How to create a radar composite?

Hi all,

as I can not find any open data for a nordic radar composite, I want to create one myself. But, I have never done this and might need some help from someone.

I have open data from each country (Finland, Sweden, Norway) as single radars and composites.

Is it possible to create a composite by merging the individual composites from each country, or must I merge the raw radar data from each country?