Is a Laser Data Link 1.5 Million Kilometers Feasible?

An anonymous reader writes "On the Canary Islands last week, a team from Oerlikon Space demonstrated the feasibility of a laser link across a distance of 1.5 million kilometers for the first time ever. In the future, laser links like this one will be able to transmit data across huge distances through the universe far more rapidly and efficiently than is possible using conventional radio links today."

Re:Never saw this coming

[LiquidCoooled]

Correction:

Voyager is 15 billion kilometres not miles as stated (about 9 billion miles)

http://voyager.jpl.nasa.gov/mission/weekly-reports/index.htm [nasa.gov]

Re:Question about lasers

[rcw-home]

Do lasers follow the inverse square law? I'm guessing it doesn't since it's focused.

Yes they do, since that focus is never perfect. A cheapie laser pointer will show a 1/8" dot at 30 feet and a 1/4" smudge at 60 feet.

Huge distances through the universe?

[JohnnyDanger]

For a sense of scale:

1.5 million kilometers = 1.6 x 10^-7 light year.

Distance to galactic center = 26,000 light years
Distance to nearest (Andromeda) galaxy = 2.5 million light years

"Faster than radio" probably refers to increased bandwidth, because light-speed is light-speed.

why didn't they use the lunar retro-reflector?

[jfb2252]

It seems strange that they didn't aim for the retro-reflector placed by one of the Apollo missions which has been used for 30+ years for laser ranging experiments. It's location is well known. That would give them a real 800,000 km beam path, roughly half of what they claimed.

Re:Aiming will be a major problem

[onion2k]

When the Apollo mission landed on the Moon they left behind a retroreflector that NASA used (still use?) to bounce a laser back and forth to measure the distance from the Earth very accurately. That's 385,000 km. If they were doing that in the late 1960s I don't see any reason why 1.5m km should be that tricky today.

Re:Question about lasers

[pedestrian crossing]

Do lasers follow the inverse square law?

No.

I'm guessing it doesn't since it's focused.

Close. It is because it is collimated. [wikipedia.org].

Re:Question about lasers

[hansraj]

I could be wrong so someone knowing better please correct me.

The inverse square law is applicable only for point sources that are radiating in every direction. The inverse square of distance d arises in the formula that you are interested in the surface of a ball centered at the source with radius d. The surface area is proportional to the square of distance so intensity in some part of the surface relates to the inverse.

Now lasers are not omnidirectional so the inverse square law is not applicable.

InterSatellite Communications

[d-Orb]

I remember this being done with Earth Observation satellites. The EO satellite beams data using an optical link to a satellite that is in geostationary orbit. This satellite then beams the information down through a microwave link. This frees the EO satellite (that producue huge amounts of data) of the need of high-power consuming RF transceivers, reduces the need for ground stations, and is seriously cool. This was done in 2001 between SPOT 4 and Artemis (Press release [esa.int]). Note that SPOT sits in an orbit around 800km, and Artemis is geostationary... They then did the same with an aircraft (see here [esa.int]).

So it is really quite useful. When you consider the amount of data the sensors on board ENVISAT (or even MODIS) produce, this is an important tool.

Re:Aiming will be a major problem

[Anonymous Coward]

The retroreflector isn't easy to hit, and they actually get back only one photon every few seconds [nasa.gov]. This would not yield much bandwidth for communications.

Re:Faster?

[frith01]

Faster Baud rate, not faster event rate. higher frequency signals can carry more information.

Re:Question about lasers

[Anonymous Coward]

I've seen some comments to this post saying that a laser beam dosen't obey the inverse square law and some saying that it does. Actually, everyone is right in a sense. Over "short" distances, laser beams expand at a rate that is slower than inverse square. Over "large" distances, the rate of expansion increases, eventually approaching the inverse square law. The distance that distinguishes "small" from "large" is called the Rayleigh range and it depends on two properties of the beam: the wavelength of the light, and the curvature of the beam's phase fronts at the reference point from which you measure the expansion.

Concrete example:
A 1 micron infrared laser has a 3mm diameter spot with flat phase fronts. The Rayleigh range is 28m
Distance: 1m, 2m, 4m, 8m, 16m, 32m, 64m, 128m
Spot Size: 3.002mm, 3.008mm, 3.030mm, 3.118mm, 3.447mm, 4.531mm, 7.425mm, 13.91mm

The same amount of power is contained within the spot, so the ratio of the intensity (power/area) goes as the inverse square of the ratio of the spot size.
Between 1m and 2m, there is essentially no change in intensity (collimated beam) but between 64m and 128m, the intensity reduces by (1/4)

-Anonymous Physicist

Re:Question about lasers

[Andy Dodd]

Wrong, they do follow the inverse square law.

