[X] My Assistant

[Harkeppler]

Hello!

I am wondering why the SAR on Cassini is producing segments of the radar path, in most cases four parallel stripes per flyby. Does anyone know how the instrumentsworks exactly?

[Roly]

This probably a really obvious question, but I'm a humble humanities student !

Is there any prospect of multiple (>2) overlapping passes for improving the characterization of particularly interesting sites; or for set of various typical terrains and features encountered across the other SAR passes? [And therefore providing an improved understanding of the single-pass data.]

Something for the extended mission perhaps? Would there be much to be gained perhaps half a dozen of the Huygens site, perhaps coupled with a lower altitude ceiling? Edit: Looks as if the altitude won't be lowered anytime soon, though this does leave the possibility open in the future I think, as variations in the atmosphere become better known (http://www.planetary.org/blog/article/00000707/).

The RADAR seems to be the instrument that has the most flexibility and scope for improvement as the mission goes on.

Roly

[JRehling]

This probably a really obvious question, but I'm a humble humanities student !

Is there any prospect of multiple (>2) overlapping passes for improving the characterization of particularly interesting sites; or for set of various typical terrains and features encountered across the other SAR passes? [And therefore providing an improved understanding of the single-pass data.]

Something for the extended mission perhaps? Would there be much to be gained perhaps half a dozen of the Huygens site, perhaps coupled with a lower altitude ceiling?

I don't think there's much point to having MANY passes of the same site (n>2), but there is a lot of value, potentially in having exactly n=2. That's not coincidentally why just about every advanced animal has two eyes but basically none have 3. You get stereopsis with 2, and 3 is redundant. With SAR, it's not quite stereopsis we're after, but the principle is analogous: We want to distinguish bright=rough vs. bright=sloped towards the spacecraft. Two perspectives provide the discrimination in most cases, although pathological cases could thwart that.

Unfortunately, Cassini has no chance of an extended mission that so egregiously exceeds the main mission as we've seen with the MERs. I think the best case scenario for additional RADAR passes in the extended mission will be no more than about the same number in the main mission, but that is far on the optimistic side. We might get as few as 6, even assuming that Titan is the sole target (and it will be a major one, by necessity).

Overlap of some stripes is basically unavoidable as mapping continues, so we will get at least some terrain that enjoys that level of scrutiny. However, we're not going to get to map the whole globe once, so overlap comes at the expense of "once-over" coverage. Also, targeted coverage comes at the expense of pragmatics (near-nadir pointing provides best resolution, and orbits must be designed to bring Cassini back to Titan). I think we'll end up seeing Titan covered about 25-30% (?) with a small fraction of that SAR'd twice.

[ugordan]

It has been reported (though I can't remember where -- possibly in a conference abstract) that some small amount of overlap was already achieved. Specifically, over the "sand dunes". A surprise at that was that although the radar illumination was from a different direction, the dunes didn't change their appearance. Bringing into question their vertical relief and topographic shading assumptions.

[Roly]

Thanks for the responses, they were most helpful, especially regarding the intrinsic properties of the surface vs. its orientation toward the beam.

I vaguely thought that performing more than two passes might allow for improvements in effective resolution through multi-sampling (maybe improving SNR), or some other kind of clever superresolution trick, but what works for optical images is probably wildly different from SAR. With such a brief extended mission (I was more optimistic), the trade between mapping and repeat coverage seems to weigh against deliberately targeted second passes (c.f. overlap.)