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?
- JPL's Aug. 7, 2023 NASA’s Deep Space Communications to Get a Laser Boost
- which links to JPL's (older but undated) Superconducting Nanowire Single Photon Detectors for DSOC