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System Description - Network

The core of the GDGPS tracking network is a JPL-owned and operated network of 75+ geodetic-quality, tripple frequency receivers, distributed globally. Additional real-time sites are contributed by a variety of U.S. and international partner organizations. The result is the world's largest real-time GPS tracking network, with more than 200 global sites. All these sites stream their GPS measurements at 1 Hz to the GDGPS Operation Centers (GOCs), where it is processed and analyzed in real-time.

The GDGPS network is highly redundant, by design, to provide a unique measure of reliability to the many critical applications that depend on it, such as real-time GPS integrity monitoring, and precise differential corrections. On average, the network is 25-fold redundant for GPS (meaning that at any given time each GPS satellite is observed, on average, by 25 ground sites), and is minimally 10-fold redundant. The network is 18-fold redundant, on average, for GLONASS.

A variety of communications channels are used for streaming the raw measurements from the tracking sites to the GOCs, including internet, dedicated land lines, and satellite links. When internet is used the data is sent in parallel to multiple GOCs to ensure redundancy of the internet channels. All the GOCs are inter-connected with dedicated high speed land lines.

The GDGPS system is proud to count 4 national timing laboratories among it contributing network partners. In particular, the United States Naval Observatory (USNO) contributes two monitoring sites driven by its Master Clock, allowing the GDGPS System to provide its global users the most accurate real-time realization of USNO UTC. In addition, many GDGPS sites (35+) are driven by atomic frequency standards, enabling robust data quality schemes.

Because we fully own and operate a vast, redundant, global tracking network, we can configure our network receivers to extract any and all GNSS data. These include all L1, L2, and L5 phase and pseudorange measurements, the navigation messages on the various channels, SNR values, and any other publicly available data.

It typically takes about 1 second for the tracking data from most of the monitoring sites to reach the GOCs, and a few more seconds for processing and quality control. The final products, such as the precise corrections to the GNSS broadcast ephemeris, are available within 5 seconds of data collection at the remote site.

To ensure the integrity of the GDGPS products, the data from JPL-owned core of the tracking network can be authenticated. Consequently, the system is immuned to data spoofing. The extremely high redundancy of the network is another powerful measure against spoofing of data from any site or region, as strong majority voting schemes are employed to detect any anomalous sites.

We continue to expand our network, and welcome contributions from interested organizations. We offer our network partners a variety of benefits, including real-time positioning, timing, and environmental monitoring, as well as data archiving and data distribution through the NASA CDDIS facility. Timing laboratories are able to monitor in real-time their frequency standards relative to some of the world's best standards. In some cases we may be able to contribute equipment and expertise toward the installation of a real-time monitoring site.

The GDGPS Network as of December 2008 (some partner sites not shown)

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