GRACE Measurements of Mass Variability in the Earth System

Monthly gravity field estimates made by the twin Gravity Recovery and Climate Experiment (GRACE) satellites have a geoid height accuracy of 2 to 3 millimeters at a spatial resolution as small as 400 kilometers. The annual cycle in the geoid variations, up to 10 millimeters in some regions, peaked predominantly in the spring and fall seasons. Geoid variations observed over South America that can be largely attributed to surface water and groundwater changes show a clear separation between the large Amazon watershed and the smaller watersheds to the north. Such observations will help hydrologists to connect processes at traditional length scales (tens of kilometers or less) to those at regional and global scales

First observations with a GNSS antenna to radio telescope interferometer

We describe the design of a radio interferometer composed of a Global Navigation Satellite Systems (GNSS) antenna and a Very Long Baseline Interferometry (VLBI) radio telescope. Our eventual goal is to use this interferometer for geodetic applications including local tie measurements. The GNSS element of the interferometer uses a unique software-defined receiving system and modified commercial geodetic-quality GNSS antenna. We ran three observing sessions in 2022 between a 25 m radio telescope in Fort Davis, Texas (FD-VLBA), a transportable GNSS antenna placed within 100 meters, and a GNSS antenna placed at a distance of about 9 km. We have detected a strong interferometric response with a Signal-to-Noise Ratio (SNR) of over 1000 from Global Positioning System (GPS) and Galileo satellites. We also observed natural radio sources including Galactic supernova remnants and Active Galactic Nuclei (AGN) located as far as one gigaparsec, thus extending the range of sources that can be referenced to a GNSS antenna by 18 orders of magnitude. These detections represent the first observations made with a GNSS antenna to radio telescope interferometer. We have developed a novel technique based on a Precise Point Positioning (PPP) solution of the recorded GNSS signal that allows us to extend integration time at 1.5 GHz to at least 20 minutes without any noticeable SNR degradation when a rubidium frequency standard is used.Comment: 33 pages, 19 figure

A Reconciled Estimate of Ice-Sheet Mass Balance

We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth's polar ice sheets. We find that there is good agreement between different satellite methods-especially in Greenland and West Antarctica-and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by -142 plus or minus 49, +14 plus or minus 43, -65 plus or minus 26, and -20 plus or minus 14 gigatonnes year(sup 1), respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 plus or minus 0.20 millimeter year(sup 1) to the rate of global sea-level rise

E/I-Vector Separation for Safe Switching of the GRACE Formation

After more than three years in orbit, a longitude swap maneuver was required to exchange the leading and trailing spacecraft of the GRACE formation. While the two satellites are nominally separated by about 220km in along-track direction, a close encounter took place during the swap sequence. Based on the successful use for co-location of geo-stationary satellites, the concept of eccentricity/inclination vector separation has been suggested for safe proximity operations in this mission phase. Taking care of the natural evolution of the relative orbital elements of GRACE 1 and 2, optimum maneuver dates were identified. By proper timing of the maneuvers a safe limit for the minimum distance during the encounter could be guaranteed even in case of arbitrary thruster performance errors. This allowed the use of a fuel optimal maneuver sequence with individual drift start and stop maneuvers. The paper provides a conceptual outline and mathematical description of the e/i-vector separation for spacecraft formations in low Earth orbit and discusses the GRACE longitude swap maneuvers

Mission design concepts for repeat groundtrack orbits and application to the ICESat mission

textThe primary objective of the NASA sponsored ICESat mission is to study the short and long term changes in the ice mass in the Greenland and Antarctica regions. The satellite was therefore placed into a frozen near-polar near-circular repeat groundtrack to ensure an adequate coverage of the polar regions while keeping the groundtrack periodic and reducing the variations in the orbital elements, and more specifically the semi-major axis of the ICESat orbit. After launch, a contingency plan had to be devised to compensate for a laser that dangerously compromised the lifetime of the ICESat mission. This new plan makes an intensive use of the ICESat subcycles, a characteristic of the repeat groundtrack orbits often over-looked. The subcycle of a repeat groundtrack orbit provide global coverage within a time shorter than the groundtrack repetition period. For a satellite with an off-nadir pointing capacity, the subcycles provide near-repeat tracks which represents added opportunity for altimetry measurement over a specific track. The ICESat subcycles were also used in a very innovative fashion to reposition the satellite within its repeat cycle via orbital maneuvers called phasing maneuver. The necessary theoretical framework is provided for the subcycle analysis and the implementation of phasing maneuvers for any future repeat orbit mission. In the perspective of performing cross-validation of missions like CryoSat using the ICESat off-nadir capacity, a study was conducted to determine the geolocations of crossovers between two different repeat groundtrack Keplerian orbits. The general analytical solution was applied to ICESat vs. several other repeat groundtrack orbit mission, including the future ICESat-II mission. ICESat’s repeat groundtrack orbit was designed using a disturbing force model that includes only the Earth geopotential. Though the third body effect from the Sun and the Moon was neglected in the orbit design, it does in fact disrupt the repeatability condition of the groundtrack and consequently implies orbit correction maneuvers. The perturbations on ICESat orbit due to the third body effect are studied as a preliminary work towards including these forces in the design of the future ICESat-II repeat groundtrack orbit.Aerospace Engineering and Engineering Mechanic

Auxiliary Space-Based Systems for Interpreting Satellite Altimetry: Satellite Gravity

This chapter outlines satellite altimetry: the mean geoid and the mass component of sea level, or ocean bottom pressure variability and discusses how the mean gravity field is determined from Gravity Recovery and Climate Experiment (GRACE) and Gravity Field and Steady-State Ocean Circulation Explorer. It also discusses how it can be combined with surface gravity from marine and airborne gravity, satellite altimetry, and terrestrial gravity to obtain even higher resolution geoids. The chapter describes how time-variable gravity is estimated from GRACE, how it is converted to mass density, and how the relatively small signal of ocean mass variability is extracted from the noisy measurement. It explores how the mean geoid and altimetry can be combined to determine the mean dynamic topography, from which the surface geostrophic currents can be derived. The chapter discusses how ocean bottom pressure variations are related to sea level measured by an altimeter and to changes in the deep ocean circulation

Combination Service for Time-variable Gravity Field Solutions (COST-G) - current status

In the frame of the European Gravity Service for Improved Emergency Management (EGSIEM), a prototype service was established to combine monthly gravity field solutions from the past US- German GRACE mission in order to deliver improved gravity field solutions for applications in Earth and environmental science research. This prototype now is in transition to the Combination Service for Time-variable Gravity Field Solutions (COST-G), a Product Center of the International Gravity Field Service (IGFS) of the International Association of Geodesy (IAG). We report on the achievements made so far and the transition of the prototype phase into regular operation. We present a comparison, validation and combination of the latest GRACE gravity field time-series of different GRACE processing centers, based on recent RL03 Level-1B GRACE observation data as well as updated background models and processing standards. A focus is laid on the effect of different background modeling strategies on the resulting gravity field models and the relative weights determined by Variance Component Estimation on solution level

GRACE Level-1 Status

No abstract availabl