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ViaSat lofts world's most powerful communications satellite into orbit

6.5 Ton to GEO, that is a big baby.

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Mixture of Chemical and Xenon?

It's been a while since Chemistry was my main thing but I thought Xenon was a chemical too? I presume the Xenon is released either as a compressed gas or as the medium in an ion drive? Moderately pedantic and mildly curious ...

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Re: Mixture of Chemical and Xenon?

Xenon here is a shortening referring to the ion drives use for altitude control.

The sat probably also has a propellant altitude control system, for when you need a faster reaction time.

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Re: Mixture of Chemical and Xenon?

The xenon gas is the medium in the ion drive (ViaSat refer to it as electric propulsion). The long-term purpose is for station-keeping, keeping the satellite exactly above 69.9°E longitude on the equator.

The chemical thrusters will be used to circularise the satellite's orbit. The launch has only put it into a Geosynchronous Transfer Orbit, an elliptical orbit that is near the final altitude at the furthest point (apogee) but still barely above the Earth's atmosphere at the closest (perigee). It typically needs around 1500 metres per second delta-V to circularise - bring the perigee up to the target altitude - and change the inclination to equatorial. The orbit that the booster left it in is inclined by about the same amount as the latitude of the launch site, so for Ariane around 5°.

ViaSat state that they have enough chemical fuel (probably UMDH/N2O4) to circularise, but electric propulsion will be used to change inclination. Source: https://www.viasat.com/news/what-happens-after-viasat-2-launches-step-1-orbit-raising

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"Smaller satellites spend a lot of time over parts of the earth that don't have demand and they can't reallocate that capacity to other areas,"

... these smaller satellites are also much close to the ground, offering much better latency. For a sufficiently large constellation it should be even possible to beat latency of intercontinental fibre connection, which opens up a whole new attractive market sector.

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Anonymous Coward

Redundant already?

The BBC article on the satellite says that it will boost WiFi on planes.

http://www.bbc.co.uk/news/science-environment-40123180

Well if 'The Donald' has his way, we won't be able to take any electronic devices into the cabin. His motley crew of so called experts are already trying to get anything larger than a Phablet banned. I'm sure that it won't be long before someone says that even a phone or an iPod Mini can pack enough Cemtex etc to bring down a plane and as a result all electronics needs to be banned.

Naturally it won't apply to the people proposing the move as they all that their own private jets.

No electronics will mean an awful lot of people won't fly. LEss Aircraft in the air means more air space for their LearJets.... Simple really.

A good few airlines will go to the wall if this move covers the world as the US people want.

Well, that's one way of removing CO2. Less Jet-A burnt means less CO2.

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Re: Redundant already?

As much fun as it is to blame trump for everything, they're considering banning laptops because the intelligence services think some scrote has worked out how to hide a bomb in one that is unlikely to be picked up by normal screening.

Not really the orange idiot's fault, (although I'm sure he'll find a way to screw things up worse anyway).

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To all space comms experts

I've set up a satellite dish for fixed domestic satellite Internet. It's a bit of a pig, to say the least, to get the alignment spot on and the signal can easily be disrupted by stupid things like leaves growing on a nearby tree.

My question then is how does a mobile terminal such as an individual sat-phone or an aircraft moving at several hundred miles per hour acquire and maintain a high bandwidth connection to either a geostationary or LEO satellite?

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Re: To all space comms experts

I don't know how they do it on planes but it seems very common on boats which are bobbing about.

Search for "marine auto tracking satellite dish" to see images of them.

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Pint

Re: To all space comms experts

A good choice is the Luneberg Lens approach. An array of four can mimic the RF performance of an 18-inch dish, but maybe 6-inches tall.

Edit: Here's a random patent link I've found.

https://www.google.com/patents/US5781163

You're welcome. ;-)

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Re: To all space comms experts

Allow me to expand further.

L-band systems, around 1.6 GHz, such as Iridium and Inmarsat, can provide low bandwidth satcom using low gain antennas that do not require aiming. When I use the word low, I mean REALLY low bandwidth. 5% of dial-up speeds. Just enough for heavily compressed voice or short messages.

Higher data rates (0.4Mbps) on L-band need medium antenna gain which implies steerable antenna systems. There are some very cute little antenna systems, one using a quad array of end-fed helical elements. Little motors aim it, but aiming is not very critical at L-band. Floodlight, not laser.

Higher ka- and ku-bands are where all the high bandwidth action is these days. The aircraft typically need a small dish with precision aiming capabilities. It's very critical because mis-aiming could interfere with the satellite in the next orbital slot, only 2 or 3 degrees away.

Unlike manually aiming a dish, these are fully automatic. They can slew into alignment in seconds.

One thing to consider is 2-axis mounts can get into lockup. So 3-axis mounts might be better.

Electronic beam steering can be mentioned, but last time I checked it wasn't quite ready for prime time. Might be by now.

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"Clarke orbit"

"high up in the Clarke orbit at 69.9 degrees west"

Thank you, Iain! That's exactly what a geosync orbit should be referred to as.

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Re: "Clarke orbit"

I refer to all of them as Kepler orbits

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Re: "Clarke orbit"

If it requires station keeping fuel, then it's non-Keplerian.

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