 Solution

Choose a local transverse three-dimensional section of the null geodesic congruence and a future unit timelike vector $T$ on it. Orient $U$ to the future and put $E=-U\cdot T>0$. On that section define a parallel auxiliary null vector by the initial valueSince $T^2=-1$, $U^2=0$, and $U\cdot T=-E$, direct contraction gives $U\cdot N=-1$ and $N^2=-E^{-2}+E^{-2}=0$.

Extend $N$ along each generator by parallel transport, solving $U^b{\nabla }_b{N}^a=0$ with those initial values. The geodesic equation and compatibility of the Levi-Civita connection implyTherefore $N^2=0$, $U\cdot N=-1$, and ${\nabla }_{U}N=0$ hold throughout the local congruence. Smooth initial data and the transport equation give a smooth field up to the breakdown of the congruence at caustics. An arbitrary pointwise choice of $T$ away from the initial section would not automatically have this transport property.