Perhaps you should try some spelling variations:
"ground penetrating radars" » "ground penetrating radar", "ground penetrating beams", "ground penetrating bodies"
"cloud penetration radar" » "cloud generation radar", "cloud federation radar", "cloud registration radar", "cloud penetrating radar", "cloud penetration raman", "cloud penetration rate"
"ground penetrating bars" » "ground penetrating beams", "ground penetrating nats", "ground penetrating brain"

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 ((("ground penetrating radars") OR ("ground penetrating bars")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"cloud penetration radar\") or (((\"ground penetrating radars\") OR (\"ground penetrating bars\"))))".
• Removing quotes may allow a broader search: (( cloud penetration radar ) or ((( ground penetrating radars ) OR ( ground penetrating bars )))).
• Adding a wildcard symbol may retrieve word variants: (("cloud penetration radar") or ((("ground penetrating radars") OR ("ground penetrating bars"))))*.