Climate Experiment Follow-On Research Articles

Tilt-to-Length Coupling in the GRACE Follow-On Laser Ranging Interferometer

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 variations. One of its main noise sources at low frequencies is the so-called tilt-to-length coupling, caused by satellite pointing variations. This error is estimated by fitting a linear coupling model, making use of the so-called center-of-mass calibration maneuvers. These maneuvers are performed regularly for the original purpose of center-of-mass determination. Here, the results of the tilt-to-length estimations for the Laser Ranging Interferometer are presented in terms of coupling factors, which are all within and thus meet the requirements. From these parameters, estimations of nadir and cross-track components of the spacecraft center-of-mass positions with respect to the interferometer reference point are derived, providing an additional method to track center-of-mass movement over time.

GRACE-FO: The Gravity Recovery and Climate Experiment Follow-On Mission

The Gravity Recovery and Climate Experiment Follow-On mission was conceived to continue the successful legacy of the recently decommissioned Gravity Recovery and Climate Experiment mission and, at the same time, serve as a platform to demonstrate the first-ever in-space intersatellite laser ranging interferometer as a technology pathfinder for future gravity mapping missions. Launched in May of 2018, the Gravity Recovery and Climate Experiment Follow-On observatory builds on the design of the original Gravity Recovery and Climate Experiment satellites, but incorporates a number of improvements based on lessons learned, and features significantly increased complexity due to the accommodation of the laser ranging interferometer. This paper provides an overview of the challenging requirements levied on the observatory, and the mission and spacecraft design necessary to meet them. As the original Gravity Recovery and Climate Experiment spacecraft design has not been, to date, broadly discussed in the engineering literature, this paper further highlights original design rationales, where still applicable. As future satellite-based geodesy missions with similar constraints and challenges emerge, the implementation of the Gravity Recovery and Climate Experiment Follow-On observatory described herein can serve as a pathfinder and guide for the successful realization of these endeavors.

Arm-Locking with the GRACE Follow-On Laser Ranging Instrument.

Arm-locking is a technique for stabilizing the frequency of a laser in an inter-spacecraft interferometer by using the spacecraft separation as the frequency reference. A candidate technique for future space-based gravitational wave detectors such as the Laser Interferometer Space Antenna (LISA), arm-locking has been extensive studied in this context through analytic models, time-domain simulations, and hardware-in-the-loop laboratory demonstrations. In this paper we show the Laser Ranging Instrument flying aboard the upcoming Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission provides an appropriate platform for an on-orbit demonstration of the arm-locking technique. We describe an arm-locking controller design for the GRACE-FO system and a series of time-domain simulations that demonstrate its feasibility. We conclude that it is possible to achieve laser frequency noise suppression of roughly two orders of magnitude around a Fourier frequency of 1Hz with conservative margins on the system's stability. We further demonstrate that 'pulling' of the master laser frequency due to fluctuating Doppler shifts and lock acquisition transients is less than 100MHz over several GRACE-FO orbits. These findings motivate further study of the implementation of such a demonstration.

What Can be Expected from the GRACE-FO Laser Ranging Interferometer for Earth Science Applications?

The primary objective of the gravity recovery and climate experiment followon (GRACE-FO) satellite mission, due for launch in August 2017, is to continue the GRACE time series of global monthly gravity field models. For this, evolved versions of the GRACE microwave instrument, GPS receiver, and accelerometer will be used. A secondary objective is to demonstrate the effectiveness of a laser ranging interferometer (LRI) in improving the satellite-to-satellite tracking measurement performance. In order to investigate the expected enhancement for Earth science applications, we have performed a full-scale simulation over the nominal mission lifetime of 5 years using a realistic orbit scenario and error assumptions both for instrument and background model errors. Unfiltered differences between the synthetic input and the finally recovered time-variable monthly gravity models show notable improvements with the LRI, on a global scale, of the order of 23 %. The gain is realized for wavelengths smaller than 240 km in case of Gaussian filtering but decreases to just a few percent when anisotropic filtering is applied. This is also confirmed for some typical regional Earth science applications which show randomly distributed patterns of small improvements but also degradations when using DDK4-filtered LRI-based models. Analysis of applied error models indicates that accelerometer noise followed by ocean tide and non-tidal mass variation errors are the main contributors to the overall GRACE-FO gravity model error. Improvements in these fields are therefore necessary, besides optimized constellations, to make use of the increased LRI accuracy and to significantly improve gravity field models from next-generation gravity missions.

