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    Re: New false horizons? Just watch satellites...
    From: Robin Jouan
    Date: 2017 Jul 23, 06:50 -0700

    Frank,

    I agree with you. Watching satellites along the background sky can provide a good navigation alternative. About accuracy, near the zenith the scale factor between measured angle and latitude is given by the satellite altitude to Earth radius ratio (roughly 1/10 with ISS). But also it directly depends on the accuracy of the ephemerids you can get for the satellite, at the moment.

    Some military applications (in USA and elsewhere) are using optical sights of GNSS /GPS to do the job, noticeably when their RF signals are jammed. Binoculars are inefficient with MEO, but automatically steered telescopes can be used (at least 15-20 cm diameter should be needed). The advantage with GNSS/GPS is that ephemerids are broadcasted with the RF signals. In a number of cases when the jam is heavy, almanach data can be used instead of current ephemerids, with an impact of the same order about positioning accuracy. So, nothing to do with an enhanced sextant, you are right. But most of time, it can be done locally on board, from a self-contained data base, updated when GNSS/GPS signals are available. As far as I know, 18 hours of autonomy are currently required under a first step of degradation. 72 hours are a target.

    Regards,

    Yves

       
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