[gurps] Multipurpose Beams

[Pauli Hakala]

Thomas Tomiczek wrote:
> 
> > Good point - as a point-to-point communicator, lasercoms
> > suffer from time lag problems just as much as anything else
> > would.  OTOH, if the ladar's "scan rate" is sufficiently
> 
> No, they are worse. A laser is POINT TO POINT. A maser or even directed
> radio is POINT TO CONE.

Actually, laser beams spread over distance too. It is just that their
cones are smaller in relation to the distance, but over interplanetary
or interstellar distances they do 'paint' a large area at once.

A fairly well focused modern laser pointed from earth to moon would have
something like one yard wide 'point', IIRC. And this is only over
a distance of about 200,000 miles. Laser beam sent to Pluto might well
spread over 16 mile radius..

> The problem with the laser is that the beam is NARROW, so you have to be
> EXACTLY on the receiver. And the receiver on the other ship is NOT a 15
> meter sensor (and even if it is, this would not matter). With directed
> radar ome microwave, GURPS Traveller Starships says something of a 20
> degree cone. This makes smaller moements by the target way more
> handlable. Actually, over 1 ls, the cone is pretty large - the ship has
> no chance to break out of it within the 2 second signal tracking delay.
> 
> The problem with lasers is that IMHO it is simply inpossible to keep the
> laser on the receiver over relevant distances UNLESS both systems don't
> move relative to each other, or only move in pretermined ways. Perfect
> for ship to ship comm (when not maneuevering). Perfect for keeping in
> touch with a satellite (oif you don't move when receiving data  back).
> But nothing for flight approaches, nothing for combat etc.

Cyberslaved lasers guided by ultra-tech computers would seem to be
easily accurate enough for in-flight communications with relatively
nearby targets, such as the spaceport flight control tower.

If you could easily shoot holes in it with the laser, you most
definitely can communicate with it.

> > high, the lasercom could be used as something more closely
> > resembling a broadcast communicator (by "sufficiently high",
> > I mean that the beam would need to sweep its entire area in
> > the time it would take to transmit a single bit of digital signal).
> 
> Well, two problems:
> 
> * You need to have a sync signal that is accurate between source and
> target, so that this can basically know when bits start or end.
> Basicall, you need to know when the signal is the next bit.

Thats why you have various communications protocols. It is not
absolutely written in stone that all lasercom links would have the
same bandwidth. Over long distances you would propably need a more
clearer signal, so you would have to cut back on the bandwidth.

User of course need not to worry about such things, TL9 hardware
could automatically optimize any given laser link in a relatively
short time (depends on the lightspeed timelag, of course).

> * Your timeframe is NARROW. Unless you want no bandwidth (throughput),
> we talk of an awfull lot of bits per second. This means that you have to
> scan the whole area hundreds os thousands of times per second, POSSIBLE
> EVEN MULTI MIULLION TIMES for larger  bandwith. I don't thing this is
> practicable. And can go ith something that is point to cone in a much
> mor efficint way by then - you loose the one advantage of laser anyway,
> which is that noone else can listen to the data.

I am not sure I get your point. Why scan the whole area, if precise
coordinates and movement vectors are known in advance for both
lasercoms in the link?

> Lasers have the advantage of requiring little energy (as the energy is
> conserved much better in the narrow beam, not spreading) and this makes
> them perfect for a lot of scenarios. But moving receipients is NOT one
> of them :-)

Starships have advanced inertial navigation systems onboard, as well
as high-end computer systems for their era (TL9 computers will propably
start to be more complex than the human brain, actually). Along with
other info, they can transmit their exact position and estimated course
data. In the event of combat or emergency situation, I doubt anyone
would want to or need to keep a commline open to Pluto anyhow..


-Pauli
-- 
"Nothing is more deceiving than a low down dirty... deceiver."
-Jayne Cobb, Firefly