Small Satellites, Big Dishes


#1

There has been a lot of talk about using micro satellites (CubeSats) for Outernet. But, tiny satellites have very limited power output, requiring much larger antennas, especially as bandwidth increases.

“The first step in designing a satellite network is performance
of a satellite link budget analysis. The link budget will determine what
size antennae to use, SSPA or TWTA PA power requirements, link availability
and bit error rate, and in general, the overall customer satisfaction with
your work.”

http://technologyinterface.nmsu.edu/spring97/communications/nsn/linkbudget.html

Also, telecom satellites require licenses. So, the idea that you can launch a satellite and broadcast whatever you want is rather detached from reality.

In general, commercial satellite operators will broadcast any content that is sent to them, without review. Again, in very general terms, most satellite data networks are considered private and not held to the same standards as terrestrial broadcast TV or radio.


#2
tiny satellites have very limited power output, requiring much larger antennas, especially as bandwidth increases.
You mean, much *higher* *gain* antennas. The size/power output of the transmitter does not determine the size of a receiving antenna - *frequency* determines size of an antenna.

The real logistical problem of using CubeSats in the UHF range is, like you said, limited power output and the altitude of most CubeSat orbits. They traverse the sky much faster than satellites in a higher orbit (like Iridium and GPS). Such a fast track would require high gain antennas (yagi’s and their derivatives) and rotators that actively track the satellite. It’s unlikely anyone besides astronomy/amateur radio satellite enthusiasts have such equipment at their disposal.


#3

I’m not arguing about the statement you presented, as link budget is definitely king, but there are some assumptions that are wrong.

Iridium is a LEO constellation. It’s not much higher than the 500km-600km sweet cubesats and microsatellites with a medium-term mission duration (greater than a couple years). I believe Iridium is at an 800km orbit. Though there is a difference in speed, it’s not monstrous.

Power is a factor, but 2W of RF can be received by a 6-inch whip on the ground. Additionally, a 6-inch UHF antenna can also be received by a microsatellite at a 600km orbit.

Data rate is a key variable. We’re talking about kbps, not Mbps. But just because the speeds are slow does not mean that the amount of information is insignificant. Over the course of a day, 2400 bits per second amounts to about 20 MB of information. That’s almost 3-million words a day. That’s the equivalent of 40 novels. A day.


#4

With a cubesat in the UHF band don’t expect to see any kbps data rate.
You are looking at rates around 1200 Baud and only 6-12 minutes of access to the satellite as it passes overhead.

Stick with geo sats like those you’ve been using. I love cubesats but they are not cut out for this unless you can attach them to a new geosat launch they just don’t have a good orbit for this sort of thing. Not to mention poor speeds.


#5

We’ll be seeing higher rates in the cubesat market though. Strand-1 was d/l at 9k6 and PlanetLabs are X-Band so maybe a piggy-back on PL cubesats? Polar orbits rather than geostationary but, as Syed highlights, lots of little bits of data adds up to a lot of usable data. INASP have been trying to focus delivery of content to developing countries universities by utilising information flow through their libraries and I would see that Outernet’s data delivery would suit this perfectly - data does not need to be live, just more current than their current book stock (often many years out of date as they are usually donated by western institutions that are clearing their older content).


#6

Actually, 9k6 is pretty normal in UHF. But you are correct, the cost and speeds from Ku-band just can’t be beat. The downside, of course, is that huge dish–and having to point it!


#7

Just come across another of your videos on youtube https://www.youtube.com/watch?v=oA76VRJO4uI that had a nifty looking portable satellite dish. Do you have further details about that too?


#8

Due to how expensive a portable prime-focus dish would be for the end user, we have paused the development of this product. We want to keep our focus on making products that are accessible to the greatest number of people.

The traditional satcom industry has tons of expensive toys for rich people. That’s not the market we want to serve.