GB2541370A

GB2541370A - Satellite beam monitoring

Info

Publication number: GB2541370A
Application number: GB1513179.0A
Authority: GB
Inventors: Cooke Jason; Bestwick David; Wilkinson Guy; Lucas Frederic; Bougouin Theo; Bonafe Bernard
Original assignee: AVANTI COMMUNICATIONS GROUP PLC
Current assignee: AVANTI COMMUNICATIONS GROUP PLC
Priority date: 2015-07-27
Filing date: 2015-07-27
Publication date: 2017-02-22
Also published as: GB201513179D0; WO2017017101A1

Abstract

A beam 402 emitted by a satellite 110 produces a footprint on the ground typically a circle or ellipse, A. The power of the beam, p(r), varies across the footprint and falls off most steeply around the periphery of the footprint, P. Multiple monitoring stations, 406(1), 406(2), 406(3), are distributed around the periphery of the footprint. Each of these monitoring stations measures the power of the beam at its respective location and the power measurements are used to detect beam misalignment. A control signal is transmitted to the satellite to correct the misalignment. Either a user beam or a gateway beam may be monitored and gateway beam power measurements may be used to correct the alignment of the user beam. There are preferably at least eight monitoring stations.

Description

SATELLITE BEAM MONITORING

Technical Field

This disclosure is In the field of satellite communication, and relates in particular to monitoring of satellite beams.

Background

Some regions of the world such as rural, developing or isolated areas often have limited communication infrastructure where high speed broadband through traditional, ground-based (i.e. wired) means is not feasible. Providing an internet link via satellite enables such regions to obtain modem standards of internet access without the need to build a large amount of new infrastructure on the ground. Furthermore, satellite-based internet access can even be used as an alternative to ground-based links in regions that do have a developed communication infrastructure, or as backup to such infrastructure in case a ground-based link fails.

Figure 1 gives a schematic overview of a system 100 for providing access to a network, which is an internet 102 i.e. a wide area internetwork such as that commonly referred to as the internet (capital I). The system 100 comprises a gateway Earth station (gateway) 104, a satellite 110 in orbit about the Earth {labelled Έ" in various figures), and one or more client systems 112 remote from the gateway 104 and iocated In a region on the Earth's surface to which internet access is being provided. The gateway 104 comprises a satellite hub 402 connected to the internet 102, and at least one gateway antenna 106 connected to the hub 402. Each of the client systems comprises an antenna 114, connected to a satellite modem 420. The satellite 110 is arranged to be able to communicate wirelessly with the hub 402 of the satellite gateway 104 via the gateway antenna 106, and with the modems 420 of the client systems 112 via the antennae 114, and thereby provide a satellite Sink 107 for transmitting internet traffic between the source or destination on the internet 102 and the client systems 112. For example the satellite Sink 107, hub 402 and modems 420 may operate on the Ka microwave band (26.5 to 40 GHz). The sateiiite link 107 comprises a forward Sink 107F for transmitting traffic originating with an internet source to the client systems 112, and a return link 107R for transmitting traffic originating with the client systems 112 to an internet destination.

The hub 402 serves (i.e. provides an internet access service to) the client systems 112 so that internet traffic can be transmitted and received between the client systems 112 and the internet 102 via the satellite link 107 and the hub 402. Though not shown in figure 1, the gateway may comprise multiple such hubs, each serving a respective subset of client systems. in one mode! the operator of the satellite 110 and/or gateway 104 provides bandwidth to a downstream internet service provider (iSP), who in turn provides an internet access service based on that bandwidth to a plurality of end users 118. The end users 118 may be individual people (consumers) or businesses. Depending on implementation, the client systems 112 may comprise a central satellite base station run by the ISP (the base station comprising an antenna 114 and modem 420), and a local communication infrastructure providing access onwards to the equipment of a plurality of users within the region in question. E.g. the Socai communication infrastructure may comprise a relatively short range wireless technology or a local wired infrastructure, connecting onwards to home or business routers or individual user devices, Alternatively or additionally, the client systems 112 may comprise Individual, private base stations each with its own satellite antenna 114 and modem 420 for connecting to the satellite 110 and local access point for connecting to one or more respective user devices. In this case the ISP does not necessarily provide any extra infrastructure, but acts as a broker for the bandwidth provided by the satellite 110, For example an individual femtocel! or pscocell could be located in each home or business, each connecting to a respective one or more user devices using a short range wireless technology, e.g. a local RF technology such as Wi-Fi,

