This is an interesting question and can be answered separately from the related questions.
Given that the planets are in known orbits, their positions can be accurately predicted... and a given planet can be fairly close to or very distant from another... (t)hey can easily be on opposite sides of the sun, making direct communication impossible...
Bottom line: is there any place a relay station could be put to simplify communications between Earth and Mars? Or do the planets simply have to hold off and wait to communicate until they are in conjunction?
Yes, there are many! The relay station would either have to have a big (but light-weight mesh) dish if it used a radio link to Mars, or an optical communications link which would be smaller and future-compatible.
- ~7° inclined orbit roughly matching Mars'. This would be slightly different in some of the orbital parameters so it was always 20 to 30 million km from Mars so as not to be too perturbed by it. It will be chosen to have maximum out-of-ecliptic separation each time there was a conjunction. Their orbital periods will be 687 days and the synodic period for conjunctions is 780 days, so you could set it up to work nicely for a decade or two at least. The advantage is that it's always relatively close to Mars physically so that engineering that leg is more constrained. From the relay to Earth would be done however it's done from a Mars orbiter, so no changes there. Much of Mars' perturbations will be perpendicular to the velocity vector, so they will partially cancel out. This only needs to last a few decades.
- Heliocentric orbit 30 million km leading/lagging Mars. Similar to 1. but now Mars will be pulling prograde/retrograde and may perturb the orbit much more.
- Same as 1., but associated with Earth (none of the benefits of being near Mars
- L4 or L5 associated with Sun-Earth. Advantages are minimal
- Mars L4 or L5, advantages are minimal, really far from Mars 230 million km!
Why solar conjunctions are a problem.
Deep space communications dishes on Earth don't want to point too close to the Sun due mostly to radio noise and the risk of heat. Most dishes are not specularly reflective to light (visible or infrared) so they won't concentrate sunlight (true also for commercial satellite dishes) but the noise from the sun and the distortion of the signals passing through the millions of kilometers of charged particles near the Sun make communications within a few degrees of the Sun difficult.
From the DESCANSO series Deep Space Communications Edited by Jim Taylor (page 126):
4.5.7 Solar Conjunction
Solar conjunction was difficult for communications because the radio signals traveling between the Earth and Jupiter passed through regions of high and variable charged-particle density close to the Sun.
From Phys.org's Mars solar conjunction puts missions on hold in June:
The teams running NASA's three active Mars orbiters and two Mars rovers will refrain from sending commands to their spacecraft from about June 7 to June 21.
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This diagram illustrates the positions of Mars, Earth and the sun during a period that occurs approximately every 26 months, when Mars passes almost directly behind the sun from Earth's perspective. This arrangement, and the period during which it occurs, is called Mars solar conjunction. Radio transmissions between the two planets during conjunction are at risk of being corrupted by the sun's interference, so NASA Mars missions have a moratorium on sending commands to spacecraft on the surface of Mars or in orbit around Mars. Credit: NASA/JPL-Caltech
From Planned Science Data During Reduced STEREO Science Operations:
Between August 2014 and January 2016 one or both STEREO spacecraft will have their high gain antenna pointed at an angle a few degrees away from Earth to keep the antenna feed horn from getting too hot. This will result in greatly reduced telemetry rates while the antennas operate on one of their lower power side lobes instead of the main lobe normally used. However, all the STEREO science instruments will be able to provide a subset of their usual data during these periods.
During 2006, three ESA interplanetary spacecraft, Rosetta, Mars Express (MEX) and Venus Express (VEX), passed through superior solar conjunction.
For all three spacecraft, the noise in the post-fit rangerate residuals from the orbit determination was analysed. At small Sun-Earth-Probe (SEP) angles the level was almost two orders of magnitude higher than normal. The main objective was to characterise the Doppler (rangerate) noise as a function of SEP angle. At least then the range-rate data can be appropriately weighted within the orbit determination so that the solution uncertainties are
For VEX, some intervals of particularly noisy Doppler
data could be correlated with unusual solar activity.
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Optical communications might be better, but...
Optical communications (instead of radio) helpful for two reasons here:
- Higher bandwidth and/or signal-to-noise ratio at a given distance
- Better immunity to distortions due to charged particles near the Sun.
On the Earth end of the link, it is likely that receivers will be space-based telescopes rather than ground based: Will future deep space optical communications "ground stations" actually be in space, or on the ground?
Without the atmosphere, a telescope could potentially look very close to the Sun to receive optical signals. The laser wavelength will be extremely narrow and in the infrared, so a filter will cut down any scattered light by a perhaps a factor of 1000 or even more, and since we look at the high frequency modulation rather than the DC component, some kinds of modulations can be pulled out of a much brighter background.
However, not sure about pulse position modulation: