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President Obama’s trip to Cuba this week marks a historic milestone in the normalization process between the U.S. and Cuba. At the same time, the two countries are working to improve maritime navigation safety and related areas of mutual interest to protect lives and property at sea.
Ambassador Jeffrey DeLaurentis, the chief of mission at the U.S. Embassy in Havana, and Col. Candido Alfredo Regalado Gomez, chief of Cuba’s National Office of Hydrography and Geodesy (ONHG), signed a Memorandum of Understanding (MOU) on maritime navigation.
The MOU calls for cooperation in the areas of hydrography, oceanography, geodesy and related services of mutual interest. One of the major focuses will be to improve maritime navigation safety including efforts to ensure the accuracy of both electronic and paper charts, eliminate charting overlaps and fill in gaps in navigational chart coverage.
In February 2015, less than two months after President Obama announced the United States’ new approach toward Cuba, the National Oceanic and Atmospheric Administration (NOAA) and the ONHG, through a set of reciprocal exchanges, launched what became a year-long effort to formulate the technical exchange that is a normal course of affairs between most of the other maritime nations of the world. Both agencies are working on plans for monitoring and forecasting tides and currents for ports and improving positioning networks among other related scientific and technical activities.
“NOAA has a strong interest in both improving navigational safety and in protecting the marine environment in the heavily travelled and vibrant waters between our two countries in the Straits of Florida,” said Russell Callender, Ph.D., assistant NOAA administrator for the National Ocean Service. “We welcome this agreement and the progress it represents.”
“Improved navigation services are important for commercial mariners and individual boaters alike,” said Ambassador DeLaurentis, “and it is particularly important as authorized trade and authorized travel increase between the two countries.”
“This MOU will allow us to fill gaps in essential navigational data, working on a practical level with our Cuban counterparts,” said Kathryn Ries, deputy director of NOAA’s Office of Coast Survey. “The U.S. works with hydrographic offices of all nations that have waters adjacent to the United States and our territories, and this agreement improves the exchange of charting information with Cuba as well.”
The MOU is the first step in what is expected to be a long-term collaboration between the two countries.
In addition to aligning each country’s navigational charts, NOAA and ONHG are sharing data for the creation of a new international chart (known in mariner’s parlance as “INT chart”) 4149, which will cover south Florida, the Bahamas, and north Cuba. NOAA plans to publish the new chart this year.
News courtesy of NASA / Goddard Space Flight Center
The surface of Earth is constantly being reshaped by earthquakes, volcanic eruptions, landslides, floods, changes in sea levels and ice sheets and other processes. Since some of these changes amount to only millimeters per year, scientists must make very precise measurements of the landscape and ocean in space and time in order to study their evolution and help mitigate their impacts.
The foundation for these precision measurements is the terrestrial reference frame, which serves the same purpose as landmarks along a trail. Earth-orbiting satellites and ground-based instruments make use of this reference system to pinpoint their own locations and, in turn, those of the features they are tracking. It is also the hidden framework relied upon by aircraft to determine their locations and by mobile phone apps that provide maps and driving directions. And it is a fundamental reference for interplanetary navigation of spacecraft.
NASA helps maintain the worldwide standard called the International Terrestrial Reference Frame, or ITRF, and recently contributed to an update issued by the International Earth Rotation and Reference Systems Service’s International Terrestrial Reference System Product Center at the Institut National de l’Information Géographique et Forestière (IGN) in Paris.
“The new release lays the groundwork for more detailed studies than ever before of global changes in Earth’s ocean, ice sheets, land and atmosphere,” said Stephen Merkowitz, manager of NASA’s Space Geodesy Project at the Goddard Space Flight Center in Greenbelt, Md.
Earth-observing satellites — such as the Jason 3 spacecraft, launched in January through a U.S.-European partnership, and the upcoming ICESat-2 mission — will be among the beneficiaries of the new standard.
Officially called ITRF2014, the update released in late January is the ninth ITRF issued since 1992. More than a thousand observing stations run by NASA and other scientific institutions worldwide contributed to it, collecting data through 2014.
Global in nearly every sense of the word, the ITRF is made up of specific geographic positions around the world, along with information about how each one drifts over time. This is important because the positions move relative to each other, with some drifting more rapidly than others. The reference frame includes details about how quickly and in which directions the positions are expected to move.
