Wireless Waffle - A whole spectrum of radio related rubbish
'Loafsat' to solve Internet Censorship Problemssignal strength
Saturday 1 March, 2014, 09:03 - Satellites
Posted by Administrator
outernet logoA consortium led by the Media Development Investment Fund (MDIF), and calling itself Outernet is planning to launch hundreds of small satellites (at 30x10x10 cm at their largest, they are about the size of a loaf of bread) to 'broadcast' the Internet. The idea is that selected portions of the internet will be broadcast using UDP-based WiFi multicasting (as well as, potentially, DVB and DRM).

Stepping aside from the political questions about who will decide which portions of the Internet will be broadcast - and which will not - there is the much bigger question of whether or not it is even possible to broadcast WiFi successfully from a satellite. There are several technical issues to overcome:
  • The satellites, presumably in low earth orbit, will be several hundred kilometers above the planet, so the path loss will be significant.
  • They will have to overcome interference from terrestrial WiFi networks on the same channel.
  • The low earth orbit means they will not be stationary in the sky, leading to Doppler shift on the received signal.
The power of the transmitter on the satellites is not known, but we can backwards calculate what it would have to be in order to deliver a service. WiFi typically needs to receive a signal of around -90dBm (1 picoWatt of power) in order to function, and preferably more (especially for faster connection speeds), but let's take that as the baseline.

loafsat 1At a frequency of 2450 MHz, the free space path loss over 500 km (a typical height for low earth orbit satellites) is just over 154 dB. In reality, atmospheric absorption will increase the path-loss, as will clouds and rain, but let's assume it's a relatively clear, low humidity day. The satellite will therefore have to have a radiate a power of 154 - 90 = 64 dBm in order to achieve the necessary signal level on the ground. This is a power of just over 1.5 kiloWatts. At a satellite height of 150 km (about the minimum possible), path loss is around 10 dB less, meaning it would have to radiate a power of 150 Watts.

7dBiIf the transmit antenna has a gain of 10 dBi, which is very feasible, the transmitter power requirements end up being 150 Watts at a height of 500 km and 15 Watts at a height of 150 km. Note that no transmitter is 100% efficient, and the satellite would have to have receivers and control systems too, so the power requirements would be greater than that which the transmitter alone requires. If it is also assumed that the satellite is over the dark side of the Earth for some proportion of time, and has to rely on batteries, the solar power generation requirements increase, or alternatively the satellites would have to switch off at night.

Of course, high gain antennas could be used on the ground, but this would then require special equipment for the satellite to be received and would go against the concept of receiving the signal on 'smartphones and tablets'.

It is not possible to easily generate 150 Watts of power on a satellite the size of a loaf of bread. A typical satellite solar panel can generate around 300 Watts per square meter of area. The total surface area of the 'loaf' would be 0.14 square meters, meaning it could potentially generate 42 Watts of power if all faces were covered in solar panels and were in full sunlight (which is, of course, impossible as at least one face would be in shadow).

Of course, the 'loaf' could have its solar panels unfold after it is launched to make a bigger panel, so the 0.5 sq metres required to generate 150 Watts might just be possible. But this would still only provide power when the satellite was in daylight. To be powered at night it would need to generate at least double the power (one lot for the transmitter and another to charge the battery) and contain a battery capable of holding the charge. This would again be difficult on a satellite of this size.

The above transmitter power calculations assume that there is no interference on the channel. If standard WiFi channels are to be used, then depending on the location it could be expected that there would be other signals around causing interference. Assuming that the main use of the satellite will be in areas where there are no other forms of Internet connection, we could take it that there would not be WiFi interference, and so arguably we could look upon the satellite kindly and disregard this effect.

On the Doppler shift issue, at 2450 MHz, the received frequency of a low earth orbit satellite will vary by around +/- 50 kHz as it passes overhead. The IEEE standard for WiFi specifies a maximum frequency error of +/- 25 parts per million (ppm) for the 2.4 GHz band. This equates to roughly +/- 60 kHz meaning that the Doppler shift of the satellite leaves it just within acceptable frequency tolerances.

So, in conclusion:girls and loafsat
  • If the 'loaf' was at 150km height it might just be able to generate enough power to transmit a WiFi signal that is strong enough to be received on the Earth. At a height of 500 km, extending solar panels would be necessary. For use at night even larger panels, plus batteries would be needed.
  • Any terrestrial interference in the band would largely obliterate the satellite signal, so it would only really be receivable in remote areas (which is, after all, it's main intention).
  • Doppler shift is just within acceptable tolerances.
So is it technically feasible. The Wireless Waffle answer is 'just about'. But if it was launched, it would increase WiFi interference levels over the majority of the planet, especially outdoors.

