Balloon Satellite - Range of Radio Waves, Visibility

Range of Radio Waves, Visibility

The Pythagorean theorem allows us to calculate easily how far a satellite is visible at such a great height. It can be determined that a satellite in a 1,500-kilometer (930 mi) orbit rises and sets when the horizontal distance is 4,600 kilometers (2,900 mi). However, the atmosphere causes this figure to vary slightly. Thus if two radio stations are 9,000 kilometers (5,600 mi) apart and the satellite's orbit goes between them, they may be able to receive each other's reflected radio signals if the signals are strong enough.

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

• the satellite must be illuminated by the sun
• the observer needs a dark sky (that is, he must be in the Earth's own shadow on the planet's twilight or night side)
• the brightness of a sphere depends on the angle between the incident light and the observer (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

Despite this there is no problem observing a flying body such as Echo 1 for precise purposes of satellite geodesy, down to a 20° elevation, which corresponds to a distance of 2,900 kilometers (1,800 mi). In theory this means that distances of up to 5,000 kilometers (3,100 mi) between measuring points can be "bridged", and in practice this can be accomplished at up to 3,000–4,000 kilometers (1,900–2,500 mi).

For visual and photographic observation of bright satellites and balloons, and regarding their geodetic use, see Echo 1 and Pageos for further information.