Satgen 474 Geosat Gyrations by GM4IHJ (BID SGEN474) 1998-04-25 In recent discussions about Amsat Oscar 10's visibility, comparisons were made with the probability of seeing geostationary satellites. There is in fact no problem with this. Geosats can be photographed with a standard camera using a modest telephoto lens. When the night sky is clear and dark, the geosats appear as discrete dots on a short time exposure, whereas the stars trace out short lines , moving as they do at about one degree every 4 minutes. If however you train a telescope on the geostationary arc, the picture becomes much more interesting, as each individual geosat is seen to trace out a partial ellipse over a period of several hours. A film or CCD record of several hours duration will clearly show that far from being stationary, each geosat has an individual track and the tracks can differ markedly one from another. The reasons for this apparent contradiction are present in the typical satellite orbit data below:- Geosat Inclination Period min Apogee Kms Perigee Kms Position Astra 1G 0.03 inc 1436.13 35,755 35,798 19.2 E GE3 0.06 1436.02 35,728 35,842 272 E APStar2R 0.10 1442.81 35,818 36,018 76 E a. As the above examples show, the satellite is never in a perfectly circular orbit . When viewed from the Equator the apogee/perigee motion is not apparent but viewed from a station in Northern Europe this motion is clearly seen . Astra is very close to circular so the motion is only about 2 mins of arc, but GE3 moves about 6 minutes as seen from Canada, and APStar moves over 10 minutes of arc when viewed from Northern Asia. b. In some geosats the inclination of the orbit is much greater than those listed here, but even these sats moving 0.1 degrees or less above and below the equator shift over 6 minutes of arc (APStar) or 2 minutes of arc ( Astra) . Which in the Astra case is a movement of around 140 kms. c. Last but not least a satellite in an elliptical orbit travels at different speeds over different parts of the orbit . So the satellite appears to sometimes lag behind earth rotation , and sometimes lead it The end result of all this non stationary motion , is the appearance on a long exposure telescope view of the geostationary orbit , of an odd mix of large and small partial ellipses with their major axes tilted at a variety of angles to the equatorial plane. None the less, the overall motion of a geosat should not quite exceed the aperture of a fixed 8inch telescope , but bigger telescopes may see only part of the movement. Please Note. The motions described above are unrelated to the longer term variations in a geosats position , which are caused by solar and lunar attraction, and, by the irregularity of the earths gravity field All these latter features produce motion off station, which must be regularly corrected by using the satellites onboard fuel and thrusters