Effect of atmospheric anisoplanatism on earth-to-satellite time transfer over laser communication links

Abstract

The need for an accurate time reference on orbiting platforms motivates study of time transfer via free-space optical communication links. The impact of atmospheric turbulence on earth-to-satellite optical time transfer has not been fully characterized, however. We analyze limits to two-way laser time transfer accuracy posed by anisoplanatic non-reciprocity between uplink and downlink. We show that despite limited reciprocity, two-way time transfer can still achieve sub-picosecond accuracy in realistic propagation scenarios over a single satellite visibility period.

Measurement of the impact of turbulence anisoplanatism on precision free-space optical time transfer

“…To mimic the beam path displacement caused by transverse motion, we implement the comb-based O-TWTFT across two displaced counterpropagating optical links rather than a single bidirectional link. Our results compare well with theory [15,16] out to averaging times of 10 to 100 s without invoking any free parameters and indicate that the degradation in optical time-frequency transfer due to turbulence-induced reciprocity breakdown should be negligible. At longer averaging times we do see a disagreement with theory, likely resulting from displacement of the terminals at our optical link ends, as explained in Section 4.…”

“…Even at the 0.5-m separation (gray, short dashes), the time deviation remains below ~2 fs over all averaging times, similar to the ~5 fs at 1000 s presented inFig. 2of[16] as expected given the similar conditions and including our measurement noise floor. If the same terminal is used for both transmit and receive (terminal separation of 0 m) the time deviation is reduced by about 6 dB, demonstrating that the ground-level atmospheric conditions are responsible for a substantial portion of the timing noise if separate, displaced ground terminals are used.…”

“…The satellite has an altitude of 9000 km, a velocity V of 5.4 km/s, and a slew rate of 400 µrad/s giving a point-ahead angle PAA  of 36 µrad. Note that these conditions are similar to those presented in [16]. The model itself includes variation of Cn 2 vs altitude given by the widely-accepted Hufnagle-Valley turbulence model and upper-altitude winds as given by the Bufton wind model [24].…”

“…For line-of-sight motion, a newly developed implementation of comb-based O-TWTFT cancels the effects of relative velocity to below 100's of attoseconds [13,14]. In the present work, we explore the effects of motion traverse to the link experimentally and compare the results to previous theoretical discussions [15,16].…”

“…Additionally, the impact of turbulence anisoplanitism on two-way time-frequency transfer has been analyzed in two recent theoretical efforts. Robert et al [15] numerically model the effects of turbulence on both the phase noise and heterodyne efficiency of an optical link to a satellite in low-earth orbit, finding a fractional [16], and extrapolate to satellite-based links.…”