Impact of a novel hybrid accelerometer on satellite gravimetry performance
Introduction
Climate change is one of the biggest societal challenges today. Understanding the underlying processes, which are most frequently related to mass variations in the Earth system, is a prerequisite for climate modelling and forecast. As changes in gravity are directly related to mass variability, satellite missions observing the Earth’s time varying gravity field are a unique tool for observing mass redistribution among the Earth’s system components including global changes in the water cycle, the cryosphere and the oceans. These observations provide essential indicators of both subtle and dramatic global change and are complementary to many other Earth observation technologies and missions. We have gained essential experience in observing the Earth’s gravity field via the successful GRACE (Tapley et al., 2004) and GOCE (Drinkwater et al., 2003) missions, the former currently being extended through the successfully launched GRACE Follow-On mission (Flechtner et al., 2018). The need for sustained observation of mass transport from space was expressed by a resolution adopted by the Council of the International Union of Geodesy and Geophysics (IUGG, 2016). Besides requesting higher spatial resolution in order to allow for more regional applications, it was in particular the need for long and consistent time series that has been expressed by an international expert panel under the umbrella of IUGG representing all relevant geoscientific applications (Pail et al., 2015).
Therefore, currently several concepts for next-generation gravity missions (NGGMs) are under investigation and discussion. One of the most promising constellations are double-pairs in Bender configuration (Bender et al., 2008), i.e. two GRACE/GRACE-FO-like pairs with high-precision inter-satellite ranging, where one pair is flying in a (near-)polar orbit and the other in an inclined one, where the inclination turns out to be optimal in a range of 60°–70°. It can be shown that with such a configuration temporal aliasing effects, which are one of the dominant error contributors of single-pair missions like GRACE and GRACE-FO and result in typical striping patterns, can be reduced significantly (in the order of a factor 5–10; Wiese et al., 2011, Daras and Pail, 2017). Another promising constellation is high-precision high-low tracking between high or medium orbiting and low orbiting satellites (Hauk et al., 2017), a concept which has been proposed to the European Space Agency (ESA) in response to the Earth Explorer 10 call (Pail et al., 2018).
From the instrument and payload point of view, currently the most limiting factor to a dedicated gravity field mission, especially to the retrieval of long-wavelength signals, is the performance of the accelerometer (Flechtner et al., 2016), which is required to measure the non-gravitational forces acting on the satellite. So far, in all gravity field missions, electrostatic accelerometers (EA) have been employed, offering some distinctive advantages such as an extremely high short-term sensitivity as well as robustness towards launch and space environment. However, they also suffer from certain drawbacks like (uncontrollable) thermal drift in the low frequency range below a few mHz and scale factor instabilities (Christophe et al., 2018, Klinger and Mayer-Gürr, 2016).
A new generation of instruments, relying on the manipulation of matter waves through atom interferometry, appears nowadays as very promising candidates for inertial measurements of high precision and accuracy. Cold Atom interferometers (CAI) have already proven on ground to be very high-performance sensors with the development of cold atom gravimeters (Peters et al., 2001), gravity gradiometers (McGuirk et al., 2002) and gyroscopes (Gustavson et al., 1997) in recent decades. This promising technology has demonstrated on-ground performances that compete with other state-of-the-art inertial sensors and is only expected to reach its full potential in space-based applications. In such a micro-gravity environment, the free fall time of the atoms in the instrument and therefore, the measurement scale factor, can be increased by orders of magnitude compared to ground-based sensors.
These two types of instruments have their own assets which are, for the electrostatic sensors, their demonstrated short-term sensitivity and their proven flight heritage, and for atom interferometers, amongst others, the time-invariant measurement stability as well as the absolute nature of the measurements, thus making calibration processes obsolete. These two technologies seem in some aspects very complementary and a hybrid sensor bringing together all their assets opens new perspectives in terms of space-based inertial measurements. Following this idea, we study an instrument configuration where the EA’s bias is periodically estimated through CAI measurements and then corrected with a specific gain. This gain then determines the rate at which the EA is recalibrated and is dependent on the EA and CAI performance. The hybridization is expected to be an important step towards significantly improving the quality of satellite-based gravity field retrieval. Its technological aspects as well as its impact on satellite gravimetry is subject of a currently ongoing joint ESA study between ONERA and TUM (Contract No. RFP/3-15194/17/NL/FF/mg).
This paper aims to evaluate and quantify the additional value of a hybrid accelerometer compared to a regular EA for the performance of satellite-based gravity field determination in the context of a GRACE-type and Bender-type mission and its corresponding requirements by means of numerical closed loop simulations which incorporate the novel instrument’s specifications.
The manuscript is structured as follows. In Section 2 the simulation environment alongside all relevant parameters is discussed. In Section 3 the simulation results based on different signal and error contributions to the observation system are presented and analysed. Also, the influence of the scale factor uncertainty is investigated. Section 4 summarizes the key findings and draws conclusions towards future developments.
Section snippets
Simulation environment and parameters
The simulations presented in this study have been conducted on IAPG’s closed-loop reduced-scale simulation software which is described in detail in Murböck, 2015, Murböck et al., 2014. Compared to a real gravity mission processing scheme this tool uses various simplifications as a trade-off for improved computation time. The most important aspects which are of relevance for the study are reviewed in the following.
First, externally generated Keplerian orbits and gravity field background models
Numerical simulations and results
In this chapter the impact of the hybrid instrument on the performance of gravity field retrieval is assessed and quantified based on a number of numerical closed-loop simulations. In a first group of experiments, we disregard temporal gravity signals and thus also temporal aliasing errors, and only take instrument errors of the accelerometer and the LRI into account. Afterwards, we include also temporal gravity to compare the error contribution related to temporal aliasing with the instrument
Summary and conclusions
In this paper we investigate the impact of a novel hybrid accelerometer consisting of a standard electrostatic component and a cold atom interferometer on the performance of gravity field retrieval in the context of a GRACE- and Bender-type satellite formation. This is done by means of numerical closed-loop simulations where four different accelerometer noise specifications are considered and different components of the total gravity field signal (static, non-tidal and tidal temporal
Acknowledgements
A big part of the investigations presented in this paper was performed in the framework of the study “Hybrid Atom Electrostatic System Follow-On for Satellite Geodesy”, ESA-ESTEC, Contract No. RFP/3-15194/17/NL/FF/mg funded by the European Space Agency.
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