| 摘要 |
A satellite communications system is of the type in which a satellite provides a plurality of spot beams 301 on the earth's surface, a respective main broadcast channel in each of the spot beams carries a signal containing geographical information relating to the boundaries of the corresponding spot beam, and a user terminal 38 is operable to learn its position on the earth's surface and to receive the broadcast channel of the spot beam currently occupied by the user terminal so that the user terminal can confirm that it is located within the spot beam boundaries, in order to maintain camping of the user terminal on the spot beam. If the main broadcast channel of a spot beam has to be turned off, for example to avoid interference with facilities such as radio telescopes or defence installations, this could result in multiple user terminals requesting a location update leading to system overload. To avoid this overload, the satellite transmits a substitute broadcast channel in a spot beam commencing a predetermined time prior to the main broadcast channel of that spot beam being turned off. The substitute broadcast channel carries the same geographical spot boundary information as the main beam, whereby a user terminal in that spot beam will acquire the substitute broadcast channel and remain camped on the same spot beam. The main broadcast channels of contiguous spot beams are all on different frequencies f1 to f7 to one another, and the substitute broadcast channel in a spot beam is on a substitute broadcast frequency which is different to both the main broadcast frequency of that spot beam and the main broadcast frequencies of all the contiguous spot beams. The substitute broadcast channel may initially be at a higher power than the main broadcast channel, but may be reduced in power after the main broadcast channel of the spot beam has been turned off. The satellites may be in medium earth orbit, and the user terminals may be mobile, semi-mobile, or stationary, radio telephones. |
