The Laser Ranging Interferometer (LRI) instrument on the Gravity Recovery and Climate Experiment (GRACE) Follow-On mission has provided the first laser interferometric range measurements between remote spacecraft, separated by approximately 220 km. Autonomous controls that lock the laser frequency to a cavity reference and establish the 5 degrees of freedom two-way laser link between remote spacecraft succeeded on the first attempt. Active beam pointing based on differential wave front sensing compensates spacecraft attitude fluctuations. The LRI has operated continuously without breaks in phase tracking for more than 50 days, and has shown biased range measurements similar to the primary ranging instrument based on microwaves, but with much less noise at a level of $1\mathrm{nm}/\sqrt{\mathrm{Hz}}$ at Fourier frequencies above 100 mHz.

The Laser Ranging Interferometer onboard the Gravity Recovery and Climate Experiment Follow-On satellites is the first laser interferometer in space measuring satellite-to-satellite distance variat...

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.

The precise tracking of distance variations between two satellites in low Earth orbit can provide key data for the understanding of the Earth’s system, specifically on seasonal and sub-seasonal water cycles and their impact on water levels. Measured distance variations, caused by local variations in gravitational field, serve as inputs to complex gravity models with which the movement of water on the globe can be identified. Satellite missions GOCE (2009–2013) and GRACE (2002–2017) delivered a significant improvement to our understanding of spatial and temporal gravity variations. Since 2018, GRACE Follow-On has been providing data continuity and features for the first time through the use of a laser interferometer as the technology demonstrator, in addition to a microwave ranging system as the main instrument. The laser interferometer provides an orders-of-magnitude lower measurement noise, and thereby could enable a significant improvement in the measurement of geoids if combined with suitable improvements in auxiliary instrumentation and Earth system modelling. In order to exploit the improved ranging performance, the ESA is investigating the design of a ‘Next Generation Gravity Mission’, consisting of two pairs of satellites with laser interferometers, improved accelerometers and improved platform performance. In this paper, we present the current design of the laser interferometer developed by us, the development status of the individual instrument units and the options available.

The GRACE follow-on satellites carry the very first interspacecraft Laser Ranging Interferometer (LRI). After more than four years in orbit, the LRI outperforms the sensitivity of the conventional Microwave Instrument (MWI). However, in the current data processing scheme, the LRI product still needs the MWI data to determine the unknown absolute laser frequency, representing the “ruler” for converting the raw phase measurements into a physical displacement in meters. In this paper, we derive formulas for precisely performing that conversion from the phase measurement into a range, accounting for a varying carrier frequency. Furthermore, the dominant errors due to knowledge uncertainty of the carrier frequency as well as uncorrected time biases are derived. In the second part, we address the dependency of the LRI on the MWI in the currently employed cross-calibration scheme and present three different models for the LRI laser frequency, two of which are largely independent of the MWI. Furthermore, we analyze the contribution of thermal variations on the scale factor estimates and the LRI-MWI residuals. A linear model called Thermal Coupling (TC) is derived, which significantly reduces the differences between LRI and MWI to a level where the MWI observations limit the comparison.

The inter-satellite distance changes of the GRACE Follow-On spacecraft are measured by the main science instrument, the K-Band Ranging (KBR) system, and by the new Laser Ranging Interferometer (LRI), which exhibits a higher precision. The official ranging data is provided by the Science Data System (SDS) in the so-called KBR1B and LRI1B data products, which are available in the latest version v04. However, the Albert Einstein Institute (AEI) in Hannover is deriving alternative LRI1B data products (v50+v52). These data sets allow us to investigate different processing strategies that might improve the quality of the ranging data.We will present the most important processing steps for the AEI-derived data products and compare them to the official SDS v04 data. The main differences in the processing regardthe deglitching algorithm, which is essential to remove disturbances induced by thruster activations or by single event upsets transformation of LRI time-tags to GPS time using a smooth and at day-bounds continuous correction formed by CLK1B, TIM1B, datation reports and empirical offsets obtained by daily cross-correlating KBR and LRI (only v50) conversion of phase-to-range using a novel formula that was published recently and that can utilize a time-depend laser frequency (scale factor) light-time-correction using alternative formulas that are expected to be more precise tilt-to-length (TTL) coupling correction, which is similar to the antenna offset correction (AOC) in KBR1B but missing in LRI1B v04 so far. Furthermore, we have implemented a quality flag to indicate special events such as sun-blinding periods or momentum transfer events that might reduce the quality of the ranging data. This allows to down-weight affected segments during gravity field recovery. By comparing LRI1B v04 and AEI-derived LRI1B data, a verification and validation of the official dataset is possible. We will conclude with some examples where the LRI1B data quality can be improved compared to the v04 results.Our LRI1B datasets are available at:  https://www.aei.mpg.de/grace-fo-ranging-datasets