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: Data relating to the status of users 12 in a mobile communications system is broadcast via satellite 8 from a network register 4 to local registers 10 in an HDLC format. Each of the local registers 10 requests retransmission of any HDLC frames which are incorrectly received. The network register 4 records the earliest transmitted frame not yet acknowledged by all of the local registers and inhibits transmission of new frames if they fall outside a transmission window relative to the earliest transmitted frame. The network register 4 polls the local registers 10 for retransmission requests and the local registers 10 also send unsolicited requests to the network register 4. The network register 4 only retransmits a frame once if multiple requests for that frame are transmitted within a predetermined period.

Abstract: A satellite radiodetermination system comprises global navigation service (GNSS) satellites 2 such as GPS satellites, which generate GNSS ranging signals Rn, geostationary satellites 6 which retransmit ranging signals Rg generated at a navigation land earth station (NLES) 8, including augmentation data A, and medium earth orbit (MEO) satellites 10 which generate ranging signals Rm 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.

Abstract: A set of formats and protocols is proposed for a satellite communications system. In these formats, a pilot signal (PS) is inserted after every 25 or 29 data symbols. The formats consist of SCPC frames (F) which may contain either data (D) and in-band signalling information (SU), or only signalling information (SU). In either case, the contents of each frame (F) are error-correction coded before transmission with the same coding rate. Each data frame (F) carries the data content of an integral number of input user data frames (M), each of which comprises four subframes. Different symbol transmission rates are used for different input data rates, the symbol transmission rates being selected so that their different synchronising clock rates can easily be obtained from a common clock. Data bursts may be preceded by a constant power level preamble (P).

Abstract: A set of formats and protocols is proposed for a satellite communications system. In these formats, a pilot signal (PS) is inserted after every 25 or 29 data symbols. The formats consist of SCPC frames (F) which may contain either data (D) and in-band signalling information (SU), or only signalling information (SU). In either case, the contents of each frame (F) are error-correction coded before transmission with the same coding rate. Each data frame (F) carries the data content of an integral number of input user data frames (M), each of which comprises four subframes. Different symbol transmission rates are used for different input data rates, the symbol transmission rates being selected so that their different synchronizing clock rates can easily be obtained from a common clock. Data bursts may be preceded by a constant power level preamble (P). The formats and protocols satisfy the requirements of a high data rate satellite communications system.

Abstract: A network station receives a principal signal and data. When the principal signal is present or contains information it is transmitted to a receiving station through a communications channel. When the principal signal is absent or does not have a significant information content, the network station transmits the data through the same communications channel in a format such that the data is received and output by a further receiving station.

Abstract: A communication system for communication with a plurality of mobile terminals. A plurality of earth stations are arranged to communicate with mobile users via a plurality of orbiting satellites. A plurality of gateway stations interconnect terrestrial equipment with the earth stations and a store stores access data for the mobile terminals. The mobile terminals are divided into first and second categories, the store retaining corresponding different status information and both the first and second categories communicating with the satellites. Also provided are first and second different communication channels between the gateway stations and the earth stations, and a route control system for selecting one of the first and second channels, in dependence upon the category of a mobile terminal.

Abstract: An interface between a data terminal and a digital communications link implements protocols and frame formats designed to reduce delays in a data communication with a remote terminal. The interface overcomes the need to send or receive an HDLC SABME or UA control signal for establishing an asynchronous balanced mode, and allows data to be sent over the digital communications link as soon as the communication parameters are established. In a non-ARQ (error correction) mode, the interface sends control signals and data over the communications link in frames subdivided into many small subframes of fixed length, each subframe having a length code indicating an amount of valid information but without CRC, address or control information. The interface, when arranged for connection to the data terminal through a telephone network, encodes call progress signals from the telephone network for sending over the digital communications link.

Abstract: A satellite communications system comprises satellites 9A-C, earth stations 7A-E, user terminals 11 and controlling gateways 3 in the form of mobile satellite switching centers 5A-E. Access with a user terminal 11 may be gained to a terrestrial telephone system 1 by earth stations 7A-E contacting the satellites 9A-C and by the satellites contacting the user terminals 11. The satellites 9A-C periodically broadcast messages indicative of the earth station 7A-E with which they are instantly in contact. The user terminal 11 listens to and logs these broadcasts to keep a list of the relative number of times each earth station 7A-E can be heard. When calling into the system, the user terminal pages earth stations in descending order of number of entries in the log. When in contact, the user terminal 11 sends the log to the system. When subsequently trying to contact the user terminal 11, the system uses earth stations in descending order of number of entries on the log, last received from the user terminal.