  Absolute distance measurement (ADM) over a long range has been studied
intensely over the last several decades, due to its important applications in
large-scale manufacturing and outer space explorations [1-5]. Traditional
absolute distance measurements utilize detection of time-of-flight information,
detection of phase shift, or a combination of the two [6-17]. In this paper, we
present a novel scheme for high-precision ADM over a long range based on
frequency detection by using two optoelectronic oscillators (OEO) to convert
distance information to frequency information. By taking advantage of
accumulative magnification theory, the absolute error of the measured distance
is magnified by about 2*10E5 times, which makes the precision of the measured
distance significantly improved. In our experiments, the maximum error is 1.5
um at the emulated ~6 km distance, including the drift error of about 1 um in
the air path due to the change in environmental conditions. In addition, the
measurable distance using this scheme could be further extended. The highest
relative measurement precision is 2*10E10 in our current system while the
actual relative measurement precision of our experimental system is limited by
the variation of atmospheric conditions and is about 4*10E9.
