Bjoern Gimle wrote in the message "RE: USA 129: no manoeuvre as of 19:50 UTC on 17 April": http://www2.satellite.eu.org/seesat/Apr-2000/0292.html > At a few previous manoeuvre instances that I checked, the > KH's raised their > orbits at a perigee pass immediately after the southbound > pass closest to > Vandenberg (or the KH control center E thereof). I only stumbled upon this over the week-end, but now looking through my archives, I find posts and e-mail from Bjoern on this subject at least as far back as 1994, so I have some catching up to do. My findings differ from Bjoern's in that I find that the node (i.e. equator) crossing can also be headed northbound (i.e. the ascending node), as was the case of 96072A's apogee-raising manoeuvre of 28 April 1998. At first, it appeared to me that only the object in the western Keyhole orbital plane (currently USA 129 = 96072A = NORAD 24680) makes its manoeuvre with the perigee at a node, but in fact it can also be the one in the eastern plane (currently USA 116 = 95066A = NORAD 23728). Since both make their major apogee-raising manoeuvres on the same date, and since their lines of apsides (long axis of the orbital ellipse) are separated by about 211 degrees (the eastern orbit's argument of perigee is about 211 deg greater than that of the western orbit) their perigee's cannot BOTH be near a node at the same time. One of them will have its perigee about 31 deg away from the nearest node. The western-plane Keyhole's perigee arrives at a node sooner than that of the eastern-plane Keyhole. The time difference is simply 31 deg divided by the rate of precession of the perigee, currently about 3.28 deg per day for USA 129, or about 9.5 days. We know that USA 129's perigee was at its descending (southbound) node at day 107.62 (April 16), which means that USA 116's perigee will be near its ascending (northbound) node at about day 117.1 (April 26). So, depending upon which satellite is to make its burn when the perigee is at the node, the manoeuvres could be expected within say, about one day (I'm not yet certain of the tolerances) of either day 16 April or 26 April. Thanks to Jim Nix, we know that USA 129 had not manoeuvred as of day 109.1618, about 1.5 days past its latest perigee node-crossing time. As I said, I do not know what the tolerances are, but it appears that in this case, USA 129 may not be the one planned to manoeuvre with its perigee at a node. If so, then both spacecraft are likely to manoeuvre at USA 116's perigee-at-node date of day 117 = 26 April, give or take a day or so. We'll know soon enough. One might ask, what is the advantage of manoeuvring when the perigee is at a node? It depends on the objectives of the manoeuvre. A simple apogee-raising can be carried out any time the satellite is at perigee, regardless of where the perigee is located, i.e. once per revolution. However, if the objective is to change the orbital inclination, then the most efficient place to do so is at a node. It could be done anywhere in the orbit, but only at the node is it possible to change the inclination without also changing the longitude, or more properly, the RA (right-ascension) of the ascending node. Moreover, when the perigee is at a node, it is possible to made a single-impulse manoeuvre to raise the apogee AND change the inclination, without affecting the argument of perigee or the RAAN. Another single-impulse option available when perigee is at a node, is to raise the apogee, and change both the inclination and argument of perigee, without affecting the RAAN. (I looked all this up in an early 1960's orbital dynamics textbook.) So making manoeuvres with the perigee at a node offers both efficiency and flexibility. One might ask, how could the Keyholes benefit from this? I believe that the flexibility is advantageous in maintaining both orbits' orientation with respect to the Earth and one another, within their design tolerances. Due to the significant atmospheric drag on Keyhole orbits, they are almost never exactly sun-synchronous, so their planes will drift over time with respect to the Earth and one another. Much of this drift will cancel out over time, as a result of the apogee-raising manoeuvres alone, but I suspect that small residual variances in argument of perigee and RAAN may build up, requiring some compensating correction. The single-impulse manoeuvre when perigee is at a node provides the flexibility to accomplish this, efficiently. We may or may not be able to deduce the mission constraints with sufficient detail to accurately predict the exact date and time of Keyhole manoeuvres, but I believe that we may already be able to narrow them down to one or the other spacecraft's perigee-at-node date, plus or minus some tolerance, as I suggested earlier. Ted Molczan ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Tue Apr 18 2000 - 08:15:24 PDT