September 27, 2012

Improvising a portable satellite internet link

We have recently deployed iRIS to a mobile clinic that can provide community-based treatment for wet age-related macular degeneration (wet AMD), a degenerative eye disease. The clinic is a mobile unit based in a lorry and therefore we need to provide Internet connectivity from wherever the clinic happens to stop!

We initially investigated using 3G devices, but we found that the network was unreliable in use and expensive to operate for the bandwidth we needed. Moreover, in some of the locations we needed to work we found that no 3G signal was available. This wasn’t a viable option.

Satellite internet connections aimed at fully mobile solutions such as boats are available. These provide global coverage but are expensive and could provide limited bandwidth. However, Ka-band satellite services from Tooway looked to have potential – reasonable cost and claiming to provide similar bandwidth to ADSL2. The system is aimed at providing satellite broadband from fixed locations – could we improvise a portable setup for it?

Hacking the portable setup

A standard installation uses a permanently mounted satellite antenna, usually on a substantial wall bracket. However, we needed to provide the capability to pack the link away ready for the unit to move to a new location.

Our first challenge was to find a portable stand for the dish. Portable stands for satellite dishes are available but these are really aimed at the caravanner wanting to receive satellite TV. This is important – the transceiver used to send and receive signals weighs several kg and is mounted on a long arm so the dish is both heavy and potentially unstable. We tried a commercially available portable stand, but it simply wasn’t up to the job so we had to improvise.

A bit of digging around found us a speaker stand from a PA system which was large enough to give us a wide base for stability and had tubing strong enough for us to clamp the antenna on. Even with our new stand we had concerns about stability so we resorted to a lo-tech solution – three 25kg sandbags across the struts at the base of the tripod finished the job and gave us a solid but portable stand.

The only remaining job was to link the satellite modem in our IT cabinet to the antenna using a coaxial cable. To make it easier to disconnect the system we added a short flylead from the antenna so that the main 20m cable can be easily disconnected when the clinic is packed up to move.

Pointing the antenna

The documentation about accurately pointing the dish doesn’t make it sound easy. After all, we’re aiming a one-metre antenna at a satellite in orbit 35,786 km above the equator. It requires calculation of elevation and azimuth angles based on current longitude and latitude, followed by micro-adjustment of grubber screws until the strongest local signal is found. Needless to say, the process sounds pretty complicated!

Fortunately, in practice it’s much easier. First, point the dish in roughly the right direction – you can even get augmented reality apps that overlay satellite locations onto your smartphones picture of the sky. Second, switch the satellite modem into pointing mode so that it starts making an intermittent tone. Then using a spanner, adjust the direction of the dish from side to side and up and down until the sound changes from a beep to a continuous tone. The antenna is now locked on, so you can let the modem spend a few minutes establishing it’s connection with the satellite.

Job done!

How well does it work?

Our experience has been that in operation the service delivered a reliable connection with performance similar to a good ADSL2 connection. This bandwidth is more than enough to operate the clinic.

After a few practices, the satellite antenna and stand can be moved into position, pointed and a working connection established in around about 10 minutes from start to finish.

This hack has worked to get the network in place for a pilot project, but lacks a little finesse! If we were to roll-out more installations then we’d look seriously at bespoke hardware or vehicle-based mounts, depending on the actual use-case.

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