A Series Look at the Earth

Abstract

Like a parent marking a child's growth, geodesy measures the earth. monitors the slip along the San Andreas fault, the uplift of the Palmdale Bulge, even the sinking of the ground beneath the White House. Field surveying and mapping are the common tools of geodesy. A network of permanent markers placed throughout the United States fixes vertical and horizontal reference points for geodesists. Computers, lasers and microwaves have lengthened the geodesist's arm and steadied his aim. Even so, such manual techniques are slow and, when an earthquake strikes or a volcano erupts, human safety preempts the need for a complete, continuous survey record. In 1974, geodesy gained another tool. Called ARIES, for Astronomical Radio Interferometric Earth Surveying, it determines distances by comparing the radio signals from quasars received by two separate antennas (SN: 8/31/74, p. 136). Unlike conventional field methods, it is an allweather system and can measure changes even across intercontinental distances. is so precise, it can measure an east to west distance of 500 kilometers within two centimeters. The unwieldy antennas, however, make the system expensive, slow and less than easily mobile. Now, Peter F. MacDoran of Jet Propulsion Laboratory in Pasadena, Calif., has another geodetic gimmick up his sleeve. Begotten by ARIES, it is called SERIES Satellite Emission Radio Interferometric Earth Surveying. So far, NASA'S Office for Space and Terrestrial Applications, which also funds ARIES, has given JPL enough money for an initial design study that may lead to a feasibility demonstration next fall. If the system works, says MacDoran, who is also ARIES project manager, It will be a revolution in earth surveying. could have the impact of learning to measure angles. Like ARIES, SERIES will depend on radio interferometry the technique of comparing radio signals received at two locations. Just as the noise of an explosion reaches observers at different locations at different times, a radio signal from a single source reaches two antennas at different times. In the ARIES system, the signal's arrival is timed by very precise atomic clocks at each antenna. The signals are recorded on tape at each location and combined later in a computer. The combined signals will produce a pattern where they are out of phase. The signal delay, and thus the distance, is computed from the fringe pattern. Though similar in principle, ARIES and SERIES differ significantly. For example, ARIES uses quasars and Seyfert galaxies for a radio source; SERIES will use signals from military satellites. The satellites are part of a Department of Defense system called Navstar-GPs (for Global Positioning System) (SN: 7/2/77, p. 6). The system will consist of 24 satellites; the fourth was recently launched. When all the satellites are operating in 1984, no fewer than four will be visible above the horizon at any given time. Navstar-GPs was originally designed solely for defense purposes. Coded radio signals from the satellites, picked up by U.S. ships or aircraft, for example, will give their precise three-dimensional global position. But whether it intended to or not, says MacDoran, the DOD has also provided SERIES with a dependable source for radio interferometry, one that is much closer and therefore about 100,000 times brighter than quasars. And SERIES need not know the code, stresses MacDoran. didn't know the 'code' from quasars, either. We just used them. It's like throwing Brer Rabbit in the briar patch: We're used to it, we grew up with it. And a different signal source means different receivers. In order to pick up the quasars' weak signals, ARIES requires large, very stable antennas. For example, to measure horizontal or vertical movement across the San Andreas fault, ARIES uses a fixed 64-meter dish antenna at Goldstone, Calif., and a transportable (as opposed to easily mobile) nine-meter dish antenna at Pearblossom, Calif. Transport-