[X] My Assistant

[hubdel11]

the DIRS have posted 2 new mosaics on their site :
http://www.lpl.arizona.edu/%7Ekholso/Titan...3K_big_ster.jpg
http://www.lpl.arizona.edu/%7Ekholso/Titan...Apr15_gnom_.png

[dvandorn]

the DIRS have posted 2 new mosaics on their site :
http://www.lpl.arizona.edu/%7Ekholso/Titan...3K_big_ster.jpg
http://www.lpl.arizona.edu/%7Ekholso/Titan...Apr15_gnom_.png

I guess I've just been spoiled by the MERs and MGS, but I am disappointed in the quality of the images from Huygens. Why the very limited number of pixels per image -- was it a matter of the width of the comm pipe between Huygens and Cassini? Would it have cost that much more in terms of a weight penalty to build five or ten times more pixels into the camera system? Even in the late 80s, when Huygens was first developed, they could build higher-resolution cameras than they ended up using...

I know there was fog and haze (more than expected), I know we lost half of the images... but the image encoding from the source has introduced so many artifacts it's really hard to analyze the images to anywhere near their theoretical resolution.

Yes, there is a great deal of good information in the images and other data Huygens returned... but it's so tantalizingly close and yet so lacking in critical definition that I just want to tear my hair out... *sigh*...

-the other Doug

[Decepticon]

It didn't help that the Earth testing images where SHARP and pixless.

I saw the link before but can't find it now.

[deglr6328]

I guess I've just been spoiled by the MERs and MGS, but I am disappointed in the quality of the images from Huygens.  Why the very limited number of pixels per image -- was it a matter of the width of the comm pipe between Huygens and Cassini?  Would it have cost that much more in terms of a weight penalty to build five or ten times more pixels into the camera system?  Even in the late 80s, when Huygens was first developed, they could build higher-resolution cameras than they ended up using...

Yes it was the com link. Because Huygens was descending on a parachute, twisting and swaying in the wind, it was of course not possible to use a high-gain directional antenna and because it was a small probe it only had so much of a power budget to make RF transmissions with. So Huygens was restricted to a mere 10 watts of transmitter power at 8,192 bits per second. Thats just 1 KBps !! I think condsidering the amount of other data they had to transmit (something like 5 other science instrument packages with several individual instruments per package with each one producing data) the images we got back were really really good. The DISR team knew the bandwidth they had to work with and they squeezed every last drop of data through it that they could.

This problem of a trade off between directionality and power got me thinking though. Maybe someone here more knowledgeable can correct me. If we're ever to increase the science returns from these types of missions there must be a way around this problem somehow. Optical transmission is out of the question right away obviously because of the even higer limit on pointing accuracy and attenuation prblems associated with the atmosphere. But what about a phased array transmitter? The problem with using directional radio transmitters to increase the signal/noise ratio on a decending atmospheric probe is obvious - conventionally, you'd need to use a dish to concentrate the beam in a particualr diretion (just like cassini's high gain antenna) and you'd need to continually re-point this dish as you're falling and turning under the parachute. You would lose track of where to keep pointed after just a few seconds of this. But what if you had a transmitter on the orbiting data recieving spacecraft that sent a pure tone to the falling probe and a small fixed position directional reciever antenna (juat a whip antenna) on the probe? It would be easy to determine at least roughly where the signal was coming from as you were moving and rotating by simply analysing the strength of the received tone and when this information is coupled to a phased array transmitter you could continually re-point the radio beam in this direction instantly, without moving any physical antenna. Phased array (digitally controlled) techniques are fairly new, I wonder, has this ever been considered before?

Incidentally there are reports on Huygens data return that I simply do not understand. For instance it was stated in a press release that 474 Mbits of data was returned by huygens over a 3.75 Hr. time. Now, every single place where I have seen it listed, the data rate from huygens is said to be 8192 kiloBITS/sec (fixed rate). Even figuring generously at a 4 hour mission that only gives you a maximum of 14,400 seconds to work with for a max of 14 MBytes (or 112 MBits) of data so where this 474 number comes from I have no clue. Once again ESA is no help in figuring this one out as they've published several conflicting values of total data returned by Huygens.