See the article you link to, which states that perfect collimation can never be achieved in reality. Thus, like any other source, laser light follows the inverse square law in the far field.

Note that in general, I believe the inverse square law only applies to a point source, or a source which is effectively a point source at the distances involved. For dealing with cases where the source can't be approximated as a point (either because it's really large, or the radiation intensity is being measured very close to the source), RF engineers use the term "near field gain reduction" for the behavior of RF field intensities in close proximity to an antenna, which probably has an equivalent term for optics. As a result, for an optical source with a large aperture in relatively close physical proximity to the aperture, the inverse square law will appear not to apply, but once the "near field" for that source is exited, the inverse square law holds.

Re:Lagrange points

[einhverfr]

The basic problem is that the LaGrange points 4-5 are stable, but require a fair bit of energy to get to in part because you have to slow down a lot more (no nice large gravity well to assume an orbit around).

In general the amount of time to get there/back would be dependant on how much energy you want to put into getting there and back.

Finally we do already have a satellite (SOHO) on the L1 point relative to Earth and the Sun. This is an unstable point so some energy is used maintaining position However it is a telescope on an L point relative to the Earth and Sun.

Re:Never saw this coming

[kd5ujz]

Your "ping" time is a measure of latency, in milliseconds.

Re:Never saw this coming

[awehttam]

http://voyager.jpl.nasa.gov/spacecraft/index.html [nasa.gov]:

"Uplink communications is via S-band (16-bits/sec command rate) while an X-band transmitter provides downlink telemetry at 160 bits/sec normally and 1.4 kbps for playback of high-rate plasma wave data. All data are transmitted from and received at the spacecraft via the 3.7 meter high-gain antenna (HGA)."

Re:Never saw this coming

[fnord_uk]

Here they are, at the home of the Delay Tolerant Networking Research Group:
http://www.dtnrg.org/ [dtnrg.org]

Whilst the Delay Tolerant Network Architecture (a store-and-forward overlay network, see RFC 4838) and Bundling Protocol are mainly there to solve different problems (mostly the potential lack of a known end-to-end path) the Licklider Transmission Protocol (LTP), is designed primarily to run efficiently over single link-hops with considerable round-trip times (typically introduced by light speed constraints) but also deals with bandwidth asymmetry and mixed reliable or best effort delivery.

Deploying BP over LTP on these kind of links seems to be the plan. Then BP can extend from the orbital node down to the ground over TCP or SCPS-TP (http://www.scps.org/) or similar. BP is designed to have a shim-like Convergence Layer (CL) to interface with the whatever stack underpins it on any specific link. The open source reference implementation (Apache 2.0 licence) currently supports TCP and Bluetooth (Linux only). I've implemented a mostly working CL for AX.25 which I hope to try out using ham radio gear soon.

Re:Never saw this coming

[wwphx]

The laser that my wife blasts the moon with [wikipedia.org] on a regular basis starts at 3.5 meters here [wikipedia.org] and I've heard is over 2km when it hits the moon. I have no idea how big it is when it finally bounces back.

Re:Never saw this coming

[wwphx]

My wife says: "One in 30 million outbound photons strike the retroreflector, after they're reflected, one in 30 million of those photons make it back to the telescope and are detected. So about one in a quadrillion of the photons sent out are returned. And that is a record-breaking rate, no other retroreflector laser experiment has come close."

She thinks the return beam diameter is probably more than double due to the lensing nature of the atmosphere, but she has no numbers off the top of her head.

Been there, done that.

[beckettmw]

JPL's been working on it too for a while now... and with similar datarates, and a ground acquisition plan to boot.

http://lasers.jpl.nasa.gov/PAGES/pubs.html#ocd [nasa.gov]

But, yes, a laser link indeed is desirable. Sure, we can still contact Voyager with radio telescopes, but even from the Mars rovers, notice how it takes so long to get from Mars to grainy B&W picture back on Earth?

Sending back live video feeds and more full colour images sets the data rate bar much, much higher. Getting this much data back quickly is limited by the frequency of the radio waves/light. Laser light has an over 1,000 times shorter wavelength than Ka band radio telescopes can manage (that's what NASA uses now to talk to the Mars probes), which increases the potential amount of data that can be sent in a given timeframe by essentially that amount.

In addition, because laser light is focused so narrowly, it wastes much less energy than a radio antenna which must spray a good portion of space with radio waves in order to hit Earth. Imagine focusing your mag-light in the dark... the narrower the focus, the brighter the beam gets, because more energy is packed into less space. The challenge though, is that you have to aim much more precisely at Earth to compensate for that more focused beam.

Here's a great overview of JPL's long-term vision:

http://lasers.jpl.nasa.gov/PAPERS/REVIEW/overview.pdf [nasa.gov]