Lab Notes

Posted August 2008

A Big Eye Sees Small Things

An upgrade to the Millstone radar antenna will ensure uninterrupted tracking of the ever more crowded geosynchronous orbit.

It takes a really big antenna to locate small, faraway objects. At 26 meters across, Lincoln Laboratory’s Millstone Hill Radar (MHR) antenna certainly fits this description. And thanks to a recent renovation, the antenna is now easier to maintain and much less likely to suffer from downtime.

Millstone Hill Radar

Thanks to a critical upgrade, the Millstone Hill Radar in Westford, Mass., rejoins ALTAIR (in the Western Pacific) and Globus II (in Norway) to monitor the increasingly cluttered geosynchronous orbit.

MHR is one of the principal tools for maintaining the Deep Space Catalog—the listing of the more than 3000 objects that are circling the earth 40,000 km away in geosynchronous earth orbit (GEO). The antenna consists of an 84-foot-diameter Cassegrain-fed parabolic reflector, together with counterweights; its motion is controlled by a set of motors mounted to the azimuth deck. In an elevation-over-azimuth axis configuration, the entire structure is supported on a 26 m tower. With its wide beam, MHR can view large regions of space and can locate objects for the first time or after they have been lost because of a planned or unplanned shift in orbit.

Installed in 1957, Millstone was the first radar system to do space surveillance (it observed the Sputnik satellite) and satellite launch tracking. But the venerable system was showing its age. The motors and motor generators replaced in this renovation were original 1950s era equipment. "They were past their end of life. The motors were worn from years of use and regular rebuilds, and the inefficient motor generators were failing frequently," says Paula Ward of the Control Systems Engineering Group. Each failure would shut down the antenna for a significant period of time.

Now the system is easier to troubleshoot, and downtime can be kept to a minimum. Jeff Dominick, the site manager of the Lincoln Space Surveillance Complex (LSSC) that includes MHR, stresses the importance of MHR to LSSC and how important LSSC is to the Air Force Space Command. "Losing MHR for any period of time would impact our ability to track in this region," he says, pointing to the arc of GEO above the United States that isn't covered by the other two surveillance radars—ARPA Long-Range Tracking and Instrumentation Radar (ALTAIR), located on the Kwajalein Atoll in the Marshall Islands, and Globus II in Norway.

As project lead of the recent upgrade, Ward was working with two extremes—very heavy and bulky motors and gear boxes, and new software controls running in a real-time embedded environment. The components that were replaced included the drive motors, solid-state silicon-controlled-rectifier power amplifiers, some of the gear boxes, the servo control unit, the programmable logic controller, and all the position and velocity sensors. Installing and aligning the new motors and gear boxes was challenging since no mechanical computer-aided-design models existed. At the other end of the spectrum, while many upgrades had been done to the radar system and associated computers over the years, the antenna control system had been upgraded only twice in the last fifty years. "The previous antenna control upgrade, completed 21 years ago, used a microprocessor for position-loop control, but most other functions were still done in hardware," says Ward. "Development for controls to drive new motors using the obsolete 386-based microprocessor was out of the question." The new servo control software, written in C, runs on a VME-based Motorola 6100 processor by using the VxWorks real-time operating system. MatLab and Simulink were used to create a detailed model of the antenna and drive train. An improved algorithm for control of the motor pairs in each axis to mitigate effects of drive-train compliance and backlash was also implemented.

"The safety interlock controls were obsolete as well," Ward says. "And safety at every level was of prime importance." Special precautions were taken for personnel safety during integration. Equipment safety was also critical since the new motors and gearboxes were interfacing with one-of-a-kind 50-year-old equipment in several places. The legacy programmable logic controller, another obsolete computer, was replaced with a new Allen-Bradley programmable logic controller to implement many of the safety functions. Extensive testing of all safety functions was performed during integration.

Prior to the upgrade, an operator needing to move the antenna for maintenance had to turn mechanical knobs to rotate the antenna and read meters on a panel indicating positions. Now the interface is more intuitive and is done on a laptop running a custom application written in LabVIEW. The operator simply sets the desired positions in azimuth and elevation, and clicks Run. A complete suite of servo and safety data is recorded automatically on the laptop whenever the motors are enabled. In addition, the upgraded system provides, for the first time, remote access to the antenna's local displays. The new maintenance laptop, combined with the servo control unit, gives unparalleled ease of use for maintenance and complete insight into antenna operations.

The Millstone team is very satisfied with the results of the renovation. With its broad beamwidth, Millstone surveys about a 400 km diameter circle at GEO altitude, and its high angle accuracy provides excellent data for the other antennas, such as Haystack, to track objects. Today, MHR and its partners, ALTAIR and Globus II, cover the entire 360 degrees of GEO as well as monitor satellite and spacecraft launches. With more than 12,000 known objects in earth orbit, the vacuum of space is quickly filling up. MHR searches for new objects and reacquires the essential information on drifting or unstable objects. Dominick concludes, "We're trying to reduce the probability of collisions. We have to rely on Millstone and Haystack to keep track of everything. This upgrade has significantly reduced downtime and maintenance tasks associated with the MHR antenna control system."


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