GPS Used to Reach the Stars

When Albert Einstein developed his theory about gravity, he probably couldn't have even guessed that a few years later, Dr. Rainer Weiss, a scientist at MIT, would come up with a method of detecting and verifying gravity waves as a measure of detecting cosmic activity. Who could have known that GPS would play an integral part in the development of a detector of gravity waves, the Laser Interferometer Gravitational-Wave Observatory (LIGO)? With LIGO, scientists have put a ruler in the heavens for the purpose of detecting gravitational waves, created by violent events in the cosmos such as the collisions of stars and the vibrations of black holes.

 

This remarkable achievement is being made a reality by CBI Services, Inc., under contract with the California Institute of Technology (Caltech) to design, build and test four giant LIGO vacuum tubes in two locations across the United States. The first two beam tubes are located at a site near Richland, Washington, and another two will be located near Livingston, Louisiana. The facilities are being built at a distance of 3,100 kilometers apart to confirm the presence of gravity waves and to rule out local vibrations such as seismic activity and weather. The project is sponsored and funded by the National Science Foundation and is a national resource for physics, astrophysics and astronomy.

What is a Vacuum Tube?

Each facility is made up of two 4-ft-diameter stainless steel beam tube modules that are constructed in an L shape with an overall length of eight kilometers (five miles). Laser systems are located at the intersection of the L shape to project a laser beam down the length of each beam tube arm. Mirrors at the end of each end station reflect the laser beam back to collectors. The arms must be straight and aligned to within a few millimeters' tolerance to achieve the needed clear beam tube aperture. This provides for a clear laser path in measuring the arm lengths. To measure gravity waves, LIGO must detect arm length changes of one hundredth the diameter of a proton, the nucleus of a hydrogen atom.

"The tubes operate at 1 x 10-11 torr hydrogen in vacuum," says Steve Hand, a project construction engineer for CBI Services. "This will give us a very high level of accuracy over the length of the tube, and since, according to Einstein, a gravity wave will distort space and time in one direction and, 90 degrees to that position, will compress space and time. Space being the operative word here, the idea is that with the 90 degrees between the two vacuum tubes, one tube will get longer as the other tube shrinks in response to the gravity wave passing through the Earth." According to Hand, scientists hope to build a third LIGO facility to allow them to triangulate from gravity wave data to accurately position disturbances in space.

Beam Tube Alignment and GPS

One of the major concerns of the LIGO project, expressed by representatives of Caltech, was the ability of scientists to measure the alignment of the beam tubes accurately. Numerous methods including optical systems and lasers were considered. Unfortunately, problems with cumulative error resulting from multiple set-ups over the length of each arm and the refraction of the line of sight resulting from the close nature of the beam tube covers made it difficult to achieve the accuracy needed for the LIGO Project.

DGPS was suggested early in the project's development as a solution for setting reference points that could be used as millimeter-level optical benchmarks. In 1993, Trimble introduced the Site Surveyor RTK system, a practical, cost-effective and off-the-shelf method of measurement with sub-centimeter-level accuracy in real time.

The use of DGPS by CBI Services engineers began in September 1996 at the Richland, Washington, facility. This site is well suited for the use of GPS because it is relatively flat, with no vertical obstructions, and only a few multipath conditions, such as road traffic and cranebooms. "Of course we all know about the use of DGPS for surveying, but there haven't been very many uses of GPS, that we know of, as an optical aid for precision leveling," explained Dennis E. Dickinson, an engineering consultant contracted to CBI Services for the LIGO Project. "The original Caltech design called for DGPS to be used in the creation of survey markers from which conventional surveying techniques would be used to determine final alignment adjustments. This didn't strike us as a method that would be nearly as accurate as using DGPS all the way through the leveling process."

A critical goal of the project was to minimize the cost of the beam tube portion by maintaining the smallest beam tube diameter possible but still meeting the required minimum aperture. Because the alignment process would contribute the largest increase in tube diameter, it would have to be reduced by determining the best alignment system available. DGPS was determined to be the logical choice to control the beam tube costs by minimizing the beam tube diameter while maintaining the maximum clear aperture.

LAYOUT Cart

As a result, engineers created LAYOUT, a moving cart that incorporates six Trimble products: a dual frequency 4000SSi receiver, TDC-1 Controller, Trimble 15" choke ring antennas, Trimble's Pacific Crest radio system, GPSurvey software, and TrimMap software.

The LAYOUT cart incorporates the establishment of the local coordinate system, laying out beam tube centerlines and generating centerlines needed to align the beam tube when the support is finally attached to the tube. Because the LIGO beam tube is straight without regard to earth curvature, a reference program was provided by Dr. Tom Herring of MIT to calculate a straight line referenced to the WGS-84 latitude, longitude and height above the ellipsoid.

Final Alignment

Final alignment is accomplished after the tube support is installed. The fixture is designed much like a high-accuracy centering head clamped on the machined beam tube support stiffener ring. The fixture is plumbed using a coincidence level. The GPS antenna is attached to a calibrated fixed-length rod and held centered and plumbed to the beam tube support stiffener diameter as a reference height to the beam tube centerline. This provides an X, Y and Z readout on the data collector. The readout is used in the Trimble RTK "stakeout" mode to position the beam tube's center to the calibrated coordinate system. Once the position is within a few millimeters of theoretical, the support is locked down. An engineer then takes multiple 30-minute control point shots and logs the data in the receiver for later post-processing with precise ephemeris for final positions. "Although construction of the vacuum beam tubes is ongoing," Hand said, "GPS is proving itself as an excellent millwright tool in situations where a high degree of accuracy is needed over a very long distance."


 

Mike Michelsen is a marketing writer for Trimble Navigation Ltd. in Sunnyvale, California.

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