11 May 2004

ESA s Proba satellite here shows a winding segment of the 7240-km long Great Wall of China situated just northeast of Beijing. The Great Wall s relative visibility or otherwise from orbit has inspired much recent debate.

The 21 hours spent in space last October by Yang Liwei - China s first ever space traveller - were a proud achievement for his nation. The only disappointment came as Liwei informed his countrymen he had not spotted their single greatest national symbol from orbit. "The Earth looked very beautiful from space, but I did not see our Great Wall," Liwei told reporters after his return

China has cherished for decades the idea that the Wall was just about the only manmade object visible to astronauts from space, and the news disappointed many. A suggestion was made that the Wall be lit up at night so it can definitely be seen in future, while others called for school textbooks to be revised to take account of Liwei s finding.

However such revisions may be unnecessary, according to American astronaut Eugene Cernan, speaking during a visit to Singapore: "In Earth s orbit at a height of 160 to 320 kilometres, the Great Wall of China is indeed visible to the naked eye."

About Proba

Proba (Project for On Board Autonomy) is an ESA micro-satellite built by an industrial consortium led by the Belgian company Verhaert, launched in October 2001 and operated from ESA s Redu Ground Station (Belgium).

Orbiting 600 km above the Earth’s surface, Proba was designed to be a one-year technology demonstration mission of the Agency but has since had its lifetime extended as an Earth Observation mission. It now routinely provides scientists with detailed environmental images thanks to CHRIS - a Compact High Resolution Imaging Spectrometer developed by UK-based Sira Electro-Optics Ltd - one of the main payloads on the 100 kg spacecraft.

Also aboard is the HRC, a small-scale monochromatic camera made up of a miniature Cassegrain telescope and a 1024 x 1024 pixel Charge-Coupled Device (CCD), as used in ordinary digital cameras, taking 25-km square images to a resolution of five metres. Proba boasts an intelligent payload and has the ability to observe the same spot on Earth from a number of different angles and different combinations of optical and infra-red spectral bands. A follow-on mission, Proba-2, is due to be deployed by ESA around 2005.

Liwei may well have been unlucky with the weather and local atmospheric or light conditions – with sufficiently low-angled sunlight the Wall s shadow if not the Wall itself could indeed be visible from orbit.

What is for sure is that what the human eye may not be able to see, satellites certainly can. Proba s High Resolution Camera (HRC) acquired this image of the Wall from 600 km away in space. The HRC is a black and white camera that incorporates a miniature Cassegrain telescope, giving it far superior spatial resolution to the human eye.

So while the HRC resolves man-made objects down to five square metres, astronauts in low Earth orbit looking with the naked eye can only just make out such large-scale artificial features as field boundaries between different types of crops or the grid shape formed by city streets. They require binoculars or a zoom lens to make out individual roads or large buildings.

SURVEYORS USE SEISMIC WAVES TO STUDY MTBE CONTAMINATION

Gregory Croftongcrofton@tahoedailytribune.com June 9, 2004

Following the contamination in water from MTBE an attempt by seismic surveyor teams to investigate has started at South Shore in the South Tahoe Public Utility District. Methyl tertiary butyl ether is a gasoline additive that has shut down 13 public water wells since it was discovered it had leaked underground at South Shore in 1997.

Paulsson Geophysical Services, of Brea, Calif., wants to send seismic waves, generated by a heavy weight landing on a 2-foot square metal plate positioned on the ground outside the well house, 205 feet down a foot-wide pump shaft.

Geophones dropped at a variety of levels within the well will collect information and may allow a geophysicist to determine the "properties of the sediment surrounding the well," and "It will also allow a better understanding of how the groundwater flows through the subsurface."

Results will be interpreted by James Rector, a geophysicist at UC Berkeley, within the next two months, said Ivo Bergsohn, hydrogeologist for the district.

This study will cost $25,000 and was paid by money available from a $69 million settlement the district received for MTBE cleanup from oil companies. The district originally had planned to conduct underground surveys on a much larger area near Barton Meadow heading toward its well on Paloma Avenue, but proposals for the study indicated a cost of more than $500,000. The experts from the Lawrence Livermore National Laboratory and UC Berkeley recommended the smaller survey to determine if seismic tests would be effective in evaluating the way things look underground.

