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In this comment below Will future deep space optical communications “ground stations” actually be in space, or on the ground? in response to a discussion about ground stations being used for tracking of deep space satellites requiring continuous, long periods of contact (transmitting to or receiving from a spacecraft's coherent transponder) I wrote:

...your comment about ranging is really important and thought provoking. Since a 1 meter aperture telescope can slew way faster than a 34 or 70 meter dish antenna, you can now think about intermittent sampling. Even though an extended timespan of data may be required, you don't necessarily need to monitor continuously as long as the phase noise of the encoded signal is sufficiently small. The bandwidth of the ranging signals must be tens of MHz at most, you need to sample a long time for accuracy, BW of an optical link can be tens of GHz so modulated Gold code can can come 1000x faster!)

Now I'd like to be shot down or perhaps agreed with.

Question: Assuming both-way optical legs of a delay-doppler measurement of a deep space spacecraft's movement is made and for some reason needed to be done over an eight hour period to get acceleration, would it have to be done continuously? If signal-to-noise issues were not there, couldn't the period be spanned by numerous short observations with the telescope slewing to other targets for contact in between?

My thinking is that the cadence that the gold codes (or similar) were clocked could be 1000 times faster since it's an optical beam being modulated (potentially at GHz) rather than a radio beam with its associated circa 1 MHz bandwidth.

Sure it would have to be reconstructed with a fancier computer and signal processing algorithm, but DSPs (hardwired or implemented in FPGAs) are awfully fast and low power these days, so I don't see any probelm with this form a signals point of view.

Question: Am I wrong? Can this be shown using mathematics and engineering and not just prose like "Clearly you are wrong because this is obviously impossible" type answers?

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  • $\begingroup$ I'm not sure if Doppler measurements (at the commonly available mm/s precision used with spacecraft) can be done at all using an optical communication. Radio-Doppler only works so well because the spacecraft sends a coherent signal back, at the same (or with a fixed ratio) frequency to the incoming one. $\endgroup$
    – asdfex
    Mar 30, 2021 at 16:38
  • $\begingroup$ @asdfex Coherent radio transponders use the carrier frequency (of order GHz) with a rational fraction offset in frequency via PLL. An optical transponder now way up at 10${}^5$ GHz could be modulated at order GHz and only the modulation need be coherent. That said, lasers can be locked to other lasers, though it's not something you'd put in a small box in a spacecraft. $\endgroup$
    – uhoh
    Mar 30, 2021 at 16:53
  • $\begingroup$ @asdfex I just ran across the linked video and remembered our discussion. I've just asked How did LADEE and LDRC measure it's distance from Lunar orbit to Earth to 1 centimeter accuracy using optical communications? $\endgroup$
    – uhoh
    Apr 8, 2021 at 15:56

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