Radio Science

Images, and other such observations that Cassini sends via telemetry, are very useful for obtaining scientific knowledge. But what if you could reach out and directly touch Saturn's rings? In a way, that's what radio science can actually occultation do. Radio science (RS) is briefly described here in this section, because it can be one of the uses of pure uplink and downlink, working closely together in flight operations, to collect scientific data in a unique way. Also, RS is related to some of the techniques used for tracking Cassini in flight. Cassini's RS experiment sends a radio beam from the spacecraft (the downlink) directly through Saturn's rings, or its upper atmosphere, or some other phenomenon under investigation. The received radio signal can then be analyzed to find out, for example, the size of particles making up Saturn's rings, or much about the structure and composition of Saturn's upper atmosphere. This kind of RS experiment is called an occultation experiment, because Cassini's radio signal becomes occulted, or obscured, by an intervening medium. There are many other experiments that RS can perform, as you will see.

Everyone loves a band

Cassini carries transmitters that beam signals out in three regions, or bands, of the radio spectrum which can be used for RS: S-band, Cassini's lowest frequency, has the longest wavelength; X-band can also be used to send telemetry; and Ka-band is the highest frequency (shortest wavelength). Cassini's S-band transmitting capability and its Ka-band receiving capability are used exclusively for RS; its X-band receiver can be used for RS, as well as for commanding and ranging. All three bands can be used for Doppler measurement, which contributes to some RS experiments as well. No matter what radio frequencies are in use, the principles are mostly the same, although slightly different information can be extracted from each of the three different bands.

Clean up your downlink

To generate a stable downlink frequency for use in RS experiments, sometimes Cassini uses the uplink, in coherent mode as described above in Tracking. Other times it uses its USO, ultra-stable oscillator, as a frequency reference aboard the spacecraft. During most RS experiments, ranging and telemetry are turned off, so that the downlink will be simply a pure tone, free of the "mess" that telemetry and ranging would make of the nice, pure downlink. (Data to one person is noise to another!). That way, any variations in the downlink received on Earth will have been caused by the object under study, such as rings or atmosphere.

Radio science experiments

In addition to measuring rings and atmospheres, Cassini's RS will make measurements of the masses of Saturn and its satellites. To measure the mass of a body, Cassini is tracked as it passes close to the body. This RS experiment is called celestial mechanics (CM). Now, the more massive a body is, the stronger the effect of its gravity. Careful analysis of the changes in Cassini's speed, caused by the gravity field of the body, can be translated into precise measurements of that body's mass. Changes in speed, of course, show up in the Doppler measurements made while a Deep Space Network Earth station is tracking Cassini. One way to measure the diameter of a solid body such as one of Saturn's moons is by flying behind it and noticing exactly how long Cassini's downlink is blocked from view (occulted) on Earth. Given that Cassini's speed, path, and distance from the moon are known, the diameter can be calculated. Of course images can provide another measurement of size to correlate with RS data. Once size and mass are known, the density of the body can be figured. And knowing the density lets you place constraints on what the body must be made of: rock, ice, iron, and in what proportions. That's a lot of basic information about the objects in the back yard of our solar system. But it's not all!

There's more

RS can probe the Sun's own outer atmosphere, called the solar corona . Once a year, as Earth orbits the Sun, Earth is on the opposite side of the Sun from Cassini as it flies toward Saturn (or when it is orbiting Saturn). That time is called superior conjunction. Some years, at solar conjunction, the straight line from Cassini to Earth happens to pass close to the Sun, or even go through it. At those times, RS experiments are undertaken to probe the corona with Cassini's downlink, in order to learn more about the star that powers life on Earth.

Also, at those times when Cassini appears to go behind the Sun, another very different kind of RS experiment can be done, testing the theory of general relativity. Albert Einstein predicted that the tremendous mass of the Sun causes light (or other wavelengths) to bend, due to way the Sun warps space-time. ( Observations made during a solar eclipse in 1922 - during his lifetime - corroborated his theory.) Cassini's RS experiments will collect high-precision data to help quantify these effects, perhaps adding further confirmation to the theory. Doppler and precision ranging, which constitute this kind of RS experiment, detect the small increase in apparent distance from Earth to Cassini caused by the Sun's mass.

Still going

So far, all the radiation we humans have ever seen or otherwise detected has been electromagnetic radiation: light, heat, radio, etc. Ever since Galileo Galilei turned his telescope to the sky, astronomers have been observing various forms of electromagnetic radiation, to learn about the universe in which we live. But there may exist a completely different kind of radiation altogether, one which would open up brand new doors of perception. General relativity predicts that massive bodies, when they are accelerated, radiate gravitational waves, warping space-time as they propagate through the universe at light speed. Gravitational waves have never been directly detected (as of the time this was written, mid 1996), although some astronomical observations of massive objects have implied that they exist. Cassini's RS can and will search for such gravitational waves. All it takes to carry out this kind of experiment is for Cassini to be tracked from very far away; the distance to Jupiter's orbit or beyond is useful. The best time of year to do this when Cassini is at opposition, that is, on the same side of the Sun, in line with the Sun and Earth. That way, the Sun is out of the way, contributing minimum noise to this sensitive experiment.

Then the spacecraft is commanded to be quiescent, so that nothing will interfere with Cassini's steady speed, like rocket thrusters firing. Coherent tracking is established. Then Cassini's X-band and Ka-band downlink signals are collected for a number of hours, to be analyzed later. If a gravitational wave passes through the solar system during that period, it might (depending on its characteristics) cause a recognizable change in Cassini's Doppler signature. Other spacecraft have done this kind of experiment in the past, and have not detected gravitational waves. Earth-based gravitational wave detectors are also being invented. Cassini's use of Ka-band will provide a new level of precision to this exciting search. Will Cassini be the first to detect a new way of looking at things? Even if no gravitational waves are detected, that fact in itself is useful, for determining constraints to apply within the theory. But it would be much more exciting to find one! Other scientific experiments, such as imaging or spectroscopy, are mostly based on individual instruments aboard Cassini. Radio Science is unique, though, in that its "instrument" comprises the spacecraft's receiver and transmitters, and the Earth-based DSN, all working together as one large system.