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Topics - Copernicious

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Flat Earth Q&A / Satellite footprints - a replicatable experiment
« on: May 17, 2012, 06:41:10 PM »
There has been lots of discussion of satellites, pseudolites, stratellites (I'll just call them 'lites in this post), and the like on this forum. One question that has, AFAIK, not yet been brought up, is the issue of 'lite beam footprints.

As you may know, if you shine a point light on the surface of a sphere, the boundary of the lit area will be a circle. The farther the point light is from the sphere, the bigger the circle will be - a point light at infinity distance will light up an entire hemisphere (the boundary will be a great circle), while a point light directly on the surface of the sphere will light only a single point.

In an equirectangular projection, a circle centered on the equator of a sphere will appear as a superellipse commonly known as a "squircle". The Azimuthal stereographic projection, on the other hand, renders all circles as circles.

With that said, here is an equirectangular footprint map of the Intelsat 10-02 'lite's "Global" beam:


A feature of this 'lite is that it is essentially an isotropic "point" microwave source - if you can "see" it, you can receive transmissions from it. The shape of the footprint is a squircle, as predicted by the model of a point light shining at a sphere. To further prove this point, here is the same map in azimuthal stereographic projection (centered on the equator, at longitude 0 - the bounds of the original map are 85N-84S, 177W-177E, for those wishing to reproduce the other projections):


Indeed, the footprint is a circle.

Yet, here is the same image in the flat-Earth projection commonly used on this forum:


I've yet to see any explanation for the shape of this footprint on a flat Earth.

And now, how to actually confirm these footprints.

You will need:
-A parabolic dish reflector with a diameter of at least 3m (7ft). Limited success should be expected with a smaller dish.
-A C-band LNB converter that supports circular polarization.
-A DVB-S tuner. This can be a conventional set-top-box with free-to-air reception support, or a computer with an add-in card.
-A GPS receiver (optional, used for obtaining geographical coordinates).

What you need to do is set up your equipment (dish+LNB+tuner) at a location near the edge of reception as shown on the map.
Install the LNB on the dish, connect the tuner to the LNB, then align your dish as suggested by http://www.satlex.de/en/azel_calc-params.html?satlo=17.0&user_satlo=1&user_satlo_dir=W&location=6.33,-10.76&la=6.33&lo=-10.76&country_code=lr&diam_w=180&diam_h=180 - set your coordinates according to the values from the GPS receiver.

Now, tune your DVB-S receiver to 4084 MHz, left circular polarization, symbol rate 3906, FEC 3/4 (some DVB-S tuners can auto-detect the FEC) and scan for channels. If you are lucky, and there is nothing blocking the path between the 'lite and your receiver, you will see a signal from Senegal TV station "RTS". The farther you are (inside) from the edge of reception, the more likely you will get a signal, and the higher you will need to align your dish.

The alignment calculator I posted uses RE math to calculate the dish settings from your coordinates - oddly enough (for FE'ers at least), it works.

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