Real-Time Net Transfer (RTNT) software allows for efficient and reliable transport of raw, GPS (Global Positioning System) observables over the open Internet. Efficiency is achieved by editing and compressing the GPS observables at the remote site, and by using User Datagram Protocol (UDP) rather than the higher overhead required of Transmission Control Protocol (TCP). However, TCP reliability is still achieved by the central server (the central collection computer) by monitoring sequence numbers of the remote client's packets. If there is a skip in sequence numbers, the central server may request retransmission of missed data packets from the remote clients. In this way over 98 percent of the data is returned to the central server. Once the central server receives the data, it may additionally retransmit the packets to other servers on the open Internet. This provides the capability of merging regional servers into a global server. There is a particular secondary server also running receiving data packets from the primary global server. If the secondary server no longer sees incoming packets, it will reroute the entire global network to itself. This provides a backup system should the primary server fail.

RTNT returns data from geodetic-quality receivers, such as Ashtech Z-12s, Turbo-Rogues, and AOA Benchmark receivers. Module construction of the s/w processes permits addition of other receivers. Five of six basic GPS observables are compressed down to 14.5 bytes per 1 Hz epoch, per GPS s/c tracked. The two phase observables (L1,L2) have a resolution of 0.02 mm, and the three range observables (CA,P1,P2) have a resolution of 1 mm.

RTNT is currently returning data with a latency of less than two seconds from a global network of 17 receivers. Latency here is defined as the time tag of the GPS observable and the time that the packet arrives at the central server. The Web page, contains the real-time operating status of the RTNT's global network. Data latencies and the number of GPS spacecraft tracked per remote site for the previous hour can be monitored through this Web page.

Once the data arrives at the central server, it is sorted by epoch, duplicate packets are rejected, request for retransmission of missed data packets are made, and the data is placed into a revolving segment of shared memory. From there, JPL's Real-Time Gipsy (RTG) software is used to compute global differential corrections to the GPS broadcast orbits and broadcast clocks. Real-time user position accuracy from this global differential system is 8 cm (RMS) in horizontal, and 20 cm (RMS) in vertical. The above Web page also contains a live demo of the receiver at JPL demonstrating these accuracies.

RTNT also provides the mechanism to distribute both the GPS broadcast orbits and clocks, and RTG's global differential corrections to the broadcast orbits and clocks over the open Internet. A server will fork dedicated TCP processes to any client making requests.

This program was written by Ronald Muellerschoen of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at  under the Software category.

This software is available for commercial licensing. Please contact Don Hart of the California Institute of Technology at (818) 393-3425. Refer to NPO-20976.

This Brief includes a Technical Support Package (TSP).
Software for Real-Time Transfer of GPS Data Over The Open Internet

(reference NPO-20976) is currently available for download from the TSP library.

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This article first appeared in the July, 2001 issue of NASA Tech Briefs Magazine.

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