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Shooting at satellites
07.08.2009 11:31 AM
Tom Roberts

By Tom Roberts
Optical Communications

It's 1:56 AM. We've been going since 8:00 AM - I guess technically that's yesterday now. In just under an hour, the Japanese OICETS optical communications satellite will rise above the horizon, and our 1-meter optical communications research telescope (OCTL) will slew across the sky to point at the satellite. It will then track the satellite as it rises and passes overhead. Once the satellite rises above the local tree line (about 20 degrees in elevation) we'll turn on our laser beacon, which consists of three infrared diode lasers 'ganged' together. The satellite's smaller telescope terminal will be looking for this beacon, and once it 'sees' us, will be able to send its narrow communications beam back to our telescope. We will then reciprocate by sending them our communications beam, in which we'll be transmitting data at about 50 million bits per second (MBPS). At least, that's what's supposed to happen.

We've been working on deep space optical communications technology for the better part of 3 decades now. The whole point of this is to open up a new high-bandwidth capability to support future deep-space exploration. Until recently, almost all space data has been sent over radio waves (Radio Frequency or RF communications). While the RF channels have some real advantages (like very low background noise and system maturity), ultimately it's quite limited in just how much data you can send over long distances. In a simplified analysis, the one-way transmission efficiency is inversely proportional to the square of the wavelength of the signal, all other things being equal, so that long wavelength (~1-10 cm) RF signals spread light over a relatively large angle on the sky. That means that the receiver at the other end can only capture a very, very small fraction of the light, especially when communicating over interplanetary distances (like hundreds of millions of kilometers). By contrast, optical wavelengths (actually they're usually near infrared) are on the order of 1 micrometer, or around 10,000-100,000 times shorter, so that they can be roughly 100 million to 10 billion times more efficient at concentrating the signal far away, assuming the same transmitter size. In reality, we would use some of this huge optical advantage to scale down the size of the transmitting and receiving apertures, but still have plenty of additional bandwidth left to make a radical increase in data-transfer rates. Just imagine that we're trying to convert NASA's current 1200 baud dial-up modem to Wi-Max.

So, the pass is completed. We had some pretty significant success. As soon as the satellite cleared the trees we transmitted our beacon, and within seconds received an extremely bright return signal. We tracked the spacecraft as it rose in the sky, and could see the data-stream they were transmitting. We'll have to wait until tomorrow to see if they got our communications signal, but right now, the experiment looks like a big success. This is a big step in demonstrating that we are ready to implement this technology. And it will help to happily ease me into some much-needed sleep.


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