  Laser frequency combs, which are composed of a series of equally-spaced
coherent frequency components, have triggered revolutionary progress for
precision spectroscopy and optical metrology. Length/distance is of fundamental
importance in both science and technology. In this work, we describe a ranging
scheme based on chirped pulse interferometry. In contrast to the traditional
spectral interferometry, the local oscillator is strongly chirped which is able
to meet the measurement pulses at arbitrary distances, and therefore the dead
zones can be removed. The distances can be precisely determined via two
measurement steps based on time-of-flight method and synthetic wavelength
interferometry, respectively. To overcome the speed limitation of the optical
spectrum analyzer, the spectrograms are stretched and detected by a fast
photodetector and oscilloscope, and consequently mapped into the time domain in
real time. The experimental results indicate that the measurement uncertainty
can be well within 2 $\upmu$m, compared with the reference distance meter. The
Allan deviation can reach 0.4 $\upmu$m at averaging time of 4 ns, 25 nm at 1
$\upmu$s, and can achieve 2 nm at 100 $\upmu$s averaging time. We also measure
a spinning disk with grooves of different depths to verify the measurement
speed, and the results show that the grooves with about 150 m/s line speed can
be clearly captured. Our method provides a unique combination of non-dead
zones, ultrafast measurement speed, high precision and accuracy, large
ambiguity range, and with only one single comb source. This system could offer
a powerful solution for the field measurements in practical applications in
future.
