Perhaps you should try some spelling variations:
"ground penetrating radars" » "ground penetrating radar", "ground penetrating bars", "ground penetrating beams"
"cloud penetration radar" » "cloud generation radar", "cloud federation radar", "cloud registration radar", "cloud penetrating radar", "cloud penetration raman", "cloud penetration der"
"cloud penetration rate" » "cloud generation rate", "cloud federation rate", "cloud registration rate", "cloud penetration a", "cloud penetration chapter", "cloud penetration tester"

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (((("cloud penetration radar") OR ("cloud penetration rate"))) OR ("ground penetrating radars")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"cloud penetration radar\") OR (\"cloud penetration rate\"))) or (\"ground penetrating radars\"))".
• Removing quotes may allow a broader search: (((( cloud penetration radar ) OR ( cloud penetration rate ))) or ( ground penetrating radars )).
• Adding a wildcard symbol may retrieve word variants: (((("cloud penetration radar") OR ("cloud penetration rate"))) or ("ground penetrating radars"))*.