WO2000025448A2

WO2000025448A2 - Satellite terminal

Info

Publication number: WO2000025448A2
Application number: PCT/SE1999/001870
Authority: WO
Inventors: Ingmar Folke Karlsson; Mats Andersson
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 1998-10-23
Filing date: 1999-10-15
Publication date: 2000-05-04
Also published as: AU1427100A; SE520787C2; SE9803649D0; WO2000025448A3; SE9803649L

Abstract

A satellite terminal having data transmission processing means and antenna means for communicating with a constellation of non-geo-stationary satellites, the satellite terminal comprising control and interface means for controlling the terminal and for providing interface to a number of peripherals, such as PC, telephone, TV-set etc has been described. The satellite terminal accomplishes significant reductions in overall cost due to the reduction of components in the sending sections of the terminal, while offering a high level of quality.

Description

SATELLITE TERMINAL

Field of the invention

The present invention relates to a satellite terminal and a method for communicating with a plurality of non geo-stationary satellites such as low-earth-orbit (LEO) satellites.

Background of the invention

In prior art document US-A-5 408 237 an earth-fixed cell beam management system for LEO-satellites has been disclosed.

In figure 1 , which relates to this document, a possible pattern of movement for a constellation of such non geo-stationary low-earth-orbit-satellites has been shown. In figure 1 the trajectory of a satellite over an earth bound grid at times t^ t₂and t₃has been shown.

One embodiment according to the above document counts 840 identical satellites occupying 21 orbital planes inclined at 98,2 degrees to the Equator the satellites being deployed at an altitude of approximately 700 km.

The satellites are equipped with communication transmission directing means, which enable the satellites to direct a number of communication channels to particular predetermined earth-fixed areas or footprints while passing over the surface of the earth.

The satellites are designed to communicate with ground terminals within a certain angular field, i.e. up to 40 degrees with respect to the horizon. Moreover, the satellites are provided with means for communicating data between them. As opposed to geo-stationary satellites, a respective satellite in the above low-earth orbit system is serving adjacent footprints in a consecutive manner.

Seen from the perspective of an earth terminal, such a terminal will be served by a number of satellites over-flying the footprint, for example from south to north. When a particular satellite is moving out of the angular field associated with the terminal, it will be served again by another satellite following either the same course as the previous satellite or following a new course relating to another orbit.

The satellites according to the above system are furthermore equipped with means, such as active antennas, for spatially directing the above mentioned communication channels to confined areas within the footprint. In this way the footprints are subdivided into supercells, which again are subdivided into cells, in a manner resembling the structure of land based mobile communication systems. This has been illustrated in figure 1.

The above described satellite system is designed to operate in the Ka and Ku band of 20 / 30 Ghz at data-rates up to 2 Mbps.

The short signal path and the limited area of the angular field ensure brief propagation delay, which is required for speech and data transmission, low power consumption for terminals and satellites, reliable and weather (rain) independent transmission and footprints with well-defined boundaries.

Another project proposed by SkyBridge TM involves 2 constellations of each 40 satellites being deployed in low earth orbit and providing for world-wide coverage between latitudes of +68° and -68°. Each satellite illuminates an area of 3000 km in radius. According to this system, there is always at least one satellite visible within the coverage area of a gateway. However, most of the time at least 2 and up to 4 satellites are visible and available to transmit traffic.

According to this project, the satellite functions as a so-called transparent link, whereby switching takes place on terrestrial gateways. A terminal for use with the above described type of systems will comprise both transmit antennas and receive antennas for communicating with the above mentioned satellites.

As a general requirement such a terminal must be able to direct its communication against a given satellite and track this satellite, that is, follow the orbit of the satellite in the sky. At the same time, the terminal should be able to direct its communication against the next or an alternative satellite overflying the footprint in which the terminal is situated, while the terminal should be able to switch between these satellites without any functional disruptions or interruptions occurring in the dataflow.

Summary of the invention

Since space and rain attenuation is relatively high at the above-mentioned frequencies, high antenna gains become key characteristics of such terminals. Therefore, antenna lobe control would typically be required for such terminal systems.

Both electrically controlled antennas, so-called active antennas, and mechanically operated antennas are widely known in the art.

Although electrically directed antennas are able to change their directional characteristics within milliseconds and therefore are highly desirable, these are currently unfavourably expensive in relation to mechanically directed antenna systems.

Especially antennas with electrically controlled characteristics require expensive components and are therefore, at present, not suitable for consumer products.

