SpaceX launch of Psyche mission to bizzare metal asteroid just 1 month away...

[server]

On Fri, 08 Sep 2023 20:43:33 +0200, jeroen <jeroen@nospam.please>
wrote:

On 2023-09-08 08:18, Jan Panteltje 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.
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\'?


I have no doubt all telescopes involved in this project are some
variant of Newtonian telescopes, i.e., they user mirrors, not lenses.

I found some more information: The laser on the probe is 4W at 1.55um
wavelength. I don\'t know about the transmitter optics.

The ground station uses a 5kW laser at 1.064um, focused by a 1m aperture
telescope. That should give us a clue as to the EIRP. Hold on...

With 1 m mirror the uplink beam is about 1 um / 1 m or 1 uradians. At
2 AU distance, the beam covers a circle of 300 km in diameter.

The probe receiver uses a 22cm aperture telescope.

The probe will capture (0.22 m / 300000 m) ^ 2 of the transmitted
uplink power.

Assuming the 22 cm telescope is used also for downlink, the beam is
1.55 um / 0.22 m so the beam is 7 uradians wide, at 2 AU the beam on
the ground is 2100 km wide, that is less than a continent.

The ground receiver uses the 5m Hale telescope at the Palomar observatory.

(5 m / 2100000 m) ^ 2 of the downlink is captured.

I still didn\'t find the link budget, but we\'re getting closer to making
one up ourselves.

You should be able to count that now.

 

[Phil Hobbs]

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