Finding a geostationary satellite

As the great hunter said, to find something one must first understand how it moves. It’s no less true with satellites

When writer Arthur C Clarke fi rst dreamt up the idea of using communications satellites in geostationary orbit in his 1945 Wireless World article, it was a stroke of genius. This provided a way (once technology had caught up) to broadcast to a huge area from one transmitter but keep the receiving antennas simple, cheap and, above all, fi xed. The only problem is knowing where to point those fi xed antennas – they must be aligned accurately because the signal is so weak, but the satellites in orbit are completely invisible to the naked eye.

1.How the satellites stay still

Geostationary satellites are not stationary at all (‘geosynchronous’ is really a better term to describe their orbit). At a height of about 22,250 miles (36,000km) above the Earth’s surface, a satellite travels at a speed of nearly two miles a second to stay in orbit. This means the time to complete each orbit is 24 hours – the same time it takes the Earth to rotate about its axis. So, if the orbit is directly above the Equator, as the satellite races around the Earth, the ground is rotating beneath it at the same speed, and so from the ground it appears that the satellite stays still in the rotating heavens.

In fact, because the orbit may not be exactly circular or exactly line up with the Equator the satellites move about a bit, so they are held ‘on-station’ – inside an imaginary box about 40 miles wide – by small booster rockets under the control of the ground station. They can move around inside the box without moving out of reach of a fixed dish aimed at the orbital position.

Different look angles: The angles to a satellite change according to your location on the Earth. The closer your longitude to the satellite’s longitude, the smaller the bearing (blue angle) is, so for Viewer 1 the bearing is just a few degrees east of South while Viewer 2 looks about 30° east of South to see the satellite. The opposite is true of the elevation (red angle). Since Viewer 1 is ‘closer’ to the satellite, the angle of elevation is greater than that for Viewer 2.

Who goes where?

There are well over 230 TV satellites in orbit above the Earth. Although some of these are in clusters of co-located birds (such as the four Astra satellites carrying Sky Digital and Freesat), most of the craft are kept apart so that they can use the same transmission frequencies without interfering with one another.

It’s preferable to position a satellite along the arc so that it’s high in the sky for the countries receiving it (so the dishes can ‘see’ the satellite above hills, buildings and trees).But some satellites are used to feed signals from one region to another – so the best position is then halfway between the two – and often there just isn’t room for all the satellites needed above the planet’s most populated areas. Compromises must be made, and the satellites are allocated their positions by an international committee, the International Telecommunications Union.

3.Orbital position

The allocated positions of the satellites along the arc are given as their celestial longitude – that is, the angle around the satellite arc. The zero (and 360°) position is in line with the zero terrestrial line of longitude, which goes through Greenwich (and crosses the Equator, directly under the satellite arc, in the Gulf of Guinea, south of Ghana).

The satellites positions are usually (in Europe) labelled in degrees east or west of this position. So the Astra 2A-2D satellites that bring us Sky and Freesat are at a position in the geostationary orbit that is 28.2° east of the zero position. That’s above a remote region of eastern Congo –quite a way from the ‘ideal’ point that’s highest in the sky for the UK.

Freesat and Sky share the same group of satellites at 28°E, so one dish can be used to feed either a Freesat or Sky receiver – or even both

Which way to the satellite?

It’s a common mistake to think that you simply use the satellite position as the azimuth (the direction along the ground) to point the dish – i.e. point an Astra 2A-2D to 28.2° east of due South. However, this is not the case (unless you live very close to the North Pole). The satellite’s azimuth depends on where upon the Earth’s surface you are. If you were at the same longitude as the satellite, then the bearing would be due South, and if you were located at a longitude more easterly than the satellite then the dish azimuth to the satellite is west of South. So if you lived in, say, Greece, the direction to the Astra satellites at 28.2°E would be a few degrees west of South – nothing like 28.2° east of South!

It’s only because in the UK we are all within a few degrees of the 0° line (the Greenwich Meridian) that the satellites’ azimuths are similar to their orbital position – similar but not the same. The azimuth also varies with your latitude, as does the elevation of the satellite (its angular height). To accurately calculate the azimuth and elevation of any geostationary satellite from any spot on Earth uses some pretty heavy trigonometrical calculations. To complicate matters, true North and true South are not shown with a compass. A compass will point to magnetic North, which is a few degrees off , and so you also need to calculate the ‘magnetic variation’ at your location to compensate.

The satellite arc: From anywhere on Earth, the geostationary orbit traces a line across the sky, (except at the poles where it’s along the horizon). Viewed from the Equator, the satellite arc passes directly overhead, stretching due East and West. From anywhere else it is a curve in the sky, to the south in the Northern Hemisphere, with its highest point due South. The closer to the Equator you are, the higher (and longer) is the arc.

5. The easy way to the satellites

You can get the results you need without all this number-crunching. For the Astra 2A-2D satellites (or Eurobird 1 at 28.5°E), you can use the map given here to read off the azimuth (in blue, east of South) and elevation (in red) for your location in the UK.

If you want these angles more accurately, or want the azimuth and elevation of another satellite, or from another country, you can have the calculations done for you by one of the many websites that provide the satellite ‘look angles’ for your location. Here are a few we accessed earlier:

www.satellite-calculations.com/Satellite/lookangles.htm
www.dishpointer.com
www.satsig.net/ssazelm.htm
www.ses-astra.com/consumer/en/how-to-receive-astra/installationassistant/index.php
www.satcom.co.uk/article.asp?article=1

Or you can download (free) software to for your PC from:

www.smw.se/FreeSoftware.htm
www.al-soft.com/saa/satinfo.shtml.

These use your latitude and longitude, postcode, place name, or maps to input your location and the normal notation of degrees east or west of South for the satellite’s orbital position. Some of these sites calculate the magnetic variation for you too, and some show you the direction to the satellite on a map or aerial photograph. Whichever you choose, it’s never been easier to pinpoint the satellite you want Geoff Bains

If you’ve already got a Sky dish for your main TV, fit a quad LNB and put Freesat receivers next to all your other TVs, or get a Freesat PVR

Glossary

Latitude

Distance on the Earth’s surface north or south from the Equator expressed as degrees from (the Equator) to 90° (the poles).

Longitude

Distance on the Earth’s surface east or west of the Greenwich Meridian expressed in degrees from the Meridian. Used with Latitude as a co-ordinate system to locate any point on the Earth.

Equator

Imaginary line round the Earth at latitude 0°, equidistant to each pole.