then squares its module and divides the whole by the duration of the window.
According to one embodiment of the invention, the average scanning speed of the antenna relative to the tracking speed of the pointing position in the sequence is less than θ/D, where θ is the width of the main radiation lobe of the antenna.
According to one embodiment of the invention, the method comprises the calculation, by the calculator, and a recording, by the calculator, of a mapping constituted by the recordings of refined power spectral densities as a function of the frequency and of the prescribed pointing position.
According to one embodiment of the invention, an image is extracted from the mapping by the calculator, wherein the image arranges side by side the levels of the refined power spectral densities, read successively at the prescribed pointing positions, in the at least one receiving frequency sub-band, the pointing orbital position and the frequency constituting, as desired, the abscissa and ordinate axes of the image.
According to one embodiment of the invention, the level of each refined power spectral density is represented by a pixel value, in particular the color or a brightness level or a gray level.
According to one embodiment of the invention, the calculator searches for peaks on part or on all of the longitudinal level curves of the raw or refined power spectral densities to determine the most probable position of the at least one emitter satellite of the read spectrum.
According to one embodiment of the invention, the probable position(s) of satellites and their associated power spectral density recording are compared by the calculator to the declarations of the operators.
According to one embodiment of the invention, the probable position(s) of associated satellites and possibly all or part of their power spectral density recording are transmitted by the calculator to an orbital tracking system.
A second subject matter of the invention is a device for recording a radiofrequency activity of at least one artificial satellite, which is emitting a radiofrequency signal, characterized in that the device comprises:
According to one embodiment of the invention, the drive device is able to point the axis of sight of the antenna following a prescribed scanning trajectory passing through the prescribed pointing positions at consecutive instants.
According to one embodiment of the invention, the antenna is chosen of just sufficient size to be able to detect the signal from a satellite emitting at the equivalent isotropically radiated power threshold to be detected.
According to one embodiment of the invention, the radiofrequency activity recording device comprises a recording device able to record a mapping constituted by recordings of the power spectral density as a function of the frequency and of the prescribed pointing position.
According to one embodiment of the invention, the recording device is able to record a history of the mapping, that is to say several power spectral density recordings made on different dates concerning the same prescribed pointing position.
According to one embodiment of the invention, the calculator and the recording device are able to constitute at least one differential image between histories of the mapping corresponding to different acquisition dates for the same sequence of prescribed pointing positions.
According to one embodiment of the invention, the spectral density recording is made by acquisition of the radiofrequency signal, which is collected by the antenna and which is digitized by the radiofrequency chain according to separate windows of a predefined minimum duration D.
According to one embodiment of the invention, to measure the power spectral density of the signal at each prescribed pointing position, the calculator for each duration window of the digital signal is configured to calculate the Fourier transform then to square its module and to divide the whole by the duration of the window.
A third subject matter of the invention is a computer program for the implementation of the method for recording a radiofrequency activity of at least one artificial satellite as described above, comprising code instructions for the execution of the pointing, collection, conversion, calculation and reprocessing steps, when the computer program is executed on one or several calculators.
The invention will be better understood upon reading the following description, given solely by way of non-limiting example with reference to the figures below of the appended drawings.
In
One or several stations located on the surface ST of the globe T can be provided, the station or each station comprising the recording device 1 according to the invention. The work of each station can be planned, for example centrally in the case of several stations, so that the work of the stations is complementary.
In a non-limiting embodiment, several antennas 2 and devices 1 according to the invention can be distributed over different points of the globe T in order to be able to cover all the orbital positions of interest and all the sub-bands Wx of target frequencies.
