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About this website: The Center for
Real-Time GPS Data and Environmental Products is being proposed as a NASA Earth Science
REASoN (Research, Education and Applications Solutions Network) Data and Information Provider.
The purpose of this website is to present information about the proposed Center,
details of the proposed products and services, and active links to relevant real-time
and near-real-time demonstrations utilizing the existing GDGPS system.
If the Center becomes operational, this website will evolve into the Center's
web-based interface.
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Overview of the Center's Products and Services
GPS is one of the most powerful Earth-sensing systems in operation. Its unique strength lies in
the diversity of its applications, including geodesy, atmospheric sensing, Earth orientation,
oceanography, and space weather. In addition, the GPS-based positioning of dedicated air-borne
and space-borne sensor platforms is often a critical component in the derivation of their science
products. However, to derive any of the GPS by-products we must first (or simultaneously)
estimate the precise orbits (and often the clock state) of the GPS satellites. This is performed
using measurements from a global network of GPS receivers. Consequently, the GPS data from
this ground network is the key to all the multiple benefits GPS has to offer. It is in this context
that the significance of the breakthrough ushered in by the GDGPS system can be understood.
By enabling the real-time collection, communications, processing, and dissemination of GPS
data from the global network of GPS receivers, nearly all of the incredible benefits from
GPS-heretofore only available with latencies of many hours to days-become available in real-
time. This is clearly the way of the future, but the GDGPS system so far remains alone in its
capability to tap into a large global network of GPS receivers and to provide precise real-time
estimates of the GPS orbit and clock states.
In addition to the raw GPS ground measurements and the core products of GPS orbits and clock
state, the center will offer valuable new environmental by-products with applications to natural
hazards, geodesy, oceanography, atmospheric processes and weather. We will also exploit the
increased availability in near-real time of science data from dedicated ESE Earth sensing
platforms carrying GPS receivers (such as Jason-1 and GRACE), to synergistically combine the
GPS data with the platform's science data, and to produce derived new environmental products
with unprecedented latency. The Center will also offer a broad range of services that will extend
the benefits from its unique capabilities to a much broader user base: small companies,
individual surveyors, and the general public. Arguably the Center's most valuable service will be
its GPS integrity monitor. No other system is currently capable of performing such monitoring
on a global scale. It will provide immeasurable benefits to operational GPS, and to all systems
and users that critically depend on GPS.
Data processing will be performed by a large and cost-effective cluster of Linux
PCs, using the GIPSY-OASIS and Real-Time GIPSY (RTG) software packages. All the core
products, and the majority of the by-products described above, have been previously demonstrated
by the GDGPS system, and the data production is well characterized.
Real-time (streaming) products
Through leverage of the real-time system developed for GDGPS, the Center
will pioneer the use of streaming geodetic data and products over the open Internet. This data
exchange technology, along with high-level WSDL web service calls, will hasten the
interconnectivity between diverse GPS centers and enable unprecedented levels of distributed
computation and data mining.
Real-time dissemination will be available for the following products: GPS data from
the ground network, GPS orbit and clock corrections, and GPS integrity monitoring.
In addition, real-time geodetic site positions, tropospheric parameters,
and ionospheric TEC measurements will be available for a network of ground sites.
The real-time data stream will be available through either UDP or TCP protocols.
Dedicated servers will receive data requests from external
Internet clients and promptly relay the data to the clients.
Experience developed with the GDGPS system has shown that one process is sufficient and
capable of responding to both protocols (UDP and TCP) and serving data to requests from
hundreds of users. Additionally, Mrouted tools will be installed to multicast UDP packets
over the Multicast Backbone (MBONE).
The open Internet has proven to be an extremely reliable and cost-effective communications option,
and will be the Center's primary communications channel for receiving and disseminating data.
The Center will retain the GDGPS communications and computing architecture, which implements
reliability through redundancy. The GDGPS Operations Center (GOC) will be maintained,
with at least dual-redundant computing capability, and at least dual-redundant
access to the Internet backbone. Additional access with dedicated land lines is also available by
special arrangement.
(However, the Internet is not fail safe, and as part of our technology program we will study the
reliability issues associated with real-time operational use of the Internet, including the issues of
failure modes and product integrity.)
Near-real-time (file-based) products
The Center's near-real-time products (as well as a component of the real-time products),
will be disseminated as files, accessible through several different means:
- Web-based download
- FTP server
- E-mail exchange
- WSDL (Web Services Definition Language) compliant web services such as SOAP
(Simple Object Access Protocol).
Depending on the nature of the data products, the transfer will either be available anonymously
or only by secure transfer for validated users. Web interfaces will provide for near-real-time
monitoring of system performance and certain ancillary products. These ancillary products include
displays of GPS broadcast ephemeris quality, statistical representations of monitored clocks in
the network, and maps of global sea surface height and the ionosphere.
The Center's Web interface will include a search engine
responsive to complex queries, as well as routinely updated monitors of system performance,
coverage, and selected data parameters. Besides interactive file transfer, file-based data
(and in principle the recent components of real-time data streams) will be available to remote clients in
several ways, including direct FTP, e-mail exchange, or SOAP calls.
The latter two methods will accommodate complex queries; the search will be designed so
that any query performed interactively can also be implemented non-interactively, hence enabling
automation by remote clients. SOAP calls (or similar procedures within the framework of WSDL,
the Web Services Definition Language) are especially convenient for data distribution to automated
client-side processes, such as continuously running global climate models.
Other Services
Besides distributing the data and products discussed above, the Center will support
the Kinematic Auto GIPSY service and several other web services.
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