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


Subscriptions
LiDAR Webinar
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


Product Reviews
Partner Sites

machinecontrolonline 

LiDAR News

symbianone
lbszone.com

GISuser.com

GeoJobs.biz

GeoLearn

 

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: Watchman on the Wall
In this issue we have Dick Elgin's open letter to NCEES which details why the proposals before us are bad ideas. We also have the NCEES response to my Fire Alarm editorial. Lately we've been asked why we are devoting so much coverage to the licensure experience requirement when many say ....
Read the Article
Dick Elgin, PhD, PS, PE 
Open Letter: Regarding NCEES and Survey Licensing
I thank the Council for its many years of work assisting state boards, preparing exams and for generally advancing the surveying profession and contributing to protecting the public. Your stated missions are worthy. However, as stated in the Council's December, 2014 "Exchange," ....
Read the Article
Wendy Lathrop
Vantage Point: When Flooding Leads to Creativity
Sometimes a disaster is the best wake up call and the prod that moves us forward from "same old, same old" practices. About 15 years ago, a colleague once noted ruefully that the best check of a Flood Insurance Rate Map's accuracy is to have a disaster: did the map predict the horizontal ....
Read the Article
Michael J. Pallamary, PS 
The Curt Brown Chronicles: Similarity of New Zealand and U.S. Laws
Within the United States it is a well established fact that old possession can stand as a monument to the original lines as marked and surveyed by the original surveyor. Fences built soon after section lines were run might stand as proof as to where future the original lines were run, especially after ....
Read the Article
Chad & Linda Erickson 
One-Room Schools, Aerial Photos, & Hokey Pokey Surveys
We found a blurb about the Idaho State School Board in the 1900's making the decree that sunlight coming over the left shoulder made the students more intelligent, or something like that, and all windows in one-room schools had to be moved ....
Read the Article
Smith, Roman, Youngman 
Recent Activities at the National Geodetic Survey - Part 3 of 4
The advent and evolution of new technology such as Global Navigation Satellite Systems (GNSS) over the last thirty years has allowed NGS, other Federal, State and Local Agencies, and the Private Sector to determine geodetic positions with greater speed, better accuracy, and less ....
Read the Article
Michael J. Pallamary, PS 
U.S. Supreme Court Introduces Confusion and Conflict
On December 15, 2014, the United States Supreme Court issued a decree in the hopes of settling a decades-old dispute over the location of California's offshore boundary, a line common with the United States of America. The conflict originated in 1946 ....
Read the Article
Brynna King 
Tunnel Vision - Excavating Subsea Roadways
Workers who blast tunnels 290 meters below the ocean's surface have plenty of "what-ifs" to consider. Technology failures and project budgets shouldn't be among them. So contractors working on Norway's Ryfast tunnel megaproject are using ....
Read the Article
 Mark Silver 
Geodetic Preppers - Surviving the Next OPUS Disaster
I don't have any inside knowledge if the US government is going to shut down again this year. But I do know if there is a shutdown like there was in October 2013 I am going to have a hard time grounding surveys without the National Geodetic Survey's OPUS products. One never knows what the ....
Read the Article
Jason E. Foose, PS 
The HP 35s Calculator - A Field Surveyor's Companion: Part 6 - Curve Traverse
This program is a curve traverse routine based upon the traditional methods of laying out a curve with a transit and tape. I assure the users of radial layout equipment and GPS that using this program is 100% compatible ....
Read the Article
David H. Widmer, PS 
Open Letter: Response to The Fire Alarm
The purpose of my letter is to dispute some information contained in your editorial mentioned above. First off, NCEES has absolutely nothing to do with any proposed legislation in Idaho to do away with experience prior to licensing. That goes against our three legged stool test of education ....
Read the Article

deliciousrssnewsletterlinkedinfacebooktwitter

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


Google
 
AMERISURV TOP NEWS

Topcon Announces
LN-100 Seminars

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