And you can bet that if it did work, those Governments that censor Internet access would find ways to jam the signal either terrestrially or by building their own 'loaf sat', increasing WiFi interference further. The loaf-sat-wars may be just about to get toasty...
add comment ( 417 views )   |  0 trackbacks   |  permalink   |   ( 3 / 633 )

Dish Of The Daysignal strength
Tuesday 26 November, 2013, 03:55 - Satellites
Posted by Administrator
dish of the dayDo a search for 'Dish Of The Day' on Google and all manner of results pop up, many cooking related, but none more dishy than sexy chef 'Pinny Rollings' here on the right. But even Pinny's dishiness pales into insignificance compared to the downright deluge of dishes on the roofs of buildings in downtown Abu Dhabi as the picture below attests (click it to see the full version and look at each building - many have dozens of dishes on their roofs).

One can but wonder whether any of these dishes actually work, or even how a situation as silly as this came about. Presumably people installed a dish to watch satellite TV and then found the service provider had moved to a different satellite. So instead of re-pointing the dish towards the new service, they installed a second dish. And then they decided they wanted services on another satellite, and instead of installing an arm capable of holding multiple LNBs, they installed a third dish. Then they found that that one of their dishes didn't work because someone else mounted a dish in front of it, so they put up yet another. And so the story goes on.

abu dhabi rooftops 2

Whatever the reason, with so many dishes in close proximity, it is almost inevitable that some won't work, some don't work and some haven't worked in a long time. Surely there's a business opportunity for someone here. Firstly to go around and remove all the unused dishes (and sell them on to other people) and secondly to install CATV type systems that allow the distribution of one dish to multiple households.

There is also another question... which of these dishes is in which satellite band. There is increasing pressure for regulatory to allocate the extended C-Band (3400 - 3800 MHz) for wireless broadband services. In Europe, this spectrum is already allocated for such services through Commission Decision 2008/411/EC. If even 10% of the dishes on the roofs of Abu Dhabi are for C-Band services, then the introduction of wireless broadband services in those frequencies is a complete no-go - not without causing harmful interference, and lots of it. So in the United Arab Emirates, there is little chance of the C-Band being used for anything other than satellite reception.

But probably the most disturbing thing, is that the proliferation of dishes means that it is unlikely that Lady GaGa will perform in the country any time soon. Ms GaGa likes to stand out and what with so many other dishes around, her own dishy outfit would go largely unnoticed...

gaga ga ga
add comment ( 172 views )   |  0 trackbacks   |  permalink   |   ( 3 / 853 )

NO-3bsignal strength
Friday 1 November, 2013, 13:00 - Satellites
Posted by Administrator
no3b1Recently Wireless Waffle reported on the launch of the first four satellites in the O3b fleet which were intended to launch a service to provide high speed wholesale broadband services in remote areas. We suggested that there were many business and technical challenges that O3b might face in commercialising their service. It seems we may have tempted fate a little - oops!

o3b launchRumour has it that a power component in all of the satellites currently in orbit has failed meaning that they will have to be scrapped. In addition, the launch of the next set of satellites will be delayed by up to a year whilst they are re-fitted. The reason for this is not certain, but might be related to the orbit of the O3b satellites being in the Van Allen belt.

Whilst the technical failure can be recovered from, there are, of course, commercial implications of these problems. Firstly there is the cost of replacing the first 4 satellites - although this should be able to be claimed from insurance (which all satellite launches have). Like any insurance policy though, the broker will want to investigate the fault in detail, identify who is culpable, and assess what payments are due and this will certainly add delays to any re-payments and may not necessarily result in the recovery of all the losses.

Then there is the also cost associated with the need to re-fit the yet-to-be-launched satellites. But perhaps the biggest loss may arise from the delay associated with the launch of the service. Like any project requiring infrastructure, O3b will be relying on funding from banks and other organisations to finance the launch of the service. And like any loan, there will be expectations on repayments. Delaying the launch will delay any revenues and thus the abililty to service debts. This might even result in O3b breaching its banking covenants.

Technical failures of satellites are not uncommon but such a major fault on multiple satellites leading to delays in the launch of a service that will, in the long term, be replaced by fibre connectivity, will make the business model look less attractive to investors. The question now is whether the service will become O3bsolete before it gets off the ground!

o3bsolete dish
add comment ( 5696 views )   |  0 trackbacks   |  permalink   |   ( 3 / 973 )

Oh, 3Bsignal strength
Wednesday 24 July, 2013, 07:44 - Satellites
Posted by Administrator
o3b logoSatellite officionados will no doubt be aware of 'O3b' but to the rest of the world, the launch of their first four satellites probably passed them by. O3b stands for 'other three billion' and is meant to highlight the plight of the significant proportion of the world's population who do not have access to high speed internet connections. O3b's role is to provide a 'backbone in the sky' which will allow places that are remote from terrestrial infrastructure to have high speed connections.

