  Astronomical observations of (sub)millimeter wavelength pure rotational
emission lines of the second most abundant molecule in the Universe, CO, hold
the promise of probing regions of high temperature and density in the innermost
parts of circumstellar envelopes. The rotational spectrum of vibrationally
excited CO up to $\varv = 3$ has been measured in the laboratory between 220
and 1940 GHz with relative accuracies up to $5.2 \times 10^{-9}$, corresponding
to $\sim 5$ kHz near 1 THz. The rotational constant $B$ and the quartic
distortion parameter $D$ have been determined with high accuracy and even the
sextic distortion term $H$ was determined quite well for $\varv = 1$ while
reasonable estimates of $H$ were obtained for $\varv = 2$ and 3. The present
data set allows for the prediction of accurate rest frequencies of
vibrationally excited CO well beyond 2 THz.
