  The Heegaard Floer correction term ($d$-invariant) is an invariant of
rational homology 3-spheres equipped with a Spin$^c$ structure. In particular,
the correction term of 1-surgeries along knots in $S^3$ is a
($2\mathbb{Z}$-valued) knot concordance invariant $d_1$. In this paper, we
estimate $d_1$ for the $(2,q)$-cable of any knot $K$. This estimate does not
depend on the knot type of $K$. If $K$ belongs to a certain class which
contains all negative knots, then equality holds. As a corollary, we show that
the relationship between $d_1$ and the Heegaard Floer $\tau$-invariant is very
weak in general.
