Telemetry

Telemetry brings many kinds of information from Cassini to Earth. Look at the word... "tele" means "at a distance" and "metry" refers to measurement: measurement at a distance. And what a distance! When Cassini is orbiting Saturn, it will be a billion miles away. Yet we'll be able to see the exact temperature of nearly every one of its major components. We'll know the voltages, currents, pressures, and other measurements throughout virtually all of its systems. We'll see the status of the programs it's running, and the attitude the spacecraft is pointing. There are thousands of such measurements telemetered to Earth. All of these measurements are first converted to digital data (ones and zeroes) in the spacecraft's computers; they are arranged into "packets," and placed on the downlink, to be received here on Earth. These telemetered measurements are called "engineering data" because they are needed by the engineers who fly the spacecraft.

But that's just the beginning! With the exception of radio science, all of Cassini's scientific instruments send their observations to us by means of telemetry. That's what constitutes the "science data." Science data are the reason for Cassini and Huygens in the first place. Pictures, for example, are one form of science data. Data for images are arranged into packets and transmitted from Cassini pretty much like the engineering data are. The Huygens probe will send its telemetry (both engineering data and science data) from Titan's atmosphere to the Cassini spacecraft, which will then send it to Earth. Technically, the process of handling telemetry is not too different from the way our web server here at JPL is transmitting text and images to you right now, which you see on these web pages. If you listen to the sounds your modem uses on your telephone line, you'll have a general idea of how Cassini modulates its downlink to transmit digital data. We'll look at the process a little more closely in a while.

The Big Picture

But there's more to talk about than how the process of receiving downlinked telemetry works. The broader definition of "downlink" embraces the whole process of receiving, deciphering, storing, routing, distributing, displaying, and archiving the telemetry data. It extends all the way to the scientists who use the data with their experiments, and publish their results in the scientific literature.

Perhaps most important, it reaches the worldwide general public: the people who funded Cassini and Huygens, and who ultimately benefit from their scientific results. Articles appear in newspapers, magazines, and on television. Over time, the knowledge is integrated into the "baseline" scientific knowledge of our time. Textbooks are rewritten; for example, if you read any current textbook concerning our solar system today, you'll find that much of the information it presents came from interplanetary spacecraft such as Voyager. And most of that information started out as telemetry.

Going from left to right in the cartoon, Cassini's scientific instruments collect data (images, spectra, etc.) from the objects of interest. The spacecraft stores and transmits these data as telemetry, which reach JPL's worldwide Deep Space Network (DSN). From the DSN, the telemetry is forwarded to JPL via land lines, earth communications satellites, and microwave links, a system known as the Ground Communications Facility (GCF). JPL processes, stores, and distributes the science data to the teams of scientists who originally commanded their instruments to collect the data. Once the scientists have made sense of their data, they present and publish findings within the scientific community. From there it reaches the worldwide general public (although JPL frequently releases exciting images at the same time the scientists are receiving them - that's the dotted line).

In flight operations, Cassini is always sending telemetry... except for a few short periods during some radio science experiments, when telemetry is shut off to provide a pure downlink tone. What is contained in the telemetry, though, varies greatly, depending on when Cassini is sending it.

The Long Haul

During long periods of cruise, the seven years Cassini will be coasting in its trajectory toward Saturn, the telemetry will consist mainly of engineering data, reporting on the health and status of all the subsystems and instruments aboard. For example, pressures and temperatures, voltages and currents, positions of deployed appendages, status of computer registers and programs are all reported in telemetry. From these engineering data, we'll be able to keep the spacecraft safe, and operating properly; we'll be able to fine-tune its trajectory, making best use of the planetary flybys Cassini needs for gravity assist. The teams of scientists will analyze telemetry from their instruments (a special selection of engineering data from the science instruments called "instrument housekeeping" data) to make sure they are configured, calibrated, healthy, and ready to study Saturn upon arrival in 2004.

At Saturn

When Cassini finally arrives at the ringed planet nearly seven years after launch, telemetry will be streaming from the science instruments, carrying information about objects of study within the Saturnian system. These are the science data. Most science data will be stored temporarily aboard Cassini's solid state recorder (SSR), under control of the onboard computer. The SSR plays back its data later, so Cassini can send it back to Earth during the periods when Earth is listening. Hardly a bit of precious science data will ever be permanently lost. If some do get lost on the way down, say because of a rainstorm over the receiving DSN station ( X-band downlink cannot be received through heavy rain!), we can command the spacecraft to replay the data from the SSR again later, when the Earth has rotated a drier DSN station into view.

A Closer Look aboard Cassini

Cassini has two kinds of scientific instruments: direct sensing, and remote sensing. A direct sensing instrument comes in direct contact with stuff, dust particles, or plasma waves for example. It analyzes the stuff it touches. A remote sensing instrument receives its information remotely from an object, by analyzing radiation (light, heat, etc) that comes from the object. A camera is a remote sensing instrument. In the cartoon below, a remote sensing instrument is receiving an image of Saturn.

