Since June, 2018, the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) is extending the 15-year monthly mass change record of the GRACE mission, which ended in June 2017. The GRACE-FO instrument and flight system performance has improved over GRACE. Better attitude solutions and enhanced pointing performance result in reduced fuel consumption and gravity range rate post-fit residuals. One accelerometer requires additional calibrations due to unexpected measurement noise. The GRACE-FO gravity and mass change fields from June 2018 through December 2019 continue the GRACE record at an equivalent precision and spatiotemporal sampling. During this period, GRACE-FO observed large interannual terrestrial water variations associated with excess rainfall (Central US, Middle East), drought (Europe, Australia), and ice melt (Greenland). These observations are consistent with independent mass change estimates, providing high confidence that no intermission biases exist from GRACE to GRACE-FO, despite the 11-month gap. GRACE-FO has also successfully demonstrated satellite-to-satellite laser ranging interferometry. Plain Language Summary Mass change is a fundamental climate system indicator and provides an integrated global view of how Earth's water cycle and energy balance are evolving. The Gravity Recovery and Climate Experiment (GRACE) mission monitored mass changes every month from 2002 through 2017. Since June 2018, GRACE Follow-On (GRACE-FO) continues this data record, tracking and monitoring changes in ice sheets and glaciers, near-surface and underground water storage, as well as changes in sea level and ocean currents. GRACE-FO instruments have been successfully calibrated and are providing new monthly mass change observations at a consistent spatial resolution and data quality with GRACE. Since its launch, GRACE-FO has measured record land water storage changes in 2018 and 2019 in response to extreme heat waves and droughts over Europe and Australia, as well as to extreme rainfall events over the United States and Middle East. In the summer of 2019, GRACE-FO measured record-level Greenland mass loss rates. A novel laser ranging interferometer was successfully demonstrated on GRACE-FO, laying the groundwork for improved future satellite gravity observations.

The International GNSS Service (IGS) has been producing the total troposphere zenith path delay (ZPD) product that is based on combined ZPD contributions from several IGS Analysis Centers (AC) since GPS week 890 in 1997.A new approach to the production of the IGS ZPD has been proposed that replaces the direct combination of diverse ZPD products with point positioning estimates using the IGS Combined Final orbit and clock products. The new product was formally adopted in 2007 after several years of concurrent production with the legacy product. We describe here the advantages of the new approach for the IGS ZPD product, which enhance the value of the new ZPD product for climate studies. We also address the impact the IGS adoption in November 2006 of new GPS antenna phase center standards has had on the new ZPD product. Finally we describe plans to further enhance the ZPD products.

[1] A ray tracing GPS signal multipath simulator which takes into account the signal reflection and diffraction from surrounding objects has been developed. By properly modeling the environment around an antenna and the GPS receiver's tracking loop, this simulator can assess the GPS signal multipath error. Thus, it can be used in the early design phase of an experiment to foretell hazardous environmental configurations that can cause severe multipath. It can also aid in finding the best antenna type, location, and orientation within a given environment, and provides a quantitative estimate of multipath errors on GPS measurements. The capability of the simulator is demonstrated by using the International Space Station (ISS) environment.

<p>The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO), launched in May 2018, provides invaluable information about mass change in the Earth system, continuing the legacy of GRACE. Fundamental requirements for successful mass change recovery are precise orbit determination and inter-satellite ranging, determination of the relative clock alignment of the ultra-stable oscillators (USOs), precise attitude determination, and accelerometry. NASA/Caltech Jet Propulsion Laboratory is the official Level-1 data processing and analysis center, and is currently processing software version 04. Here we present analysis of the aforementioned GRACE-FO sensor data, as well a preview of an upcoming GRACE reprocessing, and a discussion of measurement performance.</p>

No abstract