FR2793620A1 - Ground station/low orbit satellite communications using stationary satellite intercommunications during maximum doppler time window period. - Google Patents

Abstract

The ground station to low orbit satellite communications technique has a low orbit satellite (20) communicating with a geostationary satellite (101). Communications between the two satellites are carried out in the time window (t1,t2) where the doppler moves to a maximum value.

Description

 METHOD OF COMMUNICATION BETWEEN A LAND STATION AND A SATELLITE NON-GEOSTATIONARY. The present invention relates to a communication method between a terrestrial station and a non-geostationary satellite via at least one service providing satellite, for example a geostationary satellite forming part of a constellation of satellites providing a communications infrastructure for mobile systems.

There is currently a strong interest in developing low-cost programs for scientific and terrestrial satellite observations. A crucial point for this application is the communication between the non-geostationary satellite, in particular a LEO low orbit satellite, or a MEO medium orbit, and a ground control station, since the high speed of the LEO or MEO satellite limits the possibilities of communication. The communication time between the non-geostationary satellite and the ground station, which depends on the altitude and inclination of the satellite in orbit, as well as the latitude of the ground station, is generally very short.

In addition, the communication networks between LEO low-orbiting satellites and GEO geostationary orbiting satellites offer valuable solutions for making contacts between two fixed or mobile points on Earth. It therefore seems very appropriate to use the same network to make communication links between small satellites and their ground control stations as this would lead to substantial savings on infrastructure and mission costs. Indeed, operational costs often represent more than a third of the total budget of a satellite program.

Such a concept that one or more satellites of an MSS satellite mobile communications system are used to communicate between a terrestrial station and a non-geostationary satellite has been suggested by the article by Hans-Jurgen Fischer et al. . The Potentiality of Commercial Satellite-Based Communication Networks for Telemetry and Commanding Small Satellites, published in the 11th AIAA / USU Conference on Small Satellites, September 1997, pages 1-7.

However, this article does not provide a detailed analysis of the feasibility of the concept or its operational potential. In addition, column 2 on page 3 and column 1 on page 4 of this article report a number of significant constraints on such a system.

A similar concept has also been described in the article by William E. RYAN et al. entitled "Design of a Low-Orbit-to-Geostationary Satellite Link for Maximal Throughput" in IEEE Transactions on Communications, Vol 45, No. 8, August 1997, 988-995.

The subject of the present invention is a communication method of the aforementioned type, in which at least some of the constraints of the known methods are improved, or even eliminated, by means of one or more improvements relating to the means of communication between the non-geostationary satellite and the or service delivery satellites, especially for mobiles.

For this purpose, the invention relates to a method of communication between a terrestrial station and a non-geostationary satellite via at least one service providing satellite, characterized in that the non-geostationary satellite has a terminal of embedded communication capable of communicating with said service providing satellite, and in that it implements a communication between the non-geostationary satellite and a said service providing satellite during time windows where the shift or Doppler shift , ie the frequency variation due to the Doppler effect, has a maximum value or is close to its maximum value.

Preferably, said service providing satellite is geostationary, and may be part of a geostationary satellite system such as INMARSAT.

The method according to the invention is particularly applicable to the case of non-geostationary satellites placed in a low orbit (LEO).

It is particularly advantageous that each time window frames a time when the Doppler shift has said maximum value.

The method can implement, on the onboard communication terminal, a determination of the Doppler displacement and a compensation thereof, for example by performing a frequency shift to follow the Doppler shift, and compensate for it. The communication terminal may then have a return path to the service providing satellite which also provides a frequency offset compensating for the Doppler effect.

The on-board communication terminal on the non-geostationary satellite may be coupled to at least one fixed antenna pointed to communicate with said service providing satellite, in particular for mobiles, and in particular it comprises two antennas, one of which is pointed towards the forward and the other rearward in correspondence with the two orbital positions in which can be arranged the aforementioned time windows.

Alternatively, the communication terminal on the non-geostationary satellite can be coupled to a steerable antenna.

Other features and advantages of the invention will appear better on reading the following description, given by way of non-limiting example, in connection with the drawings in which - Figure 1 is a general diagram illustrating the concept of the invention; FIG. 2 illustrates the implementation of the concept of the invention in the context of a constellation of INMARSAT service providing MSS satellites; - Figure 3 illustrates the geometry of the system; FIG. 4 illustrates a time slot configuration according to the invention; FIG. 5 illustrates a Doppler compensation mode according to the invention.

