Circumstances and Prediction of KeyHole Satellite Manoeuvres

From: Ted Molczan (
Date: Tue Apr 18 2000 - 08:14:04 PDT

  • Next message: Ted Molczan: "Latest USA 129 elements; manoeuvre alert continues."

    Bjoern Gimle wrote in the message "RE: USA 129: no manoeuvre as of 19:50 UTC on
    17 April":
    > 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
    So making manoeuvres with the perigee at a node offers both efficiency and
    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
    Ted Molczan
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