I’ve recently been working at Goonhilly Downs, the largest satellite earth station in the world. The site, owned by British Telecom, is near the southernmost tip of the UK, at the Lizard Peninsula, and has line of sight to satellites over the Atlantic and, just barely, the Indian ocean. John Bilkey was good enough to give me the VIP tour of the site, and it was quite amazing.
You can view a gallery of photos I took while there.
The Indian ocean satellite I mentioned (actually I think there is more than one) sits on a line of sight just 5 degrees above the horizon, far off to the east. Consequently, all the dishes for the Indian ocean point not at the sky, but straight along the ground and out to sea. Line of sight is only possible because modern communications satellites are in geostationary orbits, around 25,000 miles away.
The first dish on the site (BT call them aerials) was built in 1962 to track Telstar, the first communications satellite. Telstar wasn’t geostationary, having instead a low orbit. This was probably because the early technology could not achieve successful transmission over the distance needed for geostationary satellites (but possibly just due to the higher costs of launching an unproven technology into much higher orbit). Either way, the concept of geostationary (or geosynchronous) satellites was already known from Arthur C. Clarke’s 1945 paper on “Extra Terrestrial Relays”.
A remarkable consequence of this is that Aerial 1, which was the first ever satellite dish, and which weighs around 1000 tons, can track a satellite from horizon to horizon in just 3 minutes, even to this day. Telstar used to take around 20 minutes to cross the sky, so Aerial 1 had little trouble keeping up.
In fact, fairly rapid and very accurate aiming of the antennas is required even for the geostationary satellites. Viewed from the ground, these exhibit a figure-of-eight “wobble” caused by relative differences in their orbits and the Earth’s off-kilter spin, and the antenna must track this almost imperceptible movement to maintain the signal. During my visit to Aerial 3, I saw the elevation gauge reading 5.92 degrees above the horizon. (Aerial 3 tracks an Indian ocean satellite).
Satellites move around in the sky for another reason: keeping the satellite in an exact orbit uses a lot of fuel, and each satellite is sent into space prepacked with all the fuel it will ever have. Using it reduces the satellite’s lifespan, so satellites are allowed to drift more than they used to to prolong the life of the fuel and hence the useful life of the satellite.
Aerial 1 is remarkable in another respect too: it was the first dish-shaped antenna. The equivalent French and American antennas were quite different (the French one apparently looked like an ear trumpet). Both those antennas are now gone, but Aerial 1 is still in active service, and is one of Britain’s more improbable listed buildings, which grants it protection from being modified or demolished.
Another antenna was originally built to track missiles and tracks even faster than Aerial 1, though it’s somewhat smaller (I’d estimate 50 feet in diameter). It stands on a 100 foot tower, far higher up (though not taller) than the others, and the dish moves independently of the tower.
Many experimental designs are scattered over the site, mostly fully operational, but some reserved for backups. Aerial 2 has a dish fixed on a pivot at the base, while huge hydraulic jacks push the top of the dish to control elevation. The entire building rotates on rails. From a distance its squat appearance looks odd, while from close up, the fact that almost the entire building doesn’t touch the ground seems even odder. The others look a little more typical, with a tower which offers the azimuth rotation at some point along its length, a building housing the power units and electronics at the base, and a dish with an elevation control at the top of the tower.
The aerials are designed to operate in winds up to 120 miles per hour, which is useful on the Lizard Peninsula with the wind howling in off the Atlantic, believe me. The forces involved in this must be astonishing. The dish has to point directly at an object smaller than a car over 25000 miles away while a 120 mph wind batters a wind-catching dish 100 feet across. I once heard that a truck travelling at 70 mph experiences a force equivalent to 2 tons just from the 70 mph wind blowing against its front, which puts the forces on the aerial into perspective.
Particularly impressive is the backup plan: if the wind rises, the plan involves pointing the dishes straight up into the sky, and chaining the edges to the ground 80 feet below. Everyone there seemed to agree that it was good this plan has never been put into effect, as it sounds a bit suicidal!
The dishes are also pointed straight up for cleaning and repair. You can then get into the dish via a door in the face of the dish, and wander around in relative safety - though apparently the view over the edge of the dish is pretty sickening. One story I heard told of someone taking their bicycle up there and cycling round and round the inside of the dish.
The windy nature of the location has been put to good use. Near the antennas is a moderate sized wind farm. Some people complain about these huge windmills but personally I find them serene and beautiful. Out of sight isn’t out of mind, and I would rather have my view interrupted by a windfarm than see an unspoiled vista but know there’s a nuclear power station nestling in a bowl in the hills (e.g. Sellafield).
Though beautiful, the Lizard Peninsula is also very remote. I was staying 1 hour’s drive from Redruth, the nearest major railway station, in the village of St Keverne. It’s lovely, but very very quiet. Sadly, my visit was afflicted with poor communications! A little ironic, but to be expected, as the small population of the area doesn’t justify setting up mobile phone masts. So I was spending my time in the middle of one of the world’s most important communication hubs, and not able to make telephone calls….