The recording device 1 according to the invention comprises the elements which will be described in more detail below:
In one preferred embodiment, the steps of the recording method according to the invention follow the description given below. They describe the different operations that contribute to the invention. They actually take place non-sequentially, that is to say several steps take place concurrently. One or several of these steps can be omitted or implemented according to alternatives, with reference to
The user of the recording system must plan the scans and signal acquisitions to be performed. To do so, it configures the device 1 for recording the mapping. The user first configures the specific characteristics of the station. These are useful for executing the subsequent steps:
For the step E1 of pointing the antenna,
The recording device 1 comprises an antenna 2 control unit 4, called ACU, to point satellites whose orbital position Ps is pointed at an elevation greater than the minimum elevation El0 defined in the configuration step E0. The ACU 4 which contains a calculator, controls the motors of the positioner of the antenna 2 so that its axis of sight 21 points towards a pointing position, such as for example those described below. The ACU 4 of a station according to the state of the art which would communicate with a satellite SAT located at this pointing position calculates the evolution as a function of time t of the location S(Ps,t) of the pointing position, then the angle of the axis of sight 21 β[OS(Ps,t)] which will be noted for the sake of simplification β(Ps,t), where O is the geographical location of the station. By convention, the angle of the axis of sight 21 is determined by the azimuth and the elevation of the straight line OS(t), relative to the horizontal plane and to the geographic north. Also, the ACU 4 must first convert the geographical position O of the station into position in the geocentric reference frame for which the orbital parameters of the satellite are given. It is therefore necessary to take into account for this purpose the rotation of the earth on itself and around the sun.
Within the framework of classical mechanics and gravitational attraction by a single revolution star, the orbits obey Kepler's laws. In a geocentric reference frame, as illustrated in
These five first parameters make it possible to unequivocally determine an ellipse in space, that is to say the trajectory followed by the satellite which is called the orbit. The last parameter to describe an orbital position is the anomaly that allows knowing at any instant the angular position of the satellite on the ellipse. The anomaly can be defined in several ways: true anomaly v, eccentric anomaly or average anomaly, this choice does not matter for the invention.
The orbital position of a satellite drifts slowly, under the effects of variations of the earth's gravity, of the moon and sun tides, of the general relativity, and of the solar wind. These effects are noticeable over periods of several days. Most often, a satellite is assigned to an orbital position, it maneuvers to stay there. Some satellites are intended to change their orbital position (space monitoring, on-orbit service). The invention makes it possible to identify all these movements.
In the invention, the planning programmed in step E0 provides for moving the axis of sight 21 of the antenna 2 towards a sequence of successive prescribed pointing positions Pi distinct from each other, which are for example P1, P2, P3, P4, P5, P6, P7, P8, P9, P10 in
The axis of sight 21 must necessarily evolve continuously, also the ACU must interpolate the pointing positions between the instants ti and ti+1 to create a continuous scanning curve B(t). Two scanning modes are possible. The first one, called discrete scanning mode, consists in tracking a fixed pointing position then joining the next one as quickly as possible, given the maximum speed at which the positioner can move. In this case, the pointing position stops on Pi at ti and remains there for a certain time before meeting Pi+1 as quickly as possible. The second one, called regular scanning mode, consists in gradually varying the characteristics from the target pointing position to the next one. In this case, each pointing parameter gradually evolves from the value of the parameter of the pointing position Pi to that of Pi+1 between ti and ti+1. By construction, in the two scanning modes, we have for each instant ti: B(ti)=β[OS(Pi, ti)]=β(Pi, ti).
The ACU 4 must check at every instant that the elevation of the axis of sight 21 remains greater than the minimum value El0 defined in the configuration step E0. For each pointing position, the ACU must determine whether it respects the minimum elevation. During the scanning, as soon as a pointing position causes an axis of sight 21 whose elevation is lower than the minimum elevation El0, it ignores the pointing position and moves directly to the next one. For each sequence, the scanning curve B(t) traverses an arc portion, whose developed angle is denoted α. The latter will therefore be smaller than the arc whose scan has been planned in step E0, as long as part of this arc is at an elevation lower than the minimum elevation. When the ACU has completed the entire scan of each sequence, it can start again from the beginning. The time elapsed to complete all of the planned scanning sequences is called the revisit time R.
In the most general embodiment, the prescribed pointing positions Pi of each sequence are disposed in any order, but to optimize the revisit time R, it is advantageous to define an order such that the curve B(t) presents the developed angle α as short as possible, that is to say B(t) is the shortest path connecting the pointing positions Pi. If the sequence has pointing positions of different periods of revolution, the optimal solution will be very different in each sequence because the orbital positions will have evolved independently.
