Quote:
Originally Posted by TrevorW
I watched that (it's not true video) and its just bad, surely in this day and age they could have captured video to show the tethered drop etc.
An amazing achievement no doubt but IMO they could have done more to capture public interest
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To answer why it is difficult to have real-time high-resolution video during the
descent, an Electrical Engineer would quote the
Shannon–Hartley theorem
which dictates the maximum rate at which information can be transmitted over a
communications channel of a specified bandwidth in the presence of noise.
For inquiring minds that need to know, the late, great
Claude Shannon tell us that -
C = B * log2( 1 + S/N)
where
C is the channel capacity in bits per second
B is the bandwidth in Hertz
S/N is the signal-to-noise ratio
and log2 is to denote log to the base 2.
So what does all that mean? It means that for a given bandwidth with given
noise, and for a transmitter with given power, there is a maximum data rate
that one can achieve and no more.
This theorem applies to whether the channel is a telephone line, radio waves,
light waves or even shouting out an order across a noisy pub.
To get a more complete picture with respect Curiosity, one then has to also consider
the propagation characteristics of different parts of the radio frequency spectrum.
During descent, the UHF part of the spectrum (around 400MHz) was used
to communicate back to Tidbinbilla and then to the two orbiting Mars satellites
once Curiosity dropped below the Martian horizon.
The UHF part of the spectrum is effective at propagating signals omni-directionally,
just what you need for a moving target being buffeted around in the wind. However,
this part of the spectrum also provides a narrower bandwidth that one can effectively
use compared to X-band (7 to 11.2 GHz) and because one needs to transmit
omni-directionally, much of the transmitted power is effectively "lost".
Which brings us back to Shannon. Smaller bandwidth, smaller power, the less data
you can reliably transmit in a given amount of time (and as we will discuss below,
time was also of the essence).
What type of data rate are we talking about? Around 128kbps. Though the
landing sequence was all done autonomously without requiring communication
back to Earth, that limited bandwidth was also used to transmit telemetry.
Apparently three UHF antenna were employed. One mounted on the backshell
to transmit information prior to entry until backshell separation. A second
antenna then took over on the descent stage and a third was used on the rover
itself.
The rover itself has three radios. Two of them operate in X-band. One purposely
uses a low gain omni-directional antenna whose main function is to receive
commands from Earth at a very low data rate (Shannon tells us the lower the
data rate, the more reliable the data can be recovered - important when you
are driving a billion dollar asset on Mars). A second X-band radio employs a
movable high gain antenna about 0.3m in diameter that can be pointed to Earth.
It's data rate ranges from 160 bits-per-second (bps) to 12,000 bits per second.
The 400MHz UHF omni-directional antenna is used to transmit to the orbiters
at around 128,000 bits per second.
Two orbiters are used to relay data back to Earth. These are Mars Odyssey
and the Mars Reconnaissance Orbiter (MRO). They have more powerful transmitters
that can relay data back at a higher data rate - around 2 megabits per second.
But there is a catch. These two satellites each fly over within line of sight of the
Curiosity landing site twice a day, but only for about 8 minutes a time on each pass.
So relaying all that data in a limited window of time is tricky. In fact, when Mars
Odyssey developed a fault on June 8th when one of its reaction wheels used
to position it failed, a major piece of juggling took place to correct its orbit
so that both it and MRO would magically both be at the right place
at the right time during the seven minutes of terror.
That story is a compelling one in itself -
http://www.americaspace.org/?p=23351
With limited bandwidth, limited power, omni-directional antennas and with
Earth and the Mars orbiting satellites in sight of Curiosity for only a precious few
minutes, it was not technically possible to stream real-time video. Once on
the ground, there was just the opportunity to transmit a couple of low resolution
thumbnail images before the relay satellites themselves dropped beneath the
horizon.
I guess what happens is that with the Hollywood treatment of spaceflight, many people
have become conditioned to anticipate high resolution streaming images of beads
of sweat pouring down some astronaut's face as he battles to blow up some
distant asteroid.
The practical reality is that the Shannon-Hartley theorem kicks
in and there is a limit to how much data one can reliably transmit when one has
a limited amount of power.
As a footnote, this same fundamental theorem is why optical fibers to the home in
urban settings are technically the best choice for a broadband network and that
a suggestion that copper cable or wireless could be a suitable alternative is
technically naive.
Thankfully, the genius of minds such as Shannon's helped set the basis for the
modern communications systems that we enjoy here on Earth and make
communicating to a probe on Mars technically possible.
And the photos just blew me away! And are now a fixture on my Iphone background 
Sorry, couldn't resist! Rolf's answered your question in detail. There just isn't room to take 'pretty pictures' on an entry like this, and most of the cameras were stowed away during the descent to protect them.