Referring to Figure 2 by way of example, the satellite 110 is deployed in a geostationary orbit and arranged so that its field of view or signal covers roughly a certain geographic region 200 on the Earth’s surface, Figure 2 shows South Africa as an example, but this could equally be any other country or region within any one or more countries (e.g. a state, county or province, or some other non-politically defined region).

Furthermore, referring to Figures 2 and 3, using modem techniques the satellite 110 may be configured as a spot-beam satellite, so that the communications between the satellite 110 and the client system(s) 112 in the covered region 200 are divided amongst a plurality of spatially distinct beams 202, referred to herein as “user beams”.

The term "beam11 refers to a signal emitted by a sateliite having a signai power that is concentrated within a volume of space or “lobe", typicaiiy a signal cone i.e. in which the emitted signai is approximateiy confined. The beam thus covers only a limited geographic area on Earth, whereby only receiving equipment within the geographic area can properly receive the signal. The lobe has a size and shape that is set by physical characteristics of an antenna on board the sateiiite which emits the beam, in particular the directivity of the antenna. The directivity may be determined by the geometry of the antenna, for example where the directivity is achieved using a paraboiic dish antenna on board the sateliite, or the beam may be generated based on beam forming, whereby the directivity is determined by phase and amplitude relationships between different antenna elements within a phased array antenna on board the satellite. These two approaches can also be combined. Where signals are also received by the same antenna from the ground, signal reception is approximateiy confined to the same lobe.

Each user beam 202 is directed in a different respective direction such that beams are arranged into a cluster, each beam covering a different respective (sub) area on the Earth’s surface within the region 200 in question (though the areas covered by the beams 202 may be arranged to overlap somewhat to avoid gaps in coverage). Signals transmitted from the satellite 110 to a given client system are approximately confined to the user beam which covers that client system, with only a small amount of leakage into neighbouring user beams. This is a way of increasing capacity, as the limited frequency band of the sateiiite 110 (e.g. Ka band) can be re-used separately in different beams 202 - i.e. it provides a form of directional spatial division multiplexing (though adjacent beams may still use different bands or subbands, especially if they overlap in space). By way of example Figure 2 shows five beams 2G2a-2G2e which between them approximately cover the area of South

Africa, but it wlii be appreciated that other numbers and/or sizes of beam are aiso possible. Figure 3 also shows a gateway beam 204, emitted by the satellite 110 and covering the gateway 110. Note figure 3 is not to scaie, and that in particular the gateway beam 204 may be significantly narrower than the user beams 202 as, on the Earth’s surface, it need only cover the area in the immediate vicinity of the gateway antenna 100. The gateway beam 204 may be separated from the user beams 202 in space as shown in figure 3, or it may overlap with one or more of the user beams 202 on the Earth’s surface. A beam emitted by a satellite will exhibit a power distribution over a geographic area. In practice, the power distribution has a shape such that the beam's signai power is highest when received at locations at or near the centre of the geographic area, and decreases towards the edge of the geographic area. For this reason, in practice, only a portion of the geographic area (the operating region) will be useabie in the sense that intended receiving equipment will only be able to properly receive the beam, i.e. in a way that allows them to extract any data carried by the beam, if the receiving equipment is located within the operating area. Outside of the operating region, the beam has a non-negiigible signai power, and is thus still be detectable, up to the edge of the geographic region, but it will become difficult or impossible for receiving equipment to properly receive the beam and extract any data. The operating region is generally a central portion of the geographic area. A satellite can start to exhibit beam misalignment over time i.e. various factors can cause one or more beams provided by the satellite beam to become displaced relative to Earth’s surface, so that the beam(s) shift away from the respective geographic areas they were originally covering. This can lead to loss of coverage for any receiving equipment that was already located near the edge of the operating region i.e, to the operating region effectively moving so that the receiving equipment is no longer included in it. In particular, where a user beam becomes misaligned, this can lead to a loss of or reduction in service for client systems that were already located near the edge of the operating region of the user beam originally.