[Phil Stooke]

I know nothing about this, so I'm only playing with numbers... but if we say 8000 bits/sec times 10000 sec we have 80 Mbits tansmitted.. at 6:1 compression we would have 480 Mbits when decompressed. (note your units are mixed up!) ESA could be more forthcoming but it's not too difficult to manipulate the numbers to make a reasonable guess. The images look much more than 6:1 compressed, and other data might be a lot less, but it could average about 6:1.

Phil

[Guest_BruceMoomaw_*]

Incidentally there are reports on Huygens data return that I simply do not understand. For instance it was stated in a press release that 474 Mbits of data was returned by huygens over a 3.75 Hr. time. Now, every single place where I have seen it listed, the data rate from huygens is said to be 8192 kiloBITS/sec (fixed rate). Even figuring generously at a 4 hour mission that only gives you a maximum of 14,400 seconds to work with for a max of 14 MBITS of data so where this 474 number comes from I have no clue. Once again ESA is no help in figuring this one out as they've published several conflicting values of total data returned by Huygens.

This has to do with the fact that this was actually Huygens' maximum data return rate -- initially it returned its data at several lower bit rates. It may take me a while to dig up the place where I saw the details on this (one of the ESA's web documents, I think).

[Decepticon]

Looking at the above images will they eventually clean up the lines and color the data?

The above mosaic looks very raw.

[deglr6328]

I know nothing about this, so I'm only playing with numbers... but if we say 8000 bits/sec times 10000 sec we have 80 Mbits tansmitted.. at 6:1 compression we would have 480 Mbits when decompressed.  (note your units are mixed up!)  ESA could be more forthcoming but it's not too difficult to manipulate the numbers to make a reasonable guess.  The images look much more than 6:1 compressed, and other data might be a lot less, but it could average about 6:1. 

Phil

oops! I of course meant 8,192 BITS/sec for huygens data rate not 8,192 KBits/sec!!

[alan]

It didn't help that the Earth testing images where SHARP and pixless.

I saw the link before but can't find it now.

Test images here
http://www.lpl.arizona.edu/~kholso/test_images.htm
The lowest altitude image from Huygens has so many artifacts because the frames in the upper upper part of the image had to be stretched so far just to show a few details. If they weren't stretched so much the images would look more like this.

[tedstryk]

In response to Phil's comment, that trick was used often by Galileo. Additionally, it is often said that Galileo returned 14,000 images. They leave out that the bulk of those were from the Earth/Moon flybies where it could transmit at a relatively high data rate. Also, for the rest of the images, 14,000 images includes scientifically useless opnavs, images so compressed that they are unrecognizable (often to find targets in the image to trasmit certain portions at a higher rate - which was then counted as another image). Very few images sent by Galileo from Jupiter are full 800x800 frames, and even those are compressed. So, while not technically fabricated (that is about the number of Galileo SSI EDRs you will find on the PDS), it is a bit Enron-ish.

[edstrick]

Note also that the lowest altitude frames from Huygens are mostly of nearly featureless dark plains/channel material. There almost isn't anything there to see.... without far more signal/noise than we ended up with after fiber optics relayed the image to the CCD, it was read out and the data was compressed.

Sigh.

[tty]

This problem of a trade off between directionality and power got me thinking though. Maybe someone here more knowledgeable can correct me. If we're ever to increase the science returns from these types of missions there must be a way around this problem somehow. Optical transmission is out of the question right away obviously because of the even higer limit on pointing accuracy and attenuation prblems associated with the atmosphere. But what about a phased array transmitter? The problem with using directional radio transmitters to increase the signal/noise ratio on a decending atmospheric probe is obvious - conventionally, you'd need to use a dish to concentrate the beam in a particualr diretion (just like cassini's high gain antenna) and you'd need to continually re-point this dish as you're falling and turning under the parachute. You would lose track of where to keep pointed after just a few seconds of this. But what if you had a transmitter on the orbiting data recieving spacecraft that sent a pure tone to the falling probe and a small fixed position directional reciever antenna (juat a whip antenna) on the probe? It would be easy to determine at least roughly where the signal was coming from as you were moving and rotating by simply analysing the strength of the received tone and when this information is coupled to a phased array transmitter you could continually re-point the radio beam in this direction instantly, without moving any physical antenna. Phased array (digitally controlled) techniques are fairly new, I wonder, has this ever been considered before?

