To support the processing of spatial window queries efficiently in a non-flat wireless data broadcasting system, we propose a Two-Tier Spatial Index (TTSI) that uses a two tier data space to distinguish hot and regular data items. Unlike an existing index which repeats regular data items located near hot items at the same time as the hot data items during the broadcast cycle, TTSI makes it possible to repeat only hot data items during a cycle. Simulations show that the proposed TTSI outperforms the existing scheme with respect to access time and energy consumption.