It is an object of the present invention to provide a terminal for a non-geo-stationary satellite system, which can be manufactured economically and which will satisfy a broad range of users.

This object has been accomplished by the satellite terminal as set out in claim 1. Briefly, the present invention contemplates a satellite terminal for a non-geostationary satellite system which includes first and second signal units which are associated with first and second antenna units respectively, whereby the first signal unit comprises a receiving section and a transmitting section, while the second signal unit comprises a receiving section but no transmitting section.

It is another object to set forth a satellite terminal which can identify given locations of individual satellites of a given constellation and which can track these satellites precisely. This object has been accomplished by the subject matter set forth in claims 6, 7 and 8.

Further advantageous embodiments have been defined in the remaining dependent claims.

The present invention provides a terminal, which is economical to manufacture and which provides an almost seamless coupling to the satellite for speech, Internet and other services for the user.

For the terminal according to the invention, only short interruptions will occur in the user-transmitted data-flow at regular intervals. If the interruptions occur, say at every 5 minutes and have duration of a few seconds, the interruptions will only be perceptible for services such as speech transmission and these interruptions might very well be acceptable to the user in view of the low price of the terminal. For other services such as typical Internet applications there will be no perceptible deviation in quality.

Description of the drawings

Fig. 1 shows a pattern of illuminating a footprint on earth according to a known satellite system,

Fig. 2 is an illustration of the satellite terminal according to a preferred embodiment of the invention Fig. 3 is a block diagram pertaining to a preferred embodiment of the invention,

Fig. 4 is a schematic illustration of the pattern of movements for satellites in the constellation, and

Fig. 5 is an exemplary illustration of a first preferred antenna unit movement and signal unit switching pattern relating to fig. 4.

First preferred embodiment of the invention

In fig. 2 a first embodiment of the invention has been shown which relates to a satellite terminal having two mechanically operated antenna units, each of which is adapted to follow a given satellite overflying the satellite terminal and move to a new satellite.

In fig. 2, the antenna units, A1 and A2 are shown in a situation whereby satellite SAT1 is about to leave the visible field and whereby handover has been accomplished from the first satellite SAT1 to a subsequent second satellite SAT2. In this situation, both antenna units A1 and A2 are tracking satellite SAT2.

The antenna units A1 and A2 are adapted to move omnidirectionally within a certain angular field and are controlled by an antenna actuator unit AAU, which is part of the satellite terminal. Antenna unit A1 is provided with a receiving antenna RX-A and a transmitting antenna TX-A mounted on the same base and antenna unit A2 is provided with a single receiving antenna RX-A.

In fig. 3, a block diagram of the satellite terminal according to fig. 2 has been shown.

The satellite terminal ST comprises a control and interface unit CIU, a first signal unit S1 and antenna unit A1 , a second signal unit S2 and a second antenna unit A2, an antenna actuator control unit AAU and motor units M1 and M2, which are steering the respective antenna units A1 and A2 in the appropriate directions. The control and interface unit CIU is provided with a set of terminals TRM, which are adapted to be connected to a number of peripheral devices such as an external network, a personal computer, a telephone, a multi-media system, a TV etc.. The control and interface unit CIU comprises a central control unit CRTL-U, local memory MEM, and buffers BFF for converting signals to appropriate signal levels at the terminals TRM. The control and interface unit CIU, moreover comprises means for up- and downlink processing UP-PRC, DN-PRC1 , DN-PRC2.

The central control unit CTRL-U manages the overall control of the terminal and performs protocol adaptation, that is, performing adaptations between the protocol for communication utilised by the satellite system and the protocol or protocols utilised by the peripherals, for instance the IP protocol.

Moreover, the control and interface unit CIU comprises a baseband unit B-BNDU in which appropriate queuing and scheduling of signals coming from the downlink- processors DN-PRC1 , DN-PRC2 and signals going to the uplink-processor UP-PRC are processed. The baseband unit B-BNDU communicates these signals to and from the central control unit, for the aforementioned protocol adaptation, thus providing a flow of data to and from the terminals TRM.

The control and interface unit CIU also comprises means for antenna movement control ACTL, which communicates with the central control unit CTRL-U and the baseband unit B-BNDU, and which provide two sets of target co-ordinates for the respective antenna units in order to control the directional characteristics of the receiving and transmitting antennas.

