Engineering topics
Willis, P.
Publications and source records attributed to Willis, P..
Horizontal and Vertical Velocities Derived from the IDS Contribution to ITRF2014, and Comparisons with Geophysical Models
In the context of the 2014 realization of the International Terrestrial Reference Frame (ITRF2014), the International DORIS Service (IDS) has delivered to the IERS a set of 1140 weekly SINEX files including station coordinates and Earth orientation parameters, covering the time period from 1993.0 to 2015.0. From this set of weekly SINEX files, the IDS Combination Center estimated a cumulative DORIS position and velocity solution to obtain mean horizontal and vertical motion of 160 stations at 71 DORIS sites. The main objective of this study is to validate the velocities of the DORIS sites by comparison with external models or time series. Horizontal velocities are compared with two recent global plate models (GEODVEL 2010 and NNR-MORVEL56). Prior to the comparisons, DORIS horizontal velocities were corrected for Global Isostatic Adjustment (GIA) from the ICE-6G (VM5a) model. For more than half of the sites, the DORIS horizontal velocities differ from the global plate models by less than 2-3 mm/yr. For five of the sites (Arequipa, Dionysos/Gavdos, Manila, Santiago) with horizontal velocity differences wrt these models larger than 10 mm/yr, comparisons with GNSS estimates show the veracity of the DORIS motions. Vertical motions from the DORIS cumulative solution are compared with the vertical velocities derived from the latest GPS cumulative solution over the time span 1995.0-2014.0 from the University of La Rochelle (ULR6) solution at 31 co-located DORIS-GPS sites. These two sets of vertical velocities show a correlation coefficient of 0.83. Vertical differences are larger than 2 mm/yr at 23 percent of the sites. At Thule the disagreement is explained by fine-tuned DORIS discontinuities in line with the mass variations of outlet glaciers. Furthermore, the time evolution of the vertical time series from the DORIS station in Thule show similar trends to the GRACE equivalent water height.
Europa Clipper: Selection and Characterization of Electronic Parts and Materials for the Europa Clipper Mission
No abstract available
Research Activities for the DORIS Contribution to the Next International Terrestrial Reference Frame
For the preparation of ITRF2008, the IDS processed data from 1993 to 2008, including data from TOPEX/Poseidon, the SPOT satellites and Envisat in the weekly solutions. Since the development of ITRF2008, the IDS has been engaged in a number of efforts to try and improve the reference frame solutions. These efforts include (i) assessing the contribution of the new DORIS satellites, Jason-2 and Cryosat2 (2008-2011), (ii) individually analyzing the DORIS satellite contributions to geocenter and scale, and (iii) improving orbit dynamics (atmospheric loading effects, satellite surface force modeling. . . ). We report on the preliminary results from these research activities, review the status of the IDS combination which is now routinely generated from the contributions of the IDS analysis centers, and discuss the prospects for continued improvement in the DORIS contribution to the next international reference frame.
DORIS weekly solutions: status report and open problems
Since the official start of the International DORIS Service (IDS) in 2003, several Analysis Centers have submitted time series of station coordinates in SINEX format. We discuss here the different DORIS weekly solutions available for ITRF 2004 and for the IERS Combinationa Campaign Project.
GGOS working group on ground networks and communications
Activities of this Working Group include the investigation of the status quo and the development of a plan for full network integration to support improvements in terrestrial reference frame establishment and maintenance, Earth orientation and gravity field monitoring, precision orbit determination, and other geodetic and gravimetric applications required for the long-term observation of global change. This integration process includes the development of a network of fundamental stations with as many co-located techniques as possible, with precisely determined intersystem vectors. This network would exploit the strengths of each technique and minimize the weaknesses where possible. This paper discusses the organization of the working group, the work done to date, and future tasks.
Status of DORIS stations in Antarctica for precise geodesy
In Antarctica, besides the quite numerous GPS stations, four DORIS stations are permanently operating. In addition to the permanent DORIS stations, episodic campaigns took place at DomeC/Conccordia and on Sorsdal and Lambert glaciers. In this paper, we first collect general information concerning the stations and the campaigns (location, start of measurements, etc). We then present the results of observations of the permanent stations keeping in mind that we are primarily interested here in the vertical component, which is the most uncertain component.
Scientific applications of GNSS: could Galileo really make a difference
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DORIS as a potential part of a global geodetic observing system
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DORIS as a potential part of a global geodetic observing system
We have processed all available DORIS data from all available satellites, except JASON over the past 10 years. Weekly solutions have been produced for stations positions coordinates, geocenter motion and scale factor stability. We present here the current accuracy presently achievable for all types of potential geodetic products.
The proposed international DORIS service
The goal of this paper is to discuss the status of the DORIS Pilot experiment, to analyze the progress made during the past two years and to propose the official creation within the IAG of an International DORIS Service (IDS).
TOPEX/JASON combined GPS/DORIS orbit determination in the tandem phase
This paper will present Jason-1 POD results obtained at JPL using the Gipsy-Oasis I1 (GOA). Results from standard tests (orbit overlaps, Laser control points) suggest that 1 to 2 cm radial orbit precision is already being achieved using the JPL reduced-dynamic filter approach. New DORIS POD strategies will be an emphasis of this paper.
Intial orbit determination results for Jason-1: towards a 1-cm orbit
The U.S/France Jason-1 oceanographic mission is carrying state-of-the-art radiometric tracking systems (GPS and Doris) to support precise orbit determination (POD) requirements. The performance of the systems is strongly reflected in the early POD results. Results of both internal and external (e.g., satellite laser ranging) comparisons support that the 2.5 cm radial Rh4S requirement is being readily met, and provide reasons for optimism that 1 cm can be achieved. We discuss the POD strategy underlying these orbits, as well as the challenging issues that bear on the understanding and characterization of an orbit solution at the l-cm level. We also describe a system for producing science quality orbits in near real time in order to support emerging applications in operational oceanography.
Dimensional stability of CFRP composites for space based reflectors
This paper provides a recent assessment of the main factors influencing dimensional stability of composites.
Space Applications of Polymeric Materials
Polymers constitute an extremely large class of materials generally based on carbon chemistry. Their long chain structures and unique molecular architecture make them distinct from metals and ceramics and provided them with a very wide range of properties.
Fabrication and Thermo-Optical Properties of the MLS Composite Primary Reflector
The Microwave Limb Sounder (MLS) is a limb-sounding radiometer sensing emissions in the millimeter and sub-millimeter range.
Low Mass Muscle Actuators (LoMMAs) Using Electroactive Polymers
NASA is using actuation devices for many space applications and there is an increasing need to cut their cost as well as reduce their size, mass, and power consumption. Existing transducing actuators, such as piezoceramics, are inducing limited displacement levels. Potentially, electroactive polymers (so called EAP) can be formed as inexpensive, low-mass, low-power, miniature muscle actuators that are superior to the widely used actuators.
Electroactive Polymers (EAP) low Mass Muscle Actuators
Actuation devices are used for many space applications with an increasing need to reduce their size, mass, and power consumption as well as cut their cost. Existing transducing actuators, such as piezoceramics, induce limited displacement levels. Potentially, electroactive polymers (EAP) have the potential for low-mass, low-power, inexpensive miniature muscle actuators that are superior to the widely used actuators.