D A Duff

Abstract: Part 1 of this article (Phys. Educ. 1981 16 352-6) described the AMSAT Oscar satellites, the NOAA weather satellites and the information they transmit. Part 2 deals with how to locate a satellite and the antennas and receivers required to receive information. There are two stages involved in finding the position of a satellite at a given time: (a) Calculation of the equatorial crossing time and position for the orbit required; (b) Extension of this data to the point where the satellite rises above the horizon of the listening station. As a satellite travels round the earth, its track makes an angle to the earth’s equator known as the satellite’s inclination, angle I on figure 1. The time at which the satellite crosses the equator when northbound (ascending node) is called the equatorial crossing time (ECT). The position of this crossing is usually given as degrees of longitude west of the Greenwich meridian. The orbital period of a satellite remains fairly constant, although over long periods a reduction in this time becomes apparent as the satellite height falls as a result of drag caused by the very thinly dispersed matter in its path. Each successive orbital track appears to move west by an amount referred to as the longitudinal increment, an effect produced largely by the rotation of the earth about its axis. Signals from the satellite can normally only be heard when the satellite is above the receiving station horizon and one can draw a circle on a map that