Also, in one preferred embodiment, the pointing positions of a sequence are chosen to all present the same orbital period, which guarantees that their relative distances will evolve little from one scan to another, and then makes it possible to define quasi-optimal sequences once and for all.
In one particular embodiment, quasi-circular orbits that is to say orbits with almost zero eccentricity, which concerns the vast majority of satellites, are scanned. The best-known circular orbit is the geostationary orbit. In this case, the orbital positions are on the equatorial plane and fixed for a fixed observer on the surface of the globe. The largest possible arc portion α is the measurement of the geostationary arc from its Eastern end to its Western end at the minimum elevation El0. If the sequence scans only the geostationary arc, the revisit time R as defined above is the time elapsed between two consecutive measurements of the same pointing or orbital position. For scanning a LEO orbit, the revisit time separates measurements which can be made at different anomalies because this revisit time is not necessarily a whole number of periods of revolution of the considered orbit.
Another special case consists in scanning the positions of a constellation of satellites. In general, a satellite constellation consists of a small number of periods of revolution (1 for GPS, Galileo, O3B, OneWeb, Iridium Next; 3 for the Kuiper project and the phase 2 of Starlink; 4 for the phase 1 of Starlink). At each of these periods Pr (corresponding to an altitude), the satellite constellation places a large number of satellites in circular orbits and on a series of orbital planes, all at the same inclination and evenly spaced, that is to say their ascending nodes are multiples of 360/p° with p distinct orbital planes. Here, the planned scan can consist in varying both the anomaly and the ascending node. For example, when the constellation is strongly inclined, it can be sought to scan the orbital positions corresponding to the location of each orbit which is closest to the polar axis, while traversing the successive planes, that is to say by varying the ascending node step by step in the increasing or decreasing direction. The scan obtained will form a circle about the polar axis and it has a small angular displacement to traverse the considered altitude of the constellation. When the position of the antenna is sufficiently close to the pole, then the axis of sight 21 of this scan always has a significant elevation and the sequence can be maintained indefinitely.
In
The device 1 comprises a chain 20 for radiofrequency receiving and digitally converting the radiofrequency signal X(t), wherein the chain acquires the radiofrequency signal X(t) in the form of digital samples X(n).
An example of receiving chain 20 downstream of the antenna 2 in a station is described below with reference to
The digitized signal X(n) is then recorded. The recording of the raw signal can be kept if needed, for the desired time, depending on the memory capacity 51.
In practice, the antenna 2 can generally be in the shape of a parabolic reflector 22, whose external edge 23 delimits an opening diameter 24. Of course, the antenna could be in the form of a reflector with a shape other than parabolic. The antenna 2 is not a perfect antenna with an infinitely fine beam. Also, it does not only collect the signal derived from its axis of sight 21 but a multitude of contributions coming from various directions which are weighted according to its radiation pattern at the considered frequency. The radiation pattern at the frequency f, an example of which is given in
The pattern D(γ, f) is a function of the frequency and of the depointing angle γ (which is the angular deviation with respect to the axis of sight 21, as represented by way of example in
In one embodiment, the prescribed successive pointing positions are isolated positions of interest. In another preferred embodiment, the objective of the scan is to collect in the process the signal derived from all the intermediate positions between two successive pointing positions prescribed in the sequence so as to detect active satellites that could be located there. For this to be met, these positions must be collected in the main lobe of the previous pointing position or of the next pointing position, as desired. More specifically, it is required that each intermediate position is located in the cone of lobe width θ around the previous or next pointing position, so as to guarantee a certain quality of collection of the signal which may come therefrom. Consequently, in this preferred embodiment of the invention, two successive pointing positions at which the spectral density is measured must correspond to tracking trajectories deviated by a value of axis of sight 21 less than the width θ of the main lobe, this in the time interval where we switch from one to the other. This condition is reflected by the inequality:
|β(Pi,+1,ti)−β(Pi,ti)|<θydBet/ou|β(Pi,+1,ti+1)−β(Pi,ti+1)|<θydB
When y is chosen equal to 1 or 2 dB, the considered antenna lobe is narrow and the signal collected at each intermediate pointing position is little distorted. On the other hand, with y equal to 3 or 4 dB, the antenna lobe is wider at the cost of greater distortion of the signal collected at the intermediate pointing positions.
