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Talking to a rover on Mars is like making one heck of a long distance call. When we want to chat with our robots, we have to keep several things in mind. First of all, there are others waiting to use that phone line. Well, it’s not a phone line. It’s the Deep Space Network. And, it has plenty of users these days. We also have to make sure our equipment and the planets are lined up properly, so that we can relay or receive our communications, and the network can only handle so much information at a time.

The Deep Space Network (DSN) is one of the largest telecommunications systems in the world, and it will be our link to the rovers when they are on Mars. The DSN's dish antennas have been strategically placed in Spain, Australia and California so that, as the Earth rotates, at least one part of the network will have Mars in its view at all times. You cannot detect a signal from Mars if the planet is in one part of the sky and your antenna is facing another part of the sky. It’s that simple, or that complicated. On top of that, the rovers aren't the only game in town, or on the antennas. There are other missions that need to use the Deep Space Network. By January, 2004 (when the rovers are on Mars) the Stardust mission, SIRTF, Mars Express, Cassini, Mars Odyssey, and the Mars Global Surveyor will be a few of the many missions that will be pinging the cosmos to get their time on the "line."

In short, our rovers have to share. They are limited to contacts twice a day of about one or two hours each.

Okay. We have these huge dish antennas on Earth that we use to "listen" for messages from the rovers and send information to the rovers. But what about on Mars? How do the rovers receive and send information? They can’t carry around antenna dishes the size of a couple of basketball courts.

So, each rover carries 3 much smaller antennas — one that can move in different directions called the high gain antenna, and two others that are fixed — the low gain and Ultra High Frequency (UHF) antennas. When a rover uses the high gain antenna to speak directly with Earth, it is important that Earth be in view of the rover. That's because this type of communication is a direct shot to Earth, so Earth must be above the martian horizon. The high gain antenna must also be in the correct position. It can spin around so that the entire rover doesn't have to waste energy twisting and turning to get in the right orientation to face Earth. However, in the first few hours of the mission, before the high gain antenna is up and running, the low gain (low-bandwidth) antenna is put into action. Again, Earth must be above the martian horizon as the low gain is also a direct-to-Earth communicator. But the low gain antenna does not have to be pointed at the Earth. Its signal is sent out in all directions at once. We say it’s “omnidirectional”.

Not all information needs to be sent directly to Earth. Some can be sent to orbiters circling Mars which, in turn, send the information to Earth. The UHF antenna on each rover is used to relay information to Mars Odyssey and the Mars Global Surveyor. But, again, these orbiters have to be in view of the rover that is trying to communicate. They have to be overhead — not on the other side of the planet. This means that there are specific times in the martian day when the rovers can send messages to these spacecraft, which are already up there working away. The Mars Global Surveyor and Mars Odyssey will pass over each rover twice a day (12 hours apart). They’re moving pretty fast to stay in orbit, so that allows just 10 minutes a day for the rovers to beam up their findings. The information will be stored on each orbiter and then sent to Earth sometime within the next two days. One of the perks of using the orbiters to relay information is that they each have a lot more power and much bigger antennas than the rovers.

In general, instructions to the rovers and critical information from the rovers use the direct communication route — the high gain antenna. Information from the rovers that does not need to be received immediately can be relayed through the orbiters.

In case you were wondering, all of this "talking" between Earth and Mars is done with radio waves similar to the radio waves that bring you your favorite radio station. They are at a particular radio frequency designated as the X-band. Radio waves are another form of light waves. They travel through space at the speed of light, 300,000 kilometers a second. But, the distances to Mars and back are so huge that even at that speed it takes time for any type of electromagnetic wave to make the trip. In other words, there is a lag time. And this lag time grows as the mission progresses because Earth and Mars are moving away from one another in their orbits. At the beginning of the mission, a radio signal takes about 10 minutes to travel from Earth to Mars. By the end of the mission, the distance between the planets has increased to the point where a one-way signal takes about 20 minutes. And, that’s at the speed of light.

So we have a bunch of radio waves bouncing to and from the Earth and the rovers at various times during the day. So where do they fit in the whole communication process? They are in the middle of a digital sandwich. Think of your computer modem. When you send an email, “bits” of information from your computer travel to your modem (which stands for modulator/demodulator). This digital signal is converted into an analog signal, a wave which travels through the phone line just like your voice. The signal enters your friend’s modem, where the analog wave is changed back into bits that are sent to your friend’s computer.

Now think of the rovers. They gather pieces of information, or “data” as scientists like to say. The data are broken down to their smallest unit – a bit – by each rover’s computer. In a downlink a “bitstream,” or digital signal, is converted to radio waves, the radio waves travel through space, reach a DSN antenna, and then are converted back to bits here on Earth. Think of a downlink as being like a download through your modem.

The amount of data that can be sent from the rovers to Earth, and visa versa, is not limitless. The low gain antenna (remember – this one is used in the first hours of landed operations and as a backup for the high gain antenna), can only handle 40 bits per second. That’s about 350 times slower than regular modems. However, we can use the high gain antenna to talk to the rovers at 2,500 bits per second. The return conversation from the rovers can be at 11,850 bits per second through the high gain (that’s approaching the speed of a phone modem). When the rovers talk to the orbiters through the UHF (ultra high frequency) antenna, they can do so at a respectable rate of 128,000 bits per second.

So let's walk through a typical day on Mars with one of the rovers:

It's early morning — the rover has been "sleeping" all night to conserve energy. (Remember: the rovers are solar powered). A wake up call is provided by an on-board alarm clock. It's time for the rover to do a few chores using instructions that were part of the stream of information received on the previous day. Earth has emerged above the martian horizon, so the rover soon receives a new set of commands directly from one of the DSN antennas. This is called an "uplink." The rover continues its work from this new list of commands, such as crossing martian terrain, approaching a rock target, examining a rock target, or taking pictures of its surroundings. As the rover works, it is gathering and storing data in its computer — information for engineers and scientists to study.

It is now afternoon and the rover has completed many of its tasks for the day. Earth is still above the horizon, so there is a chance to communicate directly with a waiting DSN antenna. But, this time, the rover will be doing the talking. It will "downlink" its information. After the Earth drops below the martian horizon, the rover will "talk" to the Mars Odyssey orbiter if it is available. The sun has set and the rover will "sleep" — but it won't be a deep slumber. During the night, the rover may wake up briefly to use its UHF antenna to send more information, this time to the Mars Global Surveyor as it passes overhead.

The data that are downlinked to the DSN are then relayed to waiting scientists and engineers at the Jet Propulsion Lab in California. The information is analyzed so that a new list of tasks can be created for the rover's next work day. Formulating these new commands is not easy. It takes several hours of meetings and discussions to plan a strategy. Scientists and engineers want to make sure the time they have on the "line" with Mars is quality time.

By the next martian morning, a new list of commands will have been hammered out and readied for an uplink to the rover. This begins another round of daily, very long-distance conversations between Earthlings and Mars machines.

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