WPF Datagrid Sorting on grouped rows

Hi...
I have a WPF Datagrid which I've grouped by a column ProductName. I want to perform sorting on UploadDate which is a datefield. The issue here is on clicking on sort I want the data to be sorted on UploadDate within the groups, by the datagrid sorts the data based on the ProductName, on which I've grouped the rows. Due to this the groups are re-ordered.
 
How can I stop the default sorting on group name????

Example
 
I have a datagrid with 2 groups and I've grouped them on ProductName
Group 1 : BProduct ---> This is the Product Name
There are 2 rows in this group
04/31/2012
01/31/2012
 
Group 2 : AProduct ---> This is the Product Name
There are 2 rows in this group also.
04/31/2012
01/31/2012
 
On Sorting I want the data to sort within groups, the result I expect is
 
Group 1 : BProduct
There are 2 rows in this group
01/31/2012
04/31/2012
 
Group 2 : AProduct
There are 2 rows in this group also.
01/31/2012
04/31/2012
 
But the datagrid sorts on group name as well....the actual result is
 
Group 1 : AProduct
There are 2 rows in this group
01/31/2012
04/31/2012
 
Group 2 : BProduct
There are 2 rows in this group also.
01/31/2012
04/31/2012