Cassini-Huygens questions - Radar Titan

[Boldra]

I'm wondering why the radar shots coming back from the surface of Titan (http://antwrp.gsfc.nasa.gov/apod/ap041124.html) aren't nearly as good as those taken by Magellan of Venus (http://antwrp.gsfc.nasa.gov/apod/ap030427.html). From what I read on the Cassini-Huygens site, the radar data from Titan only show areas that are very radar reflective and areas that are not. Why don't they have altitude measurements like they did with venus?

I also seem to recall seeing something saying that the landing zone for Huygens had to be chosen in late 2004. This meant that some info about Titan could be gathered by Cassini before the separation and comitting Huygens to a landing elipse. So far it seems like the info they've gathered hasn't been much use. Were the Huygens mission planners sitting up late one night debating, "Ummm, do we land on a light bit or a dark bit?" Or do they have some better idea what they're looking at now?

It makes me wonder whether Huygens isn't being released too soon. Couldn't Cassini have done some more fly-bys to get some more data before separation?

Boldra

[Guest]

Concurring to what you are saying, here, see: http://asterisk.apod.com/viewtopic.php?t=211

[Boldra]

Concurring to what you are saying, here, see: http://asterisk.apod.com/viewtopic.php?t=211

Right, Magellan was launched in 1989 and Cassini-Huygens in 1997. I would have expected the radar technology to have improved a little in that time. Also Venus' atmosophere is 90 bars compared with Titans at only 1.5 bars. I note that Magellan flew within 257km of Venus and Cassini only within 1200 km of Titan.

Is the range the one reason why pictures of Titan are so (relatively) uninformative? Or is it the time spent? Cassini had only two hours, vs more than two years for Magellan.

I'm also curious about another aspect of the Huygens probe: I've heard that the probe is designed to function whether it lands on solid, liquid or slush. I was wondering whether a landing on trees was considered? Just how optimistic were the mission planners?

Boldra

[crosscountry]

I'm also curious about another aspect of the Huygens probe: I've heard that the probe is designed to function whether it lands on solid, liquid or slush. I was wondering whether a landing on trees was considered? Just how optimistic were the mission planners?

Boldra

I bet there is much still to be learned from these two instruments. We've still got probably 5 years for Cassini to teach us new things. Remember it has to move in an orbit. It cannot just go from one moon to another.

and your second question - The Huygens probe is the size of a car. I expect it will find solid something whether it be ground or the bottom of a lake. I think they meant it could be submerged and still be considered sucessful.

I for one am extremely excited about what will happen in a couple weeks. This is going to be great for all of us if we just hold our horses a little longer.

Cross

[S. Bilderback]

It took many flybys at a much closer range to get those wonderful images of Venus, also the data must be collected from multiple angles and lots of man hours of rendering to obtain a good 3D image.

[Boldra]

It took many flybys at a much closer range to get those wonderful images of Venus, also the data must be collected from multiple angles and lots of man hours of rendering to obtain a good 3D image.

Thanks for the info; but where did you get it? Did you work on either of these projects or their instruments?

This would certainly explain the image quality, but I'm still puzzled that there doesn't seem to be any altitude data, just light patches (radar reflective) and dark patches (not radar reflective).

Boldra

[crosscountry]

Thanks for the info; but where did you get it? Did you work on either of these projects or their instruments?

This would certainly explain the image quality, but I'm still puzzled that there doesn't seem to be any altitude data, just light patches (radar reflective) and dark patches (not radar reflective).

Boldra

I think that's the best you can get? when you see something with your own eyes you can only see the reflective surfaces.

Most of our accurate altitude data on earth was done by hand.

[Boldra]

I think that's the best you can get? when you see something with your own eyes you can only see the reflective surfaces.

Most of our accurate altitude data on earth was done by hand.

Hopefully my airport control tower is a bit better than that..! They can not only tell whether the planes are radar reflective or not, they can tell how far away they are.

It seems to suggest that Cassini is using some kind of passive radar, that only collects bounced radar originally emitted from the sun. Normally a radar would also activelz emit the radio waves, and then use the echo to measure the distance. Note RADAR is an achronym for "RAdio Detecting And Ranging". I'm curious why the ranging data is missing.

Boldra

[crosscountry]

I'm curious why the ranging data is missing.

Boldra

I am too.

But, if there are non-reflective surfaces , or they bounce at a different angle, the radar cannot detect it's own signal.