Some of the drift happens because of the motion of Earth’s tectonic plates, which is well understood. Drift motions may also include the gradual rebounding of land that was covered by ice sheets during the last ice age, as well as land subsiding due to climatic effects or human activity, such as withdrawal of groundwater. Less predictable are changes due to earthquakes. Large quakes will cause a sudden shift in position and also may alter the drift rate or direction at that location. Recent versions of the reference frame have started to include these effects.
“An important feature of the latest International Terrestrial Reference Frame is that the model has a more sophisticated way of incorporating the effects of earthquakes,” said Chopo Ma, a geophysicist at Goddard who was involved in producing and analyzing data for the latest reference frame.
Helping to improve the ITRF is one of the primary goals of NASA’s Space Geodesy Project. Four measurement techniques are used by stations worldwide to collect data for the reference frame.
In Satellite Laser Ranging, or SLR, precise measurements are made by sending short laser pulses from ground stations to Earth-orbiting satellites equipped with suitable reflectors. The distance is calculated from the time it takes for the pulse to complete the round trip back to the ground station.
The second method is called Very Long Baseline Interferometry, or VLBI. Ground stations spread across the globe observe dozens of quasars, which are distant enough to serve as stable reference points. By carefully timing when the signals from the quasars are recorded by each station, the precise geometry of the antenna network can be deduced, and Earth’s orientation in space and its rotation rate can be measured.
The technique known as Doppler Orbitography and Radiopositioning Integrated by Satellite, or DORIS, takes advantage of the Doppler effect, which is what we hear when an ambulance’s siren changes pitch as it drives toward or away from us. The frequency of a radio signal from a DORIS beacon experiences the same effect while traveling from Earth to an orbiting satellite. By measuring the frequency change, it’s possible to work backward to figure out the distance from the beacon to the satellite.
The final method makes use of the Global Navigation Satellite System, known as GNSS — a network that includes GPS and other navigation satellites. Radio signals are broadcast by GNSS satellites and received at many locations worldwide.
“The big advantage of GNSS is the dense network of stations distributed around the world,” said Richard Gross, who manages the Terrestrial Reference Frame combination center at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif. “For the reference frame, on the order of a thousand GNSS stations contribute position measurements.”
Because there are GNSS receivers at the stations that perform the other three measurement techniques, GNSS also provides a method for tying together all four approaches. And when scientists worldwide want to measure how the ground is moving, they access the reference frame by using GNSS to determine their positions.
In preparation for the new reference frame, research teams worldwide carried out data analysis, looking at between 20 and 30 years of data for each method. Scientists at Goddard and the University of Maryland, Baltimore County, coordinated the data analysis for VLBI, SLR and DORIS, and JPL contributed GNSS data. All of the geodetic data for the reference frame have been archived at the NASA Crustal Dynamics Data Information System, located at Goddard, and distributed to users worldwide.
Looking forward, NASA is upgrading the stations in its Space Geodetic Network. The Space Geodesy Project at Goddard is managing these upgrades, and work is already under way at stations in Hawaii and Texas. The upgraded stations will help fill in geographic gaps in the global system, helping to improve future versions of the reference frame.
In addition, scientists are looking at other possible approaches for combining the four data types to produce an improved reference frame. Research on advancing the ITRF is conducted not only at IGN, but also at JPL’s Terrestrial Reference Frame combination center and at a similar center at the Deutsches Geodätisches Forschungsinstitut in Munich. Each center produces its own independent solution, which scientists will compare to see what they can learn from different approaches.
“We renew the International Terrestrial Reference Frame every few years because it’s more than a set of geographical positions,” said Frank Lemoine, a Goddard scientist involved in producing and analyzing data for the new standard. “It’s a projection about what will happen to those positions in the future, and our ability to extend the reference frame into the future gets better and better over time.”
UNAVCO has selected Septentrio to be the preferred vendor of next-generation GNSS reference stations for the Geodesy Advancing Geosciences and EarthScope (GAGE) Facility. The Preferred Vendor status is valid through the duration of the GAGE Facility Cooperative Agreement with the National Science Foundation (NSF).
The selection of Septentrio was made following a rigorous competitive selection process. Under the agreement, Septentrio will supply GNSS reference stations to upgrade and expand the continuous GNSS reference station networks operated by UNAVCO.
UNAVCO is a non-profit university-governed consortium that facilitates geosciences research and education using geodesy. UNAVCO’s GAGE Facility includes more than 2,000 continuously operating GPS/GNSS reference stations around the world. UNAVCO-supported networks include EarthScope’s Plate Boundary Observatory (PBO), the Continuously Operating Caribbean GPS Observational Network (COCONet), the Trans-Boundary Land and Atmosphere Long-Term Observational and Collaboration Network (TLALOCNet) and the Polar Earth Observational Network (POLENet).
