On 2023-09-08 07:18, John Larkin wrote:

On Fri, 08 Sep 2023 06:18:35 GMT, Jan Panteltje <alien@comet.invalid>
wrote:

On a sunny day (Fri, 08 Sep 2023 00:06:59 +0200) it happened jeroen
<jeroen@nospam.please> wrote in <uddhi4$34vlh$1@dont-email.me>:

I've been trying to find something about the link budget. I had
one for the Voyager or Pioneer probes somewhere, but I can't seem
to find it again. IIRC, factors going into it were transmitter power,
antenna gains, path loss, receiver S/N and bandwidth. It shouldn't
be much different for IR EM waves, although receiver S/N would here
be limited by shot noise rather than Johnson (thermal) noise.

The 'antennas' here are telescopes, of course.

Maybe I am seeing his wrong,
but for a receive antenna to collect as much signal as possible,
a focussing dish surface catches a lot more light than a telescope aperture.

Aim a laser pointer at the wall. Then try the equivalent with RF.

So for the spacecraft some sort of dish (like James Webb has) or like used for RF (Voyagers have a dish) should
work better than a 'telescope' (with lenses).
For transmission a lens system is cool, although lasers already have a narrow beam by default?
Searching finds this:
https://www.jpl.nasa.gov/news/nasas-deep-space-communications-to-get-a-laser-boost#carousel-27fdb231-a68f-434e-ad7e-5e92f104b2f1-1
you can enlarge the photo inset, shows a small mirror.
I sure hope they get enough signal.
Very small compared to the radio dish.
Now one could argue about the IR wavelength being much shorter
so for waves per square surface area more for IR..
But with such a small mirror back in a long tube, pointing becomes critical. >> ?
Seems sort of 'tucked on'?

For constant Tx power and Tx aperture, you win Rx power quadratically as
the wavelength decreases, because the beam gets narrower. It's not
quite that good in real life, because telescopes accurate enough to be diffraction limited are heavy compared with antennas.

The Rx gain doesn't have the same effect, because the output is
multimode anyway--for wavelengths short compared with the Rx aperture,
the gain is independent. (In contrast, for a single-mode receiver such
as a waveguide horn, you win quadratically with Rx aperture as well.)

You don't have thermal noise to worry about with optical comms, but your bandwidth is much much wider, and there's more ambient background to
worry about.

But the worst thing is that you're not using heterodyne detection--a photodetector is quadratic, like a crystal set. Photon counting helps
that, but makes the ambient background problem much worse.

With very dark skies, narrow interference filters, and cooled PMT
detectors, it would probably be a win. In daylight in the California
desert, not so much.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)