During step E3, the recording device 1 comprises a calculator 3 able to calculate, from the samples X(n) of the digitized signal taken in a window around a date t (in the following the terms date t and instant are synonymous), a raw power spectral density DSb(Ps,t,f) at the frequency f in one (or several) of the frequency sub-bands Wx and for each pointing position traversing the sequence of prescribed successive pointing positions Pi(ti) on the dates t=ti. The calculator 3 and the number of central processing units (CPU) thereof are adapted as needed in computing power. The calculator 3 can be implemented by one (or several) processor(s), and/or one (or several) microprocessor(s), and/or one (or several) central processing unit(s), and/or one (or several) computer(s), and/or the like.
The device comprising the down-converter, the sampler of the radiofrequency chain 20 and the calculator 3 allows, for each sub-band Wx, doing the work of a spectrum analyzer. The advantages of the architecture in
The radiofrequency activity is revealed by the presence of spectral components of the collected signal which are higher than the background noise level. The radiofrequency activity recording therefore consists in measuring the power spectral density of the signal at each prescribed pointing position. To do so, a duration window F of the signal has to be chosen, its Fourier transform has to be calculated then its module has to be squared and the whole has to be divided by the duration of the window. This provides a raw estimate of the power spectral density which has a frequency resolution of 1/F and a significant noise-related level fluctuation. To limit this effect, the power spectral density can be averaged, using two possible methods. The first one consists in averaging q adjacent frequencies, the second one consists in averaging q consecutive measurements, therefore processing a signal duration D=qF. On an analog spectrum analyzer, this is called respectively video filtering or averaging. In both cases, the effective resolution is reduced by a factor q compared to what a raw Fourier transform allows over the same signal acquisition duration D. A frequency resolution df with an averaging of depth q therefore require an acquisition duration also called exposure time D=q/df.
According to one embodiment of the invention, the calculator 3 starts by cutting the digitized signal X(n) into windows of duration Di fixed respectively on each position Pi, comprising a number Mi of samples and therefore of duration Di=Mi/fe. Typically, if the scan is discrete, the window Di begins at the instant ti and stops before the ACU tracks the next pointing position, therefore before ti+1. If the scan is regular, then the window Di will be preferably centered on the instant ti. The spectral density calculated in step i therefore corresponds to a collection of the radiofrequency signal X(t) centered at the pointing position Pi. Each raw spectral density measurement, that is to say before any reprocessing, is therefore both a function of the frequency f in the sub-band Wx and of the succession of pointing positions Pi. It was seen in the description of step E2 that the antenna 2 collects the signals coming from the cone of width θ around the axis of sight. This therefore means that θ constitutes the spatial resolution of the spectral density measurements.
Between the instants ti and ti+1, the average pointing speed is defined as the angle of sight variation divided by the time interval. This is broken down into a relative scanning speed and a tracking speed according to the following formula:
[β(Pi+1,ti+1)−β(Pi,ti)]/(ti+1−ti)=Vp(ti+1)=[β(Pi,+1,ti+1)]−[β(Pi,ti+1)]/(ti+1−ti)+[β(Pi,ti+1)−β(Pi,ti)]/(ti+1−ti)=Vs(ti+1)+Vt(ti+1).
The choice of a minimum acquisition duration D to guarantee a quality of measured spectral density, that is to say its frequency resolution and the noise fluctuation, requires that the pointing positions follow each other at instants spaced by at least the exposure time: ti+1−ti>Di>D. Consequently, in the preferred embodiment where the axes of sight of the successive pointing positions are smaller than the lobe width θ, the relative scanning speed Vs which is the difference between the average pointing speed Vp and the tracking speed Vt of the current pointing position must remain less than θ/D.
