The overwhelming loss in this calculation is from the antenna’s radiated energy spreading out over a larger and larger area (despite the directional “gain” factor).
I’m wondering: would a probe launched today instead employ a laser to communicate? This would seem to offer many orders of magnitude improvement in the directionality of the signal.
The main challenge is the earth to probe comms for distant probes, since the earth is often very close (in an angular sense) to the sun from the probes perspective, and the sun gives out a lot of black body radiation.
However, due to the shape of the black body radiation curve, the sun gives out relatively less microwave radiation than it does visible light, which might outweigh the advantages of more directionality given by using a laser.
The big dish antennas do use ruby masers, but not to transmit. The maser is used as the LNA on the receive side. Check out the picture on page 41 of the pdf, clearly this a flux capacitor, mislabeled to deceive us ;)
Not having thought this through before, I see now that while a transmit maser may have efficiency advantages, it may not improve directionality relative to a standard parabolic radio transmitter. All methods of producing microwaves will have basically the same diffraction-limited gain for a given “aperture” (dish) size. That darn uncertainty principle! (However, an optical laser would still give way better directionality.)
Further, we're good at building really big radio transceivers here on Earth, we don't have nearly the same technical experience with lasers of that scale.
Presumably though it would be useful to have a high bandwidth link back to earth even if we had to use conventional microwave transmitters to send data back.
We want to download high resolution images/spectrographs whereas we only want to upload code/instructions.
Interesting about the JPL program and I’m amazed this prototype was only launched last year! Apparently the answer to comms laser use is “not yet but soon”.
Its certainly exiting that we'll have laser based communication in the near future. its like all ingredients for interplanetary missions are slowly coming together. all we're missing now is a very high specific impulse engine that could push the several tons for human habitation to another planet.
All space agencies have optical comms in their road maps. Largely they are thinking about inter satellite communications (the atmosphere causes significant issues when going back to earth). So the main application is to have some relay satellite that can then transmit to earth via RF. The application is not mainly deep space ropes but Leo or meo satellites, the typically only have very short transit times over the ground stations, so can't get all their measurement data down. By using e.g. a geo relay they can transmit lots of data optically and the geo relay can more slowly transmit the data to earth until the leo satellite comes back in view.
I'm curious about the feasibility of combining the two problems of propulsion alway from Earth and communication with Earth into beam-powered propulsion aimed directly at Earth, pulsed for use as communication.
Probably infeasible for several reasons (only useful when accelerating DIRECTLY away from Earth, incoming light to power spaceship is probably coming from the sun and therefore likely also in the directly of Earth, so net zero acceleration at best from firing the photons back towards the sun), but it'd be pretty neat.
Assuming you don't need fast steering, is a 3.7m transmitter array doing beamforming really better than a 3.7m dish transmitting at the same power?
My intuition would have been that you are better off using a fairly standard transceiver and spending your engineering budget either increasing power or getting a bigger dish (either by launching on a wider rocket or with a folding design).
Lasers might interesting for the downlink, but receiving a laser signal on the probe sounds difficult (earth is pretty bright).
The array doing beamforming can be spread out much farther. If the DSN were being built now, I'd think its antennas would look more like the Square Kilometer Array.
Diffraction scales inversely proportional to wavelength so you gain significantly by going to optics, i.e. you can use a much smaller aperature in optics.
I’m wondering: would a probe launched today instead employ a laser to communicate? This would seem to offer many orders of magnitude improvement in the directionality of the signal.