Interspacecraft link simulator for the laser ranging interferometer onboard GRACE Follow-On.

Link acquisition strategies are key aspects for interspacecraft laser interferometers. We present an optical fiber-based setup able to simulate the interspacecraft link for the laser ranging interferometer (LRI) on gravity recovery and climate experiment Follow-On. It allows one to accurately recreate the far-field intensity profile depending on the mispointing between the spacecraft, Doppler shifts, and spacecraft attitude jitter. Furthermore, it can be used in late integration stages of the mission, since no physical contact with the spacecraft is required. The setup can also be easily adapted to other similar missions and different acquisition algorithms.

Tone-assisted time delay interferometry on GRACE Follow-On

We have demonstrated the viability of using the Laser Ranging Interferometer on the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) space mission to test key aspects of the interspacecraft interferometry proposed for detecting gravitational waves. The Laser Ranging Interferometer on GRACE-FO will be the first demonstration of interspacecraft interferometry. GRACE-FO shares many similarities with proposed space-based gravitational wave detectors based on the Laser Interferometer Space Antenna (LISA) concept. Given these similarities, GRACE-FO provides a unique opportunity to test novel interspacecraft interferometry techniques that a LISA-like mission will use. The LISA Experience from GRACE-FO Optical Payload (LEGOP) is a project developing tests of arm locking and time delay interferometry (TDI), two frequency stabilization techniques, that could be performed on GRACE-FO. In the proposed LEGOP TDI demonstration one GRACE-FO spacecraft will have a free-running laser while the laser on the other spacecraft will be locked to a cavity. It is proposed that two one-way interspacecraft phase measurements will be combined with an appropriate delay in order to produce a round-trip, dual one-way ranging (DOWR) measurement independent of the frequency noise of the free-running laser. This paper describes simulated and experimental tests of a tone-assisted TDI ranging (TDIR) technique that uses a least-squares fitting algorithm and fractional-delay interpolation to find and implement the delays needed to form the DOWR TDI combination. The simulation verifies tone-assisted TDIR works under GRACE-FO conditions. Using simulated GRACE-FO signals the tone-assisted TDIR algorithm estimates the time-varying interspacecraft range with a rms error of $\ifmmode\pm\else\textpm\fi{}0.2\text{ }\text{ }\mathrm{m}$, suppressing the free-running laser frequency noise by 8 orders of magnitude. The experimental results demonstrate the practicability of the technique, measuring the delay at the 6 ns level in the presence of a significant displacement signal.

Weak-light phase tracking with a low cycle slip rate.

The Gravity Recovery and Climate Experiment Follow-On mission will use a phase-locked loop to track changes in the phase of an optical signal that has been transmitted hundreds of kilometers between two spacecraft. Beam diffraction significantly reduces the received signal power, making it difficult to track, as the phase-locked loop is more susceptible to cycle slips. The lowest reported weak-light phase locking is at 40 fW with a cycle slip rate of 1 cycle per second. By selecting a phase-locked loop bandwidth that minimized the signal variance due to shot noise and laser phase fluctuations, a 30 fW signal has been tracked with a cycle slip rate less than 0.01 cycles per second. This is tracking at a power 25% lower with a 100-fold improvement in the cycle slip rate. This capability will enable a new class of missions, opening up new opportunities for space-based interferometry.