The maximum scanning speed is determined by the exposure time and the side lobe width. This scanning speed is by definition zero if a pointing position is just tracked. The decomposition of the pointing speed into tracking and scanning speed applies to apparent speeds which are an average, regardless of the scanning mode. This is valid whether it is performed in a discreet mode or in a regular mode. In the particular case of scan of the Clarke belt, the tracking speed is zero because the orbital positions are fixed from the geographical position O of the station, therefore the pointing speed is equal to the scanning speed and therefore must remain less than θ/D.
The tracking speed is required by the celestial dynamics of the chosen orbital positions. It strongly depends on the period of revolution of the orbital positions within a scan. For its part, the scanning speed is limited by θ/D. When the expected quality of the power spectral density measurement, therefore the exposure time D have been chosen, the scanning speed is therefore proportional to the lobe width θ. Maximizing the lobe width therefore makes it possible to maximize the scanning speed, and therefore the pointing speed, and consequently minimize the revisit time R, whatever the sequences of pointing positions planned in step E0.
Based on this observation, the invention proposes, in one preferred embodiment, that the maximum opening diameter 24 of the antenna 2 is less than or equal to 6 meters. For example, the maximum opening diameter can be greater than or equal to 1 meter and less than or equal to 6 meters.
The antenna 2 can have a maximum opening diameter 24, which is less than or equal to 4 meters, being able to be provided at least for the receiving frequency sub-band Wx=Ku or other. For example, the maximum opening diameter of the antenna 2 can be greater than or equal to 1 meter and less than or equal to 4 meters, the antenna can be provided at least for the receiving frequency sub-band Wx=Ku or other.
The antenna 2 can have a maximum opening diameter 24, which is less than or equal to 6 meters, being able to be provided at least for the receiving frequency sub-band Wx=C or other. For example, the maximum opening diameter of the antenna 2 can be greater than or equal to 1.5 meters and less than or equal to 6 meters, the antenna can be provided at least for the receiving frequency sub-band Wx=C or other.
Thus, the invention provides an antenna size large enough to identify with certainty a signal at the minimum density threshold of EIRP (equivalent isotropically radiated power) of the objects sought. The antenna 2 is chosen to be of just sufficient size to be able to detect the signal from a satellite emitting at the equivalent isotropically radiated power threshold to be read.
For example, the antenna 2 can have a maximum opening diameter 24, which is less than or equal to 2.50 meters (2 meters and 50 centimeters), and can be provided for the receiving frequency sub-band Wx=Ku and/or C or others. For example, the maximum opening diameter 24 can be greater than or equal to 1.50 meters (1 meter and 50 centimeters) and less than or equal to 2.50 meters, being able to be provided for the receiving frequency sub-bands Wx=Ku and/or C or others.
For example, the antenna 2 can have a maximum opening diameter 24, which is less than or equal to 2.10 meters (2 meters and 10 centimeters), being able to be provided for the receiving frequency sub-bands Wx=Ku and/or C or others. For example, the maximum opening diameter 24 may be greater than or equal to 1.90 meters (1 meter and 90 centimeters) and less than or equal to 2.10 meters, being able to be provided for the receiving frequency sub-bands Wx=Ku and/or C or others. In the receiving frequency sub-bands Wx=Ku and/or C or others, this makes it possible to take an antenna 2, for example of the order of 2 meters in diameter. These choices allow the shortest possible revisit time R.
The maximum opening diameter 24 of the antenna 2, described above, is particularly advantageous in combination with the recording of refined power spectral densities DSa, calculated during step E6 described below, because it is this combination that makes it possible to detect a larger number of satellites with a shorter revisit time and a lower cost.
The mapping C is organized by pointing position Pi and comprises the power spectral density DSP for one or several pointing positions Pi. For each, it was possible to record the power spectral density DSP as a function of the frequencies f with a measurement made on the date t, since it corresponds to one of the successively prescribed pointing positions Pi for i ranging from 1 to N, for each recording sequence. The frequency f is a frequency in one of the receiving frequency sub-bands Wx. The recording can be sent directly for visualisation or for use by another system, but in the preferred embodiment, the mapping C is recorded.
