Communication satellite trade-offs

[redliox]

There are many types of satellites we depend on, and if we wish to make Mars safe and comfortable at least a few around Mars required; in this case I'm referring specifically to communication satellites, a subject which I've brought up occasionally before.

Mostly, I wish to discuss where'd be the best spot to put these things, especially early on.  Let's make a couple assumptions:

1) 2 satellites, no budget for more but perhaps spares for a 3rd
2) 90% exclusively for communication purposes with perhaps room for a single science instrument, probably along the lines of a PR webcam, imaging spectrometer, or magnetometer based on whoever pays for it or sponsors the satellite.
3) Tech including ion drives, both laser and radio coms, and maybe radiation box ala-Juno.
4) Projected lifetime of ~15 years with hope of refueling or new satellites.

The main thought I usually have is where these satellites should go?  High (including the Martian moons and stationary orbit) or Low Mars Orbit?  If we have only 2 satellites to initially depend on for aiding crews and probes, what would be wise, likewise any pluses or minuses?

Personally, I favor HMO for wider coverage, but LMO would allow faster rates and more frequent coverage if an Earth analogy with megaconstellations indicates.

What needs to be considered in choosing an orbit plus any additional factors?

[RonM]

Synchronous orbit would seem like a good idea, but that's between the orbits of Phobos and Deimos, so it wouldn't be stable. If station keeping isn't a problem, one satellite in synchronous orbit would work. The second would be the spare. Not global coverage but good enough for constant communications with the Mars base.

Maybe a higher orbit than Deimos with at least three satellites to always have one visible from the Mars base. Two satellites aren't enough.

They should be combination communication and weather satellites.

[redliox]

Synchronous orbit would seem like a good idea, but that's between the orbits of Phobos and Deimos, so it wouldn't be stable.

The Tharsis bulge would actually be a greater disturbance than the moons, although their miniscule masses would likely be accounted for in regards to long-term station keeping.

Maybe a higher orbit than Deimos with at least three satellites to always have one visible from the Mars base. Two satellites aren't enough.

The more is always the merrier, but recall how NASA canceled the Mars Telecommunications Orbiter, hence why I figured two is generous enough.  I could go into Elon deploying a megaconstellation in LMO, but specifically I am referring to initial satellites in an age where NASA looms over shoulders and decides the better half of budgets and requirements.  Three or four would be an ideal number so multiple satellites could be available, but something bare-bones where one satellite rises as the other is setting could suffice.

They should be combination communication and weather satellites.

Not on a budget or if the science instrument is supposed to look in a different direction than the com-equipment.  Not impossible, but it drives up complexity.  I rarely hear of Earthly satellites doing such double-duties.

[tbellman]

Personally, I favor HMO for wider coverage, but LMO would allow faster rates and more frequent coverage if an Earth analogy with megaconstellations indicates.

What needs to be considered in choosing an orbit plus any additional factors?

For optical communication with Earth, you want a significant separation from Mars, so the receiver at the Earth end can easily get just the Mars satellite in view, and not get "blinded" by Marslight.  If I remember correctly from earlier discussions, something on the order of one Mars diameter should be enough.  (Similarly, the senders at the Earth end need to be at an altitude of several thousand kilometers to be easily distinguished from Earth itself by the receivers at the Mars end.)

And the links to/from Earth will be the bottleneck, not the links between the satellites and Mars surface.  Even at an orbit beyond Deimos, it shouldn't be too hard to get gigabit speeds between the surface and the satellite, and that should be sufficient for any surface-satellite-surface communication in the early years (you will only be servicing a handful of "customers", not hundreds).

Another consideration is shadowing by Mars.  A satellite in low Mars orbit will spend almost 50% of its time hidden behind Mars, and not have line-of-sight to Earth.  And with just two or three satellites, it won't have line-of-sight to any other satellite with line-of-sight to Earth either (I think it is reasonable to assume inter-satellite links).  This also favours high orbits, and at least three satellites.

(There's also a period of around two weeks every synodic period where the Sun lies inbetween Mars and Earth and blocks direct communication.  But I think it is a reasonable compromise to ignore that for the first few crewed missions, and just let them be isolated during that time.)

[redliox]

Personally, I favor HMO for wider coverage, but LMO would allow faster rates and more frequent coverage if an Earth analogy with megaconstellations indicates.