As shown in FIG. 1, the ground station 1 is connected by a bidirectional link 3 to a referenced service providing satellite 10, which is part of a mobile satellite system or MSS (Mobile Satellite System) for example. "), which in turn is connected by a bidirectional communication link with a non-geostationary satellite 20, the communication to the earth station 1 and the user satellite 20 being relayed by the service satellite 10. The satellite 20 has a terminal embedded communication 21, which is for example a TT & C telemetry tracking and control transponder boarded on the satellite 20. The satellite 10 can be part of an MSS system which is for example the INMARSAT geostationary system or any another geostationary satellite system capable of providing services to terrestrial terminals. t be part of a multi-satellite constellation system.

In the remainder of the description, the system will be, solely by way of example, described in the context of a geostationary system of the INMARSAT type. It will be understood that, in the context of the present invention, other satellite constellations, geostationary or not, such as EUTELSAT, and either INTELSAT, GLOBALSTAR (low orbit system) or ICO GLOBAL (medium orbit system), could be implemented. The land station can be of known type and it can even be reduced to standard terminals as simple as a mobile phone coupled to a personal computer equipped with a modem.

To return to the INMARSAT system, it provides a communications infrastructure with almost global coverage. Some of the services provided include INMARSAT-A, the original analog boat service, INMARSAT-B, the successor digital version, INMARSAT-C, which is the first personal communications satellite service, INMARSAT-M, which is the world's first pocket-sized mobile phone, and AERO-I, a telephone-fax and data service for short and medium-range planes that work with INMARSAT-3 narrow beams .

The INMARSAT-3 satellite constellation includes four operational geostationary satellites, plus one on a reserve orbit, and provides complete geographic coverage to latitudes of plus or minus 70 and introduces narrow beam technology that allows for linkages from mobile to mobile.

In the context of the present invention, however, wide-coverage INMARSAT beams are preferably used to provide complete coverage of the planet.

Each INMARSAT service has associated bandwidth, and to ensure optimal use of available bandwidth and avoid interference between adjacent channels, the carrier frequency accuracy of mobile terminals must be less than 300 Hz. For aeronautical services, Doppler compensation is implemented in the mobile terminal to correct Doppler shifts up to 2 kHz.

The user satellite 20 is registered in the INMARSAT environment as a mobile station. I1 transmits and receives data through an INMARSAT-compatible transponder 21. This concept is represented in FIG. 2. Earth station 1 represented in the form of a PC equipped with a modem is in communication with a communication network RES provided with an antenna 2 which is in communication with bidirectional links 4 in band C with one or more satellites, for example 101 and 102, of the INMARSAT system. It is also possible for the terrestrial station 1, for example a mobile telephone, to be in direct connection via a bidirectional L-band line with a satellite, for example 101, when the latter is directly accessible. In general, communications are routed through earth stations 2 which serve as a portal to the network providing bidirectional access.

The transmission between the satellite 20 and the Earth, called telemetry transmission, can be carried out as follows. The user satellite 20 selects a ground station and dials the access codes for the desired destination, adding the information of identification. The satellite 20 transmits the information on a channel that has been specially affected, and the ground station directs the call through the public telecommunications networks to the desired destination.

To allow a call between the Earth and the user satellite, a procedure is started from the ground station 1 to direct the call to a ground station belonging to the INMARSAT network. The user satellite call number 20 is a unique number that includes a satellite regional code and an INMARSAT code number for the user terminal. The INMARSAT relay station transmits this information on a channel specifically assigned by the network coordinator. The object of the invention is to establish communication intervals which take into account the Doppler effect.

The frequency variation due to the Doppler effect, here called Doppler shift, results from the relative movement between the user satellite 20 and the satellites of the INMARSAT system, for example the satellite 101. The relative speed depends essentially on the orbit of the user satellite 20 and its inclination. The geometry is shown in FIG. 3. The Doppler shift is equal to 0 when there is no "radial" relative movement between the user satellite and the satellite 101. This occurs when the satellite 20 and the satellite 101 are at the nearest point (point A) or at the farthest point A ', but in the latter case, there is an eclipse effect due to the earth T between the two satellites.

The largest Doppler displacement occurs at the edge of coverage C (instant to) when the angle between the speed vectors is minimal (or close to 180 in the symmetrical case where the satellite 20 and the satellite 101 travel in opposite directions).

In the operations implementing the INMARSAT system, the largest Doppler shift occurs at an orbital inclination of 98 when the ascending or descending node of the user satellite coincides with the longitude of an INMARSAT satellite. The maximum Doppler shift decreases slightly as the altitude of the satellite 20 and its inclination increases. These values are of the order of 40 kHz. They range from approximately 38.4 kHz to 42.4 kHz for orbital inclinations of the user satellite between 0 and 98, and altitudes between 300 km and 800 km.

The high levels of these Doppler displacements do not make it possible to use the standard INMARSAT services without Doppler compensation being applied to the user satellite 20. Such compensation can be made only in relation to a single satellite. Therefore, and to avoid interference with other satellites, it is recommended to use a directional antenna. It is possible to choose a specific frequency band which makes it possible to encompass the total variation of frequencies due to the Doppler effect of the user satellite 20. In the case where an omnidirectional antenna is used by the user satellite, this bandwidth is preferentially allocated. to adjacent ocean regions to avoid interference.

To achieve a cost-effective data link with a low or medium-low orbit satellite, it is possible to focus on the INMARSAT standard terminal services, which provide the simplest solutions.

According to the invention, the use of the standard services of the INMARSAT system is achieved by restricting the communications to portions of the orbit of the user satellite 20 where the Doppler gap is in a window of given values. Since the Doppler shift between the user satellite and the INMARSAT satellite is dependent on the selected INMARSAT satellite, this scenario implements a directional antenna on the user satellite 20 to avoid interference with other INMARSAT satellites. As has been previously shown, the Doppler displacement rate as a function of time (Doppler rate) is highest when the satellite passes the point A, whereas this rate is the weakest at the edge of coverage C. According to FIG. According to the invention, the most appropriate position for the communication window is to position it in the vicinity of the maximum Doppler displacement corresponding to the coverage edge C (instant to), and therefore to the lowest Doppler rate.

Another characteristic of this region of the user satellite orbit 20 is that the approach angle between the satellite 20 and the satellite 101 has the lowest rate of change. This is of interest if one considers the operation with a fixed antenna with medium gain to maximize the time during which the INMARSAT satellite 101 is in view of the beam of the antenna.

Figure 4 illustrates a configuration that corresponds to such a scenario. By way of example, a Doppler displacement frequency of between 2.5 kHz has been considered, namely a value which is compatible with the performance of the aerospace terminals. The center frequencies of the Tx and Rx signals are shifted by a value equal to the maximum Doppler value, minus 2.5 kHz. In this range, the residual Doppler effects are compensated by the known algorithms normally used in AFRO terminals. FIG. 4 denotes by E1 (time t1) the nearest point corresponding to the acceptable limit value of the Doppler shift (angle α1), and by E2 (time t2) the furthest point in the orbit corresponding to the other limit value of the Doppler shift (angle a2). Point E2 is likely to be beyond eclipse point F. In the most unfavorable case (altitude of 300 km, inclination of 98), a communication time of 11.2 min is obtained, which corresponds to at a rate of 12.4% of the orbit. The orbital angle range for which the INMARSAT satellite is visible by the user satellite is shown in the table below.

