102,546 research outputs found
Life with a weak Heart; Prolonging the Grace Mission despite degraded Batteries
No full textThe two Grace satellites were successfully launched on March 17, 2002 by a Russian Rockot launcher. GRACE not only was the first dual-satellite mission operated by GSOC, but it also
was the first formation-flying occurring at an altitude below 500 km. The mission is extremely successful from a scientific point of view and the originally envisaged mission duration of 5 years has more than doubled by now. A follow-on mission is planned by the same partners for 2016 and JPL projects a new generation in the twenties, so there is a strong incentive to prolong GRACE and try to bridge the gap.
Infirmity comes with age and several components have deteriorated or are defunct. Nevertheless, the scientific goals can still be obtained to nigh on 100%. The major challenge
for operations is posed by the degradation of the NiH2 batteries. These are comprised of 20 cells packaged in the common pressure vessel (CPV) configuration. However, two cells have shorted out on Grace 1 and one on Grace 2. The available capacity of the operational cells is also severely degraded. The current operational capacity of the batteries is limited to ≤ 3 Ah as compared to the original nameplate capacity of 16 Ah.
This paper describes the special operations needed to prolong the mission despite the considerable power constraints. The first Section gives a general overview with emphasis on the components relevant to this paper. The battery, its current state and the mission specific circumstances which require special handling are described in Section 2. The several threats to and failure mechanisms of NiH2 batteries are also presented here. The third Section then contains a detailed description of all measures taken to pamper the batteries. This includes
heater and parameter settings, special on-board macros, orbit-to-orbit charge regulation, but also physical actions such as turning the satellites away from the Sun to force battery
discharging and subsequent charging. The fourth Section, finally, presents conclusions, recommendations and an estimation of how long the Grace mission can be prolonged
Letter from Seth Low
No full textWhittier House scrapbooks document Whittier House programs, events, and anniversary celebrations through newspaper clippings, lecture fliers, newsletters, event programs, and ticket stubs. Newspaper clippings are primarily from the Jersey Journal. There is also Whittier House fundraising materials, including pamphlets, appeal letters, brochures, and postcards. The Whittier House Social Settlement, the first settlement house in New Jersey, was established in Jersey City, N.J. (Hudson County) in 1894. Founded by Cornelia Foster Bradford, who would remain with the organization as headworker until 1926, Whittier House was based on the settlement house, Toynbee Hall, in England. Whittier House provided various recreational and educational programs, along with much needed social services, for the immigrant populations of Jersey City. Many of these successful services were used as models for large-scale social reform movements through the state. In 1935, the Whittier House was taken over by the Boys' Club of Jersey City
Grace Lordan - Thinking big about your career
No full textMy guest is Grace Lordan. She is an Associate Professor in Behavioural Science and Director of The Inclusion Initiative at the London School of Economics and Political Science. She's also the author of Thing Big: Take Small Steps and Build the Future You Want. We have a fascinating discussion in which we explore the role of talent, hard work and luck in shaping your career. Grace explains why our emotions matter when it comes to how we receive feedback and act upon it, and how to ensure we feel like we're constantly moving forward towards our goals. All of which requires resilience and a willingness not only to accept failure but to learn from it. Finally, we discuss what the future holds with a wide lens – how, for example, people may need to accept a 'low-stuff life' – and from a personal perspective, why we should embrace uncertainty and stop striving to be in control of everything you do
Topside Ionosphere Sounding From the CHAMP, GRACE, and GRACE-FO Missions
No full textSatellites in Low Earth Orbit (LEO) are essential for sounding the topside ionosphere. In this work, we present and validate a data set of Total Electron Content (TEC) and in situ electron density observations from the Gravity Recovery And Climate Experiment (GRACE) and GRACE-Follow-On missions as well as a TEC data set from the CHAllenging Minisatellite Payload mission. Concerning TEC, special emphasis is put to ensure optimal consistency to the already existing Swarm and Gravity field and steady-state ocean circulation explorer (GOCE) TEC data sets. The newly processed satellite missions allow covering two full solar cycles with LEO slant TEC. Furthermore, the twin satellite missions GRACE and GRACE-FO equipped with inter-satellite K-band ranging allows to derive the horizontal TEC and, due to the small inter-satellite distance of the satellite pairs, an approximation for local electron density. However, the derived value of electron density is relative and requires calibration using external information. In this work, the calibration is performed using the IRI-2016 model. Radar observations, as well as in situ electron density observations available from Swarm B Langmuir probes, are used for validation. Conjunctions between satellites are used to validate the TEC time series. The newly derived data set is shown to be highly consistent with the already existing data sets with standard deviations below 3 TECU for TEC (even 1 TECU was reached for low solar flux) and an offset below 7 × 1010 m−3 with a standard deviation near 1 × 1011 m−3 for the electron density.</p
Evaluating interferometric baseline performances in a close formation flight by using relative GRACE GPS navigation solutions
No full textIn this paper the impact of relative position errors on
the interferometric baseline performance of multistatic
Synthetic Aperture Radar (SAR) satellites flying in a
close formation is analyzed and assessed. Based on accuracy
results obtained from differential GPS (DGPS) observations
between the twin Gravity Recovery and Climate
Experiment (GRACE) satellites, baseline uncertainties
are derived for three interferometric scenarios of a
dedicated SAR mission. To assess the accuracy with respect
to quality requirements of high-resolution DEMs,
topographic height errors are derived from the estimated
baseline uncertainties. The analysis reveals that the induced
low-frequencymodulation (height bias) fulfills the
relative vertical accuracy requirement (std. dev. less than 1 m linear
point-to-point error) of a Digital Terrain Elevation