[S. Bilderback]

I wish I work on one of these projects, but no I didn't.

The reason there is no altitude differentiation is the same reason why looking far into the distance your stereoscopic vision has a hard time judging distances the farther away the objects are (don't count the hazing from our atmosphere). The distance between your eyes and the distance of the object, and will determine parallax offset, that is what the brain interprets as depth perception. If you could move your eyes farther apart or take measurements from two different location the parallax offset will be larger and will allow much more information; like a boy scout using a compass and a map to find a distance to a landmark.
So if you took a radar image of a feature at the equator first by flying over the north pole and the next one flying over the south pole, perform your triangulation, you can get much more accurate measurements of its altitude. Every point on the 3D radar map of Venus was calculated this way, many fly-bys from many angles. The first fly-bys of Titan so far are like looking with only one eye. There is more than 40 passes left to make, the data will get better.

[betsy]

I worked on Magellan (Venus), and currently work on Cassini. Magellan had an altimeter, Cassini/Huygens does not. Thus the difference in ability to get altitude information.

[eyecapitain1]

Good to have a Pro here Betsy.
I would think the reasons for the poor return are many.
First we see that Cassini is doing a "drive by" when it's imaging Titan.
Magellan was orbiting Venus. Cassini just swings by Titan occasionaly and snaps "on the fly".
Sort of like taking a picture of something from a speeding car. Other cars pacing you will come out clear while stationary objects will blurr. To get a clear shot of the scenery you have to swing the camera at exactly the right pace and have a fast shutter speed.
As Cassini swings past Titan it's range is constantly variable. To me this is a nightmare of point and snap when it comes to imaging clearly. It's going very fast relative to that moon.There has to be some slurring to deal with. Remember that the spacecraft itself must rotate to focus on and object as it swings by. As far as I know the radar imager isn't gimbled to accomodate swing pointing.
So far the cassini mission has been a triumphant success. I hold the mission executers in the highest esteem. Dang those folks have done a fantastic job! What a beautifull place the Saturnian system is and a grand job the teams have done.

[randall cameron]

It is possible (now anyway) to perform mapping (either limited in area or limited in detail) with some modern types of radar on a single pass. An active electronic scanned array / phased array would allow collecting a lot of data in a short time frame, and offers weight and reliability advantages (no gimbals or other moving parts, component redundancy, ability of signal processor to compensate for individual transceiver failures).

However, a Cassini / Huygens or any deep space probe has several major limitations:

1. Size and weight - the smallest production model off-the-shelf radars with the synthetic aperture capability required are built for (15-ton plus) fighter aircraft. The radar alone probably weighs more than Huygens.

2. Power - existing AESA radars assume a large gas turbine to provide electrical power. Space probes have solar cells and tiny radio-isotope generators.

3. Budget - if a suitable production radar existed (it does not) it might cost a few million dollars. Space instruments are typically custom made and purpose designed because of the restricted weight and power available, increasing cost and engineering difficulties dramatically. Also, as puny as NASA's science budget is, ESA is an even lower budget operation.

4. Time - back when Cassini was launched, the only AESA radars were huge land- or ship-based systems -- the engineering experience with smaller units was not there yet.

5. Even if you collected the data, Cassini would still be busy transmitting it back because of bandwidth limitations.

Magellan had many major specific advantages related to its objectives and opportunity to orbit for years:

a. Even a simple doppler radar altimeter could provide detailed terrain mapping if you orbit enough times -- VOIR was simple, small and low-powered, and engineered for long term mapping;

b. The VOI radar was the primary scientific instrument and thus got a much bigger share of the Magellan financial, weight and power budgets;

c. Venus is close -- Magellan could be much heavier and still get their quicker and cheaper;

d. Venus is close to the sun -- solar panels provide loads of reliable power;

e. Data take per unit of time was modest, meaning no transmission backlog.

Of course, if you build a space probe that weighs as much as an F-15 and figure out a way to get it all the way to Saturn, you could have your radar map within weeks of arrival. But that won't happen while the Shuttle and ISS are sucking up all of NASA's budget. Things aren't likely to improve much while we try to get men to Mars.

A Titan orbiter could do the job using much simpler, low power technology. Then the question is one of science priorities.

I think Cassini/Huygens has done magnificently given the budget. Along with Galileo and Voyager 2, we have learned a ton about the outer solar system from these three (relatively) inexpensive missions. Bravo to ESA.