  We measured the ground-state hyperfine splitting of trapped
${}^{113}$Cd${}^{+}$ ions to be 15199862855.02799(27) Hz with a fractional
uncertainty of $1.8\times10^{-14}$. The ions were trapped and laser-cooled in a
linear quadrupole Paul trap. The fractional frequency stability was measured to
be ${4.2} \times 10^{-13}/\sqrt{\tau} $, obtained from Ramsey fringes of high
signal-to-noise ratio and taken over a measurement time of nearly 5 hours,
which is close to the short-term stability limit estimated from the Dick
effect. Our result is consistent with previous reported values, but the
measurement precision is four times better than the best result obtained to
date.