I’m afraid a phased array is probably not a viable solution. A phased array is just that – an array of transmitting elements which steer the beam electronically by controlled interference between the transmitters. This means:
1. a fairly large number of transmitters is required
2. the complete antenna must be large in relation to the wavelength of the transmitted signal
3. the beam can only be steered within a half-hemisphere (theoretically that is, in practice a great deal less)

Typically this means that a phased array needs four separate antennas for all-round coverage (have a look at an AN/SPY-1 radar for an example). I suppose it might be implemented in a box-shaped probe (if this is possible from an aerodynamic point of view). The sides would also have to slope quite a bit if you need to have coverage near zenith. The whole arrangement (including the steering electronics) would be rather heavy and bulky though.

Incidentally phased arrays are not a new concept, but it was quite difficult to implement it effectively until the advent of solid state transmit/receive modules.

Now that we know that a landing on Titan is survivable I suggest that a better solution would be to store an uncompressed high-definition version of the data on-board and transmit it after landing, either through a high-gain antenna or over a longer period of time (which probably implies a radioisotope power source). You could still transmit a low-res version while descending in case the landing fails.

tty

[tty]

I might add that there is a method to increase the resolution of images taken after landing that would be fairly simple to implement. If it is possible to change the direction of the camera slightly between pictures (one or two pixels is enough) it is possible to calculate a composite image with considerably better resolution than the original ones. Unfortunately Huygens was apparently rock-steady after landing. Perhaps a small pyrotechnic charge that gave the probe a nudge after say 3 images on the surface had been transmitted would have been a good idea?

tty

[Phil Stooke]

tty mentions slight movement of Huygens to increase resolution in the landed images. Unfortunately there are lots of reasons this would not have been feasible.

This method is called super-resolution at JPL and (basically the same thing) drizzling at STScI. It was invented at NASA Ames. It involves having multiple views of a scene with small (subpixel) offsets. A new image is created by enlarging all the images by a factor of 2 or 3, registering them to the nearest new pixel in the enlarged image and combining. The increase in resolution goes with the square root of the number of images: 4 images, 2x resolution. In principle... actually it's not really as good as that, but it it still useful as it slightly increases resulution and also SNR.

Tim Parker at JPL does it in Photoshop, and having told me how he does it, I have too. Excellent results, for instance, on the Voyager images of saturnian satellites which were taken in multispectral sequences. Remove noise, enlarge the images, contrast enhance each one, sharpen each one individually, then register and merge.

The trouble with tty's idea is... are... (1) Huygens was not designed to survive landing, (2) extra weight and complexity, (3) you really need a few more than 2 images to help very much, so you would need several sequential charges.

Phil

[Phil Stooke]

Here is an example of the results achievable with super-resolution, in case you have not seen this kind of comparison before. It's part of a multispectral sequence of Dione from Voyager 1, taken as Dione transits across Saturn (the grey background). The color version is well known. There is very little color variation across Dione, making the sequence ideal for super-res (Io would be useless!)

The right image is one original frame, enlarged 400% to show pixel size. The left is a composite of the entire multispectral sequence, weighted to give more prominence to the better images. The black dot (a reseau mark on the original - thankfully NASA quit using them after Voyager as they served very little purpose... a leftover of the days when images were archived on film) has been removed with a patch from another image in the sequence. I should add that other special processing to remove artifacts has also been done.

I put this here because it follows from the Huygens thread but obviously it could be in icy moons as well, or instead.



Phil