The idea is not that new. 20 years ago, Teledesic had much the same idea. The two networks share a lot of common features:
  • They both use Ka-band spectrum (around 18 and 28 GHz). At these frequencies, even small dishes have very narrow beams, requiring accurate pointing.
  • They both use low(er) orbits than the geostationary arc. The lower orbits reduce the time delay normally associated with satellite communications but require that the dishes are able to track the movement of the satellites.
  • They both have big-name backers. Microsoft was an investor in Teledesic. Google is an investor in O3b.
Unfortunately Teledesic's business model was usurped by delays in the launch of the system which led to terrestrial telecommunications quickly catching up and surpassing the capabilities of the satellite system. O3b on the other hand, is looking to offer connection speeds of up to 600 Mbps which is highly competitive in areas which do not yet have access to internet delivered via fibre. So is everything hunky dory? with O3b? Let's take a look...

Wireless Waffle has discussed the different bands used by satellites before. Ka-Band is the band of choice for new broadband satellites as it's used less than the now almost saturated Ku-Band. The thing is that there are an increasing number of satellites using the band, the majority of which are in geostationary orbits. As the O3b satellites are in a medium earth orbit (MEO), and thus move through the sky when seen from the ground, it is quite feasible that they could end up positioned between a dish on the ground, and a geostationary satellite. In these cases, there is the potential for the O3b satellite to cause interference to the dish on the ground. As there are potentially millions of such dishes, ensuring that the O3b constellation is 'switched off' when it is in a position to cause such interference is an interesting technical challenge.

get out of my cloud

Then there's the issue that as the satellites appear to move in the sky, ground stations for O3b would require tracking dishes. Much development has been done to try and develop dishes which can track the movement of satellites without the dish itself needing to move. Such dishes would be ideal for use on trains, aircraft or cruise ships to circumvent the need for tracking dishes on moving vehicles. The same idea could be applied to ground stations that are fixed but satellites that move. However this is not the plan for O3b. The plan is to have traditional steerable satellite dishes which track the satellite through the sky. And as it takes time to move a dish around, two will be used with the second picking up a connection from a second satellite as the first dish loses its connection when the satellite passes over the horizon.

Two steerable dishes makes for a complex system and one which will require significant and regular maintenance costs, and maintenance by skilled engineers who are not (currently) likely to be present in large numbers in the kinds of areas that O3b wishes to serve. There is also the question of what happens when the service is handed over from one satellite to another. Such a hand-over could introduce a variable delay in the connection (technically, known as 'jitter'). One solution to this would be to have a buffer to stabilise the connection, but adding a buffer adds a delay and it is precisely to try and reduce delay that the satellites are in a lower orbit to start with. Oops!

Current Ka-band satellite services typically offer connection speeds of around 20 Mbps and use 65 or 80 cm dishes. Delivering 600 Mbps requires a bit more effort and as such the steerable dishes for O3b will each be 4.5 metres in size! At Ka-Band this means they will have a beamwidth of less than 0.2 of a degree, requiring highly accurate tracking. Not to mention the fact that 4.5 metre dishes are large and heavy and take up a lot of space. If you think about the intended market for O3b, which is rural area far from traditional infrastructure of any kind, running two, highly accurate, steerable 4.5 metre dishes is not the kind of technology which would easily blend in with the environment. And where do you get a power source to drive them with?

dish hut

Putting these technical issues aside for a minute, there is then the question of speed to market. Many of the far flung places that O3b is looking to serve are already beginning to be connected via fibre, whether undersea or overland. Take the Pacific Islands, slowly but surely, these are being connected to the world via subsea fibre. Fiji, Guam and Hawaii have long had connections, but Norfolk Island, New Caledonia, Vanuatu, Wallis, Samoa and American Samoa are due to be connected via a project called Hawaiki Cable (though it's fair to say that a previous such projects known as SPIN did not materialise). Perhaps O3b has the upper hand here, then again if Hawaiki goes ahead, it will offer connection speeds nearly a thousand times faster than O3b.

Would you pour your hard earned money into O3b shares? Don't decide just yet as there's one other issue we need to talk about... the Van Allen belt. You may have heard of this. It's a region of space above the Earth which is full of particles charged by solar energy but trapped by the Earth's gravity. It is one of the more unfriendly space environments and the O3b satellites flying at just over 8000 km above the Earth will sit right in the middle of it. This gives them around 100 times the dose of radiation that a satellite in geostationary orbit would experience and as a result makes them more susceptible to radiation damage of various kinds. There are currently no commercial satellites (other than O3b) operating in the Van Allen belt.

Of course all these obstacles are worth the effort if the end services can be supplied more cheaply than competing technologies. O3b's pricing is not yet clear but it will have to compete with other Ka-band services such as Inmarsat's Global Xpress. This will give you a connection of up to 50 Mbps for prices purported to be around US$3000 per month. At rates of income in some countries, it would require an awful lot of users to club together to raise that kind of finance. It is therefore to be hoped that O3b can offer a service that is more cost effective. With the cost of the dishes alone likely to be at least US$50000, that seems rather unlikely.

angry dinosaur

Perhaps a better description of O3b would be 'over 3m big'. Still, it looks as if their dishes will provide good protection against angry dinosaurs!
add comment ( 2420 views )   |  0 trackbacks   |  permalink   |   ( 2.9 / 506 )


<<First <Back | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | Next> Last>>