Reason for FIA Radar 1/USA 215 retrograde orbit?

From: Allen Thomson (
Date: Fri Oct 01 2010 - 23:00:02 UTC

  • Next message: Leo Barhorst: "Re: Reason for FIA Radar 1/USA 215 retrograde orbit?"

    I've been doing a bit of head-scratching about the reason for the retrograde orbit of FIA Radar 1, aka USA 215, and have only come up with a couple of guesses.  If anybody can provide others or comment on the two presented here, it would be appreciated.
    Guess 1:  If the satellite does in fact carry a synthetic aperture imaging radar (SAR), the orbit may have been chosen to increase the doppler shift returned from objects on or near the ground vs what would be returned from a prograde orbit at the reciprocal inclination (57 deg). SAR processing exploits doppler to encode position on the ground in one dimension, and so the greater the doppler spread, the finer the spatial resolution corresponding to each frequency bin.  Retrograde orbits have greater velocity-over-ground than prograde ones, hence greater doppler and total doppler spread.
    Guess 2: Highly retrograde orbits can give greater frequency of coverage/shorter revisit times than prograde and near-polar ones. The Israeli Ofeqs are a notable example of this.
    This is all very nebulous, but there has to be a reason for the retrograde orbit. 
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