“This decision, following a highly competitive technical evaluation, is an important validation of Septentrio’s family of high-performance GNSS receivers,” said Neil Vancans, vice president, of Septentrio Americas. “Septentrio is firmly established as the preferred choice of receivers within the scientific and academic community for ionospheric observations, timing and other demanding applications, due to their superior multipath mitigation, resistance to ionospheric disturbance and in-band jamming. We look forward to working closely with UNAVCO to support its important mission of advancing geodetic science.”
“The critical technology in the new generation of reference station receivers is available in the Asterx 4 OEM boards, which also provide low and scalable power options. This technology is being extended across the full line of Septentrio products,” added Vancans.
“This Preferred Vendor relationship gives UNAVCO a unique opportunity to provide technical input during the ongoing development process of Septentrio’s next-generation PolaRx-series GNSS receivers,” said Frederick Blume, senior project manager for Development and Testing at UNAVCO.
Septentrio made the announcement during ION GNSS+, being held this week in Tampa, Fla.
INTERGEO 2015, to be held Sept. 15-17 in Stuttgart, Germany, is the world’s leading conference trade fair for geodesy, geoinformation and land management.
GeoLearn, a company focused on serving the geospatial industry with video-based online learning and continuing education credits, has added nine courses on geodesy topics by retired NGS Chief Geodetic Surveyor and GPS World contributor David Doyle.
Doyle is a contributing editor for survey to GPS World’s monthly Survey Scene newsletter. His first column appeared in May.
GeoLearn’s new introductory geodesy courses carry approval for professional development hours (PDHs) from the ABET-accredited geospatial program at Texas A&M University-Corpus Christi.
“I’ve been addressing groups of surveyors and other professionals who use NGS data. In these courses, I take those decades of interaction and try to anticipate and address the most common problems they’ve encountered and most of the questions they would ask,” Doyle said.
Doyle’s first eight courses are offered as a series, though students can pick and choose. The first is an introductory stage-setting course on geodetic fundamentals for those who have been hesitant to delve into any geodesy-related topic. It is an excellent primer for a broad spectrum of geospatial professionals and technicians in fields such as land surveying, engineering and technical GIS applications. The rest of the series includes two on classical horizontal datums and contemporary horizontal datums and two on vertical datums. He includes an additional course on future datums and another on coordinate systems.
Doyle’s ninth course uncovers the “secret sauce” to understanding and using NGS data sheets. It helps novices and experienced alike to understand all the clues and guideposts embedded in such sheets. He includes a discussion of how to understand the accuracy (horizontal and vertical) of various marks based on the metadata provided right in the data sheet. Also included is information on how to access photographs of the marks and how you can update the information using a simple program that you can download from the NGS website.
“Dave was of phenomenal service to geospatial professionals when he was with NGS,” said Joe Paiva, CEO of GeoLearn. “We are proud to be the only 24/7 education source that delivers Dave’s quality, video-based education on these needed topics.”
The National Geodetic Survey is seeking applicants for a geodesist (real-time kinematic network) with the Spatial Reference System Division. Applications are being accepted through December 4.
The individual selected for this position will:
Application links are:
The European-focused imaGIne conference will provide attendees with an opportunity learn what is going on in the geospatial sector and to network with important decision-makers. EUROGI’s “imaGIne: Opportunities Everywhere” Conference will be held October 8-9 in Berlin, Germany.
The imaGIne conference will take place at the same venue and at the same time as the InterGeo trade fair.
A key aim of the conference is to showcase the best that Europe has to offer in the geospatial field, thus the conference subtitle “Geographic Information Expertise: Made in Europe.” The aim has guided EUROGI and its member associations in the selection of themes and speakers, organizers said.
Plenary Sessions and Keynotes
During the plenary sessions, presentations will be given by top European and global experts.
Two speakers will provide a view of the state of the European geospatial industry from a global perspective, highlighting its strengths, weaknesses, opportunities and threats. Other issues which will be covered in the plenaries include the Internet of Things (billions of interlinked sensors across Europe), Linked Data (joining up data which was otherwise unconnected) and Big Data (massive amounts of data from diverse sources and across many fields).
In addition to the plenary sessions, there will be 15 parallel sessions of 90 minutes each, each of which has a specific thematic focus. The themes include Job Creation and Economic Growth, Energy, Environment, Demography, Smart Cities, Copernicus (Europe’s Earth Observation initiative), Open Data, Big Data, and Insurance. The sessions will not only provide interesting insights, but will also feature panel sessions with discussions of pertinent issues, as well as opportunities for audience engagement.
