About Amerisurv| Contact    
Magazine | Newsletter    
Flickr Photos | Advertise    
HomeNewsNewsletterAmerisurv DirectoryJobsStoreAuthorsHistoryArchivesBlogVideosEvents
 
advertisement


Subscriptions
Product Reviews
Software Reviews
Sponsored By


Continuing Series
     RTN
An RTN expert provides everything you need to know about network-corrected real-time GNSS observations.
Click Here to begin the series,
or view the Article PDF's Here
76-PageFlip Compilation
of the entire series
Test Yourself

Got Answers?
Test your knowledge with NCEES-level questions.
  Start HERE
Meet the Authors
Check out our fine lineup of writers. Each an expert in his or her field.
Wow Factor
Sponsored By


Partner Sites

machinecontrolonline 

LiDAR News

symbianone
lbszone.com

GISuser.com

GeoJobs.biz

 

Spatial Media LLC properties

Associates

ASPRS

newsnow 

Home arrow Archives   The American Surveyor     

Refinement of the Geoid—The Gravity Probe B Experiment Print E-mail
Written by Marc Cheves, LS   
Monday, 16 May 2011

Author's note: In 2004, I wrote a web-exclusive article about a science experiment that was attempting to verify Einstein's General Theory of Relativity. In addition to the cool science needed to mount the experiment, there were several aspects I felt surveyors would enjoy, such as the needed angular accuracy, etc. The results of the experiment were released last week, and can be found here: http://einstein.stanford.edu/highlights/status1.html. A 218Kb PDF of the article, complete with lots of images, can be found HERE.

Refinement of the Geoid—The Gravity Probe B Experiment
Long before GPS and the reliance on geoids to establish orthometric heights, surveyors’ plumb lines were being affected by local gravity variations, such as nearby mountains. Today, about the best angular accuracy we can achieve is on the order of tenths of a second. Stanford University’s Gravity Probe B, with its capability of resolving angles to the tenth of a millarcsecond, will explore the fine structure of the Earth’s gravitational field.

Whether you first learned it in an early science class, or recall it twirling across the opening episodes of The Twilight Zone, Einstein’s E=mc² formula expressed a special theory of relativity. Based on that theory, the development of nuclear energy was made possible, and with it, the atomic bomb.

But it is Einstein’s general theory of relativity that has been more difficult to understand. Proposed more than 40 years ago, and based on work stretching back to the 1800s and Einstein’s 87-year-old theory, the Gravity Probe B (GP-B) experiment brings together an incredible array of new technology, without which the experiment would not be possible.

From a layman’s point of view, Einstein’s lesser-known theory postulates that as an object in space (such as the Earth) travels though space, it drags space-time along with it. It may be hard to conceive of the applications for this verification here on Earth, but it has enormous implications for understanding our universe—such things as black holes, for example. The theory also suggests that the planets are not moving in elliptical orbits around the Sun, but rather are following straight lines through curved space-time.

Twisting the Fabric of Space-Time
A relatively simple explanation of Einstein’s general theory appeared in a recent issue of Popular Science (Nov 2003), called Einstein 101: “The general theory of relativity says that massive bodies distort the shape of space-time. When light from a distant star passes another star, for example, its path curves because the star’s gravity has curved the surrounding space, not because the light is being pulled inward. This warping will also affect an orbiting body. Gravity Probe B will be the first test of frame-dragging, the theory that a massive spinning body twists the fabric of space-time. Like a whirlpool in water, Earth’s rotation doesn’t stir up distant space; the effect is greatest near the planet’s surface.”

Another perspective comes from Stanford University’s website: “One way to think about space-time is as a large fishing net. Left unperturbed and stretched out flat, it is straight and regular. But the minute one puts a weight into the net, everything bends to support that weight. This is the geodetic effect. A weight that was spinning would wreak even more havoc with the net, twisting it as it spun. This is frame-dragging. The mass-energy of the planet earth represents a ‘weight’ in our net of space-time, and the daily revolutions of the earth, according to Einstein's theory, represent a twisting of local space-time. GP-B will search for this twisting effect, which has never before been measured.” (http://einstein.stanford.edu)

The seeds for the GP-B experiment were sown a half-century ago. In the late 1950s, a Stanford scientist and a Defense Department scientist came up with the idea of launching an extremely stable gyroscope into an orbit that would cross the planet’s poles. If Earth was twisting space-time, the gyroscope’s axis of rotation would tilt. By keeping the gyroscopes precisely pointed at a distant star, any variation in the axes of the gyroscopes would be detected. In polar orbit, with the axes of the gyros pointing at the star, the geodetic and frame-dragging effects would show up at right angles to the axes.

Several technologies had to be developed to make the experiment possible. First, the creation of the gyroscopes themselves. After much experimentation, scientists decided to use fused silica and single crystal silicon as the moving part, or rotor. Twenty spheres were created, of which four were selected, two of fused silica and two of silicon. The spheres were ground and polished to within 0.01 microns of perfect sphericity. If enlarged to the size of the Earth, the highest mountains and deepest valleys would be within eight feet of sea level. To create the magnetic field which could be monitored within the gyroscope, each sphere was coated with a very thin layer of niobium. To shield the gyroscopes from the effects of Earth’s magnetic field, the entire gyroscope assembly was surrounded with lead bags. For the experiment to work it would have to be in a super-cold, weightless environment.

The world’s largest dewar (essentially, a giant 9-foot tall Thermos bottle) was built and filled with 650 gallons of liquid helium. The dewar will keep the assembly in a vacuum near absolute zero (1.8º Kelvin or -271º Celsius). A special challenge was created by the fact that as the satellite passes from shadow to intense sunlight, onboard temperatures will change dramatically. If the temperature of the assembly varies by as much as one degree, it will fail, so special features have been incorporated to handle temperature stabilization. A new “porous plug” was created for the tank that allows evaporating helium gas to escape, while keeping the liquid inside. The gas is used to start the gyroscopes spinning at 10,000 rpm, and to power the satellite thrusters that keep the satellite precisely pointed at the star. Once up to speed, which will take a half-hour, the gas will be pumped out, and the resulting vacuum will be lower than that of space surrounding the satellite. The scientists estimate that the low vacuum would enable the gyros to lose less than 1% of their starting speed, even after 1,000 years. The separation between the spheres and the fused quartz block enclosing them is measured in millionths of an inch. Inside each housing, three electrodes suspend the spheres. Detectors (called Superconducting QUantum Interference Devices, or SQUIDS) in the housing can sense any change on the magnetic field created by the spinning spheres. Two of the spheres will rotate in one direction while the other two rotate in the opposite direction, thus providing canceling effects.

Assembling the telescope itself presented several challenges. 14 inches long, with a 5.6 inch aperture (focal length 12.5 feet), it will be able to pinpoint the center of IM Pegasus to within 0.1 millarcseconds. The entire probe was assembled in a Class-10 clean room, capable of eliminating any particles larger than a single micron. To put the tiny tolerances and design objectives into perspective, consider the following: the detection capability of the assembly is less than 0.002% of a degree, which corresponds to a gyro tilt of 0.1 millarcsecond. Per Einstein’s theory, the predicted amount of the geodetic effect is 6600 millarcseconds, and the frame-dragging effect is 42 millarcseonds. Accordingly, the experiment has been designed to detect twisting at the 0.5 millarcsecond level. To put this in human perspective, the distance subtended by this angle would be like looking at the edge of a piece of paper 100 miles away. Similarly, this subtended angle would result in a distance of five feet if it were extended to the moon.

The gyroscopes must provide a reference system stable to 10-12 degrees per hour, a million times better than the best inertial navigation gyroscopes. Two factors combine to make the experiment possible: the weightlessness of space and near-zero temperatures. Six requirements must be met: a drift-free gyroscope, a method for determining changes in the spin angle to 0.1 milliarcseconds, a system for referencing the gyro to the guide star, a star of which its motion and position is precisely known, a data processing technique to allow the separation of the geodetic and frame-dragging effects, and a credible calibration scheme. The last requirement is particularly interesting because once the satellite is on orbit, the experiment will require almost a year to calibrate and prepare to make the observations. After approximately two years, the satellite will run out of helium, and then become space junk.

The system developed to keep the probe precisely centered on a star in the Pegasus constellation involves beam-splitting and perfectly matching two halves of the star. Minute thruster firings will ensure that the telescope remains pointed at the star. Much more technology is involved, for instance, the joining of the telescope to the gyro assembly. Using a technique known as optical contacting, the two parts use no cement or mechanical attachment. Instead, the mating surfaces are so flat and clean that they join through molecular adhesion.

The GP-B experiment is not without controversy due to its cost of $700 million, but if it succeeds, spin-off benefits for surveyors will include a dramatic refinement of our wildly undulating geoid. Also of interest to surveyors is the incredible accuracy and precision that will be needed to detect the tiny effects predicted by the theory.

