  We review our progress in developing a frequency reference with singly
ionized lutetium and give estimates of the levels of inaccuracy we expect to
achieve in the near future with both the $^1S_0\leftrightarrow{}^3D_1$ and
$^1S_0\leftrightarrow{}^3D_2$ transitions. Based on established experimental
results, we show that inaccuracies at the low $10^{-19}$ level are readily
achievable for the $^1S_0\leftrightarrow{}^3D_1$ transition, and the frequency
ratio between the two transitions is limited almost entirely by the BBR shift.
We argue that the frequency ratio measured within the one apparatus provides a
well-defined metric to compare and establish the performance of remotely
located systems. For the measurement of an in situ frequency ratio,
relativistic shifts drop out and both transitions experience the same
electromagnetic environment. Consequently, the uncertainty budget for the ratio
is practically identical to the uncertainty budgets for the individual
transitions. If the ratios for two or more systems disagree we can be certain
at least one of the clock assessments is incorrect. If they agree, subsequent
comparisons on one transition would only differ by relativistic effects. Since
motional effects are easily assessed and typically small for a heavy ion, only
the differential gravitational red-shift will significantly contribute and this
can be confirmed by comparison on the second transition.
