Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Kryptid on 31/08/2013 03:05:55
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After watching a simulated(?) video of Huygens' descent on Titan, I couldn't help but wonder why it looked so strange and distorted: http://www.youtube.com/watch?v=HtYDPj6eFLc (http://www.youtube.com/watch?v=HtYDPj6eFLc)
Was there some reason that a high resolution "normal" camera like the kind used to film movies and television documentaries could not have been sent? Are they unable to withstand the conditions in space or the impact of landing?
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The raw data is a circular fish-eye-lens (https://en.wikipedia.org/wiki/Fish_eye_lens) view straight down ...
Was there some reason that a high resolution "normal" camera like the kind used to film movies and television documentaries could not have been sent?
As the thing is spinning during the descent the weird circular-fish-eye lens is the only practical option.
The "simulated" (morphed) version you linked to (http://www.youtube.com/watch?v=HtYDPj6eFLc) is an attempt to correct the fish-eye-lens extreme barrel-distortion (https://en.wikipedia.org/wiki/Barrel_distortion#Radial_distortion) showing how it would appear to the naked eye , ( rather than how it looks through a peep-hole-type-lens (https://en.wikipedia.org/wiki/Peephole) ).
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The fundamental constraint in communications is called the "Shannon Limit (http://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem)":
- It predicts the maximum capacity of a communications channel (eg Morse code, MP3 audio, Youtube video, Standard Definition TV, or High-definition TV)
- Based on the Signal-to-Noise ratio, which is impacted by:
- Transmit power available: Limited by the power of thermo-electric cells and/or batteries
- Size/gain of the transmitter antenna: Limited in this case because the lander was spinning rapidly
- Size/gain of the receiver antenna: probably 70m diameter on Earth, limited by gravity and winds.
- Bandwidth of the transmission: Limited by competing spectrum users on Earth
- In this case, Huygens could transmit to the orbiting Cassini probe, which had a larger directional antenna that relayed the signals to Earth; transmitting direct to Earth is much harder.
- The communications speeds that we take for granted in our homes today (perhaps 1-5km over copper wires) are thousands to millions of times faster than were possible over the billion kilometers between Saturn and Earth.
The fundamental limit in electronics is called "Moore's Law (http://en.wikipedia.org/wiki/Moore%27s_law)" (but it's not a law of nature, more a "rule of thumb" about semiconductor technology):
- The capacity of electronics doubles about every 2 years.
- This affects the ability to process and store digital data: eg can you compress and store high-definition video?
- The Huygens/Cassini (http://en.wikipedia.org/wiki/Cassini–Huygens) probe launched in 1997, and landed in 2005.
- Space probes are typically designed with the best space-rated components available 10 years before launch. So the electronics would have been selected about 1987.
- Space-rated components are typically about a decade behind commercial components in capacity and processing power, ie equivalent to PCs you could buy around 1977.
- In the intervening 36 years, PC power has increased by a factor of around 218 times, or around 250,000 times.
- Things like HDTV videos on commercial PCs that we take for granted in 2013 require far more storage and processing power than was available on Huygens/Cassini.
More recent space missions like the Curiosity rover on Mars provide much better quality stereo images, because it is able to take advantage of many years of developments in electronics technology, the ability to provide a stable base for aiming a directional antenna, and a big power boost from carrying a large thermoelectric generator. Like Huygens, it is also able to take advantage of data relay via orbiting spacecraft.