  Laser absorption Doppler velocimeters use a crossed-beam configuration to
cancel error due to laser frequency drift and absorption model uncertainty.
This configuration complicates the spatial interpretation of the measurement
since the two beams sample different volumes of gas. Here, we achieve
single-beam velocimetry with a portable dual comb spectrometer (DCS) with high
frequency accuracy and stability enabled by GPS-referencing, and a new
high-temperature water vapor absorption database. We measure the inlet flow in
a supersonic ramjet engine and demonstrate single-beam measurements that are on
average within 19 m/s of concurrent crossed-beam measurements. We estimate that
the DCS and the new database contribute 1.6 and 13 m/s to this difference
respectively.
