The need to continuously monitor dynamic Earth processes with low latency and high accuracy can hardly be overstated. Obvious examples include processes such as earthquakes, volcanic eruptions, and severe weather, all of which occur over time scales as short as seconds, minutes, and hours. Indeed, improving the monitoring and forecasting capabilities of dynamic Earth processes is a high priority of NASA's Earth Science Enterprise (ESE). A necessary component in the monitoring and forecasting of Earth processes is the timely availability of remote sensing data together with the necessary information technology and knowledge base to process the data and effectively serve the data and its by-products to the users.

The capability to process global GPS data in real-time is being demonstrated by the GDGPS system. The real-time products of the GDGPS system can be synthesized, with various other sources of GPS data and other Earth observing data types, into environmental monitoring products that are available with unprecedented latency. These low latency data and products are essential for the effective monitoring and prediction of several important natural processes and environmental phenomena. Because GPS technology and applications are now interwoven into the global economy and security, the value of the incredible multi-disciplinary information content of the GPS data extends well beyond Earth science uses and applications, and into commerce, public safety, and national security. This is a quintessential fulfillment of the ESE Applications Program goal to "expand and accelerate the realization of economic and societal benefit from Earth science, information, and technology."

We will transform the existing prototype GDGPS system and its trailblazing technology, concept of operation, and business model into a systematic, long-term, self-sustaining data service and research center, that will serve its unique products to the science community, commercial sector, and government sector. The Center for Real-Time GPS Data and Environmental Products (the Center) is envisioned as an international center of excellence in the processing, synthesis, and application of global and regional real-time GPS data and their environmental by-products. Taking advantage of the small marginal cost of any single product or service, we will realize our goals by providing a range of products and services, which offer broad and profound national benefits with minimal cost.

This work combines original research into the development of new and improved environmental products, with the application of data, products, and services to a broad spectrum of National priority areas. It includes a significant information technology development effort to address the challenges of near-real-time data processing, data mining, and information serving. The interaction between the research, applications, and technology components of the Center is necessary to ensure the success of a promising, but relatively young capability.

To foster the enhanced availability of real-time GPS and environmental data across regional and international networks and systems, the Center will promote the formulation of standards, interfaces, and data exchange protocols through participation in the Federation and SEEDS Working Groups, and through direct collaboration with its national and international partners and collaborators, such as the International GPS Service (IGS). The Center will serve as the first data hub in an anticipated international network of data centers, linked through a novel internet-based architecture to provide robust real-time data serving and exchange capability.

Two themes strongly unify the Center's diverse activities:

1. All products and services are tightly related to GPS data, which is rich with inter-disciplinary information content.
2. Only low latency products are produced, which are destined for replacement after hours or days with more accurate, long-lasting products.

In our usage, both "real-time", and "near-real-time" imply the shortest possible latency for producing the data/products. In practice, "real-time" refers to time scales of seconds to minutes, and near-real-time refers to time scales of many minutes to a few hours. The boundary is rather elastic. This work is not in competition with long-term science data archives, and the Center will only handle data and products at latencies far below what is currently available.

The tremendous richness of information embedded in the GPS data, and the breakthrough in near-real-time availability of the data from the ground network as well as from space missions, will be exploited to the fullest by the Center in deriving an array of valuable products.

The benefits of these products to NASA and society in general fall into the following three broad categories:

1. Enabling new science products
2. Enabling improved environmental forecasting and natural hazard monitoring
3. Cost savings due to the elimination or simplifications of ground operations

A detailed analysis of each of these benefits follows.

Enabling new science products

Intelligent platform control is required for several proposed ESE missions. Pointing of the science instrument in response to specific events such as volcanic eruptions and earthquakes, or controlling the orbit to follow a specific plan requires real-time knowledge of the spacecraft positions. Interferometric SAR (InSAR) missions provide an example of very demanding requirements for real-time positioning accuracy because SAR interferometry critically depends on consecutive flights along the exact same ground track. Any deviation from the repeated ground track translates to spurious fringes^[1]. This targeting operation requires real-time control and accurate real-time knowledge of the spacecraft state. The solid earth and polar science community place high scientific value on repeat SAR mapping of land surface and ice, and specifically the implementation of a tandem SAR mission that would allow the best possible interferometric reconstruction of global topography. Significant periods of overlap between successive missions would allow implementing bistatic radar interferometry, which demands very high accuracy positioning and pointing knowledge. Other mission concepts call for cooperative constellations of spacecraft, or formations. Controlling these constellations from the ground is either not feasible or inefficient. To be effective these formations would co-point their science instruments, or control their orbits to achieve their combined science goal.

Enabling improved environmental forecasting and natural hazard monitoring

Real-time processing would enable science data products to be available very close to real time. This is a key advantage of this technology and it is an important one for a wide range of natural hazard monitoring needs. Earth orbiter missions in the class of Topex/Poseidon, Jason-1, OSTM, GRACE, Icesat, and numerous atmosphere occultation missions (examples of missions in this class are SAC-C, CHAMP, and COSMIC) would all benefit from accurate orbit determination in real- time or near-real-time. These missions are unique in the sense that very accurate (< 1 meter) orbits are intrinsically required to process the science data. Currently, all the GPS tracking data as well as the science sensor data are telemetered to the ground, where complex ground operations systems must sort and organize it, and send it to various processing centers. The products of those processes are then sent to scientists. There can be a time delay of days to weeks for these processes. The capability of generating highly accurate onboard ephemerides in real-time would be the first step towards an autonomous science process onboard the satellite which would enable some level of finished or preliminary science product to be transmitted to the science investigators in real-time or near real-time. Even in the absence of onboard autonomous orbit determination capability, the availability of precise real-time GPS orbit and clock solutions reduces the latency in generating science products, and improving forecasting capabilities. In the area of natural hazard monitoring, real-time data from SAR, airborne platforms, or from a ground network of GPS receivers will provide the ability to monitor volcanic inflation precisely and in real time. Similarly, the spatial distribution of motions before, during, and after major earthquakes will be precisely known almost immediately.

Cost savings due to the elimination or simplifications of ground operations

The operational costs of maintaining constellations of orbiters will grow very large unless we enable autonomous station keeping and other routine operational tasks. The Center enables such autonomy. Ultimately, Earth orbiters carrying a GDGPS-capable GPS flight receiver (being developed under the AIST project) would require little or no ground operations for routine navigation, and even for specialized high-precision positioning needs. Very high accuracy is not necessarily needed for some applications, but the ability to support this is a key strength of this Center. With ultra-high accuracy available in real-time, NASA would be able to bypass an entire operational element, which is currently required on the ground and consumes significant resources. The savings could easily amount to between hundreds of thousands of dollars to millions of dollars per year, per mission, and certainly these savings will be multiplied by a large multi-mission factor. For remote ground GPS receivers that are monitoring natural hazards, dissemination of differential corrections enables autonomous in-situ positioning. This in turn, dramatically reduces the necessary communications bandwidth to remote sites, enabling cost-effective deployment of large self-monitoring networks.

1. Rosen, P., S. Hensley, H. Zebker, and F. Webb. 1996. "Surface Deformation and Coherence Measurements of Kilauea Volcano, Hawaii, from SIR-C Radar Interferometry." J. Geophys. Res. 101(B10): 23109-23125.^{(back to text)}