Balloon satellite - Reach of radio waves visibility - Encyclopedia II

The pythagorean theorem allows us to calculate easily how far a satellite is visible at such a great distance. The result is that when it has a 1,500 km orbit, the satellite just gets out of or into range when the horizontal distance is 4,600 km. However, the atmosphere means that this figure can vary slightly.

Thus, if two radio stations are 9,000 km apart and the satellite's orbit goes between them, they may be able to receive its reflection if the radio signals are strong enough.

Optical visibility is, however, lower than that of radio waves, because

• the satellite must be lit by the sun
• the viewer needs to have a dark sky (i.e. he must be in the earth's own shadow on the planet's twilit or night side)
• the brightness of a sphere depends on the angle between where light is falling on it and where the viewer is standing (see phases of the moon)
• the brightness of a sphere is much reduced as it approaches the horizon, as atmospheric extinction swallows up as much as 90% of the light

(translation from here)

Because of the precision of of satellite geodesy it is possible to watch missiles like Echo 1 down to 20° elevation (equals a distance of 2900 m). Therefor also abstract distances of up to 5000 km are measurable.

For visual and photographical observations of very bright satellites and balloons see Echo 1 and Pageos for further information.

Adapted from the Wikipedia article "Reach of radio waves visibility", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki/Main_Page