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Deep-Space Data Transmission = Free Storage!

September 27, 2007

Delays. Delays. Delays. Not all bad, but some better than others.

Ordinarily, in the data world, a delay is a loathsome thing. It makes your conversations on the cell phone sound strange and impersonal, it turns your favourite video first-person shoot-em-up video game skills look like a n00b, and it’s why the people across the street from you with Cable TV are jumping with joy a few seconds before you with your Satellite TV when the Blue Jays win another World Series.

What about extra-terrestrial data communications?

Canadian SAR Satellite

The satellite used for Bell ExpressVu or other TV providers is a very small distance away from earth, but even so, this adds a small delay to the digital signal you receive. A satellite in Geostationary orbit must be at an altitude of 35,786 km (22,240 statute miles).

Light travels 1,079,252,848.8 km/h, or, 17 987 547.5 km/second.

Therefore, the delay is 0.00198948745 seconds, or, about 4 milliseconds 2-way, plus processing delay. This might seem small , but keep in mind that a delay of 200 milliseconds renders 2-way conversation unusable in full duplex (ie. it’s time to talk in half-duplex like walkie-talkies). Human perception can recognise 50 milliseconds of delay.

So back to my claim of free storage. What happens to that data for the 2 seconds it is en-route from the earth to the sattelite? It’s neither at the transmitter, nor is it at the receiver. This data is in transit as an electromagnetic wave. It’s intact, coherent, but isn’t anywhere to be found!

Exactly how much data is in the “ether” between the earth and a satellite at any given point in time? Lets build some numbers to work with:

Speed of electromagnetic propogation: 17 987 547.5 km/second
Frequency of Ku Band transmission: ~ 12.7GHz (12 700 000 000 oscilations per second).

One oscillation of a frequency is equivalent to 1 bit. For our purposes, I’m going to ignore things like headers, preamble, checksums, and the “good morning vietnam” that surrounds, encompases, and trails deep-space transmission of data.

Dividing the time required to reach the Satellite by the frequency of the transmission, we can see that there are 25,266,490 bits suspended in the ether between the satellite and the ground station. Equating that to a file, we can safely say that we have”stored” a 3MB file without the use of a wire, a hard drive, a flash memory drive, or any physical device.

This is all well and good, but our “storage” seems to expire pretty quickly. In 2 ms, our data is removed from this storage medium until the next time it is spit out there. Useless, right? Not quite. What if we could position 2 satellites to constantly rebound this data off of eachother, and when we need the data, we issue a command to the satellite to copy the data over to us the next time it comes around?

—->
<—-
—->
<—-

The further we move the satellites apart from each other, the more data we can fit into this constant cyclical stream, thanks to delay.

So how far apart do we have to move the satellites in order to store, say, a DVD movie, or the entire user database of Facebook? Here’s a few examples with a relay that begins at earth. I’m going to only include the data going in one direction.

Earth to … Distance Delay (seconds) Data (Mbits) Equivalent Amount of Data
Satellite in GeoStationary Orbit 35,786 km 0.00198948745 25.2 MP3 song
Moon 384,400km 0.0213703397 256 Quicktime Program Installer
Mars 228,000,000 km 12.6754356 152105 5 DVD Movies
Voyager 1 15,500,000,000 km 861.707245 1.03404869 × 1013 Entire Wikipedia Database

So I guess that the entire contents of all the hard drives of the world (the Internet) can be transmitted before it arrives at our nearest star, which is 40,000,000,000,000 km’s away (give or take a few).

🙂

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