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As the Galileo Probe enters Jupiter's atmosphere and rapidly descends through the cloud layers, its signal will be tracked by the relay radio hardware (RRH) onboard the Orbiter. This relay link is a crucial element of Probe's mission, for otherwise there would be no way to transmit the Probe's data to Earth.
The RRH consists of a dish antenna, two receivers, and two ultrastable oscillators. The 1.1-meter-diameter (43-inch) antenna will be mounted on a moveable support boom attached to the nonspinning (or "despun") portion of the Orbiter. The antenna will remain in a stowed position until the Probe separates from the Orbiter about 150 days before arrival at Jupiter. After Probe separation, the antenna will be deployed. During the Probe's entry into Jupiter's atmosphere, the RRH antenna will be repointed to receive the Probe's signal. Use of this smaller side-mounted antenna will allow the Orbiter's low-gain antenna to remain Earth-pointed for tracking and communications.
Although the concept is simple, the Probe data relay presents special challenges that the hardware and firmware must meet. The acquisition, tracking, and transfer of Probe data to the Orbiter must occur autonomously. The receiver must automatically acquire and "lock" onto the Probe's signal within 50 seconds from the start of data transmission from the Probe. It must maintain lock for as long as the Probe survives in the increasingly hostile environment of Jupiter's atmosphere.
The first task of the Orbiter RRH will be to find the Probe's signal. It will be faint--approximately 1000 times less than the human audible level--and its frequency and signal strength will vary due to the Probe's rapid descent, as well as turbulence and chemical effects in the atmosphere. During this important phase of the mission, the RRH antenna will be receiving the Probe's signal from a distance of over 200,000 kilometers (120,000 miles), with a narrow (13 deg) beamwidth.
To ensure that the relay link is not jeopardized due to equipment failure, the RRH design is "redundant"--it has two receivers and two ultrastable oscillators, and the antenna is designed to simultaneously receive two channels of Probe data that are differentiated by frequency and polarization. The two receivers are physically and electrically identical except that each is tuned to its respective channel of Probe data transmission and each has a unique address for command and data transactions with the Orbiter's command and data subsystem computers. Both receive the same real-time data from the Probe. Each receiver uses an ultrastable oscillator as a reference to extract Doppler information from the Probe's signal. During the search and track modes, each receiver consumes 23 watts of power.
The RRH is controlled by a complex, permanently fixed computer sequence. Because events will occur rapidly, the Probe data acquisition, tracking, and transfer to the Orbiter must occur automatically.
The search for the Probe signal begins when power is applied to the receivers. In 16 seconds or less, the receivers will search the entire 70-kilohertz bandwidth several times to find the Probe signal with a high probability. At the end of 40 seconds, the signal frequency and rate are estimated, and the receiver "locks" onto the signal phase within another 8 seconds. The Probe is expected to transmit for about one hour before it is silenced by the intense heat and crushing pressure.
The receiver breadboard tests and software are complete, and the qualification and flight units and unit tester are being assembled. Parts procurement is complete except for a recent change in the random access memory to provide better radiation hardness. The spacecraft will experience the most intense radiation during its close flyby of Jupiter as it relays the Probe's signal. The antenna is in the early stages of design.
Ames Research Center developed the RRH along with the Probe system. The receivers and antenna are being developed, fabricated, and tested by Hughes Aircraft Company under contract to Ames, and the oscillators are provided by Frequency Electronics Inc. under subcontract to Hughes.
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