The NASA and ESA space agencies established the Interagency Gravity Science Working Group (IGSWG) in 2013 to advise on future gravity mission concepts. Both NASA and ESA acknowledge the need for continued global observation of mass transport. Observing mass transport with the highest possible accuracy and spatial resolution and with temporal resolutions from daily to 33 monthly is crucial for understanding the functioning and evolution of the Earth system (climate, geohazards, water cycle). Improved observation of global mass transport in terms of temporal and spatial resolution would benefit a diversity of Earth science domains (e.g. hydrology, oceanography, cryospheric sciences, solid Earth sciences, atmospheric sciences) for both scientific and application focused users, and are necessary to fully characterize diverse global change processes. The working group finalized its activities in 2016 by delivering a report with the title "Towards a sustained observing system for mass transport to understand global change and to benefit society". This report addresses the compatibility between user requirements, constellation concepts, and expected performance. In addition, a roadmap for implementation in the post-2020 time frame is included. This roadmap defines the path towards sustained observation of mass transport with the required accuracy and spatio-temporal resolution, while addressing the need for cooperation between different space agencies and/or providers, and assessing required activities such as studies and technological developments for both the near and longer terms. Coordinators ESA and NASA John Labreque, Thomas Johnson, Ben Phillips (NASA) Roger Haagmans, Luca Massotti, Christian Siemes (ESA)

N ext Generation Gravity Missions (NGGMs) expected to be launched in the mid-term future have set high anticipations for an enhanced monitoring of mass transport in the Earth system, establishing their products applicable to new scientific fields and serving societal needs. The European Space Agency (ESA) has issued several studies on concepts of NGGMs. Following this tradition, the project “Additional Constellations & Scientific Analysis Studies of the Next Generation Gravity Mission” picks up where the previous study ESA-SC4MGV left off. One of the ESA-ADDCON project objectives is to investigate the impact of different orbit configurations and parameters on the gravity field retrieval. Given a two-pair Bender-type constellation, consisting of a polar and an inclined pair, choices for orbit design such as the altitude profile during mission lifetime, the length of retrieval period, the value of sub-cycles and the choice of a prograde over a retrograde orbit are investigated. Moreover, the problem of aliasing due to ocean tide model inaccuracies, as well as methods for mitigating their effect on gravity field solutions are investigated in the context of NGGMs. The performed simulations make use of the gravity field processing approach where low-resolution gravity field solutions are co-parameterized in short-term periods (e.g. daily) together with the long-term solutions (e.g. 11-day solution). This method proved to be beneficial for NGGMs (ESA-SC4MGV project) since the enhanced spatio-temporal sampling enables a selfde- aliasing of high-frequency atmospheric and oceanic signals, which may now be a part of the retrieved signal. The