What needs to be considered in choosing an orbit plus any additional factors?

For optical communication with Earth, you want a significant separation from Mars, so the receiver at the Earth end can easily get just the Mars satellite in view, and not get "blinded" by Marslight.  If I remember correctly from earlier discussions, something on the order of one Mars diameter should be enough.  (Similarly, the senders at the Earth end need to be at an altitude of several thousand kilometers to be easily distinguished from Earth itself by the receivers at the Mars end.)

And the links to/from Earth will be the bottleneck, not the links between the satellites and Mars surface.  Even at an orbit beyond Deimos, it shouldn't be too hard to get gigabit speeds between the surface and the satellite, and that should be sufficient for any surface-satellite-surface communication in the early years (you will only be servicing a handful of "customers", not hundreds).

Another consideration is shadowing by Mars.  A satellite in low Mars orbit will spend almost 50% of its time hidden behind Mars, and not have line-of-sight to Earth.  And with just two or three satellites, it won't have line-of-sight to any other satellite with line-of-sight to Earth either (I think it is reasonable to assume inter-satellite links).  This also favours high orbits, and at least three satellites.

(There's also a period of around two weeks every synodic period where the Sun lies inbetween Mars and Earth and blocks direct communication.  But I think it is a reasonable compromise to ignore that for the first few crewed missions, and just let them be isolated during that time.)

So if optical communication is the primary means, an orbit beyond even stationary Mars orbit is preferable?  Slightly surprising, but it makes sense given how coronagraphs block starlight in exoplanet research or how the discovery of the moons' were compounded by Mars' proximity.  So Deimos' orbit could be the minimum for optical relay whereas LMO could work for radio sats, the later of course staying sun-synchronous to avoid Mars shadowing?

[tbellman]

So if optical communication is the primary means, an orbit beyond even stationary Mars orbit is preferable?  Slightly surprising, but it makes sense given how coronagraphs block starlight in exoplanet research or how the discovery of the moons' were compounded by Mars' proximity.  So Deimos' orbit could be the minimum for optical relay [ . . . ]

No, I'm mostly just saying "not a low orbit", not necessarily a very high orbit.

Mars has a diameter of about 6800 km, and areostationary orbit is at about 17000 km above the surface.  Areostationary should be plenty high to get good separation for the optical receiver at Earth.  Even down to the orbit of Phobos (~6000 km altitude) should work fine, I think.  (But this is based on my memory of earlier discussions!  I don't have much direct knowledge about this subject myself.)

Also, if you have three evenly spaced satellites (120° apart) in orbit around a body, the orbit needs to have a radius that is twice the radius of the body in order to get line-of-sight between the satellites.  I.e, the orbit altitude needs to be at least as high as the body's radius.  Areostationary fulfills that criterion as well, by a good margin.

Areostationary orbit also has line-of-sight to surface to latitudes up to cos^-1(3400 km / (3400 km + 17000 km)) ≈ 80°, which is nice.  (Although you only reach that high/low when at the same longitude as one of the satellites; inbetween, you get slightly lower.  Also, you would need to point the surface antenna parallel to the ground, and you then risk being blocked by various ground obstructions.  So in practice you won't get good coverage up to 80°.)  If the satellites are at 6000 km altitude, you "only" reach to about ±68° beyond the satellite's ground track, but I think that should still be enough for early human missions.

However, at 6000 km, with only three satellites, you may have not have a satellite more than 8° above the horizon from the base, and that's assuming the satellite track passes right over the base.  That's pretty low, so in practice there would be regular communication outages.

Areostationary of course has the advantage of not needing a tracking antenna on the surface (and by "tracking" I here also include using phased arrays).

[ . . . ] whereas LMO could work for radio sats, the later of course staying sun-synchronous to avoid Mars shadowing?

The lower you get, the shorter times the satellites will be visible from the ground.  And below 3400 km altitude, three satellites won't be able to communicate with each other.  The base would need to wait until a satellite is in view until it can send its message, and then the satellite would need to store that message until it has a direct view of Earth until it is actually transmitted to Earth.  Earth can send anytime and reach at least one of three satellites, but the satellite may need to store the message until it is in view of the recipient on Mars.  Being sun-synchronous doesn't help; the satellite still needs to wait until the combination of it going around Mars, and Mars turning around its axis takes the satellite within view of the base.