= angle d'éclipse = variation angulaire correspondant à la fenêtre

de communication Dans les conditions ci-dessus, l'angle d'approche entre le satellite 20 et le satellite 101 varie entre -27,3 et +29,6 , ce qui fait qu'une antenne fixe aurait besoin d'une largeur de faisceau >_60 . On peut utiliser une antenne à faible gain de type AERO-I qui présente un gain de 6 dBi et une largeur de faisceau à demi-puissance de 90 Dans l'exemple ci-dessus, le déplacement Doppler a été arbitrairement limité à 2,5 kHz. En relâchant cette contrainte, il serait possible d'augmenter le temps de contact entre le satellite 20 et les satellites INMARSAT, mais ceci impliquerait la mise en #uvre de bandes passantes plus larges et donc un plus faible gain d'antenne conduisant à une réduction du débit de données pouvant être transmises. Le durcissement de cette contrainte aurait l'effet inverse, c'est-à-dire de réduire les temps de contact, mais permettrait en contrepartie d'utiliser des antennes à gain plus élevé avec des bandes passantes plus étroites et des taux de transfert de données plus élevés. Tilt <SEP>% <SEP> of
<tb> of the <SEP> satellite <SEP> al <SEP> a2 <SEP> has, <SEP> Da <SEP> the orbit
<tb> 20
<tb> 1) <SEP><U> Altitude </ U>
<tb><U> 300 <SEP> km </ U>
<tb> 0 <SEP> 53.1 <SEP> 110.9 <SEP> 98.5 <SEP> 45.4 <SEP> 12.6
<tb> 28.5 <SEP> 53.3 <SEP> 110.8 <SEP> 98.5 <SEP> 45.2 <SE> 12.6
<tb> 51.6 <SEP> 53.4 <SEP> 110.6 <SEP> 98.5 <SEP> 45.1 <SEP> 12.5
<tb> 98 <SEP> 53.9 <SEP> 110.1 <SEP> 98.5 <SEP> 44.6 <SEP> 12.4
<tb> 2) <SEP><U> Altitude </ U>
<tb><U> 800 <SEP> km </ U>
<tb> 0 <SEP> 51.9 <SEP> 111 <SEP> 108.6 <SEP> 56.7 <SEP> 15.8
<tb> 28.5 <SEP> 52 <SEP> 110.9 <SEP> 108.6 <SEP> 56.6 <SEP> 15.7
<tb> 51.6 <SEP> 52.3 <SEP> 110.6 <SEP> 108.6 <SEP> 56.3 <SEP> 15.6
<tb> 98 <SEP> 52.8 <SEP> 110.9 <SEP> 108.6 <SEP> 55.8 <SEP> 15.5

= eclipse angle = angular variation corresponding to the window

In the above conditions, the approach angle between the satellite 20 and the satellite 101 varies between -27.3 and +29.6, so that a fixed antenna would need a width of beam> _60. An AERO-I low gain antenna with a gain of 6 dBi and a half-power beamwidth of 90 can be used. In the example above, the Doppler shift was arbitrarily limited to 2.5 kHz. Releasing this constraint, it would be possible to increase the contact time between the satellite 20 and the INMARSAT satellites, but this would imply the implementation of wider bandwidths and therefore a lower antenna gain leading to a reduction. the data rate that can be transmitted. The hardening of this constraint would have the opposite effect, ie to reduce the contact times, but in return would allow the use of higher gain antennas with narrower bandwidths and higher transfer rates. higher data.

The total contact time for each orbit traversed by the user satellite 20 could be increased as follows - it would first be possible to operate with more than one INMARSAT satellite. The maximum Doppler shift is a function of the angular separation between the orbital plane of the satellite 20 and the INMARSAT satellite. This would however involve selecting different transmission frequencies for each communication with a different INMARSAT satellite.

the contact time could be doubled by using for the satellite 20 two fixed antennas, one of which points forward and the other towards the rear. In this case, different transmission frequencies should be assigned to each of these antennas.

- A combination of operations with multiple antennas and multiple satellites could also increase communication time. In connection with FIG. 5, a Doppler shift compensation scenario will now be described. The bandwidth required to transmit a 4.8 kbits / second telemetry signal is 10 kHz, assuming BPSK modulation and half rate convolutional encoding, ie approximately 5 kHz for the signal data. proper and 5 kHz to take into account the Doppler shift. If we compensate for this Doppler shift, it appears that to obtain a data rate of 4.8 kbit / s, the bandwidth can be reduced by half, at 5 kHz. A Doppler compensation is currently put into a work in the INMARSAT Aero terminal up to 2 kHz values. This is implemented by using the attitude and position of the satellite to pre-adjust the transmission frequency so that it reaches the INMARSAT satellite at the correct frequency and by shifting the receive frequency to allow acquisition of incoming signals.

Such a technique can be applied to the system according to the invention for performing a Doppler displacement compensation by programming the acquisition frequency of the satellite terminal 20 in an open loop so that it follows the received carrier at its offset frequency due to the Doppler effect. For the return link (telemetry), the satellite terminal 20 shifts its transmission frequency so that it is received by the INMARSAT satellite at the desired channel frequency. In order to allow the calculation of the offset frequencies necessary to obtain the Doppler compensation, it is necessary to know the orbital parameters of the INMARSAT satellites and the user satellite 20. These parameters can be pre-loaded and updated regularly. on board the user satellite 20. The use of an on-board GPS system could also be envisaged, since it would make it possible to provide the user satellite with the information necessary for determining the Doppler shift compensation to be implemented.