- Gregory Crofton can be reached at (530) 542-8045 or by e-mail at gcrofton@tahoedailytribune.com

GPS MONITORING

Adopted by BP for their Schiehallion FPSO, BPP offers state of the art Global Positioning System (GPS) technology for accurately determining the real time position of moored vessels to a high level of accuracy.

On modern Floating Production Systems it is of vital importance to be able to accurately monitor the vessel’s mooring system in order to avoid problems owing to the interference with flexible risers and moorings. Usually the mooring lines terminate at a turret at which their close proximity leads to these design complications.

In the past several measurement systems may have been used to monitor the position including hydro-acoustic and microwave.The most recent offering from BPP is of particular interest to designers since it entirely avoids the problems of locating equipment in hazardous areas altogether and obviates the need for additional angular motion sensors, hence reducing costs. It is based on the use of 3D Differential Global Position System (DGPS). The arrangement used provides real time position to an accuracy of about 1m at the turret. The 3D GPS provides higher accuracy in yaw heading measurements than is achievable with a gyrocompass required by alternative systems. Over the 200m or so lengths of an FPSO the accuracy of this measurement is most important.

This system enables all the equipment to be located well away from the turret hazardous area, avoids cable runs or need for local radio data links, and provides previously unavailable levels of positioning accuracy.

The instantaneous position of the vessel can be monitored from various view points, typically a plan view is displayed with information relating to:

·Vessel heading.

·Velocity of excursion away from and towards the target position.

·Distance to the target position.

·Pre-set excursion limits.

The system can be easily interfaced with existing navigational equipment, and thanks to the in-built flexibility of the software developed by BPP, data can be manipulated which can provide to the operator a variety of valuable information such as:

·Statistics of the vessel movements.

·Time series of displacements.

·Simultaneous logging of coincident data.

·Maximum excursions.

For more information on this subject or related subjects please contact us.

URBAN ENVIRONMENTS

Applications in dense urban environments raise a number of concerns, including potential RF interference. Since September 2002, there has been no loss of tracking ability due to RF interference, though at times the ambiguity solution for GPS displacements cannot be achieved satisfactorily, leading to lesser reliability of displacement predictions. This results from satellite blockage at the 20-story reference station building, leading to higher GDOPs here and at times tracking of only 4–5 satellites, in comparison with the taller rover structure, tracking 8 or more satellites. System performance may be enhanced by moving the reference site to a less shielded location, though it is difficult to find such a site close to downtown Chicago. Despite this, data satisfying ambiguity checks and possessing satisfactory amplitudes of motion relative to the PQTs (Position Quality Threshold) are being evaluated.

These PQTs become quite important in distinguishing meaningful displacements from inherent errors in the GPS tracking. Consider the data sample from November 30, 2002, filtered to isolate the resonant response of the rover structure along the Northerly and Easterly axes (Figure 9). Both filtered displacements (light blue) manifest an upward ascent at the end of their records. To see if this is meaningful, the PQTs are superimposed in green. The PQTs are elevated relative to levels observed in the experimental validations: during calibrations the thresholds were on the order of 5–7 millimeters, whereas they are at times doubled in full-scale.

Due to the baseline separation and noise level in the urban environment, the PQTs observed here should not be alarming, though this situation does require motions of the building to be on the order of a few centimeters to be reasonably tracked. Note the escalating displacements at the conclusion of this record are accompanied by increased PQTs, indicating that this feature is not a multipath effect, but instead a function of satellite availability and GDOP.

Incidentally, inspection of log files for this data revealed that a satellite at the reference station was obstructed over this period of time. The gradual loss of this satellite began to degrade the GDOP, elevating the PQTs as shown just beyond the 2000th second

This demonstrates the importance of these thresholds in analyzing full-scale GPS data and determining the quality of the tracked positions.

Multipath. Another issue that must be addressed in urban monitoring is multipath interference, which can still be present even when choke-ring antennas are used, due to satellite signals reflected off surfaces above the antenna. We have initiated generation of full-scale baseline multipath signatures and a series of controlled multipath tests to identify and remove these systematic errors that may surface in GPS displacement data collected in this program. As the project progresses, multipath errors will be more accurately identified and removed to allow a more reliable correlation of GPS displacements against the predicted structure displacements under varying wind events.

Operations. The utility of the information collected in this monitoring program, particularly the GPS data, can be extended beyond the validation of modeling and analysis techniques used in design. If direct dialogs between the reference and rover enable near real-time capabilities, displacements can be streamed not only to the project team but also building owners to assist in operation of building systems such as elevators, skydecks, and coordination of rooftop and other exterior maintenance. However, this requires rectification of many of the communications issues.

