[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

[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.