Do relays communicate with Earth and rovers/satellites at the same time?

[Xavven]

How do relays work? Does it communicate with a rover 1 on 1, then hold that data in memory, point its high gain antenna at Earth, and then send what was stored in memory, then see if Earth has anything to say, then turn its antenna back to the rover and repeat the message?

Or do they point one antenna at Earth and the other at the rover and forward data both ways in real time?

In other words, does it "hang up and call the other person" or does it make a "3-way conference call"?

I'm designing some relays in KSP and want to know if it's more realistic to have 1 big dish on it, or load it up with a big dish + several other antennas.

[linuxgurugamer]

Real life, or in KSP?

 

[Xavven]

25 minutes ago, linuxgurugamer said:

Real life, or in KSP?

 

Real life.

[magnemoe]

16 minutes ago, linuxgurugamer said:

Real life, or in KSP?

 

Obviously real life, KSP relays works well with just one antenna as he stated. 
Now real time relays like communication and broadcast satellites has multiple, an broadcast satellite tend to have an small receiver who get an strong signal from an station. This can easy be an above 5 meter disc. 
It then uses its large antenna to send this to the pretty small receiver parables. 
Deep space probes like the Pluto flyby often have an fixed antenna and can not transmit while taking images, this does not matter much as its far from real time anyway and the data stream lasted months afterward. 
It took some shots up til just before timeout for communication, kind of targeting photos so they could focus on interesting features. 
Now the Mars relay also send images of the surface, I kind of suspect this is an pretty constant stream and it has an dedicated antenna for the rover.

[mikegarrison]

My assumption is that a real-time relay would need at least one antenna for reception and another for transmission. A spacecraft designed for asynchronous data transmission might be able to use the same antenna for both.

[K^2]

There are a lot of options, actually. Something like geosynchronous telecom satellite will have both the source and destination of the broadcast in the same general direction. So a single dish is just fine. If you have a bit more of an angle between the two, but it doesn't change all that much, you might use a single dish, but have transmitter and receiver in different places in the focal plane of the dish. That will allow receiving from one direction and broadcasting in another. Of course, if the angle changes, you'll need to be able to move at least one of the two, receiver or transmitter, and that's bound to have mechanical limitations as well as limited response time, but might still be better than having two separate dishes. There is also a limit to how much you can steer the beam from a dish antenna before you're no longer getting benefits of using a dish. Finally, there are antenna arrays. These can receive from multiple angles and broadcast to multiple angles, while still having all the benefits of having a focusing dish. There's also almost no time delay in steering the beam, as there are no mechanical parts to move. All you have to do is change relative phases. You can also build arrangements that can cover significantly more angles. In fact, a spherical array can basically be omnidirectional. I have no idea if anything quite like this has ever been installed on satellite, but flat arrays most certainly have, and they will likely completely displace dishes in satellites, as their only disadvantage is the cost.

[wumpus]

10 hours ago, K^2 said:

There are a lot of options, actually. Something like geosynchronous telecom satellite will have both the source and destination of the broadcast in the same general direction.

This sounds like something that would be dropped early in development.  For geostationary satellites, you want your broadcast antenna to reach a continent or more, so the focus of the dish would almost certainly reflect enough signal to saturate the incoming antenna.  For geosynchronous operation, the same is often true, especially when you want to send and receive from the same continent (acting as a deep-space relay wouldn't use the same dish) and would have all the same issues of saturating the incoming antenna.

Sure, you can do things to subtract the outgoing signal, but said outgoing signal is going to be quite a few decibels (orders of magnitude) higher than the input and would likely be the limiting factor of the design.  Exception: if for some reason you could use a much broader (or simply different) frequency or amplitude range on the uplink than the downlink (presumably for regulatory reasons), you might get away with it.

[mikegarrison]

I think we seem to be getting away from the original question. It did not seem to be in the context of a telecom sat, but rather a relay to a rover that is far from the Earth. So you would definitely have two different points to aim at -- the rover and the Earth. Probably you have some kind of antenna to point toward the Earth, and likely another antenna that you use to contact the rover.

The question was whether they would use two antennas or whether they would use the same antenna but alternately point it between the rover and the Earth.

I am fairly confident that they would use two antennas rather than trying to re-acquire both the rover and the Earth over and over.

