In reply to http://www2.satellite.eu.org/seesat/Sep-1998/0003.html (and 0005.html, 0007.html): Re: Moving satellites to another plane. Questions have been asked about using precession of the orbital plane to move an (Iridium) satellite to a different plane. The precession of the plane is approximately -10.0 * cos(i) / ( a*(1-e**2) ) **3.5 degrees/day, or at Iridium inclination -0.013345 * cos(i) * MM**(7/3) / (1-e**2)**3.5 deg/day. Assuming a typical operational Iridium orbit with i=86.4 MM=14.3422 a=1.1227, precession would be -0.4188 deg/day With Ron's pre-launch elset for the Sep.08 (04) launch, i=86.58 MM=14.796 dRAAN/dt = -0.4276 deg/day With this small difference, it would take 3300 days to precess the orbit to the next plane ! Obviously, a larger difference in inclination and/or Mean Motion would be needed. A higher MM is achieved by applying a forward thrust (velocity decrease). An inclination change can be accomplished by a thrust normal to the orbit, and horizontally, near the equator. A direct change of RAAN is possible near the apex, but very costly. A change of RAAN (or inclination) requires a velocity change of 2*v*sin(d/2), where v is the circular orbital velocity, and d the angular change. The effect of orbital maneuvres can be tested with Ken Ernandes' VEC2TLE program, using the Impulsive Delta V function. The precession rate can also be checked by applying a zero delta-V at a specific time in the future and comparing the RAAN of initially co-planar orbits at the same future time. You can also use this, or the precession formula, to verify the slightly higher inclination of the 'spare' Iridiums, to stay in the same plane as the operational ones. The apogee velocity is approximately 7908 m/s * SQRT ( (1-e)/(1+e)/a ) (reverse signs for perigee). Using this, the velocities for the circular orbit, and one with the same apogee height can be compared. For the operational Iridium, this becomes 7463 m/s. A perigee reduction to make e=0.02 decreases 'a' by 2%, and needs a 1% speed change, giving about 7% change in precession, i.e. 30 degrees in 30/0.42/0.07 days = 1020 days ! The same deltaV, 75 m/s, can produce an inclination change of 0.58 degrees (1% of a radian), which precesses to the nearest plane in about 440 days. It seems both a lower orbit, and substantially lower inclination, are required to shift plane in a reasonable time period. - bjorn.gimle@tieto.com (work) b_gimle@algonet.se (home) - - b_gimle@usa.net (temp) - - 59.2237N, 18.2286E, 44 m http://www.algonet.se/~b_gimle- - SeeSat-L / Visual Satellite Observer Home Page found at - - http://www.satellite.eu.org/satintro.html - ____________________________________________________________________ Get free e-mail and a permanent address at http://www.netaddress.com/?N=1