Speeding up interplanetary communications

[CalliArcale]

One of the biggest problems facing deep space probes today is getting the signals back to Earth. With the inherent inefficiencies in communications over such distances, they cannot transmit live images; it takes far longer to transmit the image than it takes the probe to capture the image in the first place.<br /><br /><br /><br />That may be about to change. A team from MIT has developed a light detector that can function as a receiver. It is much more efficient than previous attempts to do this.<br /><br /><br /><br />Detector may speed up interplanetary communications<br /> <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[scottb50]

Fiber-optics without the fibers. As long as the transmitters and receivers are in Space it would definitely increase capabilities. I assume you mean increasing the capabilities not the speed, though the speed of light limitation has been exceeded the usefulness is in question.<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[mikejz]

I find the article somewhat interesting. However, radio communications are only limited by power and antenna size--there have been some efforts to do inflatable parabolic dishes that would greatly increase data rates.<br /><br />Also, the data rates talked about are in the 100mbps-1000mbps. 1000mbps is equivalent to something like 50 HDTV channels. I'm not aware of any mission profile that would generate that sort of data for a prolonged period of time.

 

[mlorrey]

A big dish doesn't do anything for you if you don't have the power for it. Big dishes are primarily for improving reception, which probes don't need high bandwidth for unless they are acting as a relay for another probe. The bandwidth is needed to download large amounts of data to Earth.<br /><br />Optical communications allows you to focus your signal in a narrower beam, thus upping the signal strength within the beam locus since the same amount of power fills a smaller volume of space. With higher signal strength, higher bandwidth is attainable.

 

[mikejz]

No larger dishs on the transmit side increase the focus of the beam 'gain' this allows it focus the power towards a smaller target.

 

[mlorrey]

No, the focus of the beam gain is determined by the shape of the dish, not its size. How much of the parabolic curve the dish covers. 100 watts through a 1 meter dish shaped like a small percept of a parabolic curve produces the same signal strength as 100 watts through a 10 meter dish of the same shape scaled up.<br /><br />It is the ratio of dish diameter to the focal length that matters.

 

[mikejz]

http://sharif.ac.ir/~barkeshli/antennas/review/9510_007.htm

 

[mlorrey]

The page you linked to talked about earth based dishes needing to be larger to receive signals from satellites. It has absolutely NOTHING to do with satellite transmission antennae requirements.<br /><br />Also, if you actually understood what you were referencing, you'd see that for a shorter wavelength signal, the gain goes up without changing the dish size.<br /><br />Radio and microwave band signals are measured in meters down to micrometers (which is where MICRO-wave comes from). Visible light wavelengths are in NANOMETERS. This means a visible light laser transmitter can be much smaller AND the receiver can be much smaller for a given bandwidth (or more bandwidth for less shrinkage). <br /><br />As an example, a 1 meter wavelength signal with a 1 meter dish has the SAME EXACT gain as a 1 micrometer signal with a 1 micrometer antenna, AND the same exact gain as a 100 nanometer signal with a 100 nanometer sized receiving sensor. If you went with a 100 nanometer signal with a 500 nanometer sensor, you'd be able to receive 25 times the bandwidth.

 

[webtaz99]

"Micro"-waves transition to IR in the millimeter range.<br />The wavelength of "microwave ovens" is 2.4xxx <b>centimeters</b>. "Microwaves" seemed micro to the early radio guys.<br /> <div class="Discussion_UserSignature"> </div>

 

[mlorrey]

True, but that is picking of nits. My original point stands. Here is a graphic of the EM spectrum. <br />http://praxis.pha.jhu.edu/pictures/emspec.gif<br />You can see the change in wavelength/frequency and how microwaves and visible light are in much shorter wavelength bands, ergo the equations in the papers previously cited mean that for the same bandwidth, you need a proportionately smaller receiver antenna if your wavelength is shorter.<br /><br />What this means is that a 1 mm diameter photosensor receiving 100 nm light signals at amplitude x has the exact same bandwidth capacity as a 10 meter dish receiving a 1 mm microwave signal at amplitude x, and 1000 times the bandwidth as a 10 meter dish receiving 1 meter radio signal at amplitude x.<br /><br />

 

[nexium]

To change the subject slightly: If we have 10,000 tiny habitats in the solar system (some in the Oort cloud) each will send and receive a low data rate message daily with central control, possibly near Earth. They will also exchange data, daily, with the two or three closest habitats. In all cases more power will be used than necessary, so communications won't be lost, if the back ground noise level increases. Because very narrow beams are used, the precise location needs to be known, otherwise the beam will miss the receiving antenna/telescope. If energy is in short supply, marginal power levels will be used for typing on www.space.com etc Neil

 

[mlorrey]

Precise location (which is difficult) is why you won't directly communicate with a central control. Instead, people will have low power transponder beacons and those closest to them will maintain p2p comm beams with them, forming a solar system wide packet network.<br /><br />Packets will travel through a lot of nodes before they reach Earth Central Traffic Control. If each ship/habitat has three comm lasers, then a 10,000 node comm network will be at most 14 nodes deep from core to edge.<br /><br />Laser comm links won't suffer from significant noise issues, because coherent light signals are rather easy to distinguish from natural stellar and nebular emissions.