Description of the multi-approach gravity field models from Swarm GPS data

Abstract

Abstract. Although the knowledge of the gravity of the Earth has improved considerably with CHAMP, GRACE, and GOCE (see appendices for a list of abbreviations) satellite missions, the geophysical community has identified the need for the continued monitoring of the time-variable component with the purpose of estimating the hydrological and glaciological yearly cycles and long-term trends. Currently, the GRACE-FO satellites are the sole dedicated provider of these data, while previously the GRACE mission fulfilled that role for 15 years. There is a data gap spanning from July 2017 to May 2018 between the end of the GRACE mission and start the of GRACE-FO, while the Swarm satellites have collected gravimetric data with their GPS receivers since December 2013. We present high-quality gravity field models (GFMs) from Swarm data that constitute an alternative and independent source of gravimetric data, which could help alleviate the consequences of the 10-month gap between GRACE and GRACE-FO, as well as the short gaps in the existing GRACE and GRACE-FO monthly time series. The geodetic community has realized that the combination of different gravity field solutions is superior to any individual model and set up the Combination Service of Time-variable Gravity Fields (COST-G) under the umbrella of the International Gravity Field Service (IGFS), part of the International Association of Geodesy (IAG). We exploit this fact and deliver the highest-quality monthly GFMs, resulting from the combination of four different gravity field estimation approaches. All solutions are unconstrained and estimated independently from month to month. We tested the added value of including kinematic baselines (KBs) in our estimation of GFMs and conclude that there is no significant improvement. The non-gravitational accelerations measured by the accelerometer on board Swarm C were also included in our processing to determine if this would improve the quality of the GFMs, but we observed that is only the case when the amplitude of the non-gravitational accelerations is higher than during the current quiet period in solar activity. Using GRACE data for comparison, we demonstrate that the geophysical signal in the Swarm GFMs is largely restricted to spherical harmonic degrees below 12. A 750 km smoothing radius is suitable to retrieve the temporal variations in Earth's gravity field over land areas since mid-2015 with roughly 4 cm equivalent water height (EWH) agreement with respect to GRACE. Over ocean areas, we illustrate that a more intense smoothing with 3000 km radius is necessary to resolve large-scale gravity variations, which agree with GRACE roughly at the level of 1 cm EWH, while at these spatial scales the GRACE observes variations with amplitudes between 0.3 and 1 cm EWH. The agreement with GRACE and GRACE-FO over nine selected large basins under analysis is 0.91 cm, 0.76 cm yr−1, and 0.79 in terms of temporal mean, trend, and correlation coefficient, respectively. The Swarm monthly models are distributed on a quarterly basis at ESA's Earth Swarm Data Access (at https://swarm-diss.eo.esa.int/, last access: 5 June 2020, follow Level2longterm and then EGF) and at the International Centre for Global Earth Models (http://icgem.gfz-potsdam.de/series/02_COST-G/Swarm, last access: 5 June 2020), as well as identified with the DOI https://doi.org/10.5880/ICGEM.2019.006 (Encarnacao et al., 2019).

GOCO06s – a satellite-only global gravity field model

“…The models published so far (GOCO01s, GOCO02s, GOCO03s, and GOCO05s) were used in a wide range of applications, for example, for global and local high-resolution models (e.g., Huang and Véronneau, 2013;Pail et al, 2016Pail et al, , 2018Klees et al, 2018;Slobbe et al, 2019), geophysical studies (e.g., Rummel et al, 2011a;Abrehdary et al, 2017;W. Chen et al, 2018), oceanographic studies (e.g., Farrell et al, 2012;Siegismund, 2013), or geodetic height unification (e.g., Vergos et al, 2018).…”

GNSS Profile from the Greenland Korth Expeditions in the Context of Satellite Data

“…This one-year gap may be completed by gravity fields derived from the GNSS tracking of three Swarm satellites, in low-Earth orbit since 11/2013 [56,57]. Indeed, it was demonstrated that precise satellite positions from the geodetic-quality GNSS receivers (one receiver aboard each Swarm satellite) could be processed to produce monthly gravity fields that, although with a reduced spatial resolution of 1300 km and more noise, are able to produce reasonable estimates also for the mass trends in Greenland [58][59][60][61].…”

The Drought Events over the Amazon River Basin from 2003 to 2020 Detected by GRACE/GRACE-FO and Swarm Satellites

“…Many scholars have discussed the possibility of using the Swarm model to detect time variable gravity signals. The results indicate that the time variable gravity signals of Swarm model are mainly concentrated in the long wave part, and the Swarm model has the possibility of monitoring the regional TWSC [22][23][24][25]. Some scholars used the Swarm model to detect TWSC and droughts in the ARB [19,25,26], and the main purpose of their studies is to use the Swarm model to fill the gap between GRACE and GRACE-FO missions.…”

“…The results indicate that the time variable gravity signals of Swarm model are mainly concentrated in the long wave part, and the Swarm model has the possibility of monitoring the regional TWSC [22][23][24][25]. Some scholars used the Swarm model to detect TWSC and droughts in the ARB [19,25,26], and the main purpose of their studies is to use the Swarm model to fill the gap between GRACE and GRACE-FO missions. However, to best of our knowledge, there is no literature combining GRACE, GRACE-FO and Swarm models to construct a continuous long observations series to detect and characterize the regional droughts.…”