After almost 20 years in space, NASA's Cassini spacecraft begins the final chapter of its remarkable story of exploration. From now until September, Cassini will undertake a set of orbits that is, in many ways, like a whole new mission. Following a final close flyby of Saturn's moon Titan, Cassini will leap over the planet's icy rings and begin a series of 22 weekly dives between the planet and the rings.
Launched in 1997, Cassini arrived at Saturn in 2004. Following its last close flyby of Titan in April, Cassini began what mission planners are calling its Grand Finale. The spacecraft is on a trajectory that will eventually plunge into Saturn's atmosphere — and end Cassini's mission — on September 15.
Cassini is beaming back science and engineering data collected during its passage via the Radio Science Subsystem (RSS) and Radio Detection and Ranging (RADAR) system.
Radio Science Subsystem
Radio waves are altered as they travel through a gas, bounce off a surface, or pass near a massive object. By sending radio signals through, near, or even bouncing off objects in the Saturn system, the RSS can help scientists learn about the objects with which the radio waves interact.
The RSS sends radio signals from Cassini to Earth using the spacecraft's large radio dish — the high-gain antenna. En route, the radio signal interacts with Saturn's moons, rings, or Saturn's atmosphere. When the signals reach Earth, scientists study how the signals were altered, which helps them learn about gravity fields, atmospheric structure, composition, ring structure, particle sizes, surface properties, and more.
The spacecraft's high-gain antenna must be pointed toward Earth in order to send its data, but often the spacecraft must face a different direction because one or more of its instruments is observing a specific target. Sometimes a moon or the planet is between the spacecraft and Earth, so Cassini must store its data until it can be beamed to Earth later.
Researchers have no choice but to wait for hours, or sometimes days, before getting data that's been sitting on the spacecraft waiting to be transmitted. This is not the case for Cassini's RSS; as soon as its data is collected, it's already on Earth.
For the radio science instrument, the radio signals that Cassini transmits to Earth are the experiment, and the NASA Deep Space Network complexes in Australia, Spain, and the U.S. are part of the instrument. And despite the more than 900-million-mile journey the signal takes from Cassini to Earth, the instrument is extremely sensitive. According to Cassini radio science team member Essam Marouf of San Jose State University, “Even if you place a sheet of paper in its path, we could sense the change in the Earth-received signal.”
Cassini is the only deep space mission to transmit to Earth at three radio wavelengths — about 14 cm wavelength, designated S-band; 4 cm, designated X-band; and 1 cm, designated Ka-band) — simultaneously.
The Cassini Radio Detection and Ranging (RADAR) will be used to investigate the surface of Titan by taking four types of observations: imaging, altimetry, backscatter, and radiometry. In the imaging mode of operation, the RADAR instrument will bounce pulses of microwave energy off the surface of Titan from different incidence angles, and record the time it takes the pulses to return to the spacecraft. These measurements, when converted to distances (by dividing by the speed of light), will allow the construction of visual images of the target surface. Radar will be used to image Titan because the moon's surface is hidden from optical view by a thick, cloud-infested atmosphere; radar can “see” through such cloud cover.
Radar altimetry similarly involves bouncing microwave pulses off the surface of the target body and measuring the time it takes the “echo” to return to the spacecraft. In this case, however, the goal will not be to create visual images, but rather to obtain numerical data on the precise altitude of the surface features of Titan. In the backscatter mode of operation, the RADAR will act as a scatterometer; that is, it will bounce pulses off Titan's surface and then measure the intensity of the energy returning. This returning energy, or backscatter, is always less than the original pulse, because surface features inevitably reflect the pulse in more than one direction. From the backscatter measurements, scientists can infer the composition of the surface of Titan.
Finally, in the radiometry mode, the RADAR will operate as a passive instrument, simply recording the energy emanating from the surface of Titan. This information will tell scientists the amount of latent heat (i.e., moisture) in the moon's atmosphere, a factor that has an impact on the precision of the other measurements taken by the instrument. During imaging, altimetry, and backscatter operations, the RADAR instrument will transmit linear frequency-modulated Ku-band pulsed signals toward the surface of Titan using the high-gain antenna (HGA). These signals, after reflection from the surface, will be captured by the same antenna and detected by the RADAR Radio Frequency Electronics Subsystem. During radiometry operations, the instrument will not transmit any radar signals, but the HGA will again be used for radiometric observations.
To improve the surface coverage by radar imaging, a switched, multiple Ku-band antenna feed array structure is part of the HGA, and permits the formation of five antenna beam patterns. Each of these beams will have a different pointing angle relative to the antenna reflector's focal axis.
The major functional components of the RADAR Subsystem are the Radio Frequency Electronics Subsystem (RFES), the Digital Subsystem (DSS), and the Energy Storage Subsystem.
The RFES has three principal functions: the transmission of high-power frequency-modulated and unmodulated pulses, the reception of both reflected energy from the target and passive radiometric data, and the routing of calibration signals. The RFES has a fully enclosed structural housing and Faraday cage (i.e., an electrostatic shield). The RFES electronics units are individually enclosed and are mounted to the RFES housing wall opposite the wall that mounts to the spacecraft. For thermal control, heat flows conductively from the units to the housing wall and is then radiated away from the RFES.
The RFES digital chirp generator (DCG) generates the low-power, baseband-frequency, modulated pulse upon request from the RADAR Digital Subsystem. Both the bandwidth and the pulse width of this pulse can be varied in accordance with the parameters received from the DSS. The chirp up-converter and amplifier (CUCA) converts the baseband chirp pulse to Ku-band and provides the up-converted pulse to the high-power amplifier (HPA).
Front-end electronics (FEE) receive the high-power pulse from the HPA and route the signal to one of five different antenna ports on the RFES via an antenna switch module. The echo returns, and radiometric signals are routed from one of the five antenna ports to the RFES microwave receiver. The FEE also steers the selected calibration signal to the microwave receiver (MR) during periods of calibration mode operation.
The MR receives signals at Ku band and down-converts these to baseband so that they can be properly sampled. The sources of these signals are the echo returns, radiometric signals, and calibration signals routed through the FEE. The MR receives the re-routed chirp calibration signal from the CUCA and passes that signal to the FEE for proper routing. The MR is also the source of the noise diode calibration signal that is provided to the FEE for routing. MR gain and bandwidth information is provided to the MR from the DSS.
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