Radio Frequency Beacon
One use for radio frequency beacons is to track the position of spacecraft that are in orbit outside the earth's atmosphere. It is important to know the position of such spacecraft as exactly as possible in order to aim ground based systems that are in communication with the spacecraft. Position information is also important to accurately interpret scientific data collected by spacecraft instrumentation for transmission back to earth.
RF beacons deployed in space face tougher performance obstacles as compared to ground based RF beacons. Chiefly, an RF beacon in space must broadcast great distances through the earth's atmosphere without a significant degradation of its signal purity. Furthermore, space based systems also face restrictions on size, weight, power, and quality or ruggedness of their design.
The present invention provides an improved RF beacon having better spectral purity and frequency stability over the current state of the art, in addition to more features. The radio frequency beacon of the present invention outputs fixed tones so that the position of the spacecraft in which it resides can be determined by means of Doppler tracking. Particular characteristics of the present invention include broadcasting coherent signals at 150 MHz and 400 MHz simultaneously thereby enabling the removal of ionospheric effects. A 1067 MHz signal can also be simultaneously broadcast. The 1067 MHz signal can be modulated with a data bit stream encoded by, or mixed with a "gold code" which is a pseudo-random code modulation scheme that allows telemetry data to be placed on the carrier without disturbing the overall phase of the 1067 MHz signal.
A power supply converts unregulated spacecraft bus power to the positive and negative DC voltages needed by the beacon electronics. There are two oscillators operating at approximately 16.7 MHz. Each oscillator operates at a slightly different frequency to provide for frequency selection with respect to the transmitter frequency spectra. The oscillators are redundant meaning that only one need be operating at any given time and the other serves as a backup in case of a failure, or if a slight frequency change is desired. The RF beacon can switch between the oscillators by remote command. Each oscillator also contains a heater control element to help maintain optimal performance. Whichever oscillator is in use simultaneously provides a signal to a times-8 and a times-9 multiplier circuit. The times-9 multiplied 16.7 MHz signal is approximately equivalent to a 150 MHz signal, which is then fed to an amplifier that provides a 150 MHz output signal. The times-8 multiplied 16.7 MHz signal is approximately equivalent to a 133 MHz signal. It is multiplied again by a times-3 multiplier to produce approximately a 400 MHz signal which is then fed to an amplifier that provides a 400 MHz output signal. A 1067 MHz output signal is obtained by multiplying the original 16.7 MHz oscillator reference signal by a times-8 multiplier and then by another times-8 multiplier. The resulting twice multiplied signal is fed to a phase modulator and is modulated by a pseudo random encoded data stream. The output of the phase modulator is approximately a 1067 MHz signal that is phase modulated. This phase modulated signal is then fed to an amplifier which outputs a 1067 MHz signal. All of the amplified output signals are then fed to individual antennas and broadcast. Another variant of the present invention combines the three outputs into a single port by means of a triplexer circuit. A single multi-band antenna can be used in this case to simplify spacecraft cabling and minimize the space needed for mounting antennas.
Some of the features of the RF beacon of the present invention include redundant switchable oscillators, a 1067 MHz signal having data modulation capabilities, the ability to change power levels, improved frequency stability and temperature control characteristics, and an optional three-way combined output with a single output port.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
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