Summary

Detecting beam misalignment can be a challenge. For a sateiiite configured for bi-directionai communication, one option would be to equip the satellite within an onboard beam equipment, which is configured to detect when signais received from Earth at the sateiiite are not property aligned with the sateiiite. For example, the on board equipment couid be configured to detect when signals received from Earth arrive at a point on an on-board antenna dish that deviates significantiy from its centre. However, equipping a sateiiite with extra equipment incurs a significant cost in terms of the resources that are required to launch the sateiiite, and moreover such equipment is chalienging to maintain in working order when the satellite is in space.

For a user beam, another option would be to attempt to detect misaiignment of the user beam by statistical monitoring of data collected across a set of client systems served by the user beam. However, the client systems are unreliable sources of data, not least because they may experience a loss of coverage due to other factors such as weather, interference or improper equipment configuration etc., and at the very least a large, widely distributed user base wouid be required to compensate for this. A first aspect of the present invention is directed to a method of monitoring a beam emitted by a satellite and exhibiting a power distribution over a geographic area, the method comprising: receiving, from each of muitipie monitoring stations, an indication of a signal power of the beam as measured at a location of that monitoring station, wherein the monitoring stations are geographically distributed about a periphery of the geographic area at which the power distribution is steepest; detecting a beam misalignment from the received indications; and transmitting at ieast one control signal to the satellite to correct the beam misalignment.

The inventor has recognized that monitoring the beam at the periphery of the geographic area, i.e. locating the monitoring stations in an outer region within which the power distribution is steepest, maximizes the accuracy of the monitoring. This is because a shift of the beam relative to Earth will cause a maximum change in the local power level of the beam within this region. In practice, the periphery of the beam will normally be outside of the operating region of the beam, so that dedicated monitoring stations, i.e. each designed and built to have a main function of monitoring the power ievel of the beam at its location, will normally need to be deployed outside of the operating region. Nevertheless, the monitoring accuracy that the dedicated monitoring stations are abie to provide can make these extra depioyments worthwhile.

The beam misalignment that is corrected may be a misaiignment of the monitored beam itself, or it may be a misalignment of a different beam emitted by the same satellite which is inferred by monitoring the beam. “Misalignment" means relative to Earth, and may for example be caused by the satellite undergoing translational and/or rotational motion relative to Earth.

For example, the monitored beam may be a user beam covering one or more client systems and at least a misalignment of the monitored user beam may be corrected by the at least one control signal.

Alternatively, the beam may be a gateway beam covering a gateway Earth station and at least a misalignment of a user beam, also emitted by the satellite and covering one or more client systems, may be corrected by the at least one control signal. That is, the gateway beam may be used to detect and correct a beam misalignment of the user beam. This applies where the gateway beam and the user beam are spatially correlated (often the case in practice), so that where a drift in the gateway beam is evident from the received power levels of the gateway beam, it can be assumed that the user beam has also drifted by a similar amount In this case, it may be sufficient to only correct the misaiignment of the user beam ~ correcting the alignment of the gateway beam may be unnecessary, as the gateway will generally be located towards the centre of the gateway beam and thus the misalignment of the gateway beam may have no appreciable effect on the operation of the gateway in practice. That is, the gateway beam itseif may not be corrected (though this is not excluded). For instance, the user beam may steerable relative to the satellite, and the at least once control signal may correct the misalignment of the user beam by steering the user beam relative to the satellite. The user beam may be mechanically steerable, electrically steerable (based on beam forming), or a combination of both.

The user beam may be steerable Independently of the gateway beam, with the gateway beam being for example fixed relative to the satellite.

