Why is there no POV video of the Philae lander landing? All I can find are animations made beforehand mixed with commentary and one or two stills.
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2$\begingroup$ Without going to check and verify this, I bet that it's because there isn't bandwidth for video from the Rosetta probe itself, which I assume is relaying data from the lander. Although either way, the transmitter is probably just powerful enough to establish some low-bit-rate transmission to Earth with the signal-to-noise ratio too high for video. $\endgroup$– KirkaiyaCommented Nov 14, 2014 at 5:37
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1$\begingroup$ @Kirkaiya A high S/N ratio means much signal in relation to noise. What you meant was most likely a low signal-to-noise ratio, which makes picking out the signal from the noise difficult and thus reduces the attainable transmission speed. $\endgroup$– userCommented Nov 14, 2014 at 17:56
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1$\begingroup$ @MichaelKjörling - yah, just a typo. I'm aware of what S/N rations are, my brain just spits out the wrong words at 10 pm at night. $\endgroup$– KirkaiyaCommented Nov 14, 2014 at 18:11
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$\begingroup$ @Kirkaiya Such things happen. I assumed as much. :) $\endgroup$– userCommented Nov 14, 2014 at 18:12
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1 Answer
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According to this statement by the ESA,
Rosetta is presently sending signals to the ground stations at about 28 Kbps; Ignacio says that the spacecraft's own telemetry downlink uses about 1 or 2 Kbps of this, so the rest is being used to download science data from Rosetta and lander science and telemetry from the surface.
Given that you have 28 kbit/s for the total spacecraft data downlink, and assuming that the lander sends approximately the same amount of telemetry data as Rosetta, we are talking about approximately 25 kbit/s for science data (which I use here to mean data not related directly to the spacecraft itself or its operations, which may or may not be the same meaning as that used by the ESA).
25 kbit/s isn't a whole lot at all. If we assume that is the payload transmission rate, we are talking about roughly 3100 bytes per second. The text of this answer, not including the links or formatting, is about 3000 bytes.
Take for example this photo, taken during descent. The "high resolution" version posted on the web is 817 x 795 pixels, and it is in grayscale. If we assume one byte per pixel (8 bits shades of grey) and 50% compression (which is high for high-quality photographs, although by no means unattainable) that required downlinking about 320 kilobytes, taking about 104 seconds of payload downlink data capacity to downlink. And that's assuming that the native resolution of the imager wasn't higher and the image was later, on Earth, downsampled or cropped for posting on the web.
Even if the lander had sufficient buffer memory to be able to record even, say, one image per minute during the descent and landing, it would still have needed twice as much time to downlink the data to Earth, assuming no other measurements are transmitted during that time. Given that the landing and settling took several hours, and the downlink isn't usable all the time due to occlusion of the Rosetta spacecraft, that would have left us for several hours receiving much less scientific data than we could have, in order to have a video of the descent, most of which would probably have been rather uninteresting to everyone except the most devout followers of the project.
Probably wouldn't have been worth that tradeoff. I certainly wouldn't have made such a tradeoff.
And increasing the downlink speed isn't necessarily trivial; if increasing the downlink data transmission rate was easy, I can see no real reason why the team would have settled for 28 kbit/s.
The above assumes, too, that we are talking about 28 kbit/s usable after error correction is accounted for. I have no references to back that up, but I would expect error correction data to correspond to a significant fraction of the raw downlink data transmission rate. The last thing you want is to send a spacecraft several hundred million kilometers away and then not be able to receive the transmission because it's been garbled beyond the ability of the error correction to recover from.
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3$\begingroup$ It's also useful to bear in mind that (1) space hardware is of generally conservative design for the sake of reliability - if something fails, there's nobody on-scene with tools and spares to fix it; (2) power budgets are limited; fewer transmitter watts means poorer signal/noise ratio at the distant receiver - the higher the data rate, the more vulnerable to data loss; (3) it was launched 10 years ago, probably designed 15 years ago, and engineered conservatively for its time. POV video would be great for PR (if superfluous), but of no scientific or historic value. $\endgroup$ Commented Nov 16, 2014 at 2:34