Data model of level 3 (DTED-3) for most of the baseline constellations. DGPS can be used as an operational navigation tool for high-precise baseline estimation if a geodetic-grade dual-frequency spaceborne GPS receiver is assumed to be the primary instrument onboard the SAR satellites. To exemplarily demonstrate the error propagation into the inteferogram Orbital Phase Screens (OPS) are generated by differencing simulated interferograms. These are calculated from baselines of distinct GRACE error signatures and magnitudes
Dayside Upper-Thermospheric Density Fluctuations as Observed by GRACE and GRACE-FO at ∼500 km Height
No full textWe statistically investigate fluctuation amplitudes (normalized to the background values) of dayside low-/mid-latitude upper-thermospheric mass density as observed by the Gravity Recovery and Climate Experiment (GRACE) and GRACE-Follow-On (GRACE-FO) spacecraft at ∼500 km altitude between 2002 and 2022. There are three new findings in our results. First, the climatology closely replicates previous studies on stratospheric and upper-thermospheric gravity waves (GWs) below the GRACE(-FO) altitudes. For example, in low-latitude regions, the fluctuations are stronger above continents than in the oceanic area. Mid-latitude fluctuations prefer the local winter hemisphere to the summer, and the South American/Atlantic region in June solstice hosts stronger fluctuations than in any other low-/mid-latitude locations or seasons. Fluctuations are more intense under lower solar activity. The above-mentioned consistency of the GRACE(-FO) results with previous lower-altitude GW studies confirms that GWs can penetrate up to 500 km. Second, the anti-correlation of upper-thermospheric GW with solar activity, which has been earlier reported for multi-year time scales, can also be identified on the scale of the solar rotation period (∼27 days). Third, we demonstrate asymmetry between pre-noon and post-noon GWs. The former exhibits stronger GW activity, which may result from the colder thermosphere being more favorable for intense mass density fluctuations via secondary/tertiary GW generation.Astrodynamics & Space Mission
New thermosphere neutral mass density and crosswind datasets from CHAMP, GRACE, and GRACE-FO
No full textWe present new neutral mass density and crosswind observations for the CHAMP, GRACE, and GRACE-FO missions, filling the last gaps in our database of accelerometer-derived thermosphere observations. For consistency, we processed the data over the entire lifetime of these missions, noting that the results for GRACE in 2011–2017 and GRACE-FO are entirely new. All accelerometer data are newly calibrated. We modeled the temperature-induced bias variations for the GRACE accelerometer data to counter the detrimental effects of the accelerometer thermal control deactivation in April 2011. Further, we developed a new radiation pressure model, which uses ray tracing to account for shadowing and multiple reflections and calculates the satellite’s thermal emissions based on the illumination history. The advances in calibration and radiation pressure modeling are essential when the radiation pressure acceleration is significant compared to the aerodynamic one above 450 km altitude during low solar activity, where the GRACE and GRACE-FO satellites spent a considerable fraction of their mission lifetime. The mean of the new density observations changes only marginally, but their standard deviation shows a substantial reduction compared to thermosphere models, up to 15% for GRACE in 2009. The mean and standard deviation of the new GRACE-FO density observations are in good agreement with the GRACE observations. The GRACE and CHAMP crosswind observations agree well with the physics-based TIE-GCM winds, particularly the polar wind patterns. The mean observed crosswind is a few tens of m · s−1 larger than the model one, which we attribute primarily to the crosswind errors being positive by the definition of the retrieval algorithm. The correlation between observed and model crosswind is about 60%, except for GRACE in 2004–2011 when the signal was too small to retrieve crosswinds reliably
Global GRACE Data Assimilation for Groundwater and Drought Monitoring: Advances and Challenges
No full textThe scarcity of groundwater storage change data at the global scale hinders our ability to monitor groundwater resources effectively. In this study, we assimilate a state-of-the-art terrestrial water storage product derived from Gravity Recovery and Climate Experiment (GRACE) satellite observations into NASA's Catchment land surface model (CLSM) at the global scale, with the goal of generating groundwater storage time series that are useful for drought monitoring and other applications. Evaluation using in situ data from nearly 4,000 wells shows that GRACE data assimilation improves the simulation of groundwater, with estimation errors reduced by 36% and 10% and correlation improved by 16% and 22% at the regional and point scales, respectively. The biggest improvements are observed in regions with large interannual variability in precipitation, where simulated groundwater responds too strongly to changes in atmospheric forcing. The positive impacts of GRACE data assimilation are further demonstrated using observed low-flow data. CLSM and GRACE data assimilation performance is also examined across different permeability categories. The evaluation reveals that GRACE data assimilation fails to compensate for the lack of a groundwater withdrawal scheme in CLSM when it comes to simulating realistic groundwater variations in regions with intensive groundwater abstraction. CLSM-simulated groundwater correlates strongly with 12-month precipitation anomalies in low-latitude and midlatitude areas. A groundwater drought indicator based on GRACE data assimilation generally agrees with other regional-scale drought indicators, with discrepancies mainly in their estimated drought severity.Water Resource
GPS-Based Precision Baseline Reconstruction for the TanDEM-X SAR-Formation
No full textThe TanDEM-X formation employs two separate spacecraft to collect interferometric
Synthetic Aperature Radar (SAR) measurements over baselines of about 1 km. These will
allow the generation ofa global Digital Elevation Model (DEM) with an relative vertical
accuracy of 2-4 m and a 10 m ground resolution. As part of the ground processing, the
separation of the SAR antennas at the time of each data take must be reconstructed with a
1 mm accuracy using measurements from two geodetic grade GPS receivers. The paper
discusses the TanDEM-X mission as well as the methods employed for determining the
interferometric baseline with utmost precision. Measurements collected during the close
fly-by of the two GRACE satellites serve as a reference case to illustrate the processing
concept, expected accuracy and quality control strategies
- …
