[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?

[elakdawalla]

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?

To perform Synthetic Aperture Radar imaging, the spacecraft points its radar dish to one side of its track as it flies over Titan. To capture a SAR swath it broadcasts a burst from five different antenna beams (one at a time) and listens for the echo from the surface. The echo data is transmitted to Earth. On Earth, each burst echo is modeled in "delay-Doppler space". This is tough to describe -- but basically, the echo is returned from different parts of Titan's surface at different times depending on how far away it was from the spacecraft (that's the delay part) and the echo's frequency is shifted higher or lower depending on whether it was in front of the spacecraft's path, and hence Dopplered to higher frequency, or to the rear of the spacecraft, and Dopplered to lower frequency (the Doppler part). For this processing to work the speed and altitude of Cassini need to be known to extremely high accuracy. The resulting images are in delay-Doppler space. I've learned from the team that they always reproject their images to a resolution of 256 pixels per degree, giving a pixel size of around 176 meters per pixel. But the effective resolution is actually lower than that, ranging from around 350 to 720 meters per pixel. The Magellan instrument worked almost exactly the same way except it had only one beam in contrast to Cassini's 5 beams.

Here's a little something I wrote up for the Society website:
http://www.planetary.org/explore/topics/ca...ment_radar.html
And attached is a technical paper on the instrument.

--Emily

Attached File(s)

 cassini_radar.pdf ( 779.69K ) Number of downloads: 781