The European Commission’s Joint Research Centre will provide an opportunity to discuss the European Union Location Framework, a set of policies and measures which aim to facilitate the integration of geospatial information into e-government services and to increase alignment in and between existing and future EU policies.
InterGeo Fair. InterGeo is the world’s leading trade fair for geodesy, geoinformation and land management. With over half a million event website users, over 16 000 direct visitors each year from 92 countries and more than 500 exhibitors, it is one of the key platforms for business dialogue in the geospatial information sphere.
Registration for EUROGI’s imaGIne conference will automatically entitle delegates to visit the fair.
More information about the conference can be found at www.imagine2014.eu. The website also provides the opportunity to register. An early-bird discount rate will be available until August 31.
The Geospatial Sector — Huge and Growing Fast
Geospatial information, also often referred to as geographic information, is any information that has a location/position “tag.” The tags can take many forms, including for example, postal codes, street addresses, words that have a location/place reference (such as Barcelona, the Rhine, Slovenia, etc.), north/south coordinates, and more. Organizing and managing tags enables vast amounts of otherwise disparate information to be integrated and new and innovative insights and services to be provided.
According to a report published last year by Oxera, a leading UK economic research firm that was commissioned by Google, at $150-270 billion annually the geospatial sector globally was one third the size of the global airline industry. (See “What is the Impact of Geospatial Services?”)
The report states, “Geo services are making an important contribution to the global economy and to future productivity. The efficiency gains they create are helping to facilitate future economic activity and generate additional consumer welfare.”
Everyone is aware of the airline industry, but very few are aware of the geospatial industry, an industry that to a very large extent operates out of direct public view, but that produces products and services that impact on billions of people worldwide on a daily basis. The insurance, automotive, telecommunications, navigation, marine, agriculture, energy, utilities, tourism, and recreation and media industries are just some of sectors that rely heavily on geospatial products and services.
Apart from the enormous size of the sector, another key point highlighted in the Google-sponsored research report is that the sector is growing globally at about 30 percent annually. With overall global economic growth taking place in the lower single-digit range, growth of this nature can truly be described as explosive, conference organizers said.
The world’s leading conference trade fair for geodesy, geoinformation and land management celebrates its 20th anniversary in the German capital, October 7-9.
By James J. Miller and John LaBrecque, NASA Headquarters, and A.J. Oria, Overlook Systems Technologies, Inc.
Satellite laser ranging (SLR) and the results of combining SLR with GPS in the future will translate into significant performance advancements for generations to come, once it is fully implemented as part of the GPS III architecture. Simply put, SLR techniques will improve GPS signal performance by enhancing the accuracy of GPS orbit and clock estimates, allowing for the correction of systematic errors and limitations inherent in current GPS radiometric solutions.
This will produce higher levels of positioning and timing as new information is processed and used to update orbital models and reference frames over a period of time. Eventually this will enable user accuracy in the centimeter range, orders of magnitude better than the 1-meter average user-range accuracies accessed today. Every GNSS constellation under development will provide for SLR, because not doing so would limit their systematic accuracy and diminish the potential of their PNT services.
This SLR initiative progressed over the past decade from technical engineering exchanges to senior-level reviews and policy deliberations under the aegis of the PNT EXCOM (see Sidebar), with GPS III now poised to have laser retro-reflector arrays (LRAs) placed on board all space vehicles, beginning with number 9 (GPS-III-SV9).
The National Aeronautics and Space Administration (NASA), National Geospatial-Intelligence Agency (NGA), National Oceanic and Atmospheric Administration (NOAA), and the U.S. Geological Survey (USGS), among others, strongly support the decision by Air Force Space Command to proceed with the placement of LRAs on board GPS III satellites to enable SLR. These agencies will work together to ensure that the derived science benefits all PNT EXCOM agencies and our many constituents and users around the world.
How Satellite Laser Ranging Works
SLR to any orbiting body involves firing repetitive laser pulses towards an object equipped with some form of LRA. The laser roundtrip time is then translated into distance or range measurements (Figure 1). In our case, SLR data collected from lasing to GPS and other GNSS constellations is compared with radiometric data collected at GPS/GNSS ground monitoring stations.