With the compensation of the Doppler effect, the possibilities of communication from a fixed antenna pointing upwards were taken into account. This configuration offers the advantage of operating by using the so-called "Global" mode of the INMARSAT system while benefiting from the planetary coverage of this system. This configuration is shown in FIG. 5. The maximum contact time that can be obtained with a single INMARSAT satellite for a user satellite whose plane coincides with the longitude of the INMARSAT satellite is shown in the table below.

Altitude <SEP> Half-angle <SEP> Proportion <SEP> Half-angle <SEP> Proportion
<tb> of <SEP> of <SEP> of <SEP> of <SEP> of
<tb> satellite <SEP> coverage <SEP> the orbit <SEP> coverage <SEP> the orbit
<tb> 20 <SEP> (AFRO <SEP> I) <SEP> covered <SEP> (AFRO <SEP> H) <SEP> covered
<tb> (km) <SEP> (AFRO <SEP> I) <SEP> (AFRO <SEP> H)
<tb> 300 <SEP> 38.6 <SEP> 21.4% <SEP> 16 <SEP> 8.9%
<tb> 500 <SEP> 38.4 <SEP> 21.3% <SEP> 16 <B> 0 </ SEP> 8.9%
<tb> 700 <SEP> 38.2 <SEP> 21.2% <SEP> 15.9 <SEP> 8.8%
<tb> 800 <SEP> 38 <B> 0 </ B><SEP> 21, <U> 1 </ U>% <SEP> 15.8 <B><U> 0 </ U></B>><SEP> 8.8% The AFRO I antenna has a half-power beamwidth equal to 90, and this value is 38 for the AERO-H antenna.

With the antenna AERO-I, the maximum rate of the ascending link (command) is between 2.19 and 4.79 kb / s and that of the return line (telemetry) is between 5.06 and 8, 76 kb / s.

With the AERO-H antenna, the maximum rate of the ascending line is between 8.7 and 19.6 kb / s and that of the return link is between 19.7 and <B> 33.9 </ B> kb / s.

The Doppler offset compensation of the user satellite not only makes it possible to use the available spectrum more efficiently, but also facilitates the use of the so-called "Global" mode of the INMARSAT system. This significantly increases system performance and in particular higher rates can be achieved. In the case of this scenario, the contact time can be increased as follows - it is possible to operate using more than one INMARSAT satellite, - the contact time with a single INMARSAT satellite can be increased by using two fixed antennas, each being shifted up half of their beamwidth, with switching between these two antennas

Claims (11)

1) A method of communication between a terrestrial station and a non-geostationary satellite via at least one service providing satellite, characterized in that the non-geostationary satellite (20) has an on-board communication terminal (21). ) adapted to communicate with said service providing satellite (101), and in that it implements a communication between the non-geostationary satellite and a said service providing satellite during time windows (t1, t2 ) where the Doppler shift has a maximum value or is close to its maximum value. 2) Method according to claim 1, characterized in that said service providing satellite (101) is geostationary. 3) Method according to claim 2, characterized in that said service providing satellite (101) is part of a geostationary satellite system. 4) Method according to claim 1, characterized in that said mobile service provisioning satellite (101) is non-geostationary. 5) Method according to claim 4, characterized in that said mobile service providing satellite (10l) is part of a non-geostationary satellite system. 6) Method according to one of the preceding claims, characterized in that said non-geostationary satellite (20) is in a low orbit (LEO) or average (MEO). 7) Method according to one of the preceding claims, characterized in that each time window (t1, t2) frames a time (ta) where the Doppler displacement has said maximum value. 8) Method according to one of the preceding claims, characterized in that it implements, on the onboard communication terminal (21), a determination of the Doppler shift and their compensation thereof. 9) The method of claim 8, characterized in that the communication terminal (21) has a return path to the service providing satellite which provides a frequency offset compensating the Doppler effect. 10) Method according to one of the preceding claims, characterized in that the onboard communication terminal (21) on the non-geostationary satellite (20) is coupled to at least one fixed antenna (ANT) pointed to communicate with said satellite supplying service (101). 11) Method according to one of claims 1 to 10, characterized in that the onboard communication terminal (21) on the non-geostationary satellite (20) is coupled to an orientable antenna.