A method includes receiving a first signal and updating a bitmap index responsive to the first signal. The bitmap index includes a plurality of bit strings, where a value stored in a particular location in each of the bit strings indicates whether a corresponding signal associated with a signal source has been received. Updating the bitmap index responsive to the first signal includes updating a first bit of the bitmap index and updating first metadata values stored in the bitmap index, wherein the first metadata values comprise a plurality of sort index values indicating relative ranks of the first bit string relative to other bit strings. The method also includes outputting query results based on a query, wherein the query results identify one or more signals associated with one or more bit strings of the plurality of bit strings and one or more signal sources of a plurality of signal sources, and wherein the query results are sorted according to one of the first metadata values.