The central control unit CTRL-U may optionally be coupled to an exchangeable memory, FL, such as a flash ROM and it may also optionally comprise or be coupled to a modem, MDM, by which the satellite terminal can be connected to a conventional terrestrial network. Furthermore, the control and interface unit may comprise or may be prepared for a second optional uplink processor OP.

The control and interface unit CIU is coupled to first signal unit S1 and second signal unit S2, which again are coupled to antenna unit A1 and antenna unit A2, respectively. In operation, signal unit S1 and signal unit S2 are dealing with signals associated with the satellites appearing as illustrated in fig. 2 in an interchanging manner.

The first signal unit S1 comprises one transmit section and one receive section, whereby the transmit section comprises the following elements: Up-converter U/C, power amplifier HPA, filter F and TX-antenna A1. The receive section comprises the elements: Down converter D/C, low-noise amplifier LNA, filter F and RX antenna A1.

Thus, a signal of an intermediate frequency content coming from the up processor UP-PRC in the control and interface unit CIU is for instance fed to signal unit 1. The signal is up-converted to the satellite transmitting frequency in the up-converter U/C and amplified in the power amplifier HPA and fed through the bandpass filter F to the transmit antenna TX-A in antenna unit A1.

The design of the signal units are very similar to terrestrial radiolink apparatuses like LMDS (Local Multipoint Distribution System) or point to point links.

An incoming high frequency signal from the receiving antenna RX-A in antenna unit A1 goes through the bandpass filter F and the low noise amplifier LNA, then it is down-converted to an intermediate frequency in the down-converter D/C and passed to the down link processor DN-PRC1 in the control and interface unit CIU.

For certain frequencies and polarisations, as an alternative to the solution shown in figure 2, a common RX and TX antenna may be utilised in the antenna unit A1. In this case, an OMT (Orthomode Transducer) or a diplexer is used to separate the signals.

The second signal unit S2 comprises an identical receive section as the receive section in the first signal unit S1 , but it does not comprise a transmit section. As will be understood, this aspect of the present invention provides for a component reduction of 25 % for the antenna signal units as compared to a satellite terminal having double two-way transmission capabilities. Since the components in signal units S1 and S2 account for a large proportion of the costs, these are reduced for the terminal according to the invention.

As shown in fig. 3, the transmit section in signal unit S1 is coupled to the uplink processor UP-PRC in the control and interface unit CIU, while the receive section of signal unit S1 and signal unit S2 are connected to the downlink processor DN-PRC1 and DN-PRC2 respectively.

Thus, an incoming signal from any of the receiving antennas RX-A is going through bandpass filter F, amplified in low noise amplifier LNA, down-converted to the baseband in downconverter D/C and passed on to the respective down processor DN-PRC1 and DN-PRC2 in the control and interface unit CIU.

Baseband unit BBND-U can establish communication with both incoming channels from the down-processors DN-PRC1 and DN-PRC2, and is adapted to perform a handover or switching from one incoming channel to another.

Large proportions of the message content of the respective signals being transmitted by the satellites in the constellation are identical. The terminal will, however, receive the signals with a certain time lag and error rate depending on the actual position of the satellite and the atmospheric conditions.

The baseband unit is therefore adapted to extract information from both the down- processors DN-PRC1 and DN-PRC2 simultaneously in order to perform handover between corresponding satellites and in order to accomplish a certain degree of fault correction. In this process, the signal content of the two channels may be compared and the intended signal content may be restored.

Handover, or switching between the channels could take place almost instantane- ously, i.e. in a matter of very few data frames. Alternatively, handover may be a process, which takes place during an interval as long as two subsequent satellites are within the field of vision of the satellite terminal, in order to accomplish the fault correction mentioned above. This handover process is carried out by baseband unit BBND-U and is controlled by the central control unit CTRL-U.

In the following, special attention will be paid to the handover between incoming channels in signal unit S1 and S2.

The baseband unit may furthermore communicate on an outgoing channel, represented by uplink processor UP-PRC or optionally be prepared for communicating on two outgoing channels including the optional uplink processor OP and performing handover between two channels in a similar fashion as handover between incoming channels.

As mentioned above, the antenna control unit ACTL in control and interface unit CIU is coupled to antenna actuator unit AAU, which for instance comprises a power control (not shown) for controlling a set of actuators. The antenna actuator unit AAU is adapted for receiving data, for instance two sets of polar co-ordinates, φ, θ, which define the desired position of the two antenna units A1 and A2.

A first transport mechanism M1 , which advantageously may comprise a set of stepper motors, controls the directional movement of antenna unit A1 while a second transport mechanism M2, also based on stepper motors for instance, controls antenna unit A2.