The possibility of the at least one control signal correcting both the misalignment of the user beam and a misalignment of the gateway beam is not excluded, for example by the at least one control signal moving the satellite relative to Earth and/or by steering both the user beam and the gateway beam relative to the satellite (in the case that both are steerable),

In some cases, there may be at least eight monitoring stations, and indicators may be received from the at least eight monitoring stations and used to detect the beam misalignment.

The beam may include a beacon signal, which is used to measure the power levels.

The method may comprise a step of determining whether it is necessary to correct the beam misalignment, the step of transmitting being performed oniy if so. For example, it may be determined whether it is necessary to correct the beam misalignment by determining whether a client system is expected to experience a reduction in or loss of service due to the detected beam misalignment,

According to a second aspect a computer program product comprises code stored on a computer readable storage medium configured when executed to implement any of the methods or systems disclosed herein.

According to a third aspect a device for monitoring a beam emitted by a satellite and exhibiting a power distribution over a geographic area comprises: an input configured to receive, from each of multiple monitoring stations, an indication of a signal power of the beam measured at a location of that monitoring station, wherein the monitoring stations are geographically distributed about a periphery of the geographic area at which the power distribution is steepest; a detection component configured to detect a beam misalignment from the received indications; and a transmitter configured to transmit at least one control signal to the satellite to correctthe beam misalignment.

The device may be further configured to implement any of the method steps disclosed herein.

According to a fourth aspect a system for monitoring a beam emitted by a satellite and exhibiting a power distribution over a geographic area, the system comprising: multiple monitoring stations geographically distributed about a periphery of the geographic area at which the power distribution is steepest; and a device according to the third aspect connected to the monitoring stations.

Various embodiments are defined in the dependent claims.

Brief Description of Figures

Figure 1 is a schematic diagram of a known type system for providing internet access via satellite;

Figure 2 is a schematic diagram showing geographic coverage of a cluster of satellite beams;

Figure 3 is a schematic diagram of a part of a system for providing internet access via satellite beams;

Figure 4 is a schematic illustration of a beam emitted by a sateliite which exhibits a power distribution over a geographic area;

Figure 5 shows a schematic block diagram of a system for monitoring a beam; Figure 6 shows a fiow chart for a method of monitoring a satellite beam.

Petaiied Description of Embodiments

Figure 4 shows the satellite 110 emitting a beam 402 over a geographic area A, shown in perspective view, on Earth's surface. The geographic area A has a centre C and is approximately circular in this example, though this may not always be the case (for example, the area A may be elliptical). Orthogonal axes x,y shown parallel to Earth’s surface define a coordinate system. The beam 402 exhibits a power distribution 404 over the geographic area A, whereby the beam 402 has a signal power p(r) that varies as a function of location r = (x,y) on Earth's surface {bold typeface denoting a vector). The centre C is chosen as an origin of the coordinate system for convenience, so that C is at r = 0. For a beam projected over an approximately circular area A, the power distribution 404 has circular symmetry, whereby the signal power can be regarded as a function of radius r = jrj.

The geographic area A has a periphery P, shown as an outer region of the area A, for which RP < r < RA and which is defined by the power distribution 404. The signal power p(r) decays monotonicaliy with distance from the centre C (at least approximately) - gradually at first, but then then steeply. The periphery starts at RP, which is where the steep decay starts. That is, the signal power p(r) decays in a direction away from the centre C, having a gradient p'(r) that is relatively shallow for r < i?P, and relatively steep for RP <r < RA. The radius r = RA is where the signal power p(r) becomes negligible and so defines the edge of the geographic area A over which the beam 402 is projected. The gradient p'(r) ~ iVp(r)| where the gradient operator V ~ (dxi 9y). For a circularly symmetric power distribution 404

The periphery P is a geographic region where the power distribution 404 is steepest. In practice, this is will be beyond the 3dB point e.g. 6 to 10dB from boresight Bx. Boresight Bx is the axis of maximum gain of a directional antenna, on-board the satellite 110, that is emitting the beam 402 i.e. the axis between the satellite and the centre point C. To put it another way;