Radiometric monitoring and SLR each have their respective strengths. Radiometric monitoring stations are inexpensive and can be densely deployed, but are susceptible to systematic errors that cannot easily be identified. SLR is a high-accuracy method, independent of radiometric positioning, that can be used to identify some of these systematic errors. The two techniques in concert will provide more accuracy to the determination of satellite orbits and clocks, strengthening the societal benefits of GPS through improved performance and more precise applications over time.
Societal Benefits of Space Geodesy
Geodesy is the science of the Earth’s shape, gravity, and rotation, and their variations over time. Modern geodetic measurements rely upon GNSS technology and techniques to understand and respond to evolving geo-hazards such as earthquakes, volcanic eruptions, debris flows, landslides, land subsidence, sea-level change, tsunamis, floods, storm surges, hurricanes, and extreme weather. In recent years, GPS radio occultation data from satellites is used by weather services to improve the accuracy of forecasts. Other benefits include the use of regional differential networks to monitor crustal movements in near real time, and guide farm machinery and construction equipment with centimeter-level accuracies.
An essential element is the ability to relate geodetic measurements to one another in space and time through a stable and accurate reference frame. Most global terrestrial reference systems set their origin to the Earth’s center of mass or geocenter. Precise knowledge of the reference frame geocenter and its relative change are needed to study regional and global sea-level fluctuations and ocean-climate cycles like El Niño, the North Atlantic Oscillation, and the Pacific Decadal Oscillation.
Reference Frames
GPS satellite ephemerides are derived from ranging based on pseudorandom noise signals and carrier-phase variations, referenced to onboard atomic clocks and a ground network of GPS monitor stations expressed in the World Geodetic System 1984 (WGS 84) reference frame. The WGS 84 reference frame is determined using the analysis of GPS satellites, and must be periodically updated by the National Geospatial-Intelligence Agency (NGA) due to geophysical processes such as tectonic-plate motion. NGA works to maintain the tightest alignment between the WGS 84 and the International Terrestrial Reference Frame (ITRF) using GPS reference sites common to both.
The more ambitious ITRF is obtained using a global network of instrumentation — GPS, SLR, Very Long Baseline Interferometry (VLBI), and Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) — and geodetic satellites such as LAGEOS and LARES. These data are gathered and analyzed through an international cooperative effort by the services of the International Association of Geodesy (IAG) within the framework of the Global Geodetic Observing System (GGOS) (Figure 2).
The integration of SLR and radiometric tracking of all GNSS constellations will improve multi-GNSS performance and interoperability as tools and techniques are co-located and data combined into various products that enable PNT service providers to improve system models.
Geodetic Requirements. GPS is a critical component in the determination of the ITRF geodetic reference frame and serves as the principal means of positioning relative to the reference frame. Though the current accuracy of the ITRF and WGS 84 reference frames marginally meets most current operational requirements, emerging scientific requirements in Earth observation demand more accuracy than core geodetic systems, including GPS and the ITRF, can deliver.
There is thus a growing GPS capability gap that can only be met with systematic improvements such as SLR will enable. In this manner, today’s scientific needs for positioning and timing often become tomorrow’s operational capabilities. If GPS is to continue as the primary geodetic reference system, we must ensure that GPS continues to evolve its system accuracy as well (Figure 3).
Presently, the accuracy of both the ITRF and the WGS 84 is estimated to be on the order of 1 part per billion (6.4 millimeters at the Earth’s surface), with observed regional drifts on the order of 1.8 mm/year, and errors in the colocation of geodetic stations exceeding 5 mm/year. There is also little to verify this estimated accuracy of the reference frames, because successive estimates of the ITRF are retrospective and utilize the same historical data sets, except for the addition of more recent data and new analysis approaches. All determinations of the ITRF are therefore inter-related and not independent, allowing some errors to remain embedded.
Although such drifts and errors are acceptable for meter-level positioning, we must address these significant instabilities if we are to meet the growing geodetic requirements demanded by science and society. The GGOS and the National Research Council have called for a significant improvement in the accuracy and stability of the ITRF, including the goal for 1 mm of accuracy and 0.1 mm/year of stability.
Getting Laser Reflector Arrays aboard GPS III
In 2006, a working group of representatives from multiple U.S. civil and military government agencies identified a set of anticipated geodetic requirements for GPS to meet future geodesy and science needs. An analysis of alternatives (AoA) concluded that the only practical solution to correct for systematic errors in satellite coordinates and reference frames is optical laser ranging, as has been demonstrated on board GPS block IIA SV-35 and -36. These were equipped with LRAs thanks to the effort of Ron Beard of the U.S. Naval Research Laboratory (NRL).
