Inter-satellite laser interferometry is still in its infancy and there are improvements to be made in nearly all aspects of these systems, from the measurement concept to the signal processing. First we discuss further work in the context of GRACE Follow-On, and extend the implications to designs for future missions where there is significant scope for development.
The GRACE Follow-On interferometer design is quite mature, although there is still room for the acquisition strategy to be optimized. The speed at which the fast steering mirror can be moved to cover the uncertainty cone limits the speed of the total commis- sioning sequence, due to the finite mechanical response of the mirror. Any improvements which optimize the scanning pattern by providing more efficient coverage will directly im- prove (decrease) the commissioning time. An example would be to modify the Lissajous figure slightly so that the scan tracks are more evenly spaced over the uncertainty cone, increasing the frequency of the slow axis without sacrificing scan line separation.
The acquisition signal processing could also be refined. One straightforward improve- ment is to implement a better bin vetoing algorithm. Instead of entirely rejecting all bins outside of the cutoff frequencies, a smarter bin vetoing process could enable more bins to be utilized per FFT. One possibility is a weighted approach, where an initial calibration
§8.2 Further work 107
is performed after launch to obtain the average noise in each FFT bin when no signal is present. The maximum value per FFT would be determined by finding the bin with the highest SNR (instead of just the highest amplitude). Using more bins per FFT could allow the slave laser’s sweep rate to be increased (thus decreasing the total commissioning time).
Although unlikely for use in GRACE Follow-On, a system capable of weak-light phase- locking could significantly improve acquisition time. Last year members of our group demonstrated phase-locking to a 30 femtowatt signal [96]. If the effective received power requirement on GRACE Follow-On could be reduced from 3 pW to 20 fW, the total commissioning time would be reduced by over a factor of 4 if all other parameters stayed the same. That is, for±3 mrad uncertainty in the beam pointing, if the laser was swept at a rate requiring the spatial scan search to be repeated 100 times, the overall commissioning time would take 1.4 hours (compared to 5.95 hours).
For satellite separations on GRACE-like scales, though, the capability for weak-light tracking is simply not needed for science operation. Weak-light phase-locking becomes important in low power missions such as LAGRANGE [97], where the satellites will be separated by tens of millions of kilometers. A phasemeter capable of tracking very low powers could be key to enabling these measurement architectures.
Significant future work is needed to establish the feasibility of an inter-satellite mea- surement concept based on digitally-enhanced interferometry. As mentioned earlier, there are many open areas to be examined, such as the best way to provide the necessary back- reflections from the desired targets to serve as the measurement fiducial. A version of DI compatible with both local measurements (homodyne) and inter-spacecraft measure- ments (heterodyne) is also highly desirable. Given the performance of digitally-enhanced interferometry already demonstrated in laboratory experiments, this measurement tech- nique shows considerable promise and could revolutionize the instrument design for future inter-satellite missions.
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