It will be understood that the mechanical arrangements for the present invention can be designed in a number of known ways. Mechanically operated antennas are for instance found on current satellite terminals for TV reception and comprise a mechanical mount, which by means of electrical motors enables the antenna to be steered in a given direction. Also electro-mechanically operated antenna transport mechanisms capable of directing an antenna into a given direction by the input of for instance a set of polar co-ordinates are widely known in the art.

According to the first preferred embodiment of the present invention, the control and interface unit CIU is adapted to carry out an algorithm describing the actual and future co-ordinates for any of the visible satellites appearing over a given location on earth as a function of the time and location of the terminal. According to this first embodiment, predetermined parameters approximating the co-ordinates of the satellite constellation are software implemented on the control and interface unit CIU and may be modified and changed whenever this is needed by means of appropriate software updating. The corresponding software program may for instance be stored on the exchangeable memory FL, which may easily be replaced whenever a change in the satellite constellation occurs.

According to this embodiment, the above algorithm describing the movements of the constellation enables the satellite terminal to tune in to the satellites locations within a coarse set of tolerances, this mode being denoted as the initial mode.

The initial mode provides sufficient precision for the satellite terminal to carry out a coupling procedure with the satellite, in which communication with the satellite terminal at a given data rate can be established.

Further routines in the initial mode effect data being provided from the satellite to the terminal for on-line fine-tuning and modification of the algorithm mentioned above. The modification of the algorithm may involve that the exact co-ordinates relating to all the theoretically visible satellites at a given time and location are being provided to the satellite terminal. Communication takes place at reduced data rates allowing for reliable reception of signals from the satellite even though the antenna units are not directed in an optimum fashion.

When the modification of the algorithm has taken place, the satellite terminal is ready to enter an operational mode, in which broad band communication can take place as a result of the precise antenna unit alignment providing exact tracking of the satellites.

Now, the operational mode shall be explained further with reference to the appended figures 4 and 5. Fig. 4 is a simplified illustration of the typical course of the satellites over a given area on earth showing the variation in φ, while the variations in θ , which relate to co-ordinates in front or behind the plane of fig. 4 have not been shown. A first satellite, SAT1 , is following a given course, expressed in polar co-ordinates from (φ_{1 (} θ₁ ) 0Ver (φ₂ , θ₂ ) tθ (φ₃ , θ₃ ).

A subsequent satellite, SAT2, may typically follow either the same course as the previous course, or it may follow a completely new course.

The angles q^ and φ₃ relate to the boundaries of the actual visible field of a certain location for a certain θ -value, while φ₂ is the position of SAT2, when SAT1 leaves the visible field at φ₃.

It should also be understood that the terminology, previous and subsequent satellite, relate to the order at which the satellites of the constellation is entering the visible field associated with a given satellite terminal.

According to the satellite systems described above, many satellites will appear in a pattern, in which a subsequent satellite is following the same course or approxi- mately the same course as the previous satellites.

One possible and advantageous pattern of movement for the antenna units A1 and A2 of the satellite terminal according to the present invention relating to the situation exemplified in fig. 4 have been shown in figure 5. In this figure, the switching between the respective signal units S1 and S2 has also been shown.

In figure 5, the upper curve, indicated by a dotted line, relates to the angular movement of the first antenna unit A1 in a given plane relating to a certain θ -value, while the lower curve, indicated by another dotted line, relates to the angular movement of the second antenna unit A2 for approximately the same θ -value. As can be seen from figure 5, at time t₀ the first antenna unit, A1 , follows satellite, SAT2, somewhere over position φ₂. Antenna unit A2 may be parked at the position φ^ Transmission takes place by means of signal unit S1 , which unit has both means for sending and receiving signals.

At time t, , a subsequent satellite SAT3 enters the visible field at position φ^ and antenna unit A2 starts tracking this satellite.

At time t₂ , where SAT2 is approaching φ₃, , signal unit S2 takes over transmission whereby handover is performed between SAT2 and SAT3. Immediately after antenna unit A1 has reached φ₃, , it is carried back to a position corresponding to φ, at a certain angular speed α , which angular speed is determined by the properties of the first transport mechanism M1.

At t₃ , the first antenna unit A1 reaches φ₂ where it changes direction and follows

SAT3, and signal unit S1 is again ready to take over transmission from signal unit S2. In case that sending from the satellite terminal is wished to be undertaken, switching from signal unit S2 to S1 can be initiated as soon as antenna unit A1 follows SAT3 with the required precision.