To do so, according to one embodiment, the recording device 1 comprises a recording device 5 for recording a mapping C of the read power spectral densities DSP(Ps,f,t) having been measured by the antenna 2 to the date t while it was pointed at the pointing position Pi. The recording device 5 can comprise one or several permanent memories 51 or others to record the mapping C and/or one or several display screens 52 to display an image I of the mapping C and/or one or several physical outputs 53 to provide the mapping C or an image I thereof, and/or one or several modules 54 for processing the mapping C.
In one embodiment, the read power spectral density DSP(Ps,f,t) is chosen directly equal to the raw power spectral density DSb(Ps,t,f) calculated in step E3. In another embodiment, which will be described below in step E6, the read power spectral density DSP(Ps,t,f) is calculated by the calculator 3 with a reprocessing that refines the raw power spectral density DSb(Ps,t,f).
For each pointing position Pi of the mapping C, it is possible to keep the last spectral density recording DSP or keep any depth history, depending on the need and on the available memory capacity 51. The organization of the storage in memory 51 of the mapped pointing positions Pi is a priori independent of the succession of the prescribed orbital positions Pi. When the system has a multitude of antennas 2, the mappings established by each one are grouped together, knowing that in step E0, each one is preferably planned to track different orbits or portions of orbits, or more generally sequences of separate prescribed pointing positions Pi. Given the main lobe of the antenna, the DSP at the pointing position Pi actually incorporates spectral components collected in the cone of lobe width θ around the axis of sight 21 at the date t. The signal is therefore collected from an area of positions in the vicinity of the prescribed pointing position Pi. The satellites having an orbital period equal to that of the prescribed pointing position Pi remain in the vicinity thereof, but those having a slightly different orbital period PS drift regularly relative thereto. For example, in the particular case where a circular orbit is scanned, the mapping C incorporates satellites of the same plane having close quasi-circular orbits but with the same orbital period and it will be seen that these satellites oscillate in time around the prescribed pointing position Pi which is perfectly circular. A satellite of the same plane having a slightly lower or higher orbital period, although on a perfectly circular orbit, will appear to have an anomaly that increases or decreases in a linear manner.
A mapping C can thus be expressed graphically in easily interpretable images for objects whose relative positions vary little and slowly. The images I constituted to represent the mapping C therefore consist in forming the function of the power spectral density DSP for the last recording, otherwise a previous recording specified by the user, corresponding to a pointing position Pi of the given mapping according to a two-dimensional variable: the frequency and a variation parameter of the pointing position Pi.
In one embodiment, described below with reference to
In another embodiment, the variation parameter of the pointing position Pi is synthetically reconstituted from a mapping C which contains recordings of power spectral density DSP for a multitude of orbits at the same orbital period. For example, for a constellation as seen above, the pointing positions Pi have anomalies and variable ascending nodes, it is then possible to reconstitute an image I whose variation parameter of the pointing position Pi is the ascending node, even though the prescribed sequences Pi have been acquired in a different order, in particular by scanning each orbit.
The pixel value on each ordinate and on each abscissa of the image I represents the level A of the read power spectral density DSP, measured at the frequency f during a scan B(t). The spectral densities DSP are therefore disposed vertically in the image I, with a spectrum per abscissa of the image I. The level A of each read power spectral density DSP is represented by a color, typically according to a decreasing wavelength from blue to red, or a brightness level or a gray level, which varies unequivocally depending on the level A, for example in a monotonous, increasing or decreasing manner. For example, the level A is expressed in dBm in
As an alternative to an image where the pixel value represents the DSP function, it is also possible to make a projection of the surface of the curve of level A, function of the frequency and of the pointing position Pi. In this case, the level A is represented along a third side axis perpendicular to the abscissa and ordinate axes, which provides a three-dimensional surface which is then projected into two dimensions. It is possible to make an isometric projection of the surface in a grid, according to a succession of section lines disposed with an offset. It is also possible to constitute an image by visualizing the cutting planes that are moved forward or backward, but it is not possible to represent everything in a single image. Other image constitution techniques exist in the state of the art and are applicable.