Previous work in GPS monitoring had the added benefit of being in relatively open areas so that radio telemetry links could be readily utilized, for example transmitting over water between an instrumented long-span bridge and shoreline. Others have used direct fiber optic links to the reference, again only feasible over short, simple runs. Both these approaches would be very difficult in this application due to the poor line-of-sight and heavily-populated separation between the reference and rover. We also considered cellular links, but the poor reception quality at the rover site precluded that option.

Spread-spectrum wireless radios may present a viable solution that has already been applied in other GPS monitoring programs. Though the rover site hosts a number of two-way radio communications transmitters, interference issues can be minimized through the selection of wireless systems with occupied bandwidths distinct from other transmission sources onsite. In addition, recent advances in the technology have further improved noise immunity and transmission security. We will therefore launch a pilot study to assess the feasibility of radio spread-spectrum wireless radios to enable communications between the reference and rover stations and near real-time displacement tracking in this program.

Conclusion

Just as Chicago s skyline helped set the precedent for tall buildings, these structures continue that innovative tradition by ushering in a new era in GPS monitoring in the United States, advancing the current state-of-the art in high-rise design. Through the Chicago full-scale monitoring program, the first-ever systematic comparison of tall building response against finite element and wind tunnel models used in their design is currently underway.

In this program, we emphasize the need to calibrate and experimentally validate any GPS sensor before installation in full-scale, so that displacement tracking limitations and resolutions can be accurately benchmarked. As the accuracy of GPS continuously fluctuates throughout the day due to the position and availability of satellites, it is especially important to provide some reliability measure, such as PQTs. Ultimately, the ongoing collection of GPS data and its assessment throughout this project will address the unique challenges facing applications in dense urban environments.

Acknowledgements

We are grateful for the financial support of the National Science Foundation (grant CMS 00-85109) and the University of Notre Dame. We acknowledge project collaborators: the Boundary Layer Wind Tunnel Laboratory, University of Western Ontario in Canada; Skidmore Owings and Merrill LLP, Chicago; and Dr. Dae Kun Kwon, Tiphaine Williams, and Lijuan Wang of Notre Dame, who assisted with GPS testing and processing. This project would not be possible without the cooperation and assistance of the building owners and management of all the involved buildings. Thanks also to Mike Edwards and the Notre Dame Research Office, John Berkebile of the St. Joseph County, Indiana, Parks Department, and Leica Geosystems staff for technical advice and support.

HIGH ACCURACY REAL-TIME ENGINEERING MONITORING USING LOW COST GPS EQUIPMENT

start date: 01/08/2000, end date: 31/07/2003

Nowadays, man-made structures such as dams, regulators, locks, public and private buildings, and bridges can be very large and have very strategic high value. To sustain such strategic structures in good condition, deformation-monitoring systems should be set up in order to assess structural changes that may occur. This information is sent to decision-making agencies, which enables them to take necessary precautions.

Deformation monitoring, analysis, and prediction are of a major and ever-growing concern in practically all fields of engineering and geoscience. Safety, economical design of man-made structures, efficient functioning and fitting of structural elements, environmental protection, and development of preventative measures in the case of natural disasters (land slides, earthquakes, liquefaction of earth dams, etc.) require good understanding of the causes and mechanisms of deformation, and this can be achieved only through the proper monitoring and analysis of deformable bodies.

GPS signals suffer from many errors that potentially degrade the positioning accuracy. In order to achieve position accuracy comparable to that of conventional methods, these errors should be avoided, eliminated or corrected. Fortunately, the majority of these errors could be eliminated using double differencing techniques, especially for short base line applications, as in the case of deformation monitoring analysis. The most obvious error remaining after double differencing is multipath, as it has no correlation between receivers.

The main objective of this project is to look at how very cheap L1-only receivers could be permanently located on engineering structures such as bridges, dams, and tall buildings, in order to monitor movement with an accuracy close to that could be obtained with conventional deformation monitoring methods in real time. A key element of this work will be calibrating a site for multipath error so that small apparent changes in position due to multipath can be separated from the real movements that we are trying to detect.

In addition it is intended to study the effect of changing the environment around the receiver on the multipath behavior, in order to assess the apparent movements due to multipath as opposed that of real movement due to external actions.

Contacts: Reda Ali supervised by Paul Cross