In 2007, the geodetic requirements and AoA were submitted to the GPS Interagency Forum for Operational Requirements (IFOR), along with formal endorsement letters from NASA, NGA, NOAA, and USGS. The goal of the GPS IFOR is to ensure that new features on GPS adhere to U.S. PNT Policy objectives, and that any proposed technical enhancements do not degrade core GPS performance, schedule, signals, or services. Between 2007 and 2012, interagency IFOR discussions and studies continued and subsequently were elevated to a special multi-agency study group led by AFSPC and NASA. In December 2012, after reviewing the results of these technical deliberations, NASA Administrator C. Bolden, AFSPC Commander Gen Shelton, and U.S. Strategic Command’s Gen Kehler agreed on a plan for installation of LRAs on all GPS III vehicles beginning with SV9.
Laser-Ranging Operations
GPS laser ranging will be accomplished through the International Laser Ranging Service (ILRS), and NASA will ensure all operations adhere to a set of standards and procedures. All ILRS GPS laser ranging will use 532- or 1064-nanometer wavelengths, and the reflectivity of LRAs will be optimized for these two “colors.” To support operations and accommodate this level of control and situational awareness, the ILRS has defined minimum standards for GNSS LRA cross-sections to optimize ranging to the satellites by ILRS stations.
The design of the LRA for GPS III, funded by NASA and currently being developed by the NRL, easily exceeds the ILRS recommended standards. Some satellites tracked by the ILRS are to be ranged subject to certain basic restrictions and conditions to ensure the science data gained is optimal for all stakeholders. The ILRS has developed policies and procedures for controlled tracking, and laser ranging to GPS III will be performed on a schedule issued by the ILRS Central Bureau located at the NASA Goddard Space Flight Center in Greenbelt, Maryland.
The laser-ranging schedule will be coordinated considering ground-network capabilities, GPS operational requirements, and the tracking frequency required for accurate orbit determination. Only certified/approved ILRS stations will be authorized to perform laser ranging following a predetermined assessment, using approved laser-ranging stations operating within set technical parameters (color, power, and so on). The ILRS will issue digital keys once confirmation is received that all conditions have been met, with AFSPC and NASA maintaining a role.
Summary
A positive way forward has been established to allow for the implementation of laser ranging to the GPS-III constellation beginning with SV-9 in the 2019 timeframe. The laser ranging to GPS III, followed by post-processed analysis and mitigation of systemic errors, will contribute significantly to achieving the goal of a more accurate ITRF. These applications will also be augmented by an ongoing and significant international investment in the global geodetic infrastructure of the GGOS observing networks and analysis systems. Laser ranging of GPS III will also encourage further international investments and industry innovations as higher precisions are further introduced to the world community.
The U.S. National Space Based, Positioning, Navigation, and Timing (PNT) Policy, formally unveiled in December 2004 and supported through two administrations, strengthened GPS by creating a deputy-secretary-level PNT Executive Committee (EXCOM) to coordinate federal agency oversight of this critical national asset. The PNT EXCOM is co-chaired by the Department of Defense (DoD) and Department of Transportation (DOT), with representation by the deputy secretaries, or their equivalents, from other agencies and departments. The PNT Policy maintains the U.S. Air Force (USAF) as the DoD Executive Agent for Space.
This policy also designated newer responsibilities for other agencies. The NASA administrator, in coordination with the Department of Commerce and DOT, is responsible for developing requirements for the use of GPS and its augmentations in support of civil space systems. This level of collaboration is enabled by high-level interagency stakeholder discussions on all aspects of civil GPS activities. This is vital in the age of GPS modernization among other emerging constellations, as it allows individual PNT EXCOM agencies to develop and fund new capabilities. This multi-agency collaboration is very appropriate for GPS, since PNT is a suite of services used by all federal agencies to serve the public, providing greater safety, efficiency, and economy for a multitude of governmental missions.
Collaboration through the PNT policy has allowed NASA to optimize the use of GPS-based PNT services to fulfill a variety of science missions with ever-expanding societal benefits, ranging from space operations, exploration, Earth observation, and weather forecasting, to all manner of environmental monitoring including ice-melt and sea-level fluctuations. These data are increasingly important for governments to be able to plan for and respond to changes affecting human health, economy, and security. NASA therefore continues to work closely with the USAF and other PNT EXCOM agencies to improve the performance of GPS and its products through science initiatives.
One such initiative is known as GPS Satellite Laser Ranging (SLR), and is described here, along with its implementation aboard GPS III satellites.