P(RP) dB - P(0) dB < -3 dB and typically the periphery region P between RP and RA is such that:

P(RP) dB - P(0) dB « ~6 dB; P(RA) - P(0) dB * -10 dB

In other words, the monitoring stations 408 are iocated within a periphery region P, in which the signai power p(r) is at least 3dB down from the boresight Bx, e,g. between about 8dB and 10dB down from the boresight Bx. Note these values are exemplary, and in general the periphery region P, at which the power distribution 404 is steepest, wiii have an extent that is context dependent e.g. dependent on the configuration of the antenna on-board the satellite 110 and/or other factors that may affect the precise shape of the power distribution 404.

An operating region O of the beam 402 is shown as an inner region for which r < R0, with R0 < RP so that the periphery P is outside of the operating region O. As indicated, the operating region O corresponds to the useable part of the beam i.e. it is the region where the signal power p(r) is high enough that the beam 402 can be properly received by any receiving equipment intended to do so. The beam 402 may be one of the user beams 2G2a,,..,2G2e, with intended receiving equipment in the form of the one or more client systems 112 distributed within the operating region O, or it may be the gateway beam 204, with intended receiving equipment in the form of the gateway 104 located in the operation region O, generally at or near the centre C of the geographic area A. Note that the size of the operating region is determined to some extent by the capabiiities of the intended receiving equipment (ciient system(s) 112 or gateway 104), and thus the size of the operating region O wiii in practice be context-dependent to some extent. What constitutes the operating region O of the beam 402 wiii be readily determinable in any practicai context.

The signai power p(r) drops by about ΔΡ1 across the operating region, i.e. from the centre C to r - R0i and by about ΔΡ2 across the periphery P, i.e. from r = RP to r = Ra. As wiii be appreciated, where the boundaries R0 and RA lie is highly context-dependent as it depends on the service offered in speed and quality; also as one moves further from the boresight Bx, the resulting power drop can to some extent be compensated for by using a larger user antenna. By way of example, very approximate values might be: * ρ(βο) around 0 to 4dB from the boresight Bx; » ρ(βρ) around 3 to 6dB from the boresight Bx; » p(®a) beyond 8dB from the boresight Bx.

For the purpose of monitoring the beam 402, and in particuiar for detecting any spatiai drift of the beam 402 relative to Earth’s surface, N monitoring stations 406(1),,.,4Q8(N) are distributed within the geographic area A. Each monitoring station 406(n) is a ground station located at a respective geographic location rn on Earth’s surface. Figure 4 shows M™3 monitoring station 406(1), 406(2), 406(3) at iocaiions γ1}γ2 r3 respectiveiy though this is just exemplary. in practice, it is expected that N>8 is though to be a suitabie number to provide accurate monitoring, though in some contexts a smailer number can be used.

The locations rlt of the monitoring stations 406(1),,,,406(N) are distributed about the periphery P of the geographic area A. That is, each monitoring station 406(n) is iocated within the periphery P, Le, with jr„j > RP, where the power distribution 404 is steepest. The locations r1>.,.,rN may be uniformly distributed about the periphery P, or they may not be (for example, their distribution may be more concentrated at a certain place(s) to provide more intensive monitoring of beam drift in a particular directions}}.

Each of monitoring station 406(n) has a respective antenna 407(n) by which it can collect signal energy of the beam 402, It is thus possible to use each of the monitoring stations 406(n) to measure a respective signal power p(rn) of the beam 402 at the respective location rn of that monitoring station 406(n), for example by measuring an amount of signai energy collected the respective location rnat via its respective antenna 407(n) over a known interval. In some cases, the beam 402 may include a beacon signal, otherwise known as a “satellite beacon", which is used to perform the local signal power measurements. The beacon signal may for instance have a known (e.g. fixed) transmissions power, frequency or frequencies, and/or polarization so that it is easily distinguishable from other signals or noise.

