The two-color EDM is an ultra-precise distance measuring instrument with a precision of 0.5 to 1.0 mm for ranges between 1 and 12 km. This instrument is used to measure crustal deformation along faults and near volcanoes. Currently, we use this instrument to monitor the deformation of the Long Valley Caldera east of Yosemite, and at Parkfield, California along the San Andreas fault. To take advantage of the instrument's high precision, these measurements are made frequently, typically several times each week. In addition, we make infrequent measurements of distances at other sites in California to measure strain accumulation within the San Andreas fault zone.
This instrument is unique for laser distance measuring instruments because it uses two colors to measure the transit time of light through the atmosphere. Commercially available electronic distance measuring instruments (EDM) use one laser, usually red or infra-red, as a carrier. By modulating the laser, the instrument measures the round-trip travel time of light through the atmosphere for that particular wavelength between the active instrument and its remotely located reflector. If the index of refraction for the atmosphere is known through measuring its average temperature and pressure, then the velocity of light is known, and the distance is calculated by multiplying the measured travel time by the velocity. To be able to measure distances to a 1 mm precision over a 10 km long baseline, or 0.1 part-per-million, the average temperature and pressure along the 10 km path need to be known to better than 0.1 degree C and 1 mb. In practice, this is difficult to achieve without instrumenting an aircraft with temperature and pressure probes to obtain a profile of these quantities.
However, the two-color EDM measures the travel time of light for two wavelengths, red and blue. Because the atmosphere is dispersive, there is a difference in travel time which is a direct function of temperature and pressure. The difference in travel time is used to measure the average temperature and pressure in the atmosphere for calculating the index of refraction. With the index of refraction, the distance is computed from the travel time of one of the colors. A mechanical analog is the bi-metallic thermometer where the temperature is measured by the bending of two bonded pieces of metal having different coefficients of thermal expansion. For two-color measurements of distance to achieve a 0.1 ppm precision, pressure needs to be known to within 50 mb, and there is essentially no requirement to know the temperature. However, the partial pressure of water needs to be known to 1 mb, but this is not too difficult to achieve by measuring the relative humidity near the instrument.
During the past 15 years, Global Positioning Systems (GPS) have been developed as another method to measure crustal deformation over long baselines. This system relies on a suite of 24 satellites and many receivers on the Earth's surface that are used to measure the location of each receiver. The precision of horizontal, relative positions with GPS is approximately 3 mm for baselines in excess of a couple kilometers. The advantage of GPS is its relative ease of operation in the field since receiver sites need not be intervisible and the receivers are relatively simple to use. Although more precise than GPS at ranges less than 10 km, the two-color EDM has some disadvantages because its range is limited (due to scattering of the short-wavelength, blue, through the atmosphere), the sites must be intervisible, and the crew operating the system must be highly skilled. Although the two-color EDM was available commercially for a few years in the early 1980s, only a few were made and the cost was high, $250K. In contrast, GPS has proliferated and the receiver cost for a high-precision unit is less than $15K.