Acknowledgments
The authors thank these individuals for their contributions in developing a way forward for the implementation of LRAs on GPS III, clearly showing the high level of interagency interest and coordination required to make this initiative happen overly nearly a decade of work. We are especially grateful to the U.S. Department of Defense, and in particular to U.S. Air Force Space Commander General Shelton, for leadership and support in enabling NASA and our partners to realize this important contribution to GPS in years to come: Honorable Charles Bolden, Honorable Lori Garver, Gen William Shelton, Gen Robert Kehler, Letitia Long, Maj Gen Martin Whelan, Chris Scolese, Badri Younes, Michael Freilich, Jack Kaye, Barbara Adde, Norm Weinberg, Craig Dobson, Mike Moreau, David Carter, Stephen Merkowitz, Yoaz Bar-Sever, Scott Pace, Ray Yelle, Scott Wetzel, Major Janelle Koch, Col (Ret.) David Buckman, Col (Ret.) Allan Ballenger, Col (Ret.) David Madden, Col (Ret.) Bernard Gruber, Col James Puhek, Steve Malys, Thomas Johnson, Ron Beard, Linda Thomas, Mark Davis, Larry Hothem, Ken Hudnut, Hank Skalski, James Slater, Vaughn Standley, Mike Pearlman, Erricos Pavlis, Kirk Lewis, Maj Gen (Ret.) Robert Rosenberg, and the National Space-Based PNT Advisory Board co-chaired by Honorable James Schlesinger and Col (Ret.) Bradford Parkinson.
James J. Miller is deputy director of the Policy & Strategic Communications Division with the Space Communications and Navigation (SCaN) Program at NASA. He is a commercial pilot with master’s degrees in public administration from Southern Illinois University and international policy and practice from George Washington University.
John LaBrecque is lead of the Earth Surface and Interior Focus Area within NASA’s Science Mission Directorate, managing NASA’s Global Geodetic Network that provides PNT products in support of NASA’s Earth Observation program. He received his doctorate in marine geophysics from Columbia University.
A.J. Oria works for Overlook Systems Technologies, Inc., supporting NASA headquarters in the area of GPS and PNT technology. He has a Ph.D. in astronautics and space engineering from Cranfield University, UK.
Related article (PDF): “Innovation: Laser Ranging to GPS Satellites with Centimeter Accuracy,” by John J. Degnan and Erricos C. Pavlis, published in GPS World, September 1994.
By Chris Rizos
Geodesy is a suite of powerful Earth-observation techniques, associated methodologies, and analysis tools that today are making a vital contribution to science and society. Yet geodesy is not a new, child-of-technology sciaence. It dates back hundreds of years — some would claim thousands of years, and that the ancient Greeks and other pre-Christian cultures shaped its direction. This is illustrated by its classical definition as the science of measuring and mapping the geometry, orientation in space, and gravity field of the Earth; these days we also include their variations over time. At a practical level, geodetic practice forms the foundation for surveying, navigation, and mapping, and the digital datasets underpinning these activities.
What has enabled geodesy to change from an esoteric natural science that underpins the making of maps to today’s cutting-edge geoscience? There are a number of reasons for this transformation. Firstly, modern geodesy relies on space technology, and enormous strides have been made in accuracy, resolution, and coverage due to advances in satellite sensors and an expanding portfolio of satellite missions. Secondly, geodesy can measure Earth parameters that no other remote-sensing technique can, such as the position and velocity of points on the surface of the Earth and the shape and changes of the Earth’s ocean and land surfaces, and it can map the spatial and temporal features of the gravity field.
These geodetic parameters are in effect the “fingerprints” of many dynamic Earth phenomena, including those that we now associate with global change (due to anthropogenic as well as natural causes). The challenge is to invert the outward expressions of these global-change phenomena in order to measure and monitor over time the underlying physical causes.
Finally, what relentlessly drives geodesy into the future is the innovative use of signals transmitted by global satellite navigatiaon systems such as GPS and GLONASS.
Space-geodetic techniques such as GNSS, satellite, and lunar-laser ranging; very-long-baseline interferometry; Doppler orbitography and radiopositioning integrated by satellite (DORIS); satellite sea and ice altimetry; satellite gravity mapping; and satellite interferometric synthetic aperture radar mapping have revolutionized the geosciences. They have significantly improved our understanding of how the solid Earth, atmosphere, and oceans work as a system, giving us new insights into atmospheric and oceanic circulation, the global water cycle, the waxing and waning of ice and glaciers, sea-level rise, global tectonic motion and local earthquake fault mechanisms, to name a few of geodesy’s Earth-observation applications.
