  The Cadmium Zinc Telluride Imager (CZTI) aboard AstroSat has good sensitivity
to Gamma Ray Bursts (GRBs), with close to 600 detections including about 50
discoveries undetected by other missions. However, CZTI was not designed to be
a GRB monitor and lacks localisation capabilities. We introduce a new method of
localising GRBs using "shadows" cast on the CZTI detector plane due to
absorption and scattering by satellite components and instruments. Comparing
the observed distribution of counts on the detector plane with simulated
distributions with the AstroSat Mass Model, we can localise GRBs in the sky.
Our localisation uncertainty is defined by a two-component model, with a narrow
Gaussian component that has close to 50% probability of containing the source,
and the remaining spread over a broader Gaussian component with an 11.3 times
higher $\sigma$. The width ($\sigma$) of the Gaussian components scales
inversely with source counts. We test this model by applying the method to GRBs
with known positions and find good agreement between the model and
observations. This new ability expands the utility of CZTI in the study of GRBs
and other rapid high-energy transients.
