This invention relates to a low earth orbit satellite suitable to deliver electronic mail to hand-held terminals.
Communication between low earth orbit satellites and ground terminals has proved extremely difficult and expensive due to problems of radio power limits, satellite tracking mechanisms, Doppler shift, and licensing of a global frequency.
According to the present invention the low earth orbit satellite is equipped with a high gain directional antenna, the beam direction of which is arranged to oscillate so as to sequentially illuminate only those points of the earth's surface which would see the satellite at a low angle above the horizon. The satellite is equipped with a mechanism for adjusting the frequency and power of the radio transmission in synchronism with the direction of the beam so as to effect correction of Doppler shift and to select frequencies and power licensed by the administration for that part of the earth's surface illuminated by the beam at any particular instant. The oscillation causes the beam to scan a circle on the earth's surface but with a variable angular velocity. A mechanism is provided to control the amplitude of the oscillation and the direction of the beam corresponding to minimum angular velocity such that this minimum velocity occurs when the beam is pointing at the parts of the earth with a relatively high data traffic requirement.
A specific embodiment of the invention will now be described. The low earth orbit satellite is equipped with a parabolic dish antenna. The antenna has freedom to rotate between 360 and 720 degrees about a vertical axis (an axis intersecting the centre of the earth). The beam of the antenna is at an angle of about 50 degrees to the vertical axis (or such other angle required to give the ground terminal a low angle view of the satellite depending on the height of the orbit.). The mass of the antenna is coupled to a spring, torsion bar or magnet system arranged to form an oscillating spring-mass system. A mechanism such as an electro-magnet is provided to controllably excite or damp this oscillatory system up to a maximum of 720 degrees of angular movement peak to peak. The oscillation will cause the beam to scan all points on a circle on the earth's surface at least once per cycle of the oscillation and the beam will also have two regions of slowest angular velocity. Standard computers, sensors, actuators and radio frequency electronics co-ordinate the frequency, power, oscillation amplitude, and satellite orientation to effect optimum radio frequency, power and beam dwell time to best suit a terminal or terminals at particular parts of the earth's surface. The orbit of the satellite, the rotation of the earth, and the circular scan of the beam combine to give a regular low noise communication path to all parts of the earth's surface for a brief period sufficiently long and regular to support electronic mail transfers. Computers predict when a communication time window will occur for a particular ground terminal and schedules the mail transfers to occur during that time window.
A low earth orbit satellite equipped with a high gain directional antenna the beam direction of which is arranged to sequentially scan a circle of points on the surface of the earth, such points seeing the satellite at a low angle above the horizon. Means are provided to control the frequency, power and dwell time of the antenna beam so as to comply with a multitude of earth administration licence requirements, to correct Doppler shift and enable electronic mail exchange with a hand-held terminal.
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