Global Geodetic Observing System. GNSS today plays a crucial role in geodesy; however, we will see a massive increase in capability. Geodesy strives to increase the level of accuracy in the determination of these geodetic parameters by a factor of 10 over the next decade.
The Global Geodetic Observing System (GGOS) is an important component of the International Association of Geodesy (IAG). GGOS will integrate all geodetic measurements in order to monitor the phenomena and processes within the Earth system at far higher fidelity than at present. This integration implies the inclusion of all relevant information for parameter estimation, the combination of geometric and gravimetric data, and the common estimation of all the necessary parameters representing the solid Earth, the hydrosphere (including oceans, ice caps, continental water), and the atmosphere. GGOS is geodesy’s contribution to the Global Earth Observing System of Systems (GEOSS) initiative.
Although GPS is popularly associated with the WGS84 datum, an important GNSS contribution to geodesy is its role in the definition of the International Terrestrial Reference Frame (ITRF, itrf.ensg.ign.fr). In addition, high-accuracy differential GNSS techniques — which have been refined over several decades — provide the day-to-day means of determining point coordinates in the ITRF. This reference frame is nowadays the basis for most national and regional datums for mapping and science.
The International GNSS Service (IGS, igs.org) was established in 1994 as an IAG service to the geosciences, providing high-accuracy orbit and clock products as well as open (and free) access to measurements made by a dense ground network of continuously operating GPS/GNSS tracking stations. The IGS therefore supports ITRF maintenance and densification. The IGS nowadays supports many more user communities, such as navigation, surveying, machine guidance, atmospheric remote sensing, and others, both directly and indirectly.
GNSS’s utility includes the role that it plays in precise orbit determination of Earth observation, geodetic, and environmental satellites. GPS receivers onboard almost all such satellites ensure that the data from the satellite sensors can be correctly processed and interpreted. Consider how sea-level rise is measured by satellite-borne radar altimeters. The measurement of the time taken for a radar pulse from satellite to the ocean surface and back is made by the altimeter and converted to distance, but it is knowing where the satellite is in three dimensions to centimeter accuracy that allows the ocean surface to be mapped to extraordinary resolution. Millions of such measurements, over many years, referenced to the ultra-stable ITRF, enable scientists to determine with confidence the 3D position of a grid of points on the ocean surface and its rate of change, not just as a single average rise in sea height, but in its full spatial complexity.
The Challenge. Can GNSS and the IGS rise to the GGOS challenge? Although GPS is currently the only fully operational GNSS, the Russian Federation’s GLONASS is being replenished, and the IGS currently also generates GLONASS products. The European Union’s Galileo is planned to be deployed and operational by 2014 (although that date may slip several years), and China’s Compass is likely to also join the club by 2020, after first deploying a regional navigation satellite system by 2012. Together with dozens more satellites from other countries and agencies, it is likely that the number of GNSS satellites useful for geodesy will increase to almost 150, with perhaps six times the number of broadcast signals on which geodetic measurements can be made.
Simultaneously, the IGS is evolving to a multi-GNSS service, and at the same time improving the quality and timeliness of its products. Real-time IGS products will soon be available to all users.
In summary, geodesy faces an increasing demand from science, engineering applications, the Earth-observation community, and society at large for improved accuracy, reliability and access to geodetic services, measurements, and products. Thus, geodesy must maintain the ITRF at the level that allows, for example, the determination of global sea-level change at the sub-millimeter per year level; determination of the glacio-isostatic adjustments due to deglaciation since the last glacial maximum and to modern mass change of the ice sheets, at millimeter-level accuracy; pre-, co-, and post-seismic displacement fields associated with large earthquakes at the sub-centimeter accuracy level; early warnings for tsunamis, landslides, earthquakes, and volcanic eruptions; millimeter- to centimeter-level deformation and structural monitoring; and more.
In response, the IAG established in 2007 the GGOS, to unify all the geometric
and gravity services of the IAG so as to support the ambitious goals of modern geodesy. Through the IGS, GNSS will play an indispensible role in GGOS. However, the Earth-observing techniques of modern geodesy are but one — albeit under-appreciated — set of applications of GNSS technology. As GNSS performance improves, and as it becomes more and more pervasive, our society’s reliance on this critical utility grows exponentially.
CHRIS RIZOS is professor and head of the School of Surveying & Spatial Information Systems, University of New South Wales, Sydney, Australia. He is vice president of the International Association of Geodesy. He will assume the presidency from mid-2011 for a four-year term.