Patent GB-1086924-A (GB)
Title: Tracking system for a communication satellite
IPC: H01Q5/45
Filed: 1965-05-10
Abstract: 1,086,924. Radio tracking systems; aerials; satellite communication systems. HUGHES AIRCRAFT CO. May 10, 1965 [May 21, 1964], No. 19702/65. Headings H4A, H4D and H4L. The invention relates to a system for acquiring and then tracking a communication satellite of the type which receives signals at a high frequency, and retransmits the signals at a lower frequency. An antenna arrangement of a ground station comprises a dish antenna arrangement, automatically or manually movable, having a feed arrangement comprising two coaxial feed horns, Fig. 5 (not shown), the inner smaller horn 98, Fig. 4, transmitting to the satellite at said high frequency, with a narrow beam 290, Fig. 7, and the outer, larger horn 100 receiving the retransmitted signals from the satellite at said lower frequency, with a broad beam 292. The amplitude of the received signal is dependent on the amplitude of the transmitted signal as received by the antenna which in turn is dependent on the angle between the boresite 302 of the antenna 9 and the direction 304 of the satellite. Thus said amplitude can be used for the acquiring and tracking purposes. In the described embodiment, the ground station transmits to the satellite at a frequency of 7360 Mc/s. and the satellite retransmits the signal at a frequency of 1815À794 Mc/s. Another station may transmit to the satellite at a frequency of 7361À275 Mc/s., giving a response signal, received at the ground station, at a frequency of 1814À069 Mc/s. Both the station transmitted signals are frequency modulated. The satellite also transmits a beacon signal at a frequency of 1820-177 Mc/s. which is received by the ground station. The three received signals are detected in the receiver horn 100 by probes 106, and 108, Fig. 4, and are fed to a hybrid junction 114 to give a sum signal which is fed, via transmitting frequency band rejection filter 118 and parametric amplifier 96 to receiver 68. At the receiver, Fig. 6 (not shown), the signals are heterodyned down to a first I.F. range and then amplified. The signals are then heterodyned a second time, using a voltage controlled crystal local oscillator, controlled in frequency and phase by a signal from a phase comparison circuit to be described later, such that the I.F. for the satellite response to the ground station is always 9À383 Mc/s., that of the other station response is always 5À0 Mc/s., and that of the beacon signal is 11À108 Mc/s. The ground station I.F. signal is fed via a filter with a 10 kc/s. bandwidth for normal conditions, or via a filter with a 2 kc/s. bandwidth for noisy conditions, to an A.G.C. controlled I.F. amplifier. The amplitude of the I.F. amplifier output is thus constant having a noise content decreasing as the boresite axis 202 nears the satellite direction 304. The F.M. information is extracted and the A.G.C. signal generated in a demodulator wherein the phase of the amplifier output is determined with respect to the output of a phase-locked voltage controlled oscillator by means of a phase detector. The output of the phase detector containing the FM information is fed to a pair of earphones and to an integrating amplifier, to provide a signal for controlling the phase and frequency of the voltage-controlled oscillator such that it is locked to the I.F. amplifier output. The output of the integrating amplifier is also fed to an audio system. The A.G.C. signal is produced by a synchronous detector, detecting the I.F. amplifier output phase shifted by 90 degrees with the output of the voltage-controlled oscillator as reference. The A.G.C. signal is inversely proportional to the amplitude of the received satellite response to the ground station signal, which in turn is inversely proportional to the angular separation between the boresite 202 and satellite direction 304. The A.G.C. signal is therefore displayed on a meter and a minimum indication thereon indicates that the narrow beam 290 is accurately directed on the satellite. A similar arrangement is provided for using the received satellite response to the other station, the A.G.C. signal is now dependent however on the broad beam 292, and the indicator is used to provide an initial rough acquiring facility. The beacon signal is also fed via an A.G.C. controlled I.F. amplifier to a phase detector and a synchronous detector. The synchronous detector uses a constant-frequency reference source and provides the A.G.C. signal which can provide a second rough acquiring indication. The phase detector also uses the constant-frequency reference source, and provides a signal which controls the voltage-controlled crystal oscillator. Thus if there is no other station transmitting to the satellite, the beacon signal enables the broad beam to be roughly aligned on the satellite and the satellite response to the ground station transmission enables the narrow beam to be accurately aligned on the satellite.