Overview

IGDG is geared toward users carrying dual-frequency receivers. These high-end users typically require high-accuracy positioning. Having eliminated the ionosphere as an error source using dual-band receivers, these users are still susceptible to errors in the GPS ephemerides and clocks. Ground-based users and aircraft must also deal with errors due to the troposphere. Accurate correction for the GPS ephemeris and clock errors requires a network of GPS reference sites. With JPL's patented Global Differential GPS (GDGPS) architecture, a well-distributed global network of about a dozen sites is sufficient for continuously providing GPS ephemeris and clock corrections for GPS satellites. We have taken advantage of the NASA Global GPS Network (GGN), which is operated and maintained by JPL. The GGN consists of approximately 60 sites which have traditionally been operated in batch mode. A subset of the GGN is equipped with computers and internet connection. IGDG has been designed to return GPS data in real time from remote receivers. IGDG collects, edits, and compresses the raw GPS observables at the remote site. It then transmits the packetized data over the open internet to the processing center. At the processing center the global data is analyzed by IGDG to produce precise GPS orbits and clocks. These are formatted as corrections to the GPS broadcast ephemerides, encoded, and are provided over the internet to authorized users.

IGDG has uniquely powerful orbit determination and orbit propagation capabilities, whether it is run on the ground or as embedded flight software.

Accuracy is measured by performing real time positioning (at 1 Hz) of a stationary user, and comparing the estimates with its surveyed location. The position estimates do not assume any knowledge of the user dynamics, and thus are valid for users in motion.

Real time estimates of tropospheric delay are useful meteorological products. Zenith tropospheric delay can be determined to better than 1 cm with IGDG

The performance of JPL IGDG system compared to that of operational GPS and that of commercial differential services:

Look for even better accuracy in the near future. As our ground reference network of real time GPS receivers become denser, and due to additional tuning of our estimation strategies, we expect to deliver an accuracy of 10 cm 3D RSS for real time positioning anywhere, anytime.

A patented state-space global differential system using real time data from NASA 's Global GPS Network (GGN), provides corrections to the GPS orbits and clocks that are globally uniform and seamless. Users anywhere on the ground, in the air, or in near space can have access to the world's most precise differential corrections.

We use the open internet to collect real time GPS data from NASA's GGN sites, and to disseminate the differential corrections to authorized users.

We achieve robustness through redundancy. Our ground network is large enough to sustain the failure of one or two sites without adverse impact on the performance. Similarly, we can afford to lose certain percentage of the GPS data that is transmitted over the internet.  We operate multiple processing centers  in parallel (they can be at different parts of the world) for uninterrupted processing in case one of them crashes.