Multiple boxes appear in the cartoon to illustrate that Cassini carries multiple science instruments. Each instrument takes care of converting analog signals (light levels, for example) into digital data: ones and zeros. The instrument's internal computer groups its data into packets. A data packet is just a certain number of ones and zeros from the analog signal, accompanied by a set of ones and zeros which identify the instrument, and include other information about the observation. The packets are sent to Cassini's data processor illustrated in blue. That processor is Cassini's CDS (Command and Data Subsystem) computer. The Huygens Probe is not depicted in this cartoon, but it too will send its data to CDS over a separate radio link.

There are many Remote Engineering Units (REUs) throughout the Cassini spacecraft. Each REU continuously receives inputs from various sensors, such as temperature, voltage, or position (there are other kinds of sensors, too). REUs spend their days receiving analog measurements, converting them into data packets, and sending them on to CDS.

Engineering data are normally of a repetitive nature, and if some are lost, it's usually no big concern, because the same measurements will probably be seen again in a short time. Except in cases of spacecraft anomalies or critical tests, the science data are always given a higher priority than engineering data, because science data are a mission's end product, while the latter are just the data used along the way, carrying out spacecraft operations to be able to collect the science data.

Analog versus digital

Let's digress a bit here (sorry pun intended) and explore the difference between analog and digital signals, using the example of a microphone (say in an artist's recording studio). It outputs an electrical signal which varies according to the sound coming in. If it's a high-pitch note being sung, it's a high-frequency electrical signal coming out. If it's a louder note, there's a stronger (higher voltage) electrical signal. That's analog: the electrical signal is directly analogous to the music. You can send it straight to an earphone, and you'll hear music. Not so with digital signals. Binary digital signals are tightly constrained to be in only one of two possible states: "on" or "off" (also called "high" and "low"). In a typical digital electrical circuit, positive five volts on a wire represents "on," and zero volts represents "off." On an audio disk, there are two kinds of optical markings, one for "on," one for "off." But there are LOTS of them. Taken in the proper context, a series of ones and zeros represents a larger binary number, and a series of those numbers describes how to approximately reconstruct an analog waveform (the electrical signal that originally came from the microphone). Although the reconstruction is always approximate, it can have very high fidelity. The real advantage is that binary digits, ones and zeros, (called "bits" for BInary digiTs) are very simple, and thus they can easily be transmitted, stored, and processed without adding ANY errors or noise.

Back to Outer Space

According to how CDS is being commanded to operate at a given time, it picks and chooses incoming packets, throws out any empty ones, and assembles the packets into bigger packets (called transfer frames). CDS stores some packets, for example from the science instruments, Huygens Probe, and REUs, in its solid-state recorder (illustrated as a dark blue box), for later access, again depending on what CDS is commanded to be doing.

In any case, all the valid science data packets, and a good sampling of the engineering data packets, eventually go from the CDS to the Radio Frequency Subsystem (RFS). There, they receive coding which will help ensure error-free transmission to Earth. It's the same kind of mathematical coding your compact disc audio player probably uses to ensure error-free transfer of data bits from the disc to the audio electronics (Reed-Solomon coding). An additional layer of coding is applied (called convolutional coding), and then the RFS modulates the data onto the downlink.

Modulation

Your modem MOdulates and DEModulates. You can listen to its signals on the telephone (or those produced by a fax machine), but it probably goes too fast to make any sense to you. It starts out with a pure audio tone, and then changes it repeatedly. Each change it makes in the audio tone represents a binary digit (one or zero), or a group of them, which can be recognized by the modem at the other end of the line. You can produce a fairly pure note with your voice: say "Aaahhhh." You could then modulate the note: say "AaaaaaaWaaaaaaWaaaaahh." By doing so, you change the waveform of the audio signal coming out of your throat... modulation! In Cassini's case, the radio signal gets modulated by changing the phase of the downlink waveforms. Each change can be recognized on Earth, and translated (demodulated!) into the ones and zeros intended.

Earthly Things

The first thing to do on Earth is to capture Cassini's radio signal. It's pretty weak, having travelled a long distance, so it takes large aperture radio telescopes to collect and concentrate the signal. The antenna has to be pointed in exactly the right direction to locate Cassini in the sky; that's done by using predictions made by processing tracking data gathered previously (see the blue line in the cartoon below). Once Cassini's signal has been funneled in, it is amplified by a low-noise amplifier (LNA) which is kept at very low temperatures so that it contributes a minimum amount of noise. Typical LNAs used for receiving Cassini's signals are wide band MASERs and HEMTs, which are cooled to the temperature of liquid helium. A MASER is a vacuum tube. Its name stands for Microwave Amplification by Stimulated Emission of Radiation. A HEMT is a solid state device which is slightly less efficient than a MASER, but smaller and easier to keep cool. HEMT stands for High-Electron-Mobility Transistor.

