Technical considerations

If we are to find the satellites we need to know where they are located. Each satellite can be uniquely identified solely by its angular position around the geostationary orbit.

The geostationary orbit is a circular path around the earth about 35000 Km. above the equator, and is the only path that the satellites can follow which makes them appear to be in a fixed position when viewed from the surface of the earth. This path is popularly known the Clarke belt, and readers wishing to know and understand more about this are recommended to read Arthur C Clarke's original article, first published in 1945, here http://www.rocketrange.no/narom/kurs/fo/Relays.PDF - the concept of steam powered satellites is an intriguing one!

To a first approximation, the geostationary orbit, or Clarke belt, is located in an arc across the sky, very similar to the path that the sun appears to take during the spring and autumn. The rises somewhere in the east and then climbs upwards and moves southwards during the morning, reaching its highest point at local noon, before it starts to sink and eventually set in the west.

We can visualise its path to be like this.

For those living near the poles, the arc doesn't rise nearly so high in the sky, whilst for those nearer the equator, it appears to rise much higher. (Those living in the Southern Hemisphere will of course see the same thing, but the sun will be to the north of you and all my drawings will be mirror images of what you will see).

During summertime the sun's path will be higher in the sky than the Clarke belt, and in the wintertime lower.

This is, of course, an idealised picture, as there are no houses, trees, mountains etc. getting in the way and spoiling our fun. What you are more likely to see if you are a city dweller is something like this.

As you can see, some rotten swine has put buildings in the way! Country dwellers can imagine trees or mountains; the effect is just the same. Satellites hidden behind buildings, trees etc. will not be receivable.

Sometimes, if we are lucky, we can shift our point of view and get a better view of the arc where the satellites are located. This is much more likely to be effective if the obstructions are close to us. For example, this is what might happen if we moved a little to one side, a little further back, and a little higher too.

Now we can see a lot more of the arc to the east, with no worsening of our view to the west. By performing this simple site survey, with no investment in surveying equipment of any kind, we have found a better position for our antenna.  If you know where the satellites that you are most interested in watching are located in the sky, then this simple site survey will save you a lot of pain and heartache later.

After we have done our quick and dirty site survey, and found out whether or not it's likely that we can see a worthwhile number of satellites, it's time to get a little more accurate.

Performing a more accurate site survey.

Let’s imagine that you really want to see Fox Sports on the 43º W satellite, a pretty difficult one for people in Europe, as it's quite far round to the west.

If we imagine the arc of the satellites in the sky then this satellite is likely to be pretty close to the horizon, and consequently could very easily be hidden behind a tree or a building. I'm sure those of you not in Europe can imagine a similar difficult satellite to receive.

Our quick and dirty site survey suggests that receiving this channel is a very definite possibility, but before spending any real money it is as well to be sure.

If we are to find out exactly where this satellite is in the sky, we first need to know exactly where we ourselves are, in terms of latitude and longitude. Be as accurate as you can without being obsessive about it, knowing your position within half a degree will be quite good enough! Once we know our position, we can tell in which direction the satellite is from our location, and incidentally, how far away it is from us too.

You can find out your own latitude and longitude by

1.        Rushing out and buying a GPS receiver - (not a particularly cost effective thing to do!).

2.        Consulting a good map.

3.        Finding out the position of a nearby important navigational feature. For example, I have a small airstrip very close by whose position is published on the Internet.

4.        If you are in the UK, then you can convert your postcode into latitude and longitude using this website http://www.upmystreet.com/latlong.php3.

Once we know where we are, then we can plug our co-ordinates and the orbital position of our satellite into the smwlink software available from this website http://www.smw.se/smwlink/smwlink.htm, or better still, download Satmaster Pro from this website http://www.arrowe.com/download/smdemo.exe.

Satmaster Pro has some very powerful features that will become handy later, and keep you amused for hours just playing about with it's advanced features.

Let's have a worked example.

Imagine that you are Tony Blair, living at
10, Downing Street,
London,
SW1A 2AA

You visit http://www.upmystreet.com/latlong.php3 and type in your postcode to find out your latitude and longitude.

You discover that your location is as follows

Then you run the Satmaster Pro program that you downloaded, type in your latitude and longitude and, as if by magic, the Satmaster Pro program produces the following results.

So now we know exactly where the satellite is supposed to be, but how do we locate it in the sky?

Whatever you do next, do not run out and buy one of these.

You might think that you need a compass, but you don't.

·         Firstly how accurately do you think that you can read one of these things? After all the trouble we went to getting an accurate location, I doubt that you could read it any better than plus or minus a couple of degrees or so.

·         Making life even more difficult is the magnetic variation, which has to be added (or is it subtracted from?) the compass reading to find the true bearing, as opposed to the magnetic bearing.

·         Then, of course, you may live near a magnetic anomaly caused by underground iron ore deposits or something similar. If that is not bad enough, there may be nearby metal objects that can throw the reading off.

Read on and you will discover how to find true the north-south direction or any other direction along the satellite arc with unparalleled accuracy, for nothing! All you need is a watch and a sunny day!

Visit this website, http://susdesign.com/sunangle/ and your problems are over.

First we need to find out the satellite's bearing in terms of being west or east of south.

In this case Satmaster Pro says that the bearing to the satellite is 229.87º.  So, as due south is a bearing of 180º, then the direction west of south will be 229.87 - 180 = 49.87Wº.

Then we type into the fields on the web page our location, and our height above sea level, if we know it, the date, our time zone, and whether daylight savings time applies.

Next we guess the time of day that the sun is going to be approximately in the direction we need. As the satellite is located west of south, this will obviously be some time in the afternoon. Let's guess about 3pm.

We plug these values into the web page and get the following result.

A pretty accurate result for a guess! This shows the azimuth angle to be 49.64 degrees, very close to what we were looking for.

A quick refinement of our initial guess gets us this result.

So at one minute past three in the afternoon, British Summer Time, on the 26^th May 2003 the sun is predicted to be pretty well exactly on the same bearing as our wanted satellite! And all done without spending a penny, or a cent, or whatever.

Satmaster Pro told us that the satellite is at an elevation of 18.95 degrees.

All that remains now is to check that we have a clear view in that particular direction at an elevation of 18.95 degrees above the horizon.

You will need to make yourself a little sextant to measure the elevation of any obstructions in this direction.  Something as basic as this will do, using an old  protractor from your old school geometry set.

As an aside, if you are willing to wait until autumn, then you can use the http://susdesign.com/sunangle/ website to find the exact position of the satellite. Twice a year the sun and the satellite will appear to be in near enough exactly the same position in the sky.

But as this screenshot shows, you will have to wait until October 12^th for this to happen!

You can repeat this exercise for any other satellites that you suspect that you might have trouble receiving. Once you are sure that you can receive a worthwhile number of satellites, you can go out and buy your mount, motor and hardware.