Abstract: A satellite communications system provides a communications service to a mobile terminal on which different communications applications may be run. Calls are set up between any of the applications via a satellite to a network management centre which provides different service adaptors which adapt the calls to different types of service provided over terrestrial networks, such as telephony, facsimile, internet or ATM services. The bandwidth allocated to each call over the satellite link may be varied during the call according to demand either from the relevant application or from the network management centre. Multiple calls may be connected concurrently to or from different applications running on the mobile terminal. A maximum bandwidth is allocated to each call. Efficient use is thereby made of the limited bandwidth available over the satellite, according to the instantaneous bandwidth requirements of different applications.

Abstract: In a messaging system, message traffic is transmitted on one or more traffic channels T1, T2 and the allocation of groups S1, S2 of receivers to the traffic channels T1, T2 is controlled by information transmitted on a bulletin board channel BB. A predetermined number of frames before a change in the traffic channel allocation, a countdown value is transmitted in the relevant traffic channel T1 and the countdown value is decremented in each subsequent frame. Before the countdown value reaches zero, the groups of receivers S1, S2 retune to the bulletin board channel, receive their new channel allocations and retune to the traffic channel indicated for their group.

Abstract: A satellite communications system provides a communications service to a mobile terminal (2) on which different communications applications (4a to 4d) may be run. Calls are set up between any of the applications (4a to 4d) via a satellite (12) to a network management center (18) which provides different service adaptors (20a to 20d) which adapt the calls to different types of service provided over terrestrial networks (22), such as telephony, facsimile, internet or ATM services. The bandwidth allocated to each call over the satellite link may be varied during the call according to demand either from the relevant application (4) or from the network management center (18). Multiple calls may be connected concurrently to or from different applications running on the mobile terminal. A maximum bandwidth is allocated to each call. Efficient use is thereby made of the limited bandwidth available over the satellite (12), according to the instantaneous bandwidth requirements of different applications (4).

Abstract: A method and apparatus are disclosed for a system where a user terminal 11 can communicate with satellites 9A-C, the satellites 9A-C can communicate with earth stations 7A-E which in turn can communicate with the terrestrial telephone system 1. The user terminal can register with one of the earth stations 5C which has an associated mobile satellite switching center 5C which acts as a gateway into the terrestrial system 1. When attempting to contact the user terminal 11 from the terrestrial system 1 the mobile satellite switching center 5C of visitor registration, if it cannot make the contact itself, instructs other earth stations 5, 7, in turn, according to different methods of alternative earth station 7 selection, until contact is made, and then routes all voice messages 21 through itself 5C to retain the single and original gateway into the terrestrial system 1.

Abstract: In a messaging system, message traffic is transmitted on one or more traffic channels T1, T2 and the allocation of groups S1, S2 of receivers to the traffic channels T1, T2 is controlled by information transmitted on a bulletin board channel BB. A predetermined number of frames before a change in the traffic channel allocation, a countdown value is transmitted in the relevant traffic channel T1 and the countdown value is decremented in each subsequent frame. Before the countdown value reaches zero, the groups of receivers S1, S2 retune to the bulletin board channel, receive their new channel allocations and retune to the traffic channel indicated for their group. The frame timings of the different traffic channels T1, T2 are staggered to reduce the peak power required to transmit message bursts in these channels.

Abstract: Digital audio recording equipment is used to record modulated signals, such as the RF test signals generated by test modems. If the modulated signals are outside the audio frequency range, they are down-converted into the audio frequency range using a mixer, local oscillator and a filter. The digitally converted data are recorded onto a suitable medium, such as a DAT, CD or a computer memory. The data is then disseminated for reproduction by a suitable digital audio player. The reproduced audio signal may be up-converted to the desired radio frequency using a mixer, local oscillator and a filter. The reproduced signal may be used to test demodulators, codecs or transceivers. The signals output by the transceiver and the reproduced signal may be recorded on different channels of the digital audio recording equipment. The recording equipment may be used to capture RF signals for later analysis. The player may be used for reproducing modulated signals for broadcast.

Abstract: A store-and-forward image communications system for transmitting an image signal from a first terminal to a second terminal in one of a plurality of transmission modes which comprises a transmission path between the first and second terminals, a first image signal interface in said transmission path at which said image signal is received, a message store connected to store and subsequently recall signals received at the first interface, a data store containing at least one record comprising preference data relating to preferred said transmission modes for at least one of said first and second terminals, a transmission controller selecting from said transmission control modes in accordance with said preference data and a data update arranged to update the or each said record.

