Communications test tools: a study of the Cassini-Huygens mission.(AEROSPACE/DEFENSE TEST)(Product/service evaluation)

For aerospace engineers, 2010 marks the tenth anniversary of a technical triumph. Ten years ago, Boris Smeds made the discovery that would eventually make him a hero. What he uncovered was a design limitation on the communications link between the Huygens probe and the Cassini Spacecraft. To make matters even more interesting, the spacecraft was already many years into its journey toward Saturn. Without Smeds' discovery, a substantial portion of the Huygens mission would not have come to fruition.

Smeds discovered that, while the communications system worked fine in static testing, it would fail when Huygens separated from Cassini because the planned relative velocities between the two would render the communications link inoperative. The issue at hand, Doppler shift, was well known, but until recently, practical electrical testing was not readily available.

We can learn a great deal about Doppler shift and the tools available to communications system developers by recreating critical parts of the Cassini-Huygens mission using today's innovations in simulation and test. In this way, we can highlight the subtle, yet key, aspects of Doppler shift and how Smeds' discovery could have been easily missed given the state of available test tools more than a decade ago.

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The first challenge in recreating the Cassini-Huygens mission is calculating and visualizing the multi-year mission's path to Saturn. While the software was available to engineers designing the mission, it was specialized and custom built.

Today's engineers have access to very powerful off-the-shelf, physics-compliant, dynamic analytical software. For this recreation, I used Satellite Tool Kit (STK) Simulation Software from Analytical Graphics, Inc. (AGI) to visualize and model the mission's conditions.

It is important to note that I am not an aerospace engineer. I point this out because, like many engineers specializing in other areas, I need the data generated by such software to perform my job. It is the availability and usability of STK software that have allowed me to accurately model the Cassini-Huygens mission.

Once a mock-up of the mission's circumstances was modeled and understood, I then coupled the data output by STK to a new class of test instrument called a channel simulator. The T400CS Channel Simulator from RT Logic uses modeling data to recreate, in a physics-compliant way, the electrical perturbations that this unique transmitter-receiver pair experienced in January 2005. The end result is an illustration of how, even with a clear understanding of Doppler effects (for example, the designers used Doppler to measure atmospheric conditions on Titan), it actually was the subtle differences between carrier Doppler and signal Doppler that brought on potential disaster. …

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