The ill-fated Mars Climate Orbiter was designed to talk to the Earth and also to be a relay for a lander. Here's what Wikipedia says about it:

Quote

The spacecraft included a 1.3-metre (4-foot-3-inch) high-gain antenna to transceive data with the Deep Space Network over the x-band. The radio transponder designed for the Cassini–Huygens mission was used as a cost-saving measure. It also included a two-way UHF radio frequency system to relay communications with Mars Polar Lander upon an expected landing on December 3, 1999.

So we see that in this case, there was one antenna designed to talk to the lander, and a separate antenna designed to talk with the Earth.

[kerbiloid]

A Venusian orbiter may have just one antenna and use it twice, storing in memory the received data.

A funny fact: both orbiter period and lander lifespan were about 1 hour long. Brevity is the soul of wit, especially when the craft is so short-term.

Edited May 25, 2020 by kerbiloid

[K^2]

15 hours ago, wumpus said:

Sure, you can do things to subtract the outgoing signal, but said outgoing signal is going to be quite a few decibels (orders of magnitude) higher than the input and would likely be the limiting factor of the design.  Exception: if for some reason you could use a much broader (or simply different) frequency or amplitude range on the uplink than the downlink (presumably for regulatory reasons), you might get away with it.

You wouldn't use the same, or even similar frequency for uplink and downlink even if you had separate antennae. You never do that when relaying signals. I'm not aware of a single telecom sat design that uses separate dishes for uplink and downlink. There are satellites with multiple dishes up there, but all of the ones I've looked at simply use it to increase the coverage area. My sample size was quite small, though, so it's entirely possible there are designs that split them up, but it's certainly not a requirement.

[Nuke]

seems for the sake of weight savings you would want to use one main dish for everything. but if you got a rover/relay pair it makes sense to have a long range high gain antennea and then a short range omnidirectional antennea for the downlink. you can get full duplex communication on a dish, but the fact that its directional means that if its pointing at earth, it cant talk to the rover. now it can do a thing where it points the main dish at the rover while it doesnt have line fo sight with earth, uplinks a lot of data, and then transmit from storage when line of sight is restored. usually this also means more bandwidth between rover and relay than what you would have with the omni. this also comes with the added advantage that you can keep comms open with the rover durring this, so it can recieve and relay commands from mission control to the rover while moving the data.

 

[natsirt721]

I can't speak as to the nature of the 'conference call', but in the case of e.g. a spacecraft around Mars relaying for a lander, you would probably have two different antennas on two different bands. The long-range antenna will likely use a high frequency with a moderate-to-large diameter in order to get a high enough transmission power to communicate with DSN. When using this antenna, the vehicle will have to orient the dish towards the earth, which heavily constrains the attitude of the vehicle.

For the rover antenna, you would use a much less powerful antenna with a more symmetric radiation pattern, allowing the orbiter to communicate with the lander regardless of the vehicle's attitude. I would think that uplink data would be timed to arrive at a time when the orbiter and lander have line-of-sight, but it wouldn't surprise me if the orbiter has on-board memory capable of storing a few commands to relay later. I would almost certainly expect that the orbiter would have memory designated for stashing telemetry data from the lander.

Interestingly enough, Curiosity has a high-gain antenna that it uses for direct-to/from-earth communications, but also has a low-gain antenna for relaying data via Odyssey or MRO. The link below is worth a read, and talks about curiosity, MRO, Odyssey, and the various communication technologies they use.

https://sandilands.info/sgordon/communications-with-mars-curiosity

Edit: from the above source, a direct answer to your question:

Quote

MRO is the primary relay node for Curiosity to communicate to Earth (Orbiter is the backup relay). Communications is using a store-and-forward mode. For data from Mars back to Earth, Curiosity uses the UHF band to transmit to MRO while within range. Curiosity has about a 6 to 9 minute window for communications about 2 times per day. The data received by MRO is stored in the on-board memory. MRO has allocated 5 Gb per day for storage for all landers. When MRO can establish an X-band link, the data is relayed back to Earth. A similar approach is used for the forward direction, Earth to Curiosity, i.e. sending command and control messages. For this, MRO has allocated 30 Mb per day for storage.

 

Edited June 2, 2020 by natsirt721