Antenna unit A2 may follow the subsequent satellite SAT3 for a period of time, such that handover from signal unit S2 to S1 may be carried out during t₃and onwards until t₄ , for example for allowing fault corrections to take place. When handover is completed, antenna unit A2 is driven back to φ, at a speed β given by the second transport mechanism M2.

The latter transport mechanism M2 may not necessarily provide for as high a speed as the speed α of the first transport mechanism M1.

It appears, that signal unit S1 is only inactive in the relatively short interval from t₂ to t₃. Thus, the possible time an interruption in sending ability will last is dependent on the speed α and the angular area between φ₃, and φ₂. Therefore, on average, signal unit S1 can be rendered active the majority of the time under which communication with the satellites takes place.

It should be noted that figures 4 and 5 are schematic and that the angles and events in time are chosen so as to illustrate the function of the satellite terminal.

In practice, the time relating to signal unit S1 being inactive may be very short. It should also be noted that if the trajectories of the satellites are different or crossing one another an even shorter interruption interval could be accomplished.

In addition, the angle φ₂ where transmission to two or more satellites would be possible could be located in the opposite direction of what is shown in fig. 4. It is seen that the situation shown in fig. 4 is a worst case situation (concerning φ), because the antenna unit A1 has to reverse over almost the entire visible field.

For situations where a subsequent satellite is following a different course, or even more advantageous an opposite course, the duration of a possible fall out of transmission capability of the satellite terminal relating to times t₂ to t₃ becomes even smaller.

A special interruption warning feature has been implemented in the satellite terminal, whereby the user is informed that a brief interruption in the sending ability is about to occur. For instance, the users in a speech transmission session will receive a warning signal that a short interruption is about to occur, by means of an appropriate auditory or visual warning signal. Advantageously, the peripheral equipment to which the satellite terminal is connected presents the warning signal.

The satellite terminal according to the invention is advantageously designed in a modular fashion, such that it would be possible to upgrade the terminal for contin- uos sending ability. The signal unit S2 is simply provided with an extra sending section similar to the one, which is comprised in the signal unit S1 and the control and interface unit CIU is provided with the optional uplink processor OP, which may also be provided in advance. For this purpose, control and interface unit CIU could be provided with appropriate connectors and antenna unit A2 provided with appropriate mechanical coupling means, such that the user can easily expand the satellite terminal without professional assistance.

Second embodiment of the invention

According to a second embodiment of the invention, the control and interface unit houses a modem MDM, by which the satellite terminal is adapted to be coupled to an external terrestrial network such as a public switched telephone network or a mobile phone network.

As an alternative to the data stored in the exchangeable memory FL, the algorithm pertaining to the initial mode could be up-dated in a network assisted session with a service provider.

The user simply contacts the service provider through modem MDM. In an interactive session between user and service provider, the exact location of the satellite terminal can be derived from information such as street and house number.

According to this embodiment, an error finding function and an adjustment routine relating to the algorithm used for the predetermined antenna control could also be implemented. Other parameters could be changed in a similar way.

Both providing the satellite terminal with the modem MDM and the exchangeable memory FL would of course also be possible.

It should be understood that the embodiments of the satellite terminal described might form part of the peripheral devices, which the terminal is supposed to be coupled to. The control and interface unit CIU could for instance be formed as a plug-in module fitting in a PC-slot or a similar adapter. In this case, the signal units S1 and S2 and the antenna actuator unit AAU would be housed in another unit close to the antennas. Likewise, the antennas may not form part of the satellite terminal.

It should furthermore be understood that the present invention would be advanta- geous with various antenna designs, for instance a hybrid type antenna, based on a mechanically directed phase array, enabling a mechanical scanning in one direction and an electrical scan in another direction.