The high values of Level A, well above the background sky noise level, reveal the presence of active satellites. The formation of the images I therefore constitutes a preferred embodiment to represent the mapping, because it makes it possible to easily visualize the use of the radiofrequency spectrum by the artificial celestial objects. Typically, when the constitution of the image is a very rapid process, it is not necessary to record the result, but in the preferred embodiments, a history of the last displayed images I is recorded.
The exploitation and the processing of the mappings with the aim of identifying satellites can be done by a human operator, but can also be largely automated. These optional processing operations of the raw mapping DSb can be performed by one or several mapping processing modules 54 in
A sending to the tracking system can be provided to refine the precise orbital position of each spectrum portion and thus separate the co-located objects, that is to say whose orbital positions are close enough to be captured in the main lobe of a single spectrum measurement.
Successive mappings and differential processing In one embodiment, called consecutive mapping embodiment and described below with reference to
According to an improvement of this embodiment, called differential mapping improvement embodiment, described below with reference to
In one embodiment, the calculator 3 and the recording device 5 are configured to discriminate the spatial lines of the power spectral density DSP by peak detection, that is to say local maximum detection, algorithms. The peaks with their A-level can be used to summarize the content of the longitudinal curve of level A of power spectral density, which is illustrated in
The peaks at a frequency f are the best estimation of the position of a satellite emitting a signal at that frequency f. It is possible to attempt to approximate these results with the declarations provided by the operators in order to name the satellites discovered and indicate those that are not declared.
In one embodiment, called spectral density recombination embodiment, the calculator 3 and the recording device 5 are able to reprocess the mapping by transforming the raw power spectral density DSb(Pi,ti,f) into a refined power spectral density DSa by application of an inverse convolution processing aimed at reducing the effect induced by the radiation pattern of the antenna 2. Several methods are possible for this purpose. Thus, in a non-limiting implementation described below, the calculator 3 will proceed by linear combination of several successive raw power spectral densities DSb(Pi,ti,f). Thus, the refined power spectral density DSA or DSa(Pi,ti,f) is a linear combination of the raw power spectral densities DSb(Pi+k,ti+k,f) taken at the pointing positions Pi of the instants ti+k, where k takes the values ranging from −m to +m.
It was seen in step E2 that the antenna pointed at an axis 21 of sight β collects the signal from a satellite S emitting from the direction of angle γ according to a received power proportional to Pe·D(γ·β,f), where Pe is the EIRP of the satellite and D is the pattern of the antenna 2 of the station. According to this formula, the antenna pattern has the effect of spreading the contribution of the satellite over the width of the main lobe θ and creating responses that are artifacts at the positions of the side lobes. When it comes to a group of satellites Sj of EIRP being Pj for j varying from 1 to P, a power is received which is the sum of the contributions of each satellite Sj, i.e. DSb(β,f)=ΣPj·D(γj·β,f). In the mathematical sense of the distributions, it is possible to represent the spectrum of the signal derived from the sky at a frequency f as a series of Dirac lines, positioned at the axes of sight γj and whose amplitude is the EIRP Pj. The distribution P=ΣPj·δ(γ·γj), where δ is the Kronecker symbol designating a Dirac at position 0, illustrated in
∫ΣPjδ(γ−γj)D(γ−β)dγ=ΣPjD(γj−β)
where δj(γ) is the Dirac impulse at the axis of sight γj. Formally, the integral calculates a convolution at the position β of the ideal mapping formed by Dirac lines with an impulse response which is the radiation pattern of the antenna 2, flipped by 180°.
The convolution of the equation above applies with an integration variable which is the angle of sight of the satellites and not the pointing position Pi. When all the satellites collected during the scan can be considered as being on the scanning curve B(t), then the convolution which is natively two-dimensional just like the angles of sight, can be compared to a one-dimensional convolution, performed along the scanning curve B(t). This is considered to be true in the case of scan of a single orbit, by varying the only anomaly, when the satellites collected in the considered sub-bands Wx are necessarily on this same orbit because they obey the international regulations of the ITU preventing the interference. In this case, the impulse response of another one-dimensional convolution, called inverse convolution is determined, which in a way makes it possible to neutralize the effects of the convolution by the antenna pattern, that is to say the combination of the two convolutions approximates the convolution by a Dirac and presents a finer main lobe as well as significantly reduced side lobes.
