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.