List of refei 'ence signs

ST satellite terminal

CIU control and interface unit

TRM terminals

BFF buffer

CTRL-U central control unit

MDM modem

FL exchangeable ROM

B-BND-U baseband unit

MEM local memory

UP PRC up-link processor

DNPRC1 first down-link processor

DNPRC2 second down-link processor

OP optional up-link processor

ACTL means for antenna movement control

S1 first signal unit

S2 second signal unit

U/C up-converter

D/C down converter

HPA high power amplifier

LNA low noise amplifier

F filter

TX-A transmit antenna

RX-A receive antenna

A1 antenna unit 1

A2 antenna unit 2

AAU antenna actuator unit

M1 first motor unit

M2 second motor unit

SAT1 first satellite

SAT2 second satellite

SAT3 third satellite

Claims

Patent claims

1. A satellite terminal (ST) for a non-geo-stationary satellite system comprising

a control and interface unit (CIU) controlling the satellite terminal and providing coupling to a set of terminals (TRM), the control and interface unit (CIU) processing an incoming flow of data originating from at least one receiving antenna (RX-A) to at least some of the terminals (TRM) and processing an outgoing flow of data from at least some of the terminals destined for at least one transmitting antenna (TX-A),

the control and interface unit (CIU) furthermore being adapted for controlling the directional characteristics of the at least one receiving antenna (RX-A) and the at least one transmitting antenna (TX-A) respectively in order to track sat- ellites overflying the satellite terminal, whereby the satellite terminal (ST) furthermore comprises

a first signal unit (S1) being associated with a first antenna unit (A1) having a receiving and transmission antenna (RX-A, TX-A), the first signal unit (S1) having means (D/C, LNA, F) for receiving and processing signals from the receiving antenna (RX-A) and means (U/C, HPA, F) for transmitting and processing signals to the transmission antenna (TX-A),

the second signal unit (S2), being associated with a second antenna unit (A2) having a receiving antenna (RX-A), the second signal unit (S2) having means

(D/C, LNA, F) for receiving and processing signals from the receiving antenna (RX-A).

2. The satellite terminal according to claim 1 , whereby, during communication with the satellite constellation, the first signal unit (S1) is interrupted by the second signal unit (S2) corresponding to a handover to a following satellite and that the first signal unit (S1), on average, is active longer than the second signal unit (S2).

The satellite terminal according to claim 1 , whereby the first signal unit (S1) is active the majority of the time under which communication with the satellites take place.

4. The satellite terminal according to claim 3, whereby the control and interface unit (CIU) comprises a means for up-link processing (UP-PRC) and a means for downlink processing (DN-PRC1 , DN-PRC2), and whereby

the means (U/C, HPA, F) for transmitting and processing signals in the first signal unit (S1 ) is coupled to the means for up-link processing (UP-PRC), while the means (D/C, LNA, F) for receiving and processing signals in the first and second signal unit respectively (S1 , S2) is coupled to the means for downlink processing (DN-PRC1 , DN-PRC2) in the control and interface unit (CIU).

5. The satellite terminal according to claim 1 , whereby

the antenna units (A1 , A2) are being adapted for tracking the course of satellites between a first position (φ, , Θ over a second intermediate position (φ₂, θ₂ ) to a third position (φ₃ , θ₃ ) relating to the visible field of the trajectories for the satellites overflying an area associated with the terminal, whereby

the first antenna unit (A1) moves within a first intermediate range within the angular field from an intermediate angular position (φ₂ , θ₂ ) to the second position (φ₃ , θ₃ ) or a position close by (φ₃, , θ₃, ),

the second antenna unit (A2) moves within a second intermediate range, within the angular field from the first position (φ₁ , Θ to the intermediate position (φ₂ , θ₂ ) or further and whereby the angular movement between (φ_{1 (} Θ and (φ₂ , θ₂ ) is much smaller than the angular movement between (φ₂ , θ₂ ) and (φ₃ , θ₃ ).

6. The satellite terminal according to claim 3, whereby the satellite terminal is adapted to enter an initial mode in which an algorithm based on predetermined parameters is used to approximate the co-ordinates of the visible satellites, permitting communication at a lower data rate to be established with any one of the occurring satellites,

followed by an operational mode in which the algorithm is based on information about the co-ordinates of the visible satellites provided by the satellites, providing for exact tracking of the satellites and enabling higher data rates to be transmitted.

7. The satellite terminal according to claim 4, wherein the satellite terminal (ST) comprises an exchangeable memory (FL) and wherein

the parameters for the algorithm defining the visible satellites are stored in the exchangeable memory (FL).

8. The satellite terminal according to claim 4, wherein the satellite terminal comprises a modem (MDM) and wherein,

the parameters for the algorithm defining the visible satellites are gained from an interactive session with a service provider.

The satellite terminal according to any preceding claim, whereby the satellite terminal is adapted to transmit and receive an online service, such as speech, and whereby the terminal comprises means for providing a warning signal to users utilising the service that a brief interruption, relating to the first signal unit (S1) becoming in-active, is going to occur, the warning signal being an audible and / or visible signal.