On Sun, 28 Jul 2002, The Silent Observer wrote:
> > Continuous coverage is nice, but for a brief asteroid encounter, you can
> > plan the encounter so it's within view of your main ground station.
>
> In fact, enough flash memory to hold a hundred or so megapixel 24-bit
> images need cost only a few hundred dollars and weigh under 100 grams --
> so you time your encounter when you can (with short notice, you'll have
> to take what you can get) and download the images when your antenna can
> see the spacecraft...
I was assuming data dump after the encounter, actually. The reason for
needing contact at encounter time is navigation: the last few hours of
the approach will probably be a nonstop sequence of taking a navigation
image, downloading it, and executing a course correction based on it.
This is particularly important when the asteroid is a new one and its
orbit is not known all that precisely.
This also drives the communications data rate. Encounter data can trickle
back at your convenience, but navigation images can't wait. (Mind you,
they can be compressed heavily.)
> Also lets you avoid the mass and reliability issues
> of a movable antenna; you aim the entire spacecraft for imaging, and
> then reorient for data transfer.
The disadvantage of this, however, is that if something goes wrong, you
may have no inkling what it was, which makes fixing it for the next one
difficult. As witness Mars Observer and Mars Polar Lander, there is a
whole lot to be said for continuous contact.
> ...megapixel class cameras, OTS for under $500,
> could be stripped of case and flash (but autofocus left in place)...
No need for autofocus -- focus is always at infinity! And anything which
has moving parts requires a change of lubricant, to something vacuum-
compatible, so eliminating autofocus simplifies life.
Henry Spencer
[EMAIL PROTECTED]
_______________________________________________
ERPS-list mailing list
[EMAIL PROTECTED]
http://lists.erps.org/mailman/listinfo/erps-list
