Stellar Surveyor
The quasar-signal survey system is so precise that is will be able to detect imperceptible land creep caused by stresses in the Earth, and may lead to earthquake predictions. For now, the result will be an unexcelled accuracy in locating this state's survey markers.
Faint radio-frequency emissions from quasars, some of the most distant and mysterious objects in the universe, are allowing distances of more than 2,500 miles - Seattle to New York City - to be measured with an error of less than an inch.
The system is so precise that, over time, it will be able to detect the imperceptible land creep caused by stresses in the Earth and may even someday lead to predictions of imminent earthquakes. In the immediate future, the information will result in an unprecedented accuracy in locating this state's survey markers.
The program is a complex mix of high technology, the analysis of radio signals that have been traveling at the speed of light for billions of years, and old-fashioned geometry, devised by the Greeks thousands of years ago.
It's the latest development in the science of determining distances on land that only a few decades ago depended on surveyors tramping around with measuring chains, measuring angles to calculate distances through triangulation.
The National Geodetic Survey recently positioned a dish antenna for five days at Sand Point near Lake Washington over one of the state's primary triangulation stations, established more than half a century ago. It had been four years since the latitude and longitude of the brass marker had been determined.
``If there's been a change, it will probably be because the Earth has moved slightly,'' said Bill Strange of the Geodetic Survey, an agency of the National Oceanic and Atmospheric Administration (NOAA).
The dish, 10 feet in diameter, is ``your basic backyard antenna converted to our use,'' said Scott Riley, a technician with Bendix Field Engineering, the Barstow, Calif., contractor conducting the program for NOAA.
Computers aimed four widely scattered antennas - in Alaska, Massachusetts, Southern California and at Sand Point - at the quasars, puzzling starlike objects with the energy of large galaxies. The four antennas ``stared'' at the same source of radio signals for 10 minutes, and then the computer shifted the antennas to another quasar. Riley said the four antennas collected signals from 25 quasars in 72 hours of observing.
The aim was to calculate the distances between the land stations. The key to the process is the fact that the four antennas scattered across North America were at slightly differing distances from the quasars at he edge of the universe.
So even with the quasar's radio signal traveling at the speed of light, its arrival time at each Earth station varied by a fraction of a second. With that time difference and the speed of light, it is possible to calculate the distance the radio signal traveled from the time it reached the first station until it reached the second one. That distance becomes one leg of an imaginary triangle constructed between the stations.
As the ancient Greeks knew, if the length of one side and two angles of a triangle are known, the lengths of the other legs can be calculated. In a vastly oversimplified description, it becomes a matter of geometry to determine the distance between two ground stations.
``You can't do that with just one measurement,'' said Strange. ``But we get signals from many quasars in many directions. And if you do it enough times with enough different triangles you can put it all together and eventually come up with the answer.''
The process must compensate for the fact that the Earth is revolving and the solar system is moving in relation to the quasar while the antennas are aimed at the distant object.
Even determining the differences in arrival time of the quasar's signals at the various Earth stations required complicated computer processing.
``If the quasar's signal was just a big single blip, and if you have precise clocks at each station, you could time the signal's arrival at each station,'' Strange said. ``But it's not that way. It's a random, jumbled signal that makes a funny-looking zigzag line if you draw it out.''
The trick is to determine the arrival time at each station of a unique part of the jumbled signal.
``You know about what the time difference should be so you know what part of that squiggly line to look at,'' Strange said. ``So you take the signals and (in a computerized process) move them back and forth on a time line until you get a perfect match.''
The distance moved on the time line to get a match can be translated into the difference in arrival times at the stations.
It will be a month or more until it's known whether the latitude and longitude of the primary marker at Sand Point will change. Then a joint state-National Geodetic Survey program will begin to recalculate, in relation to the Sand Point marker, the positions of 220 survey markers in the state grid, making possible more accurate surveys all over the state.
Another federal project, which will begin operation in late 1992, is intended primarily to learn more about quasars, but also may provide information so precise that the diameter of the Earth can be measured to an accuracy of half an inch and will be sensitive enough to detect earth creep that might signal imminent earthquakes.
The $82 million project, known as the very long baseline array, consists of 10 radio telescope antennas, each 82 feet in diameter, scattered from the Virgin Islands in the east to Hawaii in the west. When simultaneous observations from the 10 antennas are combined, the result will be the same as from a single telescope 5,000 miles in diameter.
``We wanted to put them as far apart as we could and still be on United States territory,'' said Peter Napier, construction project manager for the National Radio Astronomy Observatory, in Socorro, N.M.
One of the antennas is nearing completion near Brewster, along the Columbia River in north-central Washington.
``The best optical (light) telescopes can achieve a resolution of about one second of arc, equivalent to seeing a dime a mile away,'' Napier said. ``With this telescope, we'll be able to resolve objects 10,000 times smaller than that.''
A primary aim of the National Science Foundation-funded project is to investigate the source of energy of the mysterious quasars.
``Quasars are hundreds of millions, or billions, of miles away and still are very bright, which indicates enormous energy,'' Napier said. ``The key question is the source of that energy.
``The best theory is that it's a giant black hole (a place of gravitational collapse where nothing, not even light, can escape) and what we detect is energy radiated as matter falls in.''
Even though the diameter of the very long baseline array will be nearly the diameter of the Earth, some scientists have even bigger dreams. They envision antennas on satellites in orbit around the sun that could provide an effective radio-telescope diameter of several hundred million miles, providing even more power to investigate the far reaches of the universe.
CALCULATING DISTANCES WITH QUASAR SIGNALS
Radio-frequency signals from quasars, among the most distant and mysterious objects in the universe, are being utilized to calculate distances on land with unprecedented accuracy.
1. QUASAR -- Radio signal from a quasar millions of light years away travels toward Earth at the speed of light.
2. STATION A -- Radio signals arrive here first because it is closer to the quasar than Station B.
3. STATION B -- A fraction of a second later the same signal arrives here.
4. DIFFERENT ARRIVAL TIMES -- In a computerized process, the quasar's signal is analyzed to determine the difference in arrival times at the two stations. The computer, comparing a segment of the quasar's unique signal as it arrives at the two stations, determines how far one signal must be ``moved'' along a time scale to make a perfect match.
5. CONSTRUCTING A TRIANGLE -- Multiplying the difference in arrival times by the speed of light gives the length of one side of an imaginary triangle. With two angles and one side known in the triangle, the distance between Station A and Station B can be calculated.