Too much redo

Dear communityI'd appreciate thoughts from anyone in the community on this subject.

First some details: Oracle 11.2.0.3  on Linux RHEl5 x86 -64.

The patch 14331813 (Excessive redo generation) has been applied.

The task:
We have a requirement to refresh data on one of our survey pages every 20 minutes.

What we do:
1.select the relevent rows and insert into a global temp table using a join to get some spatial boundary data.
2. truncate the summary table and drop all indexes including spatial
3. Insert the data from the global temp table.
4. Rebuild the indexes.

Typically 500,000 rows are involved, the task completes in seconds but it generates a lot of redo.