Comparing GRACE-FO KBR and LRI Ranging Data with Focus on Carrier Frequency Variations

Abstract

The GRACE Follow-On satellite mission measures distance variations between its two satellites in order to derive monthly gravity field maps, indicating mass variability on Earth on a scale of a few 100 km originating from hydrology, seismology, climatology and other sources. This mission hosts two ranging instruments, a conventional microwave system based on K(a)-band ranging (KBR) and a novel laser ranging instrument (LRI), both relying on interferometric phase readout. In this paper, we show how the phase measurements can be converted into range data using a time-dependent carrier frequency (or wavelength) that takes into account potential intraday variability in the microwave or laser frequency. Moreover, we analyze the KBR-LRI residuals and discuss which error and noise contributors limit the residuals at high and low Fourier frequencies. It turns out that the agreement between KBR and LRI biased range observations can be slightly improved by considering intraday carrier frequency variations in the processing. Although the effect is probably small enough to have little relevance for gravity field determination at the current precision level, this analysis is of relevance for detailed instrument characterization and potentially for future more precise missions.

Alternative Approach to Tilt-to-Length Coupling Estimation for Laser Ranging Interferometers in Future Gravity Missions

“…)dt = BPF(δρ TTL ) + ϵ err (32) in which a LOS,i (i = 1, 2) denotes the projection of the measured linear accelerations on the LOS for SATi. The enlarged low-frequency error due to the integration, as well as the constant terms, are filtered out.…”

“…The master (SAT1) laser frequency can be divided into three parts, as shown in the following equation [31,32]:…”

MaQuIs - Mars Quantum Gravity Mission

“…For frequencies below 30 mHz gravity dominates the ranging signal and it is not possible to directly evaluate the interferometer noise floor. However, thorough modelling of the LRI and comparison with GRACE-FO microwave ranging data suggests that the well-understood tilt-to-length coupling effect, which can be subtracted in post-processing, and laser frequency noise remain the LRI's limiting noise sources in the µHz − mHz frequency range (Müller et al, 2022).…”

“…• Due to the unequal arm lengths of the laser interferometer that shall be used for the inter-satellite distance measurements, laser frequency noise dominates the range measurement. (Bachman et al, 2017;Müller et al, 2022) MaQuIs will deploy optical frequency references to increase the laser stability.…”

Satellite Gravity Field Recovery Using Variance‐Covariance Information From Ocean Tide Models

“…On GRACE‐FO, which has an identical orbit configuration to the GRACE mission, ll‐SST observations are operated by two independent systems in parallel. In addition to KBR, laser ranging interferometry (LRI, Kornfeld et al., 2019; Ghobadi‐Far et al., 2020) is used as a technology demonstrator with increased nanometer precision compared to micrometer precision KBR (V. Müller et al., 2022). However, the potential of the LRI cannot be fully exploited since errors in sub‐optimal modeling of high‐frequency geophysical phenomena, such as non‐tidal atmospheric and oceanic mass variations as well as ocean tide signals, dominate the total error budget of the gravity field recovery process in terms of temporal aliasing errors (Flechtner et al., 2015).…”