When the scanned orbit is the geostationary arc (Clarke belt), the angle of sight γ of a pointing position Pi is fixed, there is therefore a one-to-one relationship between the two. The instant t at which the antenna 2 made the collection at the axis of sight β does not matter. The impulses D(γ·β) as a function of the variable γ translate when the angle of sight β of the antenna varies but deform very little. The inverse convolution therefore gives excellent results as shown in
In one embodiment called inverse spatial convolution embodiment, which is a sub-mode of the recombination of spectral densities, described below with reference to
In practice, the improvement of the response by this inverse convolution operation is significant, with side lobe artifacts which disappear and a main lobe whose width is almost reduced by half. It is seen in
The mapping C can be reprocessed by tracking, which makes it possible to improve the spatial resolution. Thanks to the calculation of correlation of the signals picked up by antennas remote from each other, the orbital tracking system allows a very accurate resolution of the orbits, which approximates the resolution of perfect Diracs since accuracies of the order of 150 meters at the geostationary arc are possible.
Once the spectral lines have been identified in the mapping C by the device 1 according to the invention, the individual characteristics of these spectral lines can be transmitted to the known orbital tracking system which will be able to perform new remote acquisitions, thus making it possible to separate all the celestial objects by processing operations based on the correlation. It then possible to individually track these celestial objects using techniques requiring more computing power or acquisition duration, in particular by the correlation of remote acquisitions used in the known orbital tracking system. The spatial separation of the objects detected at a pointing position Pi by the device according to the invention can thus be performed by the orbital tracking system. The technique of determining orbits by the orbital tracking system can be used to separate the objects detected at a given pointing position Pi by an acquisition at this pointing position Pi alone, which minimizes the duration of the orbit determination process. The known orbital tracking system will perform the correlation of remote acquisitions made at the tracked orbital positions alone.
The orbital tracking system and the radiofrequency activity recording device according to the invention can use identical antennas 2, or share the same antenna farm. Indeed, the constraint of a sufficient signal-to-noise ratio to detect the signal derived from the satellites is the same for these two systems. The mapping established by the system for recording the radiofrequency activity provides a spatial resolution which, once refined by one or several methods of step E6, makes it possible to locate the emission in a cone of width smaller than the main lobe width of the antenna. The tracking is therefore immediate, the orbital tracking system simply collecting this pointing position Pi alone from its remote sites, without having to perform any scan, unlike the radiofrequency activity recording system. The acquisition by the tracking system is therefore optimized in time. Finally, the mapping advantageously replaces the use of operator declarations.
The combination of the two systems argues for the choice, according to one embodiment of the invention of small antennas 2. The device is then cheaper, faster and ultimately more accurate than a single antenna thanks to the reprocessing by the orbital tracking system.
Of course, the embodiments, characteristics, possibilities and examples described above can be combined with each other or selected independently of each other.
The invention thus allows space monitoring by observation of the use of the radiofrequency spectrum and makes it possible to map the spectral situation in space. The method and the recording device 1 according to the invention allow a detection of any earth-orbiting active emitting object without using extrinsic information from the operators. The invention makes it possible to track the activity of satellites, whether LEO, MEO or geostationary satellites. The invention makes it possible to determine the life cycle of the space objects, that is to say the evolution of their use of the frequency spectrum. In terms of performance of the device, it is possible, if necessary, to improve the spatial resolution (obtained by a sensor composed of a single measuring antenna 2) by application of complementary processing operations, in particular those using the inverse convolution. The orbital positions emitting the signal can be tracked by a system using the same antennas 2 at remote sites. The mapping system described by the invention can substitute the use of the declarations of operators, making both services autonomous without reducing the performance of the known orbital tracking system.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2200924 | Feb 2022 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2023/050136 | 2/2/2023 | WO |
