back to article Boffins' satcomms rig uses earthly LEDs to talk to orbiting PV panels

As low-cost satellites become more common, researchers are turning their attention to improving their communications capabilities without adding crushing costs. A laser might, as NASA demonstrated earlier this year, be able to hit a gigabit per second – but the kit's expensive. So a pair of researchers from Florida-based …

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Pint

Very nice! I've taken to paying attention to various NASA channels. It's surprising that many of the various projects live right now don't use all that much bandwidth. True, if you want all the data immediately you need a stupendous link. Most things don't though. Anyway, worth a pint or three.

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FAIL

Signal to noise ?

The PV panels on a satellite accept a wide range of light frequencies and have a wide acceptance angle so as to get power even if the panel is not perfectly aligned with the sun. This implies that along with the desired signal from the LEDs on the ground there will be a huge amount of background noise from all the artificial lights on Earth (in nighttime) or from the reflection of the Sun's light (in daytime). The resulting signal to noise ratio will be horrible. (Look at pictures of nighttime Earth from space to see the amount of light emitted.) A lens and a narrow band optical filter could improve the S/N ratio by 40dB or more compared to the PV panels.

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Re: Signal to noise ?

Yeah, but you'd have to point your lens in the right direction, which is hard.

Picking the AC signal out of the mush is 'just' analogue and processing, no movement. (and blocking the DC from the sun is trivially easy).

I'm impressed that they got this much bandwidth, though. Good stuff.

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Boffin

Re: Signal to noise ?

Stick an aerial on an oscilloscope and see the noise present.

Pass the signal through a narrow bandpass filter and you'll see an actual signal.

Do the same thing but with the signal coming from the PV panels.

If you're looking at a particular modulation frequency, you can pick out the right signal even if there's a lot of noise elsewhere.

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Boffin

Re: Signal to noise ?

Ham radio operators have been operating digital transmissions at (sometimes below) the noise floor for years albeit mostly at slower speeds. Various modulation and encoding schemes allow differing combinations of SNR requirements and data rates. What these guys are doing sounds like some variation and improvement on MFSK-16, DominoEX, Olivia or similar. Those date from the early 2000's and there are likely newer modes with even more capability, I just haven't kept up.

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Anonymous Coward

Re: Signal to noise ?

As I understand it satellites normally attempt to orient their solar cells towards the sun so they can generate electricity. Presumably this means that communication using this method will be limited to when the satellite is in the Earths shadow?

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Pint

Re: Signal to noise ?

Similarly, GNSS (e.g. GPS) have been operating digital transmissions (always) below the noise floor for decades, albeit (always) at slow date rates.

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Joke

missed headline opportunity

PV Panels in SPAAAAAAAACE!

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Anonymous Coward

DoS

How do they address the potential for a DoS attack by some twerp with a green laser pointer?

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Boffin

Re: DoS

The same way they avoid the potential for a DoS by the sun. Signal processing.

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A truly cunning plan my lord

Lateral thinking at its finest. Only one thought - surely the PV panels on a satellite will be aligned to point towards a bright light source, e.g. the sun, rather than pointing at the ground and the transmitter?

+1 ZU DAS SUB FUR DAS BLINKENLIGHTS

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Boffin

Re: A truly cunning plan my lord

Try not to run afoul of the Blue-Green Laser Mafia ...

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Re: A truly cunning plan my lord

Things like cube sats don't bother with the weight and complexity of steerable panels - they just wrap panels on every surface.

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Pint

Thought I was reading WIRED ... again

But saw Ackerman on the telly tonight, so can't be the really good stuff ...

"Aphelion Orbitals design uses a simple QAM-16 modulation scheme to make it easier to correct atmospheric distortion in the signal."

As for AO, that one sentence explains all of their FSO expertise.

Been told drinking causes brain damage, or did I remember that wrong ...

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Did I read that right?

Are they getting more bandwidth to a cubesat using LEDs than at least 5% of the UK get over wires?

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Re: Did I read that right?

A slightly unfair comparison. To be fair you would need a system with thousands of satellites. The other down side of using light is how will your weather satellite tell you if it is cloudy?

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Trollface

Re: Did I read that right?

The other down side of using light is how will your weather satellite tell you if it is cloudy?

In much the same way as your computer tells you it has no power?

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Anonymous Coward

Heatsink

Looks like a big heatsink on the top of the unit in the picture. I'm guessing that's only for ground testing, or will an aluminum heatsink radiate an appreciable amount of energy in space?

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Re: Heatsink

It appears that the pictured device is the uplink.

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Re: Heatsink

That, and electronics-produced heat is rarely an issue in orbit.

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vir
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Re: Heatsink

Looks like a big heatsink on the top and a bigger heatsink on the bottom, with the copper heat pipes leading away from the high-power LED into the bottom heatsink. My guess is that the bottom heatsink at least is for ground testing as the primary method of rejecting heat in a vacuum is radiation, and having a lot of parallel surfaces very close to each other is less than optimal.

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Terminator

Having read the paper...

The LEDs are only used for the downlink. (A 140W laser is used for the uplink.) They transmit using 1000 lumens worth of LEDs with a peak power of 140W and use lenses to focus the beam. It is reckoned it will emit a 19.9 dBW signal -- that's visual magnitude of 1, so naked eye visible under ideal conditions.

Those fins are a conventional aluminium heat sink. But the LEDs only transmit for 2min/orbit and then they let the heat dissipate. Radiation benefits from surface area every bit as much as convection, so perhaps.

The signal is received using a commercial 30cm telescope. Accounting for losses that's a -107.3 dBW signal giving a speed of 8.85E4 bit/s or 700 photons per bit. But they plan to use "deep learning" on both ends to develop a modulation scheme -- they haven't done so,.

They don't say much about the uplink to solar panels, referring to other work. But it uses a laser mounted on the receiving telescope.

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Pint

The following quoted sentence seems to be nonsense...

"...reduce the accuracy you need to aim a signal at the satellite, by using a large photon-collection surface that's already common in space, the satellites' photovoltaic (PV) panels, as the receiver."

You'd only believe this claim if you image that a laser beam aimed from Earth to a satellite arriving at the satellite as a tiny red dot; the system struggling to maintain precise aim on the tiny phototransistor on the satellite.

If you understand real world dispersion of laser beams (small, but vastly non-zero), then you'll understand that the mythical phototransistor is *effectively* the same size as the PV array. Because the beam width of the laser beam is hugely vast in comparison, over such distances.

Especially on a tiny cubesat. Less so on the ISS, where this technique is not applicable anyway.

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CALYPSO acronym

Cubesat's Additional Light Yields Pleasing Synchronicity - Oy!

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