The DSN has three Deep Space Communications Complexes (DSCCs) located around the world, so that at least one of them can track Cassini and other interplanetary spacecraft as the Earth rotates. At each DSCC, there are several large radio telescope-like antenna dishes, and a building, called the Signal Processing Center (SPC) which houses all the computer equipment and a few human operators. Cassini's amplified microwave signal is sent to the SPC where it enters a receiver. The receiver must be configured to tune in Cassini's signal, based on predictions generated previously, which include signal strength, frequency, and modulation parameters. Once the receiver has locked onto Cassini's downlink, the signal is passed to a selected group of telemetry processing equipment (see the Telemetry Processor in the cartoon), where the modulated symbols are recognized and turned back into binary digits, the familiar ones and zeros. Errors are corrected, as the error-detection coding is removed. The result is Cassini's telemetry, exactly as it was when CDS passed it to RFS inside the spacecraft.

On to JPL

The SPC transmits the telemetry bits to JPL via the GCF. Other data types, by the way, can also be extracted from Cassini's downlink in the SPC: ranging, Doppler, and radio science, which are also sent to JPL via the GCF.

In the cartoon above, we see telemetry coming in to JPL, illustrated as a green line. It goes through a few preliminary processing steps to remove information the GCF had placed on the data stream, and to decommutate the channels.

What ?

Decommutate the channels, he said! The spacecraft placed its telemetry data into packets, remember. But each packet might have data from several different things on Cassini: a science instrument, and an REU or two. The science instrument added information to its data to identify it (part of what's called a "header"). When you read the header information at JPL, you know what kind of data it is (Hmm. Says here, Imaging Science Subsystem... so we'll have to treat this data as pictures!) And, remember, each REU aboard the spacecraft was contributing data from (typically) more than engineering measurement. Gotta look up the packet definitions to find out whether it's supposed to be the temperature of a reaction wheel motor, or the position of a propellant valve. Decommutating is the process of doing just that: determining what kind of data is in each packet. "Channels" are just the names given to each different kind of data: Instead of always calling it "the temperature of reaction wheel motor number one," we just call it something like "Channel E-1654." E-channels are for engineering data (temperatures, pressures, etc.), and S-channels are for science data. The imaging data mentioned above would be labeled channel S-1023 or something.

Delivery Truck

Anyway, the telemetry, which has now been "channalized" to identify its contents, is stored within the Telemetry Delivery System (TDS), where all telemetry is kept for the entire life of the Cassini mission. TDS also sends out telemetry to the people who want to receive it in real time (well, pretty close; within seconds, usually): the realtime flight controller who is watching out for surprises, the engineer who needs to watch that propellant tank temperature (computers on the left side of the cartoon), the scientist who wants her science data (computers on the right side of the cartoon). Users may go back to TDS and retrieve past telemetry, to their specifications, at any time. Science teams also receive other data products besides telemetry, which support analysis of their science data. One set includes DSN monitor data which indicates the performance of DSN receivers, tracking and telemetry equipment. Another set of supplementary data includes selected spacecraft engineering data, spacecraft ephemeris and pointing data. This permits reconstruction of science instruments' "footprints" on the objects being remotely sensed, as well as the locations of phenomena being directly sensed.

Dissemination of Results

Once a team of scientists has correlated all the telemetry and supplementary data, and has done analysis of them, they will be in a hurry to publish what they found, and present their results at scientific meetings.

Places you will typically expect to find results from Cassini's science investigations include the journals: Science (American Association for the Advancement of Science, AAAS), Nature, the international weekly journal of science, JGR (Journal of Geophysical Research), and Icarus a publication of the Division for Planetary Sciences (DPS) of the American Astronomical Society (AAS). Once Cassini is orbiting Saturn, presentations can be expected to be made at virtually every annual convention of various scientific societies, such as the AAS and its DPS by the science investigators who obtain data from Cassini's experiments.

The news media, and several magazines, keep a close eye on many of these journals and proceedings, and they report items of discovery from them. The thin weekly magazine Science News is a notable example, as is the amateur astronomers' monthly Sky & Telescope magazine. Splendid photography from JPL's missions occasionally appears in National Geographic magazine, and many a JPL mission has enjoyed very good treatment in public television's science series NOVA. There is every reason to expect similar treatment of Cassini's results.

Photos on File

Regional Planetary Imaging Data Facilities ( RPIF) are operated by NASA at over a dozen sites around the United States and overseas. Each maintains a complete photographic library of images from NASA's lunar and planetary missions. They are open to members of the public by appointment for browsing, and their staff can assist individuals in selecting and ordering materials. All of NASA's planetary imaging data is made available for researchers who are funded by NASA, in photographic format and digital data format, via the Planetary Data System (PDS). The PDS consists of a central on-line catalog at JPL, and a number of nodes located at various research facilities from which data may be retrieved on line.

Educators may obtain videocassettes and a wide variety of other material and information from NASA's flight projects through the network of Teacher Resource Centers (TRCs) in cooperation with educational institutions around the country. Each TRC also supports a center for distribution of audiovisual materials called the Central Operation of Resources for Educators (CORE).

Other members of the public may purchase photographic images and videotapes through contractor facilities associated with JPL's Public Information Office (PIO). The PIO can serve as a clearinghouse for information about access to all of the various avenues for dissemination. And, of course, increasing use is being made of the World-Wide Web to disseminate scientific results.