[X] My Assistant

[djellison]

Reading thru lots of little PDF's I've found, i get a figure that mentions up to 12 Gbits for a single full HiRISE image.....BUT - look at this...



To me, it would seem that they might get as few as one full HiRISE image per day

BUT - at the best data rates of say, up to 6Mbits/sec - 12 Gbits is only 34 minutes of downlink.

Can anyone sync those two facoids? It seems that the suggested performance, and the expected return, don't match by an order of magnitude.

Doug

[mcaplinger]

To me, it would seem that they might get as few as one full HiRISE image per day

BUT - at the best data rates of say, up to 6Mbits/sec - 12 Gbits is only 34 minutes of downlink.

Can anyone sync those two facoids? It seems that the suggested performance, and the expected return, don't match by an order of magnitude.

For starters, the maximum data rate that is being assumed is more like 4 Mbps. For an 8-hour pass, the s/c is only in Earth view about half the time, so two passes a day at 4 Mbps would return about 115 Gbits assuming no time to lock up, no retransmits, etc. So the graph seems roughly correct.

But the max data rate is only doable when the Earth-Mars distance supports it, which is why the graph goes up and down. And yes, at the low data rate periods you shouldn't expect to see many full-size max res images from HiRISE.

I suspect that most HiRISE images will be summed down from the maximum resolution, both to save volume and because the HiRISE MTF may not be all that great at full-res.

[Guest_Sunspot_*]

I suspect that most HiRISE images will be summed down from the maximum resolution, both to save volume and because the HiRISE MTF may not be all that great at full-res.

May not be that great?.....huh Do you know someting we dont lol

[mcaplinger]

May not be that great?.....huh Do you know someting we dont lol

No, just look at the diffraction limit. HiRISE has a 50 cm aperture and is sampling 30 cm from 300 km. MOC has a 35 cm aperture and is sampling 140 cm from 400 km. All other things being equal, you can't get 3.5x better resolution at the same image quality by increasing the aperture by less than 2x. (HiRISE has 12 micron pixels and MOC 13 micron, but that only helps HiRISE a little.) HiRISE will have better image quality than MOC, certainly, but not 3.5x better; it's not physically possible.

[SteveM]

No, just look at the diffraction limit. HiRISE has a 50 cm aperture and is sampling 30 cm from 300 km. MOC has a 35 cm aperture and is sampling 140 cm from 400 km. All other things being equal, you can't get 3.5x better resolution at the same image quality by increasing the aperture by less than 2x. (HiRISE has 12 micron pixels and MOC 13 micron, but that only helps HiRISE a little.) HiRISE will have better image quality than MOC, certainly, but not 3.5x better; it's not physically possible.

You're assuming the MOC camera is diffraction limited; the material from the HiRISE team implies the limit on MOC is the low signal to noise ratio resulting from scanning single pixels for the extremely short exposures needed for high resolution. They plan to improve the signal to noise ratio by clocking multiple pixels in the scanner to integrate the signal as the image drifts across the scanner. "Each CCD has 2048 12 x 12 um pixels in the crossscan direction and 128 TDI elements (stages) in the along-track direction." The 128 lines of time delay and integration (TDI) are used to get a very high (100:1) signal to noise ratio.

For more details see the HIRISE Instrument Development Report.

PS
Just saw the following in the last page of the report: "The nominal high resolution image is 20,000 pixels by 40,000 lines and can take from 4 to 48 hours of transmission time depending on range to earth and compression factors."

Steve

[mcaplinger]

You're assuming the MOC camera is diffraction limited...

Because it is, pretty much; at least as closely as an R-C Cassegrain system can be. As for your point about HiRISE having better SNR; that's certainly true, although the TDI will degrade the MTF at Nyquist, especially if the spacecraft attitude control has any jitter in it at all. I stand by my original assertion: HiRISE will have better image quality than MOC, but not by as much as a simple ratio of their ground sample distance would suggest.

[Bob Shaw]

I think you're all comparing apples and pears - size of data product vs optical resolution!

Bob Shaw

[djellison]

I think you're all comparing apples and pears - size of data product vs optical resolution!

Bob Shaw

Ahh - BUT - if the actual resolving power of HiRISE turns out to be only, say, 60cm/pixel - then onboard downsampling can produce a data product 1/4 the size, so we can have 4x as many of them

Doug