Satgen419 Missions of Gravity by GM4IHJ (BID SGEN419) 5 April 97 Space Shuttle STS 85 , due for launch in mid 1997, is scheduled to deploy a 40km tether with SEDSAT mode A amateur radio payload, at the upper end. This technique calls for gradual unwinding of a tether between two weights, the lower of which is the heavier one. When deployed, the extra weight should stabilize the combination in a straight line , pointing at the centre of the earth. At some point in the experiment, after deployment and stabilization, the tether will be released to propel the upper unit into a higher orbit. This is what happen to the Italian test in 1996, when its tether broke accidentally. Just how long the deployment and initial stabization will take , is uncertain. Previous Pentagon experiments ( also in 1996), took some time to stabilize, because the initial deployment introduced a considerable swinging motion which took months to settle down. But with luck we should have the SEDSAT transponder ready for operation some time before the end of 1997. Another aspect of gravity, which has recently caused some confusion , is the irregularity of the earths gravity field , which differs considerably from one place on the earth to another. Oscar 10's old Keplerian elements were getting way out of date recently because NORAD cannot track a Molniya very well in daylight, and AO10's orbit is affected by the Sun, the Moon, and, the actual strength of the earths gravity field in those regions which it overflies, particularly at perigee. Failure to allow for these features causes tracking problems Unlike Ao10, LEO low earth orbit satellites cover almost all the earth surface every day, so the gravity anomalies even themselves out. But Oscar 10 with only two orbits per day overflies a strictly limited track which can result in the well know line ups " resonating" with the Sun and the Moon, and,the less well know over flight of sections of the earth with gravity anomalies. One major source of anomally affecting the Keplerian elements occurs because the density of the rocks under the oceans is greater than the density of the continental rocks. So if Ao10 is experiencing perigees which continuously descend over the Atlantic and rise over the west Pacific in say the morning , then descend over the Pacific and rise over the Atlantic in the evening , the affect is quite markedly different from what it would experience if the perigees were over lower density continental rocks. Putting algorithms into tracking software to deal with these Sun, Moon and surface gravity sectional and tesseral anomalies , makes the program longer and slower. But with todays fast Pentium processors this hardly matters. So if you want real long term accuracy for Oscar 10 or Phase 3D. You should avoid programs which offer a choice of " the quick and dirty" standard algorithms which lack these corrections but run very fast, and go for, the slightly slower NASA SGP4/SDP4 algorithms which have all the corrections you should ever need.