The same problem applies for communication between parties on Mars.  If an away party has a problem, do they really want to wait until one of the satellites pass above them, and then further wait until that very same satellite passes above the base, until their message can be received?

So all in all, I would say that at least 6000 km altitude, regardless of if communication with Earth is done optically or via radio.  (But at this point, I really think optical is the way to go.)  Areostationary seems pretty good, assuming station-keeping is not a problem, with one satellite parked at roughly the same longitude as the main/largest base.  And inter-satellite links (also most likely optical).

[MickQ]

What about a comm relay on Phobos ?  The moon is in a well known orbit and is above the horizon for more than 4 hours twice per sol.  A good compliment to satellites IMO.

[tbellman]

By the way, here is a related thread:
• NASA considering commercial Mars data relay satellites

I'm pretty sure I have read some other thread with more detailed calculations about optical communication between Mars and Earth, but  the above was what I found with just a quick look through topic names in the Space Science Coverage board.

[tbellman]

What about a comm relay on Phobos ?  The moon is in a well known orbit and is above the horizon for more than 4 hours twice per sol.  A good compliment to satellites IMO.

What would be the advantage of putting a relay actually on Phobos, instead of having a free-flying satellite in the same orbit (if that orbit is deemed to be a good orbit)?

It seems to me that being on Phobos would just cause problems.  E.g, the antennas for communicating with the surface of Mars would need to be situated on the side facing Mars.  Luckily, Phobos is tidally locked to Mars, so that part is easy, but what about the antennas/telescopes for communicating with Earth?  Phobos itself will often block the view towards Earth regardless of where on its surface you are.  A free-flying satellite can be built to swivel its Earth-facing antenna/telescope without the view being blocked by the satellite itself.

Likewise, power.  The Mars-facing side of Phobos is in the shadow of Mars at precisely the times that side faces the Sun, so it receives fairly little sunshine.

[Jim]

What about a comm relay on Phobos ?  The moon is in a well known orbit and is above the horizon for more than 4 hours twice per sol.  A good compliment to satellites IMO.

Not really.  It would require multiple equipment locations for the earth transmitter

[MickQ]

Sorry guys, I was only considering Mars surface Comms via Phobos.

[txgho]

Optimally a relay / multiuse craft stationed in the stable Lagrange points would ensure the peek around the corner when the orbits radi the sun.

There would be arguments for L4 & L5 use for observatory bases as well as comm relays.

[DanClemmensen]

Optimally a relay / multiuse craft stationed in the stable Lagrange points would ensure the peek around the corner when the orbits radi the sun.

There would be arguments for L4 & L5 use for observatory bases as well as comm relays.

To peek around the Sun, put your relays in the Venus-Sun L4 and L5 Lagrange points. Lower latency. I Merucury's L4 and L5 would be even shorter but The Venus points have "historical" significance.
    https://en.wikipedia.org/wiki/Venus_Equilateral

[Barley]

Optimally a relay / multiuse craft stationed in the stable Lagrange points would ensure the peek around the corner when the orbits radi the sun.

There would be arguments for L4 & L5 use for observatory bases as well as comm relays.

To peek around the Sun, put your relays in the Venus-Sun L4 and L5 Lagrange points. Lower latency. I Mercury's L4 and L5 would be even shorter but The Venus points have "historical" significance.
    https://en.wikipedia.org/wiki/Venus_Equilateral

What's the advantage to using these Lagrange points rather than other heliocentric orbits?   Satellites will last decades, not millennia.  Almost all heliocentric orbits are fully predictable on this time scale.

(And it's unclear to me if Mercury's L4 and L5 are stable anyway.  The Lagrange points are a solution to a three body problem, and in this case the gravitational attractions of Venus and Earth are significant.)

[DanClemmensen]

Optimally a relay / multiuse craft stationed in the stable Lagrange points would ensure the peek around the corner when the orbits radi the sun.

There would be arguments for L4 & L5 use for observatory bases as well as comm relays.

To peek around the Sun, put your relays in the Venus-Sun L4 and L5 Lagrange points. Lower latency. I Mercury's L4 and L5 would be even shorter but The Venus points have "historical" significance.
    https://en.wikipedia.org/wiki/Venus_Equilateral

What's the advantage to using these Lagrange points rather than other heliocentric orbits?   Satellites will last decades, not millennia.  Almost all heliocentric orbits are fully predictable on this time scale.

