Satellite Link Simulation Testing

Posted on Thursday 11 January 2007

Satellite communications have long been subject to noticeable signal delays. The lags in satellite TV broadcasts and phone calls are examples that many of us have experienced first hand. Traveling at the speed of light, it takes over 200 ms to reach geosynchronous orbit (GEO) and back. Even using satellites at low earth orbit (LEO) of 200 to 400 km, transmission times between two terrestrial locations can be significant.



Next generation satellite communications technologies are now being developed using much higher bandwidth than ever before. Defense applications, in particular, will use gigabit, and even ten gigabit, data rates. In some cases, “free space optics” is being researched as a way to transmit high bandwidth signals between satellites in space. As a result of higher data rates, emulating and testing satellite communications using fiber optic cables in a traditional lab environment is increasingly common. Anue Emulators, with their precise, controllable and enormous delay capabilities fit this requirement perfectly.

Applications such as collaborative computing, distributed CAD/CAM, scientific visualization, remote sensing data relay, messaging and navigational services, electronic publishing, and others, create demand for new satellite telecommunications networks. The effects of dynamic variation of data path length and related jitter must be
considered and tested for with each new application to assure it will work before released to consumers.

Anue Network Emulators are an essential tool for satellite communications testing, providing variable path delays, jitter simulation, and dynamic error introduction, acting just like a real satellite network link. High-bandwidth satellite networks provide a growing variety of network services. Before these services can be deployed they must be thoroughly tested on the ground. Since satellites are frequently in orbits that are great distances apart, it is essential to emulate those distances while testing on the ground.

Round-trip transmission times to and from geosynchronous orbit – where satellites park some 22,300 miles above the equator – are approximately 250 msec, a considerable delay for a communications network. The satellite’s position may not be constant and, in fact, because they communicate with both ground stations and other satellites, there will be transmission paths of many different distances to consider. Unlike a fixed length of fiber optic cable, the Network Emulator is like a variable-distance cable without dB loss, so it can be used for all-distances testing rather than specific distance testing.

The communication satellite may be a moving object with a continuously varying distance to the point that it communicates with. A Doppler effect simulation feature requested by Anué customers emulates the variations in distance with a continuous smooth slew of delay distance. One of the Doppler effects is receiver signal jitter. A receiver expects a pulse of data at very regular intervals. The receiver’s eye pattern determines which signals it finds acceptable. Should the edge of a signal change at a time inside the eye pattern the receiver may go out of synchronization and communication might be temporarily lost. As the transmitter receiver pair move relative to each other the regular intervals may not be regular enough.

The Network Emulator forces the jitter to see how the receiver and in fact the application reacts. The impact on applications of long delays, jitter, Doppler effects, and other impairments, can be critical and must be tested before any satellite-based communication system is launched. After launch may be too late, that is why an Network Emulator is critical to the satellite communications testing strategy.



Since 1997, Gillaspy Associates has built a solid reputation for developing strong relationships with our customers by providing quality solutions and ongoing support.

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