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.
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.
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.
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 PNT EXCOM
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.
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.