How to make the header of an ASP.NET GridView fixed?

I have a gridview that shows me a list of data. When scrolling with the vertical bar of the GridView the header is not fixed so the display is lost when I scroll. How can the header be fixed?

<asp:GridView ID="GV_UNO" runat="server" ShowHeader = "true" AutoGenerateColumns="false" Font-Size="X-Small" >
<Columns>
    <asp:BoundField HeaderStyle-Width="47px" DataField="Dato1" HeaderText="Dato1" SortExpression="Dato1"  />
    <asp:BoundField HeaderStyle-Width="65px" DataField="Dato2" HeaderText="Dato2" SortExpression="Dato2" >
</Columns>
</asp:GridView>