Figure 5 shows a computer device 502 to which each of the monitoring stations 408(n) is connected via a network 500. The network 500 may for example be a iocai, private network (e.g. intranet) or it may be a public network such as the Internet 102. The computer device 502 comprises a processor 504, and a memory 506 and a first and a second interface 510, 514 connected to the processor 504,

The computer device 502 connects to the network 500 via the first interface 510, which is a network interface, and to a sateliite transmitter 512 via the second interface 512. The memory 508 hold sateliite control code 508 for execution on the processor 504, The sateliite transmitter 512 is capable of sending controi signals ::cfri” to the sateiiite 110 to controi operations of the sateliite 110, under the controi of the processor 504.

Figure 8 shows a flow chart of a method of monitoring the beam 402. At step S2, the monitoring stations 408(1406{N) are provided about the periphery P of the geographic area A covered by the beam 402, by deploying them at the locations rls respectively and connecting them to the network 500.

Each monitoring station 408{n) measures the (iocai) signal power p(r„) of the beam 402 at the location rn of that monitoring station 408(n) using its respective antenna 407(n) (S4), and transmits an indication of the measured iocai signal power rn to the computer device 502 via the network 500. The indication indicates the iocai signal power p(rn), for example as an absolute or relative value e.g. relative to a previously-measured local signal power so that the indication expresses an observed change in the local signal power. Such measurement are performed continuously i.e, repeatedly over a series of time intervals. How often the measurements are performed is context specific - for instance, in some cases such measurements could be performed every few seconds or minutes; in others, longer intervals (e.g. daily or weekly measurements) may be sufficient.

The computer device 502 receives the indications from all of the N monitoring stations, and uses them to monitor the beam 402 (S6), for the purpose of detecting a beam misalignment in particular.

Movement of the beam 402 relative to Earth’s surface, by an amount Sr, effectively shifts the power distribution 404 in space i.e. p(r) p(r - Sr). The movement can thus be detected by the computer device 502 detecting a change in some or ali of the local signal powers indicated by the monitoring stations 408(1),...,406(N). For example, a spatial drift of the beam in a direction towards the monitoring station labeiied 802(2) will manifest as an increase in the iocai signai power p(r2) as measured by that monitoring station 602(2), and a corresponding decrease in the local signai powers pOyJ/pCry) measured by the other illustrated monitoring stations 406(1), 408(3). Because the monitoring stations 408(1),...,4G8(N) are distributed about the periphery P of the geographic area A where the power distribution 404 is steepest, the accuracy of the monitoring is maximized as this is where smail dispiacements cause the most significant, and thus most detectable, changes in local signal power p(r).

If and when a beam misalignment is detected (SB), at step S10 it is determined whether it is in fact necessary to correct the beam misalignment. For example, if the detected beam misalignment is of a user beam 202, it may only be necessary to correct this misalignment if there is currently a client system(s) 112 located in a particular place(s) towards the edge of the operating region O that will actually be affected, if not, the misalignment of the user beam 202 may be deemed acceptable and monitoring simply continued without correction, and corrected iater if and when it becomes unacceptable. If on the other hand it is necessary to correct the misalignment, then at step 812 the satellite transmitter 512 is used to send at least one control signal is sent to the satellite 110 to correct the misalignment. For example, where the misalignment is of the user beam 202, this may be deemed necessary if the detected misalignment is such that at least one client system near the edge of the operating region O is expected to experience a loss of or reduction in service as a result of the misalignment. it is again noted that the beam that is corrected is not necessarily the beam 402 that is monitored, though this is not excluded. Monitoring of one beam 402 (e.g. a gateway beam) can he used to directly detect a misalignment of that beam 402, and/or to indirectly detect a misalignment of a different beam (e.g. a user beam) emitted by the same satellite 110. That is, a beam misalignment that is detected and corrected through monitoring may be of the monitored beam and/or of a different beam. The misalignment of the different beam can be indirectly detected from a detected movement of the monitored beam, from which the misalignment of the different beam can be inferred. Gniy the different beam may be corrected, or both may be corrected.

