Re: [FT] Sensor Range Question

On Mon, 26 Apr 1999, djwj wrote:

> Dealing with space sensors can be tricky. First is where are you using
your
> sensors? Deep space use will give you a better range than in-system
use. The
> planetary bodies emit and/or reflect enough radiation that they can
mask
> anything near them.  

I assume you mean it's difficult to locate a ship in close orbit around
a
planet, right?	You being in close orbit around a planet won't really
have
much effect on your seeing. Your analogy of getting out of the city is
true, but keep in mind that the reason you want to get out of the city
is
that the city lights scatter in the atmosphere and wipe out your seeing.
Being in orbit nicely avoids that problem. And really, even when trying
to
find things near a planet, the planet doesn't have -that- much of an
effect.  The reason you can't see the small Jovian moons is more because
of the great distance and their small size than because of any
interference from Jupiter (which is mainly in radio anyway).

> As far as how well we watch our own solar system: Yes we do have very
good
> data about other planets, we even have reliable information about what
some
> of them (mostly the gas giants) are composed of. But this kind of data
takes
> months to gather. A spectral analysis must be averaged againtst time ,
the
> combined orbits of the observer and the observed, rotation speeds,
eclipses,
> ect. to compensate for a host of possible errors, the least of which
not
> being the infamous doppler shift.

Not so!  Doppler shift is exactly -how- we determine planetary orbits!
Give me a single doppler reading (which gives me velocity and position
at
a given time) and I can give you all six orbital elements for the body.
The rotation rate of Mercury, for example, can also be measured with
doppler radar. Spectral analysis (which my wife does for a living) can
take a few weeks and isn't terribly difficult, actually.

> If you want to see something the size of a fleet carrier, in time to
do
> something about it, you are limited to the distance between the earth
and
> the moon. This kind of detection is only practical for ground and
space
> stations, a moving ship causes too much distortion, mostly in
parallax, to
> warrant having those kinds of sensors, unless it is a dedicated
science
> vessel, which usually would end up in geosynchronous orbit anyways.

I agree detection over distance is hard, but not that parallax is a bad
thing - in fact that's probably the only way you'll get a good distance
measurement without going to active sensors.  

> As far as sensor "Evading": Very few modern astrophysical sensors are
> directional.

Hmm.  -All- astronomical sensors are directional if you have three of
them...

> Then there's the manuver itself. I assume that space combat occours at
the 1
> inch = 1 kilometer scale. That makes the average table 60 kilometers
by 40
> kilometers across (30 mile by 20 mile) a respectable distance for
space
> combat, and more than most space sci-fi shows give themselves. Trying
to
> "Jink" off even an obsolete directional sensor at these ranges is
nearly
> impossible. 

At these ranges (even shorter than Solar Thrust's 80 km, I'm
impressed!),
I'd have to agree.  It's pretty much impossible to miss with a laser at
less than 30,000 km, assuming you have a good targetting solution in the
first place.  Keep in mind though that most FT'ers use a range much
greater than a few km's, though.  

TTYL..
Keith

kwatt@astro.umd.edu
Univ. of Maryland Astronomy