  The calibration and traceability of high resolution frequency modulated
continuous wave (FMCW) ladar sources is a requirement for their use in length
and volume metrology. We report the calibration of a FMCW ladar length
measurement system by use of spectroscopy of molecular frequency references HCN
(C-band) or CO (L-band) to calibrate the chirp rate of the FMCW source.
Propagating the stated uncertainties from the molecular calibrations provided
by NIST and measurement errors provides an estimated uncertainty of a few ppm
for the FMCW system. As a test of this calibration, a displacement measurement
interferometer with a laser wavelength close to that of our FMCW system was
built to make comparisons of the relative precision and accuracy. The
comparisons performed show ppm agreement which is within the combined estimated
uncertainties of the FMCW system and interferometer.