The to-be corrected beam may be a steerable beam, misaiignment of which is corrected by steering it relative to the satellite 110. For example, the to-be corrected beam may be mechanically steerable whereby an antenna emitting it is mounted on a mechanicaily steerable mount, which is mechanically steerable to change a pitch and/or a roll of the beam 402 relative to the satellite 110. Aiternatively, the to-be corrected beam may be electrically steerable i.e, emitted by a configurable antenna array on board the satellite 110. As another alternative, these technologies can be combined whereby steerability is provided by a combination of beam forming and mechanical components.

As a particular example, where misaiignment of a user beam is detected indirectly by monitoring a gateway beam, the user beam can be re-steered relative to the satellite 110 to effect the correction where necessary, without correcting the gateway beam if the latter is unnecessary (though correcting the gateway beam aiso is not excluded). The correction can alternatively or additionally be corrected by moving, i.e, relocating and/or rotating, the satellite 110 relative to Earth,

Steps S8-S12 are implemented by the code 508 when executed on the processor 504, though the possibility of dedicated hardware Implementations or implementations based on a combination of dedicated hardware and software are not excluded.

The above embodiments are exemplary, and other variants or applications may be apparent to the skilled person in view of this disclosure. The scope of the present invention is not limited by the described examples, but only by the following claims.

Claims

Claims:

1. A method of monitoring a beam emitted by a satellite and exhibiting a power distribution over a geographic area, the method comprising: receiving, from each of multiple monitoring stations, an indication of a signal power of the beam as measured at a location of that monitoring station, wherein the monitoring stations are geographically distributed about a periphery of the geographic area at which the power distribution is steepest; detecting a beam misalignment from the received indications; and transmitting at least one control signal to the satellite to correct the beam misalignment.

2. A method according to ciaim 1 wherein the monitored beam is a user beam covering one or more client systems and at least a misalignment of the monitored user beam is corrected by the at least one control signal.

3. A method according to claim 1 wherein the monitored beam is a gateway beam covering a gateway Earth station and at ieast a misalignment of a user beam, also emitted by the satellite and covering one or more client systems, is corrected by the at least one control signal

4. A method according to claim 2 or 3 wherein the user beam is steerable relative to the satellite, and the at least once control signal corrects the misalignment of the user beam by steering the user beam relative to the satellite,

5. A method according to claim 4 when dependent on claim 3, wherein the gateway beam is not corrected.

6. A method according to ciaim 3 wherein both the misalignment of the user beam and a misalignment of the gateway beam are corrected by the at least one control signal.

7. A method according to any preceding claim wherein there are at least eight monitoring stations, indicators received from the at least eight monitoring stations being used to detect the beam misalignment.

8. A method according to any preceding claim wherein the beam includes a beacon signal, which is used to measure the power levels.

9. A method according to any preceding claim comprising determining whether it is necessary to correct the beam misalignment, the step of transmitting being performed only if so.

10. A method according to claim 9 wherein It is determined whether it is necessary to correct the beam misalignment by determining whether a client system is expected to experience a reduction in or loss of service due to the detected beam misalignment.

11. A computer program product comprising code stored on a computer readable storage medium configured when executed to implement the method of any preceding claim.

12. A device for monitoring a beam emitted by a satellite and exhibiting a power distribution over a geographic area, the device comprising: an input configured to receive, from each of multiple monitoring stations, an indication of a signal power of the beam measured at a iocation of that monitoring station, wherein the monitoring stations are geographically distributed about a periphery of the geographic area at which the power distribution is steepest; a detection component configured to detect a beam misaiignment from the received indications; and a transmitter configured to transmit at least one controi signai to the satellite to correct the beam misaiignment.

13. A system for monitoring a beam emitted by a satellite and exhibiting a power distribution over a geographic area, the system comprising; multiple monitoring stations geographically distributed about a periphery of the geographic area at which the power distribution is steepest; a device according to claim 12 connected to the monitoring stations.