On August 24,1989 Voyager 2 flew by Neptune, giving us our first real look at this distant planet. Skimming a mere 3,000 miles above Neptune's cloud tops, Voyager 2 sent back images of the planet that orbits the Sun at an average distance of 3 billion miles. Even at the speed of light (186,000 miles per second), it took four hours and six minutes for Voyager's radio transmissions to reach Earth. And when they arrived on Earth, those images were received and processed by a most unusual antenna-the Very Large Array (VLA) radio telescope.

Although its powerful signal reception capabilities were trained on the Voyager mission, normally the VLA observes far more distant celestial objects, including many undetectable in the visible light range. The VLA represents the latest generation of radio observatories- radio interferometers, and according to Bob Havlen, head of observatory services "The VLA is the premier instrument for radio astronomers throughout the world."

Running the VLA is an incredibly complex control task. Nothing ever stops. Twenty-seven antennas located on a continually revolving and rotating Earth, in a moving Solar System, are aimed at moving objects many light years away. To keep those objects in focus, each antenna must be minutely adjusted every few milliseconds. Additionally, every ten seconds, a radio "snapshot" of the sky is taken. This requires compilation and analysis of massive amounts of data. These tasks are far beyond the capabilities of human operators, so the VLA was designed to be computer driven.

Specialized systems process the raw data from the 27 antennas, converting it into a single image that can be further processed and enhanced into a form usable by astronomers. Other computers run the actual drive motors and electronic equipment that control the antennas. Yet all of these systems must answer to the same master-a 32-bit CLASSlC superminicomputer from MODCOMP, Fort Lauderdale, Florida.

"The MODCOMP computers are absolutely essential to the operation of this array since they run it. One cannot conceive us running such a complex instrument without computers," says Perley "They are the on-line, real-time computers responsible for control of the antennas, and the collection and recording of the data."

"To control the VLA, there are many real-time tasks in the MODCOMPs, many of which must be executed with strict and severe time constraints," says Kenneth Sowinski, who runs on-line systems at Socorro. "They must run to completion in 10 or 20 milliseconds or when they are told to-not just anytime."

The success of the VLA has led scientists to pursue the next logical step, a Very Long Base Line Array (VLBA) interferometer using the same principles as the VLA, but linking antennas located around the world. The experience of the VLA team in using powerful real-time processors such as MODCOMP's CLASSIC system in combination with specialized signal and image processing machines has laid the groundwork for the VLBA design. The JPL-NRAO-MODCOMP partnership is successfully forging ahead with ensuring present and future space exploration and radio astronomical observations.