GP-B, with its capability of resolving angles to the tenth of a millarcsecond, makes our work today look coarse, to say the least. The experiment will explore the fine structure of the Earth’s gravitational field. As we learn to deal more and more with a geoid which undulates across the landscape, any refinements in the geoid will be welcome.

Marc Cheves is editor of The American Surveyor magazine.

 
< Prev   Next >

 American Surveyor Recent Articles
Editorial 
Editorial: GLONASS Has a Hiccup
On the evening of April Fool's Day, as I was writing this editorial, I received an urgent phone call from the folks at JAVAD GNSS. They had been receiving reports that the GLONASS satellite constellation was off the air. Sure enough, when I went to the Russian Federal Space Agency's 24 ....
Read the Article
Michael J. Pallamary, PS
Angle Points: Land Surveyor Liability
A recent article by Geneau Thames, a brilliant attorney with the law firm of Niles, Barton & Wilmer, LLP out of Baltimore, Maryland, raises some troubling issues with regards to an increase in activity for real estate professionals. Ms. Thames notes that with the increase in real estate activity ....
Read the Article
Wendy Lathrop, PS, CFM 
Vantage Point: Letting Go
It's an inevitable cycle of the business world: people are hired; people leave (voluntarily or involuntarily). Only the terms of engagement and disengagement vary from one firm to another. Recently more companies are requiring new employees to sign what amounts to an oath of ....
Read the Article
Rachel Schowe
New Record For Southeast Asia's Highest Peak?
Since 1925, the people of Myanmar have been taught from an early age that the highest peak in all of Southeast Asia was the great Mount Hkakabo Razi. Often swathed in stormy clouds obscuring it from view, Hkakabo Razi lies in the Himalayan ....
Read the Article
Marc Cheves, PS 
Smithsonian X 3D
In 1985 I attended the ACSM/ASPRS annual conference held in Washington, DC. After returning home to Oklahoma I remarked to my wife that the Smithsonian museums in DC would be great for our three children. Little did I know that less than two ....
Read the Article
Chad & Linda Erickson 
Land Locators & Claim Jumpers Part 1
True West Magazine has kindly given us permission to abridge and reprint a remarkable, autobiographical article of August 1970 titled "Claim Jumpers" by Walt Coburn. We have not yet found a more detailed account of a Land Locator's activities ....
Read the Article
Larry Trojak 
Upgrade Up North
As the economic recovery plods along in the U.S., growth in the Canadian economy is continuing at the brisk rate that has been evident for quite some time. While certainly good news, there is something of a downside to that steady, ongoing progress. Companies working in support of it ....
Read the Article
Marc Cheves, PS 
3D Pioneers—A Visit to Optech
It has been my privilege to visit the folks at Optech on three occasions, twice for company visits and once for the company's user conference. My first visit was in 2002, when the company was located in a business park mostly composed of one-story nondescript buildings. Subsequently ....
Read the Article
Curt  Brown 
The Curt Brown Chronicles: Ownership Boundaries along Seashores
Circa 1965. As of the present time, the limit of demarcation between the riparian owner and the state can be in any one of several locations depending upon the general laws in force at the time of the alienation from the sovereign to the individual. These locations are: 1. Mean high ....
Read the Article
James J. Demma, PS, Esq. 
Surveyors & Law: The Owner of the Property Is?
One aspect of my law practice has always fascinated me, and that is: The process which one must go through in order to determine the true owner of a parcel of land. It may sound easy--but it is not. What has brought me to write this particular article is an inquiry from one of my favorite ....
Read the Article

deliciousrssnewsletterlinkedinfacebooktwitter

Amerisurv Exclusive Online-only Article ticker
Featured Amerisurv Events
List Your Event Here
please
contact Amerisurv


Google
 
AMERISURV TOP NEWS


Spectra Introduces
MobileMapper 20

GOT NEWS? Send To
press [at] amerisurv.com
Online Internet Content

Sponsor


News Feeds

 
Subscribe to Amerisurv news & updates via RSS or get our Feedburn
xml feed

Need Help? See this RSS Tutorial

Historic Maps
Careers

post a job
Reach our audience of Professional land surveyors and Geo-Technology professionals with your GeoJobs career ad. Feel free to contact us if you need additional information.

 

Social Bookmarks

Amerisurv on Facebook 

Amerisurv LinkedIn Group 

Amerisurv Flickr Photos 

Amerisurv videos on YouTube 

twitter

 




The American Surveyor © All rights reserved / Privacy Statement
Spatial Media LLC
905 W 7th St #331
Frederick MD 21701
301-620-0784
301-695-1538 - fax