Satellite Solutions

Technology is always in the news these days, but there's one spectacular device out there you've probably never heard of.

ViaSat-1, launched in late 2011, provides more capacity in one satellite than all other communications satellites over North America combined. It will help 1,000,000 technologically deprived web surfers on the ground, in the air, and on the high seas join the 21st Century.

If you rummage around online forums about broadband services you'll find grumps who complain that satellite services can be interrupted by something as common as rain. That used to be true, but engineers have found a solution so the ViaSat Exede service will do just that— exceed your expectations. Now it's just silly to worry about weather effects except under the most extreme circumstances.

Jim Petranovich, MIT grad and Chief Geek at ViaSat subsidiary Enerdyne, wrote a technical  whitepaper that discusses the ways ViaSat reduces and prevents the affects of weather for users of the company's very special Ka-band (high frequency) satellites. I’ll translate the paper for you, but first a little background.

Bit buckets and fire hoses

ViaSat's high-capacity satellite uses spot beams to concentrate energy into smaller areas than conventional satellites. ViaSat-1 has 72 spot beams supported by 20 gateways. Each spot beam supplies nearly as much bandwidth as one entire old satellite. That means they can send more data, at a faster rate, to smaller user antennae.

Four spot beams on the satellite are usually supported by one ground station called a gateway. The gateways, connected to the Internet, beam data up to ViaSat-1. The satellite acts like a U-shaped pipe that collects a stream of data coming up and shoots it back down to you, wherever your bit bucket (antenna) is located to catch the data. In fact, ViaSat-1 is referred to, technically, as a bent-pipe satellite.

As an aside, the amount of energy an antenna can collect is based on size or aperture. The bigger the antenna—the bigger the bucket—the more energy they collect. So for very strong sources you don't need much aperture. A fire hose will fill a milk bucket, real quick like.

Telescopes work the same way--in a sense they're antennae too, photon buckets, receiving light energy instead of radio energy. That means I can use my little 3.5" telescope to look at the literally blinding Sun, and even need to add a special solar filter, like super-strong sunglasses. My 8" telescope collects over five times more light, so I can use that to observe dim deep-sky objects.

What's all that mean to you? ViaSat-1 has powerful transmitters, brighter or "hotter" than old satellites'. So your antenna only needs to be two and half feet in diameter--a Very Small Aperture Terminal or VSAT. In fact, ViaSat has a battery powered portable terminal for news-gathering organizations that packs into a suitcase that weighs under 50 pounds, antenna and all.

And, now, back to the weather. ViaSat can't make it stop raining, of course, but they have done something about how weather affects broadband satellite services.

Heavy rain,  I mean heavy rain such as in Hurricane Sandy, or a blizzard of wet snow (such as in Hurricane Sandy), can be an issue—light rain and dry snow not so much. Gases in the atmosphere, clouds, and weird changes in the ionosphere, called scintillation, can reduce the strength of a satellite signal too, but those effects are small.

Singing in the rain 

So how does ViaSat make sure even the worst weather doesn't affect your Facebook or Twitter experience?

To begin with, ViaSat-1 is "hotter" than older satellites, as we've said. With stronger down link signals, the reduction in strength caused by weather is less noticeable. Think of weather as blue food coloring and the satellite beam as a stream of water. If you add the coloring to a dribble from a faucet the water will be very blue. Put the same amount in our fire hose, and you won't even notice. Strong satellite signals simply overwhelm small weather effects.

Rain (virga) near Sedona AZ                                    © T Harnish
This whole issue of the effect of rain on radio signals (satellite or terrestrial) has been studied to death. Researchers can tell you how raindrop size, drop oscillation, the angle the rain comes down, and temperature, impurities and host of other nitty-gritty details can affect reception.

But, to be clear, you'll seldom, if ever, notice weather issues using ViaSat's Exede service.  One reason is that really heavy rain, strong enough to be a problem, is a rare event. Another reason weather is seldom a problem is that fancy network protocols (software sleuths) are used to detect and retransmit missing data.

In the case of streaming video, Internet applications expect micro-interruptions anyway, so the computer programs create a buffer that can be emptied while data is resent. They work like your water heater where a tank (buffer) of hot water is created so you can drain it off when needed without waiting for it to get hot. That's why you often have to wait for a YouTube or Netflix video to start, they're filling a buffer.

Okay, you say, but what if the weather is really awful, hurricane awful, and they're looking at the real possibility of signal fade? ViaSat has gotcha covered there too, with some really high tech solutions such as adaptive power control and adaptive coding and modulation.

Adaptive power control generally is only necessary when data is being sent by you to the satellite. Your little dish antenna and small amplifier have to yell most of the time for the satellite far away to hear you. If weather interferes with the satellite, and it's having trouble hearing you, it tells your system to take a deep breath and scream twice as loud for a while to make sure your email to finicky Aunt Martha isn't garbled.

Modern magic

Adaptive modulation, on the other hand, is one of those technologies that, when it's shown in a system diagram, deserves an arrow with a caption that says, "Here there be magic." One of the incantations the magicians use goes something like this: "Euler, Euler, phase-shift, quadraphase; sine wave, sine wave, lowpass, carrier. . .and . . . demodulate." Presto, you have double the bandwidth. But that's just a small part of the show that all happens at the speed of light out in space.

Adaptive coding, including forward error detection, means weather threats just evaporate. Forward error detection sounds like they're not just magicians but psychic too, correcting problems before they happen. But really they're just adding redundancy so your data doesn't have to be retransmitted. It's kinda sad all that magic is going on up there and we can't see it.

Like all magic, there's slight of hand at work behind the scenes at ViaSat, and it's fully understood by those in the know. All you need to know is that fancy software can adapt to changing conditions to adjust your instantaneous data rate and the amount of time the satellite allocates to your messages. You're not even aware anything changed.

Weather can potentially affect gateways, too, of course; and that can be a big problem because everyone supported by the gateway would be affected. But gateways use adaptive power control and adaptive modulation and coding, too. In some cases multiple gateways serve the same spot beams and end users. If weather threatens one gateway, another can be pick up the load without any interruption—more magic developed by ViaSat.


Another nifty trick that ViaSat uses is to put their ground stations outside the coverage area of the spot beams. So while it may be pouring where you are, say in the Gulf Coast or Florida, your gateway might be basking under clear skies in Albuquerque or Tucson.

The moral of the story

Even if it is raining on the gateway, by the way, signal strength is only loosely related to rain intensity. The signal path is generally on a diagonal up from the ground to the satellite located over the West Coast. In any event, the weather at your location has more effect on performance than at the gateway because they have big honking antennae and you just have the little dinky dish.

So you may have filed away in your brain that satellite broadband really isn't a good option because of weather problems. That's not the case anymore. That thought is just brain sludge, taking up space in your noggin along with the 1962 hit tune, Telstar performed by The Tornadoes.

2 comments:

Satellite Communications said...

This is really good news from ViaSat. I heard they're really expanding beyond non-government satellite services too.

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