(And it's unclear to me if Mercury's L4 and L5 are stable anyway.  The Lagrange points are a solution to a three body problem, and in this case the gravitational attractions of Venus and Earth are significant.)

Sure, you can pick a lot of perfectly good heliocentric orbit pairs for relays that let you maintain communications. However, Venus Equilateral was one of my favorite Science Fiction story series and It made quite an impression on me. The stories were written from 1942 to 1945, before I was born.

[redliox]

Some thoughts on a possible constellation for Earth-Moon-Mars communication.

Bare Bones: 2 satellites
                 1-Lunar Lagrange 4/5
                 1-Deimos Orbit

Enhanced: 5 satellites
                 1-Lunar Lagrange 4/5
                 3-Deimos Orbit
                 1-Aerostationary Orbit

In this case, I favor Deimos orbit for easier pinpointing from Earth, using Deimos itself to pinpoint satellite(s) position, long linger time over Mars and drifting around planet in case different regions need a relay.

[Jim]

                 1-Lunar Lagrange 4/5

Any near earth doesn't provide any benefit.

[JohnFornaro]

Optimally a relay / multiuse craft stationed in the stable Lagrange points would ensure the peek around the corner when the orbits radi the sun.

There would be arguments for L4 & L5 use for observatory bases as well as comm relays.

To peek around the Sun, put your relays in the Venus-Sun L4 and L5 Lagrange points. Lower latency. I Merucury's L4 and L5 would be even shorter but The Venus points have "historical" significance.
    https://en.wikipedia.org/wiki/Venus_Equilateral

Why not use Earth-Sun L4 & L5?

And thanks for the reference to "Venus Equilateral", a series I missed, growing up.

[DanClemmensen]

Optimally a relay / multiuse craft stationed in the stable Lagrange points would ensure the peek around the corner when the orbits radi the sun.

There would be arguments for L4 & L5 use for observatory bases as well as comm relays.

To peek around the Sun, put your relays in the Venus-Sun L4 and L5 Lagrange points. Lower latency. I Merucury's L4 and L5 would be even shorter but The Venus points have "historical" significance.
    https://en.wikipedia.org/wiki/Venus_Equilateral

Why not use Earth-Sun L4 & L5?

And thanks for the reference to "Venus Equilateral", a series I missed, growing up.

The Earth-Sun L4 and L5 result in longer transmission paths and therefore longer speed-of-light delays when routing around the Sun. L4 is at the third apex of an equilateral triangle, so the distance from Earth to Earth-Sun L4 is 1 AU or about 8 minutes at the speed of light.  Venus L4 will be much closer to the direct path from the far side of the sun to Earth. You want one station at L4 and another at L5 so that one of them will always be visible from both ends of the path when the other one is behind the Sun from one end of the path. In the Venus Equilateral stories, the satellites above Venus itself served as the alternate path, but unless we have those satellites for other reasons, that location is not as good as a point far away from a planet.

[redliox]

To peek around the Sun, put your relays in the Venus-Sun L4 and L5 Lagrange points. Lower latency. I Merucury's L4 and L5 would be even shorter but The Venus points have "historical" significance.
    https://en.wikipedia.org/wiki/Venus_Equilateral

Why not use Earth-Sun L4 & L5?

And thanks for the reference to "Venus Equilateral", a series I missed, growing up.

The Earth-Sun L4 and L5 result in longer transmission paths and therefore longer speed-of-light delays when routing around the Sun. L4 is at the third apex of an equilateral triangle, so the distance from Earth to Earth-Sun L4 is 1 AU or about 8 minutes at the speed of light.  Venus L4 will be much closer to the direct path from the far side of the sun to Earth. You want one station at L4 and another at L5 so that one of them will always be visible from both ends of the path when the other one is behind the Sun from one end of the path. In the Venus Equilateral stories, the satellites above Venus itself served as the alternate path, but unless we have those satellites for other reasons, that location is not as good as a point far away from a planet.

Any near earth doesn't provide any benefit.

I agree with the utility of Venus L4 satellites, especially to get around solar conjunction.  As for why I brought up the Lunar L4/L5 for assisting a Martian network, they're only a light second away from Earth while a decent distance away to allow distinct targeting from either the Earth or Mars.  I.e. as an alternate relay if the main station on Earth is facing away from Mars.