Abstract: A mobile terminal (6) communicates via a satellite (8), which projects a number of overlapping spot beams (10), with a fixed earth station (2). To determine which spot beam (10) is to be used for communication, the mobile terminal signals in a common signalling channel which is simultaneously received by all, or a subgroup of, the spot beams (10). The satellite (8) retransmits to the fixed earth station (2) in separate channels the signal as received by each of the spot beams (10) which are able to receive the common signalling channel. The relative strengths of the signal as received in the different spot beams (10) are compared at the fixed earth station and a communications channel is assigned to the mobile terminal in one of the spot beams (10), selected according to the comparison of signal strengths.

Abstract: A satellite communications system provides a communications service to a mobile terminal (2) on which different communications applications (4a to 4d) may be run. Calls are set up between any of the applications (4a to 4d) via a satellite (12) to a network management center (18) which provides different service adaptors (20a to 20d) which adapt the calls to different types of service provided over terrestrial networks (22), such as telephony, facsimile, internet or ATM services. The bandwidth allocated to each call over the satellite link may be varied during the call according to demand either from the relevant application (4) or from the network management center (18). Multiple calls may be connected concurrently to or from different applications running on the mobile terminal. A maximum bandwidth is allocated to each call. Efficient use is thereby made of the limited bandwidth available over the satellite (12), according to the instantaneous bandwidth requirements of different applications (4).

Abstract: In a radio frequency communications system, a data carrier activation method is implemented such that the carrier is switched off when no data is available for transmission. If repeated signalling information is required to be transmitted, only a predetermined number of repeats is transmitted before the carrier is switched off. The data input for transmission are compared with an idle sequence with different bit alignments to detect the presence of idle signalling, and the carrier is switched off if a match is found. When more user data is received, the carrier is switched on and frames (SM) are transmitted in synchronization with the timing of frames (SM) transmitted before the carrier deactivation. After carrier reactivation, a constant power preamble (P) may be transmitted to assist in level control in the transmitter (27; 40).

Abstract: Digital audio recording equipment (12) is used to record modulated signals, such as the RF test signals generated by test modems. If the modulated signals are outside the audio frequency range, they are down-converted into the audio frequency range using a mixer (4), local oscillator (6) and a filter (8). The digitally converted data are recorded onto a suitable medium, such as a DAT, CD or a computer memory. The data is then disseminated for reproduction by a suitable digital audio player (12′). The reproduced audio signal may be up-converted to the desired radio frequency using a mixer (16), local oscillator (18) and a filter (20). The reproduced signal may be used to test demodulators, codecs or tranceivers. The signals output by the transceiver and the reproduced signal may be recorded on different channels of the digital audio recording equipment (12). The recording equipment (12) may be used to capture RF signals for later analysis.

Abstract: An earth station receives a return signal via more than one satellite link from a mobile terminal using TDMA. The earth station selects one or more of the satellite links for transmitting a forward signal on the basis of the quality of signal received via each link. The earth section allocates frequency channels to the mobile terminals according to their location on the surface of the earth, so that the propagation time to and from those mobile terminals which share the same frequency channel is approximately the same. The satellite includes an antenna which generates an array of beams which are individually pointed to fixed regions of the earth, until the elevation of the satellite relative to a fixed region falls below a minimum value, in which case the corresponding beam is redirected to a new area, while the other beams remain pointed at the corresponding fixed areas. In this way, beam-to-beam handover is reduced, while maintaining the boresight of the antenna pointing at the nadir.

Abstract: In a mobile communications system 10, a message is sent to a mobile terminal 16 which transmits an acknowledgement signal if the message is received. If the system 10 does not detect an acknowledgement signal, it transmits a high margin notification signal to the mobile terminal 16 to notify the terminal that an attempt was made to send the message. When the mobile terminal 16 has received a notification signal and subsequently comes within range of lower margin signals from the system, it sends a registration signal to the system. The system may then retransmit the message to the mobile terminal 16.

Abstract: A data interface apparatus (10) provides an interface between an analog communications link (a2) and a digital communications link (d2). Analog dialling codes on the analog communications link (a2) are converted to control codes for controlling a digital communications interface (12) attached to the digital communications link (a2) for providing access to a digital communications network (16, 18, 20). Access to the digital communications link (a2) is selectively enabled by the data interface apparatus (10) in response to a password encoded as an analog dialling sequence on the analog communications link (a2) The data interface apparatus (10) delays call set-up with a modem (6) connected to the analog communications link (a1,a2) so as to synchronize completion of call set-up with another call set-up procedure performed on a remote analog link (a3,a4) connected to the digital communications network (16, 18, 20).

Abstract: A communication station for generating or receiving an angularly dispersed array of radiation beams, comprising an array of radiating elements (100) defining an array antenna (1); and a beam former (120), the beam former comprising a passive network having a first side (X) carrying a plurality of beam ports for electrical signals corresponding to the beams and a second side (Y) carrying a plurality of antenna ports corresponding to the radiating elements, each beam port being connected to a plurality of antenna ports via a network (50) of power divider (52) and phase shifter (53) components, the phase shifts of which are in integer multiples of a predetermined constant, so as to generate the array of beams; in which the array antenna (1) has triangular or hexagonal symmetry and the array of beams has triangular or hexagonal symmetry.

Abstract: A method and apparatus for preventing interference between a first satellite and a second satellite which share frequencies for their up or down links is disclosed. A ground station receives downlink interference above an acceptable threshold from the first satellite if the first satellite and the second satellite are separated, relative to the ground station, by less than a minimum discrimination angle. Likewise, the first satellite receives unacceptable uplink interference from the ground station in that situation. To overcome the problem of interference, the second satellite is prevented from transmitting to or receiving from a forbidden area of the earth's surface.

Abstract: In a satellite communications network, local pools of available carriers are assigned to each ground earth station GES by a network channel management station NCMS, which stores a pool of unallocated carriers. If the number of unused carriers in a local pool falls below a predetermined level, additional carriers are allocated from a regional pool in the network channel management station NCMS. If the number of carriers in a local pool exceeds a predetermined level, the excess carriers are released to the regional pool so that they can be allocated to local pools in other ground earth stations GES.

Abstract: A communications link for data communication via satellite includes interface apparatus comprising a satellite link interface for connection to a satellite link and a PSTN interface for connection to a PSTN. The interface apparatus supports different data compression modes in the communication link. Mode A allows data compression over the PSTN but not over the satellite link. Mode B allows data compression over the PSTN and over the satellite link, but enables decompression and recompression of data flowing in each direction through the interface apparatus. Mode C allows compressed data to pass transparently between the PSTN and the satellite link, while the interface apparatus performs error correction on the satellite link and on the PSTN.

Abstract: A satellite radiodetermination system comprises global navigation service (GNSS) satellites 2 such as GPS satellites, which generate GNSS ranging signals R.sub.n, geostationary satellites 6 which retransmit ranging signals R.sub.g generated at a navigation land earth station (NLES) 8, including augmentation data A, and medium earth orbit (MEO) satellites 10 which generate ranging signals R.sub.m including regional augmentation data RA transmitted from a satellite access node (SAN) 14. The regional augmentation data RA is supplied by regional augmentation systems 21a, 21b.A navigation receiver 11 receives the ranging signals R.sub.g, R.sub.m, R.sub.n and calculates ionospheric delay values for those ranging signals which are provided on dual frequencies. Using these ionospheric delay values, and optionally the regional augmentation data RA and the augmentation data A, the navigation receiver estimates ionospheric delay values for those ranging signals which are provided on single frequencies.

Abstract: A map is provided for indicating the direction of a fixed point (S), such as a sub-satellite point, from any point (P) within a specified area which is a substantial fraction of the earth's surface. The map projection is such that the actual direction to the fixed point (S) is the same as the direction shown on the map, when the map is aligned with a reference direction such as magnetic north. Optionally, the distance to the fixed point on the map represents the elevation of the satellite. Apparatus for using the map includes a holder (10) for the map and for a magnetic compass (18). A scale (22) which rotates about the fixed point on the map has a marker (32) which is positioned over the user's location on the map. The azimuthal direction of the satellite is indicated by an arrow (28) on the scale, while the elevation of the satellite is read off the scale by means of a pointer (34) located on the fixed point on the map.